A Question For The Lone Ranger

[Shelli]

Dear Lone Ranger;

I'm presently healing from a cat fight that I got caught in the middle of a few nights ago which prompted me to question why wounds itch when they're healing.

Do you know, and if so, could you please tell me how the body works in regards to that?

Signed;
Itchy in Springfield :itchyscratchy:

[The Lone Ranger]

Hi Shelli!

You might be surprised to know that there is no universally agreed-upon explanation for why healing wounds itch. The most widely-accepted explanation has to do with secretion of histamine by damaged tissues.


When you're wounded, mast cells in the damaged tissue release histamine. Histamine has a number of effects. It makes nearby blood vessels dilate and it makes capillaries become more porous. The result is that more blood flows to the site of the injury, causing reddening, heating and swelling. Histamine also stimulates nerve endings, causing pain.

That much is clear.


It's commonly thought that as the wound heals, low levels of histamine secretion by mast cells and basophils (basophils are a type of white blood cell) cause low-level excitation of pain receptors -- and that's why the wound itches. That would explain why antihistamines are often effective in relieving itching. Since they block production of histamine, they prevent the stimulation of nerve endings by histamine that seems to cause itching. (If histamine is directly applied to the skin, it causes an itching sensation, which lends support to the hypothesis that it's continued low-level histamine secretion that causes a healing wound to itch.)


Hope that helps!

Cheers,

Michael

[LadyShea]

Interesting! Our bodies are so fucking cool.

[Shelli]

aHA! Histamines.. that makes sense. :yup:

Thanks, TLR. :thankee:

* Shelli gets up to take an antihistamine :giggle:

[Stormlight]

Bollocks. It itches because angels lick the wound. :unangel:

[Shelli]

:giggle:

[Watser?]

Bollocks. It itches because angels lick the wound. :unangel:

Now I have an image of angels licking bollocks :P

[Stormlight]

:giggles:

[viscousmemories]

I was wondering why the doctor prescribed Benadryl when I had that mysterious rash on my arms recently.

[livius drusus]

That's fascinating, TLR. This should become the masked man's version of the Auntie Unmentionables thread. :thumbup:

[Shelli]

That's fascinating, TLR. This should become the masked man's version of the Auntie Unmentionables thread. :thumbup::yeahthat:

[Crumb]

And he should totally use the Unmentionables to answer the questions! :eager:

[Uthgar the Brazen]

Dear The Lone Ranger,

It hurts when I do this.

Sincerely,

Admiral Fulton J Boondesvill XIV, Esq, Mrs

[JoeP]

Dear Lone Ranger:

Talking about wounds and histamine making blood vessels dilate, what's the reason for inflammation in general? It seems like the benefits of getting more blood to the wound, joint, or diseased area are outweighed both by the psychological impact of the pain and the long-term (oxidant?) damage caused by chronic inflammation.

Yours,
Inflamed* of Johannesburg

* Metaphorically only

[livius drusus]

:immolate:

[JoeP]

:gooduse:

And btw, the Wikipedia page on inflammation (http://en.wikipedia.org/wiki/Inflammation) has some truly lovely photos. You should not visit it.

[The Lone Ranger]

Dear Lone Ranger:

Talking about wounds and histamine making blood vessels dilate, what's the reason for inflammation in general? It seems like the benefits of getting more blood to the wound, joint, or diseased area are outweighed both by the psychological impact of the pain and the long-term (oxidant?) damage caused by chronic inflammation.

Yours,
Inflamed* of Johannesburg

* Metaphorically only


Unpleasant as inflammation is, we'd have rather shorter life expectancies if it weren't for inflammation.

Inflammation is all about increasing blood flow to the site of an injury. This has the immediate effect of causing it to bleed. That's absolutely important. A wound, especially a puncture wound, provides an ideal means of entry into the body by bacteria, viruses and parasites. The initial bleeding of a wound helps to clean it out and drastically reduces the likelihood of bacteria, etc. getting into the general circulation, where they could do tremendous damage.

Because blood flows into the site of injury faster than it flows out, and because the capillaries in the injury site become porous and so leak fluid, there is swelling. It actually works rather well to isolate the site of injury, making it very difficult for bacteria, etc. to get out of the injury site and into the general circulation.

There are several other important components to the inflammatory response. The increased flow of blood to the area of injury brings in complement proteins and antibodies that help fight infection. The increased blood flow also brings in neutrophils and other white blood cells that help to fight infection. The increased porosity of the capillaries makes it easier for macrophages and other specialized white blood cells to move into the injury site, where they'll destroy bacteria and damaged or dead body cells.

The release of histamines and other chemicals by affected tissues not only stimulates inflammation, but also helps to attract white blood cells to the injury site.

Another reason inflammation is a good thing is that the increased flow of blood to the injury site causes the temperature of the area to rise. (The blood coming from the interior of the body is warmer than is blood in the outer portions of the body, so increasing blood flow to an injury site will cause it to heat up.) Increasing the temperature by just a few degrees increases the metabolic rate of (surviving) cells, speeding the repair of injured tissues and increasing the efficiency with which the various white blood cells can function to dispatch invaders. Also, many bacteria are quite temperature-sensitive, and raising the temperature of damaged tissues by just a few degrees can seriously slow down bacterial reproduction, giving the body's defenses a better chance of fighting off the infection.


The fact that inflamed tissue is so painful is probably adaptive, too. The very fact that inflammation is so painful encourages us not to put any pressure on an injury site or to otherwise use it. That reduces the chance of further injury, speeds healing time, and reduces the chance that pressure on the injury site might force pathogens into the general circulation.



So, while the inflammatory response is most-definitely unpleasant, it's definitely a good thing.

Cheers,

Michael

[Shelli]

Wow! That is so cool! :joecool2::joecool2::joecool2:

Thanks, TLR. :=)

[D. Scarlatti]

This should become the masked man's version of the Auntie Unmentionables thread.

"Ask Auntie Histamine."

[livius drusus]

:rofl: Now that is a quality pun.

[The Lone Ranger]

The subject of nut allergies came up in chat yesterday, and it occurred to me that a discussion of anaphylaxis might be in order.


We discussed above some of the components of an immune response. Any chemical that triggers an immune response (and in particular, production of disease-fighting proteins called antibodies) is an antigen.

You're born with innate immunity; that is, from the time of your birth, your immune system "knows" how to defend you against some diseases. For instance, the blood proteins that determine your blood type (ABO) help protect you against some diseases. People who are Blood Type A or O have greater resistance to the bubonic plague than do people who are Blood Type B, to take an example. This is probably why it's relatively rare to find someone of northern or western European descent who has Type B blood. The bubonic plague killed millions of Europeans during the 13th and 14th centuries, but especially people with Blood Type B.

Indeed, it has been suggested that one of the reasons the plague outbreaks had virtually disappeared by the 15th century was because most of the people in Europe with Type B blood had been killed off, making it more difficult for the disease to spread.

It has been shown that people with Blood Type O are more resistant to SARS (severe acute respiratory syndrome) than are people of other blood types.

If you have Type A blood, it tends to clot more readily than blood of Types B or O. On the other hand, the very fact that it tends to be more prone to clotting means that people with Type A blood are at higher risk of blood clots forming inside the blood vessels and blocking blood flow, causing a myocardial infarction (heart attack).



Anyway, the point of all this is that you're born with some ability to defend yourself against disease. (There are over 300 known blood antigens; at least some of them help protect against disease.)

Most immunity, however, is acquired. Your immune system has to "learn" to protect you against most antigens. When your system encounters such an antigen for the first time, it takes a relatively long time to mount a full immune response, because the specialized white blood cells (lymphocytes) that produce protective antibodies (B lymphocytes) and that initiate and coordinate the immune response (T lymphocytes) must first go through a process in which they "learn" to recognize the antigen and produce antibodies that will be effective against it.

After that first encounter, the presence of antibodies circulating in the blood, plus the presence of "memory cells" (B- and T-lymphocytes that can defend against that specific antigen) ensures that the next time the antigen is encountered, the immune response will be quicker and (hopefully) more efficient.




Now, of course, the immune system doesn't always work perfectly. Sometimes, the immune system responds to substances that are perfectly harmless. When an immune response is mounted against a substance that is not harmful to the body, a substance that does not trigger an immune response in a person with a normally-functioning immune system, that is an allergy.

Normally, an allergy is little more than an annoyance. You remember that part of a normal immune reaction is release of histamine, which causes dilation of blood vessels. As blood vessels in the affected area become dilated and more porous, more blood flows to the area, and the increased porosity of capillaries in the area means that fluid leaks out of the blood and into the affected tissues. (This accumulation of fluid in body tissues is edema, and is one of the principle causes of swelling of the affected tissues.)

Histamine causes the muscles surrounding the bronchial tubes to contract, and so these tubes narrow. Naturally, this makes breathing more difficult, but if you're breathing something in that irritates the respiratory passages, the narrowing of the bronchial tubes means that more of the air moving through the respiratory system is in direct contact with the mucous membranes lining the airways. This increases the efficiency with which smoke, bacteria, and other irritating or harmful substances are removed from the air before it reaches the delicate and easily-damaged alveoli in the lungs, where gas exchange occurs.


So, this explains what happens if you're allergic to something. If you're allergic to some substance and it comes into contact with your skin, then release of histamine and other chemicals at the site of contact will cause an inflammatory response. The increased blood flow to the contact site will cause redness and swelling (edema) of the skin, not to mention itching.

If you breathe in a substance to which you're allergic (pollen, say), then the histamine reaction will cause narrowing of the respiratory passages and therefore difficult breathing. It's also likely to trigger increased mucous production, which will have the effect of trapping more of the offending substance before it can reach the lungs -- but will also cause a runny or stuffy nose. It's also likely to trigger sneezing and coughing, both of which are mechanisms that help clear the respiratory passages.


It's worth keeping in mind that this sort of thing will probably not occur on your first exposure to the substance in question. But after the first exposure, your immune system is sensitized to the substance, and if it is hypersensitive to it, you'll suffer from an allergy. (Lots of people make the mistake of assuming that they're insensitive to the toxins secreted by Poison Ivy because there's little or no reaction the first time they touch the plant. But just because there's no reaction the first time doesn't mean there won't be a reaction on subsequent exposure.)



It seems to be the case that certain allergies are becoming more common. Recent research suggests that one reason this is true -- paradoxical though it might seem at first -- is because we expend too much effort to keep our environments clean. It appears that if a young person's immune system is not sufficiently challenged, it doesn't "learn" to properly distinguish between harmful and harmless substances, and so is more likely to show an allergic reaction.

So, it seems that trying too hard to make sure your kids are never exposed to dirt and germs can be counterproductive, because it increases the likelihood that the immune system will be unable to distinguish between harmful and harmless substances, and so increases the likelihood of allergies. Letting your kids get dirty and grimy -- up to a point, anyway -- is probably good for them.




This leads us to the subject of anaphylaxis. In the event of a severely malfunctioning immune system, exposure to a substance that triggers an immune response (even if the substance is perfectly harmless) can cause a systemwide release of histamine. Remember that histamine causes dilation of blood vessels at the site of an injury or infection and constriction of respiratory passages. Now imagine what would happen if histamine were simulataneously released throughout the body. It will kill you PDQ.


A hyperallergic reaction leading to a systemwide response is anaphylaxis. A severe allergic reaction to nuts (for example) can cause anaphylaxis, and anaphylaxis can be fatal if the victim doesn't receive prompt treatment. (Incidentally, there are different kinds of nut allergies. People who are allergic to peanuts aren't necessarily allergic to "tree nuts" such as Brazil nuts, and vice-versa.)

Constriction of the respiratory passages will cause someone experiencing anaphylaxis to have difficulty in breathing. In severe cases, the respiratory passages can become completely blocked, causing the person to suffocate.

Dilation of blood vessels in the skin will cause redness and swelling in the skin (urticaria or hives), especially the skin of the face, lips, neck and throat (angioedema). Angioedema can be life-threatening, because rapid swelling of the mucous membranes of the throat can block the air passages and cause suffocation.

But perhaps the most dangerous thing about anaphylaxis is that dilation of blood vessels throughout the body causes a sudden, drastic drop in blood pressure (anaphylactic shock). This very low blood pressure (hypotension) is life-threatening because it means the brain may not receive enough blood -- and therefore not enough oxygen -- to survive.

This, incidentally, is why it's important to keep a person's head lower than his or her heart if (s)he is in shock. It's vital to ensure that as much blood flows to the brain as possible.



Anaphylactic shock can be treated by rapid administration of epinephrine (adrenaline). It causes relaxation of the muscles surrounding the respiratory tubes, and so eases the person's breathing. If the anaphylaxis has caused the person to stop breathing, epinephrine can at lease keep him or her breathing long-enough for the hypotension to be treated. That's why people who are hypersensitive to such things as bee stings or nuts sometimes carry epinephrine with them. (It won't do much for the hypotension, but it'll hopefully keep the person alive long-enough for medical help to arrive.)


Cheers,

Michael

[Ensign Steve]

It's worth keeping in mind that this sort of thing will probably not occur on your first exposure to the substance in question. But after the first exposure, your immune system is sensitized to the substance, and if it is hypersensitive to it, you'll suffer from an allergy. (Lots of people make the mistake of assuming that they're insensitive to the toxins secreted by Poison Ivy because there's little or no reaction the first time they touch the plant. But just because there's no reaction the first time doesn't mean there won't be a reaction on subsequent exposure.)
Is that why I didn't have any allergies for an entire year after I moved to Georgia? Oh what a blissful year that was.

[LadyShea]

Thanks TLR, very informative :)

Incidentally, I took some flack for not being Mrs. Clean regarding dog hair and etc. when we brought the baby home, and then for letting him run around outside barefoot and play in the yard, and taking him swimming in the river and ocean at a few months old. I told them it was good for him to be exposed to such things...good thing I was right ;)

[JoeP]

As always - or perhaps even more so - there's a beautiful flow to your article, TLR.

You mentioned keeping a shock victim's head lower than the body to cope with blood pressure. How does the human body's circulation cope with gravity - are there particular differences compared to quadrupedal mammals, for example? What happens in zero gravity?

[The Lone Ranger]

Perhaps a brief discussion of how the mammalian circulatory system is laid out would be helpful? The heart, of course, is the muscular pump that is the centerpiece of the circulatory system. Arteries are blood vessels that carry blood away from the heart and veins are blood vessels that carry blood toward the heart. As arteries carry blood away from the heart, they divide into smaller vessels, known as arterioles. Ultimately, the arterioles divided into capillaries, which are the smallest of blood vessels. The capillary walls are only a single cell layer thick, and so substances such oxygen, carbon dioxide, nutrients and metabolic wastes can move across capillary walls from the blood and into surrounding tissues, or vice versa.

The microscopic capillaries join to form venules, which carry blood back toward the heart. The venules join to form veins, and almost all of the veins in the body ultimately empty into one of two large veins, the superior vena cava or the inferior vena cava. The SVC and the IVC empty into the heart.


The heart consists almost entirely of muscle tissue and is divided into four chambers – 2 atria and 2 ventricles. The atria receive blood from the veins and then contract, pumping it into the ventricles. The ventricles then contract and send blood out of the heart, into the great arteries. Four one-way valves in the heart (2 atrioventricular valves and 2 semilunar valves) prevent backflow of blood in the heart, ensuring that the blood flows in only one direction. (The distinctive “lub-dub” sound you hear when listening to someone’s heartbeat is the sound of the valves closing as the atria and ventricles contract.)


Because of the heart’s four chambers, mammals can (and do) have two completely different circulations. Blood in the systemic circuit travels to and from the body under high pressure. Blood in the pulmonary circuit travels to and from the lungs under lower pressure.

The delicate tissues of the lungs cannot tolerate high-pressure blood flow, and this is one reason why high blood pressure (hypertension) is dangerous. Pulmonary hypertension can cause fluid to leak out of the blood and into the lungs. This is pulmonary edema, and it can kill very quickly, since the person can essentially drown in his own body fluids.



The normal flow of blood in mammals works like this:

Blood from the systemic circuit enters the right atrium of the heart through the superior vena cava and the inferior vena cava. This blood, having come from the body tissues, is low in oxygen and high in CO2. When the atria contract, the contraction of the right atrium forces blood through the (right atrioventricular valve – also known as the tricuspid valve) and into the right ventricle.

When the ventricles contract, the contraction of the right ventricle puts the blood inside under pressure. This pressure forces the tricuspid valve shut and prevents blood from flowing back into the right atrium. The blood is therefore forced out of the right atrium through the pulmonary semilunar valve and into the pulmonary trunk.

The pulmonary trunk is the beginning of the pulmonary circuit. It splits almost immediately into the pulmonary arteries, which carry the blood the short distance to the lungs. As soon as the ventricles of the heart relax, the backpressure causes the pulmonary semilunar valve to slam shut, preventing blood from flowing back into the heart and ensuring that it goes to the lungs instead.

The pulmonary arteries divide and subdivide into capillaries in the lungs. There, CO2 leaves the blood and enters the lungs to be exhaled as O2 enters the blood from the air in the lungs, ultimately to be transported to body tissues.

The capillaries join to form venules, which join to form the pulmonary veins, which transport the now oxygen-rich blood back to the heart, entering into the left atrium. (In a non-pregnant adult, the pulmonary arteries are the only arteries that carry deoxygenated blood, and the pulmonary veins are the only veins that carry oxygenated blood.)

When the atria of the heart contract, blood from the left atrium is forced through the left atrioventricular valve (also known as the bicuspid valve or the mitral valve) and into the left ventricle. When the ventricles contract, the increased pressure in the left ventricle forces the bicuspid valve shut and so blood cannot flow back into the left atrium; instead, it exits the left ventricle through the aortic semilunar valve and enters the aorta, the main artery of the body. When the ventricles relax, the backpressure forces the aortic semilunar valve shut and prevents backflow, so the blood flows from the aorta into the smaller arteries that branch off it, and ultimately to the tissues of the body.


http://cache.eb.com/eb/image?id=85505&rendTypeId=34
[I]A cutaway view of the human heart. The right atrioventricular valve is
between the right atrium and the right ventricle. The left atrioventricular
valve is between the left atrium and the left ventricle. The pulmonary
semilunar valve is visible between the right ventricle and the pulmonary trunk.
The aortic semilunar valve is not visible in this diagram.



It’s important to keep in mind that the myocardium (cardiac muscle tissue) that makes up the left side of the heart is much thicker and stronger than is the myocardium of the right side of the heart. This is why the blood exiting the right ventricle and going to the lungs through the pulmonary arteries is under much lower pressure than is the blood exiting the left ventricle and going to the body through the aorta.

It’s our 4-chambered hearts that allow two completely separate circulations – a low-pressure pulmonary circulation to and from the lungs and a high-pressure systemic circulation to and from the body – that give mammals so many advantages over amphibians and most reptiles. The high pressure of the systemic circuit means that oxygen is delivered quickly and efficiently to body tissues, allowing us to maintain high metabolic rates for extended periods of time. If a turtle, for example (which has a 3-chambered heart, and therefore cannot isolate the pulmonary and systemic circulations), had a high-enough blood pressure to deliver oxygen to its tissues as quickly and efficiently as a mammal can, that same pressure would seriously damage the animal’s lungs.

Birds have 4-chambered hearts too, by the way, so they too enjoy the advantages that come with having separate pulmonary and systemic circulations. Birds and mammals are descended from different reptile lineages however, and so birds’ circulatory systems are laid out somewhat differently from ours.



Now let’s consider the difference between arteries and veins. Blood flows through arteries because of the pressure exerted by the heart. That pressure has more or less completely dissipated by the time the blood exits capillaries and enters venules, so the heart does not pump blood through the veins.

When the heart contracts (contraction of heart muscle is known as systole), a “slug” of blood is forced out and into the arteries. Because arteries must be able to resist the pressure exerted by the heart (the pressure of the blood in the arteries when the heart muscle is contracted is the systolic pressure), the walls of arteries are thick, elastic, and muscular. When the heart is in systole, the “slug” of blood that enters the arteries causes them to expand, and this expansion you can feel where an artery is close to the surface of the skin is your pulse.

When the heart muscle is relaxed between contractions, it is in diastole. When the heart relaxes, the elastic walls of the arteries rebound and squeeze the blood inside, keeping it moving between heartbeats. (The pressure exerted by the blood in the arteries when the heart is in diastole is the diastolic pressure.)



Blood moves through the veins for two reasons. First, gravity will tend to draw blood through any veins that are elevated above the heart. In the case of veins that are below the heart, contraction of skeletal muscles compresses the veins and moves blood through them. (Veins have one-way valves in them, preventing backflow.)

Because the blood in the veins is typically under much lower pressure than the blood in the arteries, the veins are generally much thinner-walled, weaker, and less elastic than are arteries of the same size.



Because flow of venous blood depends upon skeletal muscle contractions, blood tends to pool in the lower extremities if you remain immobile for an extended period of time. That’s one reason why a bedridden person often requires the help of a physical therapist who flexes and extends the patient’s limbs, to ensure continued blood flow.





We humans are bipeds, which puts some stress upon the circulatory system that few quadrupeds have to face. In most quadrupeds, few regions of the body are above the level of the heart. Consequently, the heart has to do little or no work against gravity in order to pump blood to the brain. The situation is very different in humans, since the heart must work against gravity to get blood to the brain. Our erect postures also mean that the human circulatory system has to deal with an increased tendency for blood to pool in the legs.


Consider: In a standing human of average height and physical condition, the mean arterial pressure in the brachial artery of the arm is about 100 mm Hg (that is, the pressure would be sufficient to pump a column of mercury to a height of 100 millimeters). By the time the blood has reached the head, the mean arterial pressure is only 60 – 75 mm Hg. In the head, pressure in the veins is 0, because the blood is “falling” toward the heart.

In the feet, mean arterial pressure is 180 – 200 mm Hg due to the effects of gravity, and venous pressure is 85 – 90 mm Hg. If the person does not move for an extended period of time, blood will begin to pool in the legs. (This is one reason why people with impaired circulation often develop swelling of the legs and feet.)



If your brain is deprived of blood for just a few seconds, you will lose consciousness. We do have mechanisms that serve to increase blood flow to the brain, as it turns out, and that’s a good thing. Were it not for these mechanisms, pooling of blood in the legs would lead to a decrease in blood flow to the brain sufficient to cause you to lose consciousness after just a few minutes of standing at attention.

The fact that soldiers rarely faint after standing at attention for 5 minutes or so means that we must have some mechanisms to compensate for the reduced flow of blood to the brain when we’re standing, compared to when we’re lying down. (When you’re lying down, the heart and the brain are at the same level and so the heart doesn’t have to work against gravity to pump blood. Also, the fact that your heart and your legs are at the same level when you’re lying down means that pooling of blood in the lower extremities is much less of a problem.)



Strange as it might seem at first, snakes illustrate the circulatory problems associated with an erect posture quite well. Snakes that spend a lot of time climbing trees have the same sorts of circulatory problems that humans do; they must pump blood to their brains against gravity and they must prevent blood from pooling in their lower bodies due to gravity. Arboreal snakes tend to have relatively large hearts, and their hearts are typically located close to the head, making it easier to pump blood to the brain. These snakes tend to have thin bodies with thick, tight-fitting skins. The tight-fitting skin acts just like the pressure suit worn by a fighter pilot. It squeezes the body and therefore the blood inside, and the pressure exerted by the skin prevents blood from pooling in the lower part of the body.

(When someone is suffering from severe internal bleeding and/or serious loss of blood, paramedics will often put a pair of pressure pants on him or her. When the pants are inflated, the pressure they exert artificially raises the person’s blood pressure, hopefully preventing the person from losing consciousness – or worse, having a stroke.)


Terrestrial snakes don’t have to worry so much about gravity making it hard to pump blood to their brains or causing blood to pool in the lower body. As such, their hearts tend to be located further down the body.

For sea snakes, gravity isn’t a concern. Plus, they’re almost always surrounded by water, and the pressure exerted by the water serves the same purpose as the tight skin of an arboreal snake – it prevents pooling of blood. Unsurprisingly, a sea snake’s heart is usually located almost exactly in the center of its body.




But how do our circulatory systems cope with the sudden changes of blood pressure that occur when we stand up or lie down? Embedded in the atria of the heart, the aorta, the vena cava and the carotid arteries of the neck are specialized receptor cells called baroceptors. These receptors are pressure-sensitive; specifically, they monitor the blood pressure.

If you’re lying down and you sit up or stand up suddenly, the pressure of blood flowing to the brain will suddenly drop. When baroceptors in the carotid arteries detect a sudden drop in blood pressure, they stimulate the heart to beat faster and more forcefully, so more blood is delivered to the brain.

Of course, this is not an instantaneous process. If you’re lying down and you leap up suddenly, the heart may not be able to adjust to the sudden change in blood pressure quickly enough, resulting in an interruption in blood flow to the brain. If that happens, you’ll experience a syncopal episode – that is, you’ll faint.


In the opposite case, if you lean over so that your head is lower than your heart, blood will tend to accumulate in the head and blood pressure in the head will rise. (You can easily see how red someone’s face turns when (s)he is doing a handstand.) When the baroceptors detect the sudden rise in blood pressure, the heart is stimulated to beat less forcefully, and constriction of blood vessels in the neck reduces blood flow to the head, lowering the risk of a cerebral hemorrhage.





There are a few quadrupeds that have to deal with the same sorts of circulatory challenges that humans do. The most obvious example is the giraffe. Giraffes have a considerable problem with blood pooling in their legs. More to the point, a giraffe risks losing consciousness every time it raises its head, and it risks a cerebral hemorrhage every time it lowers its head to drink.

A giraffe’s brain is some 2 meters above its heart, meaning that a giraffe’s heart must generate enormous pressure (about twice that of a typical mammal) to get blood to the brain against the pull of gravity. Unsurprisingly, then, a giraffe has a very large heart, even for an animal of its size.

A giraffe has a complex network of arteries and veins in its neck called a rete mirabile) that functions to reduce blood flow to the brain and thus prevent the animal from dying of a cerebral hemorrhage every time it bends down to take a drink. Arteries in the rete mirabile contract when baroceptors detect a sudden rise in blood pressure, shunting blood into the veins and away from the brain, preventing a cerebral hemorrhage.

Another trick employed by giraffes is that they have very tight-fitting, very tough skin on their legs. The pressure this tight-fitting skin exerts prevents blood for pooling in the legs.




Given what we now know about how gravity affects blood circulation, the effects of weightlessness on circulation should be fairly intuitive. Remember that the heart normally has to exert enough force to get blood to the brain. That’s the critical thing. Since the head is the highest part of the body, if the blood pressure is sufficient to get blood to the head, it’s sufficient to get it to the rest of the body – particularly since gravity will assist the flow of arterial blood below the level of the heart.

It’s also important to remember that the heart’s output is automatically adjusted so that there’s enough pressure to get blood to the brain, but not too much more. (This reduces the likelihood of cerebral hemorrhage.)


In a weightless environment, the heart doesn’t have to work against gravity to get blood to the head. But it also doesn’t have the help of gravity in getting blood to the lower extremities. The result is that long-term weightlessness can result in too much blood flowing to the head and too little blood flowing to the lower extremities.

It’s much easier for the circulatory system to cope with too little blood flowing to the head than too much. In a weightless environment, if the pressure exerted by the heart is high-enough to pump blood to the lower extremities without an assist from gravity, that means blood flowing to the head is under too much pressure, since gravity isn’t causing a reduction in blood pressure as it flows upward. Astronauts and cosmonauts tend to develop headaches due to the increased pressure of blood flowing to the brain. Fortunately, the pressure doesn’t seem to be sufficiently high to put astronauts at serious risk of cerebral hemorrhage.

Decreased blood flow to the lower portions of the body has proved to be a serious concern during spaceflight. American astronauts wear “low-pressure pants” during spaceflights. These pants work in exactly the opposite way that high-pressure pants do. They create a partial vacuum around the astronaut’s lower extremities; the decreased pressure on the lower half of the body helps draw blood down into the legs.


Cheers,

Michael

[JoeP]

Strange as it might seem at first, snakes illustrate the circulatory problems associated with an erect posture quite well.

Thanksssssss!
:rattler:

The giraffe information is fascinating too.

[JoeP]

Dear Lone Ranger

We have a problem in fluid dynamics. If water, or another fluid, flows out of a tap, or other opening, at various pressures, will it always form droplets? Or will it sometimes form a continuous stream for a certain height (and maybe then break up into droplets)?

I'm sure the latter is true but my correspondent claims to have seen high-speed photos that prove the droplet nature of fluids. We don't seem to be able to find relevant web pages.

Yours, Perplexed of Purley.

[The Lone Ranger]

When water is flowing through a pipe under relatively low-pressure and at relatively low speed, assuming there are few or no air bubbles in it, it will be experiencing laminar flow. In laminar flow, all the water molecules are moving in more or less the same direction.

This is an "ideal" situation for moving water through a pipe. If you increase the pressure/speed, the water may begin to exhibit turbulent flow. In this case, though the net movement may be in a particular direction, the individual molecules are moving in different directions, making for a very chaotic flow.


To move fluid through pipes efficiently, you want to avoid turbulence at all costs, so it's important to design both your pipes and your pumping apparatus so as to minimize the chances of flow becoming turbulent. (Indeed, if blood flow in any of the major arteries becomes turbulent, the ability of the blood to deliver adequate oxygen to body tissues is severely compromised, and the result is likely to be fatal.)


So, water flowing through water pipes is normally flowing in a laminar fashion. If the tap doesn't have an aerator (which is designed for the express purpose of breaking up the water flow), water will normally exit in a laminar flow if the pressure is low.

If you turn up the tap so that more water flows out at higher pressure, the flow will break up and become turbulent. You can easily see the difference between turbulent and laminar flow.


Lots of hospitals and laboratories have faucets that don't have aerators in them and that are designed to produce laminar flow over a wide range of pressures. The goal is to have laminar flow, because laminar flow is far less prone to picking up airborne bacteria, doesn't splash as much when it hits the sink, produces a constant volume of water at a given pressure, and is more efficient at wetting surfaces.

Here's a picture of a faucet with water flowing out of it in a continuous, laminar stream.

http://www.cora.nwra.com/~werne/eos/images/laminar.jpg


If the water falls far-enough, even if it started out flowing laminarly, it will eventually become turbulent and then break up into droplets. This is a consequence of conservation of mass.

As soon as the water leaves the tap, it is in free fall. So, it will accelerate. Because of the cohesion of water molecules (that is, because of hydrogen bonding between the polar water molecules), they tend to stick together unless enough energy is supplied to pull them apart. You'll notice that as the laminar stream falls from the tap toward the sink, it gets thinner.

This is necessarily so because as the water falls it accelerates. Since the volume of water exiting the faucet is constant but it's moving faster as it moves away from the tap, the diameter of the water stream must decrease as the water's speed increases.

This can go on only so far before the tension caused by the gravitational acceleration of the water causes the stream to break apart into individual droplets. (You can see the same sort of effect in waterfalls, though that water was already flowing turbulently. If the water falls far-enough, the stream of water has completely broken up into individual droplets by the time it reaches the ground.)



So, the short answer is: Yes, many faucets can and do produce laminar flows. But falling water will eventually break up into droplets if it falls far enough, whether it was flowing in a laminar fashion or in a turbulent fashion. (And quite a lot of faucets are designed to break up the water into turbulent flow made up largely of individual drops, so as to aerate it.)


Cheers,

Michael

[But]

Hmm.

One of these days we will find a question that he can not answer.

:plotting:

[JoeP]

Thanks Michael!

You seem to be saying that slow flow / low pressure allows for laminar flow, but stronger flow will be turbulent - fair enough. Will turbulent flow always separate into disconnected droplets? And will this happen at the tap opening or some distance below?

(In fact very slow flow could also separate into drops due to surface tension - one drop at a time.)

Assuming the fluid in question is electrically conductive, the basic question is under what circumstances could current flow up the stream of fluid, and under what circumstances can you safely piss on an electric fence/electric eel/other hazard? (http://www.freethought-forum.com/forum/showthread.php?p=413952#post413952) :wink:

[The Lone Ranger]

Will turbulent flow always separate into disconnected droplets?

No. Otherwise a fast-flowing mountain stream (which is most-definitely turbulent) would consist of disconnected water droplets. Most faucets produce turbulent flow, but it isn't necessarily broken up into individual droplets.

And will this happen at the tap opening or some distance below?

If you turn your kitchen tap to a very slow, laminar flow, such that there's a very small volume of water flowing, you can see it shatter into individual drops maybe 6 inches to a foot below the tap, depending on the flow volume. This assumes that the flow is sufficient that the water does not separate into drops because of surface tension (as you pointed out), and come out one drop at a time.

If your tap allows for laminar flow with a greater volume of water (i.e., it doesn't have an aerator), the column of laminar-flow water will remain intact all the way down to the sink. What causes the column of water to eventually "shatter" into droplets as it falls (whether the flow is turbulent or laminar) is mostly the tension created by gravity pulling on the water column, though air resistance doubtless also plays a role, since it will magnify any deviation from perfect smoothness in the column of water, making the flow more turbulent and hastening the "shattering" of the column.


Assuming the fluid in question is electrically conductive, the basic question is under what circumstances could current flow up the stream of fluid, and under what circumstances can you safely piss on an electric fence/electric eel/other hazard?

Electricity would flow through any electrically conductive fluid that was in laminar flow quite nicely, since the molecules are more or less in contact. It would also flow quite nicely though any conductive fluid that was in turbulent flow, so long as it hadn't shattered into droplets.

If you wanted to safely urinate on an electric fence, the trick would be to be far-enough from the fence that the urine stream had shattered into individual droplets by the time it reached the fence -- and that the droplets had moved far-enough apart, on average, to prevent the current from "jumping" from one droplet in the stream to the next.

If you're standing on an elevated platform and urinating onto a fence 20 feet below you, my guess is that you'd be perfectly safe. I wouldn't want to try it from a distance of 2 feet, though.

It depends on the voltage of the fence, too. The greater the voltage and the amperage, the larger a gap the electricity can jump. Urinating on a high-voltage electric line -- even from a considerable distance -- would be a very bad idea.

Cheers,

Michael

[ceptimus]

On Mythbusters, they tried pissing on an electric fence and an electrified 'third rail' (as used by some subway trains).

The conclusion was that you wouldn't get electrocuted unless you were very close, as the stream of piss breaks up into droplets after just a few inches (due to gravity accelerating it as TLR described).

[JoeP]

That must have been what Ding was referring too.

... How close to reality were the mythbuster experiments? :whoa:

[ceptimus]

They equipped 'Buster' (a crash test dummy) with a 'bladder' and a valve. The diameter of the urethra was deliberately made on the large size to give 'a really good flow rate' at the upper end of the range for a large human male. Then they propped him up next to a railroad track and arranged for him to pee on a live rail. He was equipped with a sensitive ammeter that could measure any current that flowed up the urine stream (they used real human urine, I think). There was no measurable current while the crash test dummy remained in a standing position. Slow motion photography showed that the urine 'stream' actually consisted of separate droplets, and that the gaps between the droplets formed a total air-gap that was large enough to withstand the several thousand volts available at the rail without arcing across. The program did stress that although they had 'busted' the myth, this was still not a sensible thing for anyone to actually try for real.

For the electric fence test, Adam did actually pee against a live electric fence. He eventually got real close, and then did get an electric shock, though he said it "wasn't too bad". From 'normal' range (whatever that means) he didn't get a shock.

[The Lone Ranger]

By the way, here's a neat video that demonstrates an important difference between laminar flow and turbulent flow.

http://www.youtube.com/watch?v=X4zd4Qpsbs8


The reason it works is this: the fluid into which the dye is placed is very viscous. So long as you stir it slowly, flow is laminar and all the molecules are moving in the same direction. So, if you're careful, you can reverse the direction of stirring and get your original drops of dye back.

This works because by stirring in reverse, you're reversing their direction of movement and returning the molecules to where they started. So long as you stir slowly and in a precisely reverse pattern to the original stirring that dispersed the dye throughout the fluid, fluid flow remains laminar and so you can reverse the process by which you dispersed the dye through the viscous fluid and get your dye drops back.

We used to do this trick in a Biomechanics class I took; we'd put drops of dye into corn syrup, stir it up to disperse it, then reverse the direction of stirring to get the drops of dye back. It was always a fun trick, and it really impresses people who aren't expecting that sort of thing. (You can win money off people by betting them you can "unstir" a mixture.)


The trick, though, is to use a very viscous fluid and not to stir too fast. Otherwise, fluid flow becomes turbulent and the molecules are not all moving in the same direction. You cannot unstir the mixture if flow becomes turbulent.

Cheers,

Michael

[Dingfod]

Our electric fence has high voltage, something like 24,000 Volts, but very low amperage. It won't kill you, but I do know from experience direct contact is unnecessary, it will arc to anything halfway decently grounded within a 1/4 to 1/2 inch or so of the wire. Even though I now know Mythbusters busted that myth, I'm still not going to test the theory. It might shock my peener, making it go hide up inside. It took years to get it out of hiding after living though four Wyoming winters.

[But]

It is probably highly dependent on the voltage, and third rails have voltages of not more than about 1000V. I wouldn't try it with overhead lines.

[Angakuk]

My original cautionary comment (http://www.freethought-forum.com/forum/showpost.php?p=413296&postcount=9) in the other thread was based on personal experience. I don't know what the voltage of the fence was, but here is what happened.

I was visiting a friend in rural Alabama. I went behind the barn to take a leak. I really had to go quite badly, so the initial stream of urine was quite strong. There was a sheet of roofing tin leaning up against the fence (which I did not know was electrified). I pissed on the tin from a distance of probably not more than two or three feet. I received a very definite and painful shock. Not severe enough to cause me any real harm, or immobilize me (I backed off damn quick I can tell you) but still quite painful and unexpected, comparable to getting kicked in the balls. This is definitely not an urban myth. If it were a myth at all it would be a rural one.

[But]

We could investigate this issue in numerous trials with different voltages, volunteers, pressures, peeing techniques, etc. Maybe circumcision plays a role in determining the shape of the flow too. We could get the Ig Nobel Prize for this.

[Watser?]

I am currently reading Darwin's Children by Greg Bear. It's a sequel to Darwin's Radio (http://www.amazon.com/Darwins-Radio-Greg-Bear/dp/0345435249). In the books a new disease is caused by Endogenous Retro-Viruses (ERVs) and turns out to be a new step in the evolution of humanity. Meanwhile the mummified remains of two Neandertals are found together with a Homo Sapiens Sapiens child and it is suggested that this is their child as a result of the same rapid evolution through ERV.
Anyway, my question is: is it true that parts of our 'junk DNA' consists of 'captured' viruses? That these viruses can become active again and cause diseases?

[The Lone Ranger]

Anyway, my question is: is it true that parts of our 'junk DNA' consists of 'captured' viruses? That these viruses can become active again and cause diseases?
The short answers are: yes, probably not, and probably not.

And not all ERVs are part of “junk DNA”; some of them have evolved to become not just functional, but vital to our survival.

Perhaps a brief discussion of what DNA and RNA are and how they function is in order, as well as what viruses (specifically, retroviruses) are, and how they function.


For all the hideous complexity of molecular biology, the central concepts regarding DNA, RNA and proteins are actually very simple indeed; almost everything boils down to the complementary nature of the nitrogenous bases.


First, we need to know what proteins are, and why they’re so important. Any molecule that is made by stringing together many similar smaller molecules (these small molecules are called monomers) is called a polymer. A protein is a polymer of monomers known as amino acids. There are only about 20 amino acids in living things, so what distinguishes one protein from another is simply the number and the sequence of the amino acids. Here are the 20 amino acids found in living things, shown below. Notice that the only difference between any two of these amino acids is what happens to be bound to the “alpha” carbon’s 4th bond. (These 20 different chemical assemblages are known as the amino acids’ side chains, so the only difference between 2 amino acids is the makeup of the side chains.)

http://www.biocrawler.com/w/images/c/c5/Amino_acids_2.png


If you bring two amino acids together, the amino group on the “left” side of one molecule (the H3N+) can bond to the carboxylic acid on the “right” side of the other molecule (the COO-). This bond is called a peptide bond. Consequently, a long chain of amino acids bound together by peptide bonds is known as a polypeptide.

Once a polypeptide is formed, further bonding between different portions of the very large molecule (a polypeptide can be made of many thousands of amino acids) causes it to fold up into a distinct, three-dimensional shape. This molecule is a protein.


The importance of proteins to living cells cannot be overstated. That’s because proteins are the primary structural molecules of cells. That is, they make up a great many of the internal structures of cells. Further, many proteins function as catalysts, and can greatly speed up the rates at which chemical reactions take place in cells. (Proteins that function as catalysts are known as enzymes.) A cell could not survive without these enzymes, since they regulate the chemical reactions that keep the cell alive.



What has this to do with DNA? DNA is the molecule that stores the “information” the cell needs in order to manufacture proteins. A section of a DNA molecule that contains the “information” for making a particular protein is called a gene.



DNA is a molecule known as a nucleic acid, as is RNA. Nucleic acids are polymers of subunits known as nucleotides. Each nucleotide has three components, a phosphate group, a pentose (a 5-carbon sugar), and a nitrogenous base.

In DNA, the pentose in each nucleotide is a sugar called deoxyribose. (So, DNA stands for deoxyribonucleic acid.) There are four different nitrogenous bases that can be present in any DNA nucleotide. They are adenine, cytosine, guanine, and thymine. So, the four different nucleotides found in DNA molecules look like this:

http://138.192.68.68/bio/Courses/biochem2/GeneIntro/GeneIntroResources/dNTPs.gif


RNA is only a little different from DNA. Instead of deoxyribose, RNA has ribose for its pentose. (Hence, RNA stands for ribonucleic acid.) The four bases present in the nucleotides of RNA are adenine, cytosine, guanine, and uracil. Here are the four nucleotides of RNA:

http://138.192.68.68/bio/Courses/biochem2/GeneIntro/GeneIntroResources/NTPs.gif



Now, about the complementary nature of those nitrogenous bases. A strand of DNA (or RNA) consists of a whole bunch of nucleotides bound together. Because of their structures, adenine is strongly attracted to thymine (and also to uracil), and cytosine is strongly attracted to guanine. This is what is meant by the “complementary” nature of these molecules.

So, if you assemble a strand of DNA, it will more or less automatically assemble a second, matching (but “mirror-imaged”) strand. This is true because the nucleotides containing adenine will attract and bond to complementary nucleotides containing thymine, the nucleotides containing thymine will attract and bond to complementary nucleotides containing adenine, the nucleotides containing guanine will attract and bond to complementary nucleotides containing cytosine, and the nucleotides containing cytosine will attract and bond to complementary nucleotides containing guanine.

The reality, of course, is a bit more complicated, but that’s the gist of it. That’s what makes DNA such an excellent “information-storage” molecule; it’s very easy to copy. Just split a DNA molecule down the middle – this is called “unzipping” it – and each half assembles the missing half. So you wind up with 2 identical DNA molecules.

Also, because of the complementary nature of the nucleotides, DNA is more or less self-repairing if damaged.


When the “information” in DNA needs to be copied in order to manufacture a protein, the relevant section of the DNA molecule is “unzipped” and this time, the nucleotides of RNA bond to the DNA. So, the relevant “information” is copied to make a molecule of RNA (the uracil in RNA bonds to the adenine in DNA, just as thymine does). After the RNA molecule is assembled, it is released. It then travels out of the nucleus of the cell (where the DNA is stored) and to a [I]ribosome, where the “information” it contains is used to assemble a polypeptide chain.


But how is this “information” encoded, you ask? Each sequence of three nucleotides in a DNA molecule forms what’s known as a codon. Each of these codons specifies a particular amino acid. This is what’s known as the genetic code.

Since there are four different nucleotides in DNA, and since each codon contains three nucleotides, there are 43 = 64 possible combinations of codons. Since there are only 20 amino acids but 64 codons, we say the genetic code is redundant. That’s because each amino acid is coded-for by more than one codon. (The exception is tryptophan; for whatever reason, there’s only one codon that codes for it.) The genetic code is not ambiguous, however, because each codon codes for one and only one amino acid. That, again, is because of the complementary nature of the bases – the fact that adenine will normally bond only with thymine (or uracil) and cytosine will normally bond only with guanine.

Incidentally, there are three codons that don’t code for any amino acids. Since they don’t code for any amino acids, the assembly of a polypeptide chain stops when one of these three codons is reached. They are, therefore, referred to as “Stop” codons.



Now we’re in a position to understand how proteins are made. Basically, there are two major steps – transcription and translation.

In transcription, a gene is unzipped and the nucleotide sequence in one of the DNA strands is copied to make a molecule of RNA – specifically, a molecule of what’s known as messenger RNA or mRNA for short. Each DNA codon, naturally, has a complementary codon of mRNA.

After the mRNA is made, the DNA releases it and zips itself back up. The mRNA travels out of the nucleus of the cell and binds to a structure called a ribosome. (The ribosome itself is largely made of RNA.)

At this point, yet another type of RNA, transfer RNA (tRNA for short) comes into play. A given tRNA molecule has an anticodon at one end, which can bind to a complementary mRNA codon. At the other end, it has an amino acid.

Since a given tRNA has a particular anticodon at one end (an anticodon that will bind only to the appropriate complementary mRNA codon) and a particular amino acid at the other end, the sequence of codons in the mRNA determines the sequence of tRNA molecules that will bond to it – and, therefore, the sequence of amino acids that will be assembled. Since the tRNA molecules, when they bond to the appropriate mRNA codons happen to bring amino acids into close proximity, those amino acids will form peptide bonds, and so in bonding to the mRNA, the tRNA assembles a polypeptide chain. Since there are no tRNA molecules that will bind to a “Stop” codon, when one of those is reached, the polypeptide chain is released. It then folds up to become a protein.

This process, in which the “information” in the mRNA is used to assemble a polypeptide chain, is translation.



Okay, so that’s a quick and dirty summary of the importance of DNA and how proteins are made. What does any of this have to do with viruses?

A virus is not a living thing. It is a bit of nucleic acid (either DNA and/or RNA) surrounded by a protective protein coat. Most viruses contain DNA. If a virus gets into a living cell, it can “subvert” the cell’s own machinery by integrating its DNA into the host cell’s DNA and thus “forcing” the cell to make viral proteins and DNA – that is, to make many copies of the virus. Often, it kills the cell in the process.

A retrovirus is sneakier. It has RNA instead of DNA. It also has an enzyme called reverse transcriptase. When a retrovirus gets into a living cell, it uses reverse transcription to convert its RNA to DNA (the reverse transcriptase allows it to do this, naturally), and then it splices that DNA into the host cell’s DNA. After that, it does what a virus normally does – it forces the cell to copy its genetic material.


A virus isn’t necessarily harmful to a cell. It may do nothing more than quietly co-opt the cell into making a few proteins and nucleic acids that it wouldn’t have made otherwise, without harming the cell in the process.

So, viruses can become incorporated into an organism’s cells without harming them in the process. They can even be passed on from parent to offspring, if they’re in the host organism’s gametes (sperm or ova).



An endogenous retrovirus is a retrovirus that long-ago infected organisms of a given species and become incorporated into that species’ genome, and so is passed on in the sperm/eggs to offspring. Obviously, the original virus must not have been too harmful, or the original hosts wouldn’t have survived long-enough to pass it on to their offspring, and so it wouldn’t have become incorporated into the genome of the entire species. Once a virus has managed to become incorporated into a species’ genetic makeup, it’s subject to mutation just like any other gene.

As such, virtually all ERVs have long-since suffered “knockout mutations” that rendered them incapable of “escaping” and becoming infective again. It’s exceedingly unlikely that any sort of “back mutation” could occur that would once-again make the virus infective.


Interestingly, once the virus has become part of a species’ genetic makeup, it’s just as likely that it will be subject to a beneficial mutation as is any other bit of genetic material. As such, not only have many ERVs been incorporated into the genetic makeup of humans and other species, some of them now perform important – even essential functions for us.

For instance, in all placental mammals, ERVs function to strengthen and maintain the placenta. Of equal importance, several ERVs seem to function to suppress the mother’s immune system during pregnancy; this prevents the mother’s immune system from attacking and destroying the developing embryo. As such, it’s unlikely that any of us would survive long-enough to be born were it not for ERVs.

In that sense, ERVs do play a function (sometimes a major function) in evolution. It’s likely that placental mammals arose when – quite by accident – some early mammals were infected by a retrovirus or retroviruses that a.) happened to suppress females’ immune systems, allowing them to carry embryos inside their bodies longer and thus give birth to larger, better-developed offspring, and b.) happened to get itself incorporated into its bearers’ genomes and so was passed on to those offspring.

On the other hand, as mentioned above, it’s exceedingly unlikely that an ERV could re-evolve infectiousness.


Hope this helps!

Michael

[Watser?]

Thanks Michael :thankee: that was very interesting (though I'll probably only remember a bit of it).

BTW: this bit Interestingly, once the virus has become part of a species’ genetic makeup, it’s just as likely that it will be subject to a beneficial mutation as is any other bit of genetic material. As such, not only have many ERVs been incorporated into the genetic makeup of humans and other species, some of them now perform important – even essential functions for us.

For instance, in all placental mammals, ERVs function to strengthen and maintain the placenta. Of equal importance, several ERVs seem to function to suppress the mother’s immune system during pregnancy; this prevents the mother’s immune system from attacking and destroying the developing embryo. As such, it’s unlikely that any of us would survive long-enough to be born were it not for ERVs.

In that sense, ERVs do play a function (sometimes a major function) in evolution. It’s likely that placental mammals arose when – quite by accident – some early mammals were infected by a retrovirus or retroviruses that a.) happened to suppress females’ immune systems, allowing them to carry embryos inside their bodies longer and thus give birth to larger, better-developed offspring, and b.) happened to get itself incorporated into its bearers’ genomes and so was passed on to those offspring. is explained in the second book, it is pretty high in science (though I keep forgetting most of that too).

[The Lone Ranger]

You're most welcome, Watser?!


Aw, come on, nobody thought the "unzipping of genes" joke was funny? I may have to go pout now.


Cheers,

Michael

[cappuccino]

Molecular Biology's Central Dogma

Youtube has a movie about replication and transcription, it's one of my favorite science movies. Like many other molecular biology movies, it shows as though molecules themselves were magically seeking out each other even that's not actually the case. But then shortcuts have to be taken to show the really important features.

Does the movie get all of the process details correct? I have a couple of questions, I've noticed in my molecular biology reading that sometimes it seems like they're talking about different kinds of DNA replication machinery all doing the same thing, I was wondering are they comparing various different replication enzymes from different species or are multiple redundant replication machinery at work in human cells? The other question, are the mRNA strands driven by random Brownian motion to move by themselves out of the nucleus into the cytoplasma to be united with a ribosome? Or are there helper molecules which ferry the mRNA from the transcription grounds to the ribosomes?

Your talk about ERVs made me think of a certain company's research aimed at eradicating HIV. They've developed a drug designed to accelerate the rate of mutation in HIV by tricking the reverse transcriptase into incorporating nucleotide analogues in the DNA chain. Fascinatingly, the drug causes random multiple point mutations in order to drive the virus into extinction by causing multiple knockout mutations over successive generations.

If the clinical trials bore out, then HIV could become another ERV in the humans infected by it. Who knows maybe it'll end up driving another evolutionary phase in humanity in the distant future.

[The Lone Ranger]

Youtube has a movie about replication and transcription, it's one of my favorite science movies. Like many other molecular biology movies, it shows as though molecules themselves were magically seeking out each other even that's not actually the case. But then shortcuts have to be taken to show the really important features.
I left out a lot of details regarding DNA Synthesis and Protein Synthesis for that reason too. I figured that, for example, an explanation of how mRNA is processed before it's released and allowed to leave the nucleus would be pointlessly complicated and wouldn't be helpful in understanding the basic processes involved.


Does the movie get all of the process details correct?
From what I can tell, it's pretty good, aside from some forgivable oversimplification.

I had trouble with it on my computer; it doesn't play very well. It appears, though, not to have any narration. That may be because it's intended to be played while someone explains what's going on. Otherwise, some of what you see won't make any sense. For example, DNA polymerase can only copy DNA in the 5' → 3' direction. This means the leading strand and the lagging strand of a DNA molecule are copied differently during DNA replication. They actually show that in the animation, but if you didn't know it beforehand and therefore what to look for, you'd never be able to make sense of it.

I have a couple of questions, I've noticed in my molecular biology reading that sometimes it seems like they're talking about different kinds of DNA replication machinery all doing the same thing, I was wondering are they comparing various different replication enzymes from different species or are multiple redundant replication machinery at work in human cells?
I'm not entirely certain I can answer. As far as I know, DNA polymerase works more or less the same in all eukaryotes. As mentioned though, the leading strand and the lagging strand are copied differently. You have to keep this in mind when thinking about DNA replication; that might be a source of confusion.

Also, DNA replication works a bit differently in prokaryotes than it does in eukaryotes. (Also, transcription and especially translation work a little differently in prokaryotes and eukaryotes. This is largely because prokaryotes don't process their mRNA after it's synthesized. Eukaryotes, by contrast, heavily process their mRNA after it's synthesized, but before it leaves the nucleus.)

In any event, DNA polymerase and RNA polymerase are far from the only enzymes that take part in DNA synthesis/protein synthesis, so that might be a source of confusion. If a textbook or an instructor makes a random reference to helicase, for example, without explaining its role in DNA replication, that could easily lead to confusion.

It's also worth keeping in mind how big a molecule DNA is. Several different DNA polymerase molecules will be simultaneously operating during the replication of a DNA molecule -- otherwise, copying the molecule would take much more time. So you'll have lots of (more or less) identical molecules performing the same tasks during DNA replication; if this isn't understood, random reference to "the other" DNA polymerase molecules will be terribly confusing to the listener.

The other question, are the mRNA strands driven by random Brownian motion to move by themselves out of the nucleus into the cytoplasma to be united with a ribosome? Or are there helper molecules which ferry the mRNA from the transcription grounds to the ribosomes?
Random Brownian motion plays a big role, but it's a bit more complicated than that. "Mature" mRNA (that is, mRNA that has been processed and has had its introns spliced out, etc.) can bind to nuclear export proteins (is there anything proteins can't do?). Unprocessed mRNA typically won't bind to nuclear export proteins.

The mRNA apparently can't pass through the nuclear pores and thus into the cytoplasm unless bound by nuclear export proteins ("exportins"). Unbound mRNA is far too large a molecule to have much of a chance of passing through a nuclear pore on its own. I don't know off the top of my head how exportins work, but I'd guess they cause the mRNA to fold into a more compact shape that can more easily pass through a nuclear pore. The mRNA/exportin complex diffuses out into the cytoplasm (as far as I know, no active transport is involved), and the proteins are eventually released. The mRNA, should it encounter a ribosome, can then be used for protein synthesis. If it doesn't encounter a ribosome, it will sooner or later be degraded by RNases and broken down into its component nucleotides.


Your talk about ERVs made me think of a certain company's research aimed at eradicating HIV. They've developed a drug designed to accelerate the rate of mutation in HIV by tricking the reverse transcriptase into incorporating nucleotide analogues in the DNA chain. Fascinatingly, the drug causes random multiple point mutations in order to drive the virus into extinction by causing multiple knockout mutations over successive generations.

If the clinical trials bore out, then HIV could become another ERV in the humans infected by it. Who knows maybe it'll end up driving another evolutionary phase in humanity in the distant future.

I've read of this. It's a brilliant idea, in my estimation, and may prove the best way to deal with HIV.

Cheers,

Michael

[SharonDee]

According to local folklore, we need a cold winter to cut down on insect pests during the summer months. If the winter is too mild, we get swarmed by the pesky critters. If the winter gets and stays cold enough, we'll get a normal number of them in the summer.

First, how much truth is there in this belief?

Second, if there is any truth to it, how cold does it have to get and for how long?

[The Lone Ranger]

There is truth to the belief.

Insects and other small arthropods have various ways of dealing with winter cold. The main problem, of course, is preventing their body tissues from freezing – or, from freezing in such a way that the freezing process will kill. There are conditions in which an animal can freeze without being killed in the process.


If you dissolve a substance into water, this causes freezing point depression. That is, the temperature at which the water freezes is lowered. This is why, if you live near the ocean, the salty seawater can remain a liquid at temperatures that cause nearby freshwater ponds and lakes to freeze. That’s the same principle behind antifreeze in your car. Ethylene glycol (commercial antifreeze) has a freezing point of about -13 degrees Celsius. Water, of course, has a freezing point of 0˚ C. Mix the two and you depress the freezing points of both liquids, so you get a mixture that won’t freeze unless its temperature is lowered to considerably less than -13˚ C. (The exact freezing point, of course, will depend on the proportions of water and ethylene glycol in the mixture.)


It’s worth keeping in mind that there’s no such thing as “cold.” “Cold” is merely a relatively lack of heat. The warmer something is, the faster its molecules are moving; the colder it is, the slower its molecules are moving. A refrigerator or freezer, therefore, doesn’t work because it adds some magical substance called “cold” to your food. It works because it removes heat from it. Similarly, it gets colder in the Winter because less energy is being delivered by the Sun than is escaping to space.

As a substance cools, its molecules move more and more slowly. Eventually, if it’s a liquid, the molecules will be moving slowly-enough that, when they happen to bump into each other, they won’t have enough energy to overcome their electrostatic attraction. So, they’ll stick together, forming crystals. As more and more molecules stick together, the liquid freezes.

It’s thought that the reason for freezing-point depression is that as you dissolve one substance into another, the molecules of the solvent necessarily become more widely-spaced. This means the molecules of the solvent are less likely to bump into each other and start sticking together to form ice crystals.



So, many small “cold-blooded” animals secrete sugars and/or proteins into their body tissues during cold weather that have the effect of lowering the animals’ freezing points. As such, they can avoid freezing in sub-zero (Celsius) temperatures. As an illustration, just a couple of weeks ago, while out for a hike in sub-freezing temperatures and with nearly a foot of snow on the ground, I counted three spiders and a couple of moths walking along (slowly, to be sure) on the snow.

So, the widely-believed notion that “cold-blooded” animals cannot survive – much less function – in sub-freezing temperatures simply isn’t true. Most people don’t notice spiders and moths and whatnot when hiking in the winter because they don’t think to look for them.

Even so, sufficiently prolonged exposure to sufficiently cold temperatures will kill any animal.



Most arthropods don’t remain active during the winter months, of course. In many, many species, the females lay eggs in the autumn, preferably in some well-insulated place. The eggs, jam-packed full of sugars and proteins that will prevent them from freezing in all but the most extreme of conditions, will survive over the winter and hatch out new critters in the spring.


A variation on this is that many insects enter a pupal stage during the autumn. Again, the insect generally seeks out a well-insulated place (say, under the bark of a tree or down in the ground) where temperatures are unlikely to fall too low.

Some insects (e.g. dragonflies) have aquatic larvae. The young insects will survive just fine through the winter, so long as it doesn’t get cold-enough for the ponds or lakes they’re living in to freeze solid.

Many insects can survive the winter as adults by going dormant. It’s not all that uncommon to find adult insects hibernating under tree bark or in the soil, for instance. Again, they tend to choose well-insulated sites, and so long as the temperature doesn’t go too far below freezing, they’ll survive just fine until warmer temperatures when they can again become active.



Some insects and even some small vertebrates (e.g., some amphibians and reptiles) can be supercooled to well below 0˚ C without freezing. This only works for relatively small animals, however. If it isn’t disturbed water that’s pure or nearly so can be chilled to nearly -40˚ C without freezing. This is called “supercooling.” So long as nothing disturbs the supercooled water, it will not freeze. The reason that water normally freezes at 0˚ C is because “impurities” in the water act as “condensation nuclei.” Water molecules will adhere to small bits of matter suspended in the water column, other water molecules will stick to those water molecules, and so on.

But if you can purify water sufficiently and keep it still, even at temperatures of -20˚ to -30˚ C, water molecules will collide with each other and stick together too infrequently for ice crystals to form, and so it won’t freeze. But even a slight disturbance – say, shaking the container or dropping in a single crystal that can act as a condensation nucleus will cause the entire mass of water to freeze almost instantly.

Supercooling normally works for only relatively small volumes of water because, the greater the volume of water, the greater the chance that some water molecules will happen to collide and stick together, thus initiating rapid freezing. That’s why insects and frogs can be supercooled, but not bears.


How to accomplish this? Some animals can actively transport large amounts of glucose or similar sugars into their body cells (glucose cannot cross cellular membranes on its own, and so will not “leak” out again), until their body cells have practically no water in them. By transporting sugars and proteins out of the blood and into body cells, they convert the blood to nearly pure water. So long as the animal remains absolutely still, neither the body cells (which have very little water in them, so don’t freeze because water molecules are too widely-separated to come into contact) nor the blood (which supercools because of the lack of condensation nuclei) will freeze. Of course, anything that disturbs the supercooled animal will cause it to freeze more or less instantly, killing it in the process.




Anyway, regardless of how they manage to beat the cold, insects can only carry this so far. Exposure to sufficiently-low temperatures for sufficiently lengthy periods will kill any animal.


Where the animal in question is and what the weather conditions are will play a major role. Animals that burrow deep underground to overwinter will rarely experience lethal temperatures unless it’s an exceptionally cold and snow-free winter. (Snow acts as an insulating blanket. A prolonged cold spell accompanied by snow on the ground will kill many fewer burrowing insects than will a cold spell in which the ground is bare.)

Similarly, for those of us living more than 23.5˚ north of the equator, the Sun is always in the southern part of the sky, especially in winter. This means south-facing hills, trees, etc. receive more sunlight (and, therefore, more warmth) than do those facing north. So, an insect that burrows into the bark of a tree on the south-facing side will survive a much colder winter than will an insect of the same species that happens to choose the north-facing side of the tree for shelter.


***

So, the short answer to the question is that there’s no way to say exactly how cold the weather must be or for how long. Each species’ tolerances will be different. However, as a general rule, the colder the winter and the less snow there is, the fewer insects will manage to survive it, including those that plague us in the spring and summer months.

Cheers,

Michael

[ceptimus]

How about bacteria? I thought they could survive for many thousands of years at very low temperatures? Also plant seeds?

[The Lone Ranger]

Under the proper conditions, you can freeze single cells for indefinite periods of time. Bacteria have been found in permafrost that were at least 10,000 years old. Upon thawing, they immediately began to swim about.

Small-enough organisms can be frozen without killing them, under the proper circumstances. It generally has to do with avoiding the formation of ice crystals, since those crystals will rupture cells, killing the organism in the process.

Generally, the trick is to freeze the organism fast-enough that the water solidifies without the formation of sharp crystals that will rupture the cells. Since a living thing is made mostly of water and since water has a tremendously high heat capacity, this means that it's not physically possible to remove enough heat from a sufficiently-large organism fast-enough to freeze it without ice crystals forming that will rupture its cells and kill it. That's why you can freeze an embryo but not an adult human.


Some plant seeds have virtually no metabolic activity if either frozen or dehydrated, and so can survive for years or even centuries under the proper circumstances, only to sprout when conditions are favorable. Many seeds have impermeable, waterproof coats, for instance, and the embryo inside remains in a dormant, dehydrated state with virtually no metabolic activity for years. If a fire then sweeps across the soil and heats the seeds sufficiently to weaken the protective coating, water can then infuse into them. Metabolic activity will then resume and the seeds sprout. That's one reason why it's not unusual to see plant species growing after a prairie or forest fire that haven't been seen in the area for decades. (Plant conservationists often use fire as a tool for restoring grasslands, since so many grass species have fire-resistant seeds that can remain in the soil for years until they're "activated" by a surface fire.)

Cheers,

Michael

[curses]

But if you can purify water sufficiently and keep it still, even at temperatures of -20˚ to -30˚ C, water molecules will collide with each other and stick together too infrequently for ice crystals to form, and so it won’t freeze. But even a slight disturbance – say, shaking the container or dropping in a single crystal that can act as a condensation nucleus will cause the entire mass of water to freeze almost instantly.

l
Cool! (no pun intended)

Does the same thing apply to heating a liquid?

[Ensign Steve]

I've heard of superheating water in the microwave.

Superheated Microwaved Water (http://www.snopes.com/science/microwave.asp)

[curses]

That's exactly what I was thinking of, ES. Great minds, eh?:chestram:

Edit: Oh, and I'd forgotten how much I love these "Ask TLR" threads.

[Ensign Steve]

I know. I want to go to school where he teaches.

[The Lone Ranger]

Yes. Under the proper circumstances, you can heat a liquid to well above its boiling point without it boiling. Then even the slightest disturbance will cause it to boil in an instant.

One of my chemistry professors claimed he once accidentally did this with a microwave oven. He said he put a mug of water (evidently, very pure water) into a microwave and, unsure of the microwave's power, let it run for several minutes. He then tried to pour some cocoa powder in to make some hot chocolate, but it boiled over as soon as the first flakes of powder hit it, scalding his hand.




As an aside, it would rain and snow a lot less often were it not for dust particles in the atmosphere. Dust particles act as condensation nuclei for water droplets/ice. That's why "seeding" clouds with silver nitrate or other such chemicals will sometimes trigger rain.

Ironically, if the air is too clean, it can't rain or snow.

Cheers,

Michael

[ETA: Oops, too slow! ES beat me to it.]

[The Lone Ranger]

In my Zoology class, I've been explaining about reproduction. I mentioned lizards of the genus Cnemidophorus in which no males are known to exist, yet the lizards nonetheless must actually go through the mating process in order to produce eggs. So, two female Cnemidophorus will take turns mating with each other, that each of them can produce eggs -- first one plays the "male," then the other.

Also, in explaining anisogamy, I quipped that "a sperm cell is just genetic material with a motor attached."


I heard later that the students have been enjoying telling "Dr. P" (one of the Chemistry professors) about the "lesbian lizards," "sperm as genetic material with a motor attached," and other such things they're learning in Zoology.

Cheers,

Michael

[Ensign Steve]

Can you imagine if you could get pregnant from lesbian sex! :ohnoes:

[Qingdai]

Whoa, the mind boggles.:whoa:

Purple baby clothes would be more common...

[The Lone Ranger]

Here are a couple of neat videos illustrating supercooling and superheating.

In the first video, a bottle of water has been cooled to below its freezing point. Watch what happens when it's shaken. As soon as the guy shaking it agitates it enough to get an air bubble into the water, it freezes:

YouTube - Fiji water supercooled


In the second video, some water has been heated in a microwave to above its boiling point. Watch what happens when it's disturbed:

YouTube - Microwave Superheating

Cheers,

Michael

[Ari]

Yes. Under the proper circumstances, you can heat a liquid to well above its boiling point without it boiling. Then even the slightest disturbance will cause it to boil in an instant.
I accidentally made a coffee cannon this way. The cup that I was making foldgers crystal coffee (eww, I know) in was to tall to put in the microwave so I boiled a cup of water to poor in it. After forgetting it once I added time to it, took it out and dumped it into the tall slender coffee cup and boom out comes coffee all over the counter and the wall. Luckily it was pointed away from me.

[Ensign Steve]

That is so awesome!

I like the quick freeze on the fiji bottle, too. Question: Does the ice look different or have a different structure when it freezes that quickly?

[cappuccino]

I was wondering that too, I thought that perhaps it ends up becoming a vitrified amorphous structure.

How do you supercool a bottle of water? Do you dip it into an extremely cold solution for a few minutes?

[The Lone Ranger]

When water freezes slowly -- as tends to happen when it's at or only a little below the freezing point, it tends to form sharp, needle-like crystals. Those will easily pierce cellular membranes and kill any organism that freezes slowly.

By contrast, when supercooled water freezes, it tends to form flat, hexagonal crystals that are much less likely to pierce and rupture cells. That's why you can drop a very small organism (say, an embryo) into liquid nitrogen and "instantly" freeze it without causing formation of the needle-like ice crystals that will rupture cellular membranes and kill the organism.

Some small animals actually take advantage of this phenomenon. Some insects can empty their "blood" (strictly speaking, insects don't have blood) of almost all "impurities" and then allow themselves to supercool. When they eventually freeze, the freezing happens so fast that the needle-like crystals don't form, and so the animal's cellular membranes aren't ruptured. When it thaws in the Spring, it's good to go.

***

The way to supercool water is to purify it as much as possible (any impurities can form condensation nuclei), to put it into as smooth a container as possible, and to prevent any sort of disturbance. Sufficiently pure water in a sufficiently smooth container can be cooled to as much as about 40 degrees C below freezing without actually freezing.

You can do it in your freezer, if you're careful.

Cheers,

Michael

[Ari]

A number of videos I've seen are people that live in evil cold places (like up north in the US) and just leave the bottle out overnight or however long it takes the water to get below freezing.

I've tried it in the freezer but except for a few accidents I normally get a bottle of frozen water and not super cool liquid.

[livius drusus]

I posted a story (http://www.thehistoryblog.com/archives/196) in my blog yesterday about a block of beeswax from a 300 year old Spanish shipwreck that washed up on the Oregon shore. The article says that Spain had to import beeswax to the New World because there were no native honeybees.

"The Catholic church required the use of beeswax," he said. "There were no native honeybees in the New World. The churches in Mexico had to get wax from someplace and the large Asian honeybees produced a lot of beeswax."

Is that true, do you know? It boggles my mind because they're such an integral part of the ecosystem now that their recent mysterious decline has a lot of people dismayed.

[The Lone Ranger]

There are some 20,000 or so species of bees, but the true "Honeybees" comprise just 7 currently-recognized species in the genus Apis. The genus appears to be native to southeastern Asia. The most commonly domesticated species is Apis mellifera, the Western or European Honeybee.

There are about 4,000 species of bees native to the New World, but honeybees aren't one of them. Since none of our native bee species have the favorable traits that honeybees do (living in large colonies that will produce economically-viable amounts of honey and wax, relatively benign temperaments), there's been no real effort to domesticate any of them.

The first honeybees were brought to the Americas in the 1600s. Since then, they've become quite important in North- and South-American ecosystems, though that's not entirely a good thing. Many of our native flower species evolved to be pollinated by native bees, and cannot be pollinated by the non-native honeybees. That wouldn't be a problem, except that our native bee species have been devastated, and many are in serious danger of extinction.

Between losing most of their habitat to agriculture and other human activities, being killed by pesticides, and competition with the non-native honeybees, a great many of our native bee species are in serious trouble. This is a matter of considerable concern not just to entomologists and ecologists, but to lovers of our native wildflowers. There are efforts to try to preserve habitat for native bee species, and one commonly-cited reason some people encourage "natural landscaping" and planting of wildflower gardens is to help preserve native bee populations. The Xerces Society (http://www.xerces.org/Pollinator_Insect_Conservation/generalplantsforbees.htm) is an example of an organization dedicated to educating people on the value of native invertebrates, and to promoting conservation of native bee species.

Cheers,

Michael

[livius drusus]

Fascinating, Michael, thank you. I've been lumping all bee species together, I see. I'd love to have a native flora garden. Not only do the plants provide for local creatures, vertebrate and non, but they require much less maintainance and watering, something that's hugely important right now in my droughty neck of the woods.

[The Lone Ranger]

Yup! Planting native flora is an excellent idea for a number of reasons. You can also buy or make nest boxes for native bee species.

Cheers,

Michael