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Old 07-26-2007, 03:51 PM
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Question A Question For The Lone Ranger

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:
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Old 07-27-2007, 02:41 AM
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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
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Old 07-27-2007, 03:25 AM
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Default Re: A Question For The Lone Ranger

Interesting! Our bodies are so fucking cool.
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Old 07-27-2007, 11:49 AM
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Default Re: A Question For The Lone Ranger

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

Thanks, TLR. :thankee:

* Shelli gets up to take an antihistamine :giggle:
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Old 07-27-2007, 12:57 PM
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Default Re: A Question For The Lone Ranger

Bollocks. It itches because angels lick the wound. :unangel:
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Old 07-27-2007, 01:15 PM
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:giggle:
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Old 07-27-2007, 01:34 PM
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Originally Posted by Stormlight View Post
Bollocks. It itches because angels lick the wound. :unangel:
Now I have an image of angels licking bollocks :P
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Old 07-27-2007, 01:35 PM
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:giggles:
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Old 07-27-2007, 04:13 PM
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Default Re: A Question For The Lone Ranger

I was wondering why the doctor prescribed Benadryl when I had that mysterious rash on my arms recently.
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Old 07-27-2007, 04:38 PM
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That's fascinating, TLR. This should become the masked man's version of the Auntie Unmentionables thread. :thumbup:
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Old 07-27-2007, 04:49 PM
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Originally Posted by livius drusus View Post
That's fascinating, TLR. This should become the masked man's version of the Auntie Unmentionables thread. :thumbup:
:yeahthat:
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Old 07-27-2007, 05:08 PM
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Default Re: A Question For The Lone Ranger

And he should totally use the Unmentionables to answer the questions! :eager:
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Old 07-27-2007, 05:37 PM
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Dear The Lone Ranger,

It hurts when I do this.

Sincerely,

Admiral Fulton J Boondesvill XIV, Esq, Mrs
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Old 07-29-2007, 11:53 AM
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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
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Old 07-29-2007, 12:01 PM
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:immolate:
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Old 07-29-2007, 12:12 PM
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:gooduse:

And btw, the Wikipedia page on inflammation has some truly lovely photos. You should not visit it.
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Old 07-30-2007, 12:53 AM
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Default Re: A Question For The Lone Ranger

Quote:
Originally Posted by JoeP View Post
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
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Old 07-30-2007, 02:27 PM
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Wow! That is so cool! :joecool2::joecool2::joecool2:

Thanks, TLR. :=)
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Old 07-30-2007, 02:40 PM
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Quote:
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This should become the masked man's version of the Auntie Unmentionables thread.
"Ask Auntie Histamine."
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Old 07-30-2007, 02:52 PM
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:rofl: Now that is a quality pun.
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Old 08-23-2007, 11:31 PM
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Default Re: A Question For 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
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Last edited by The Lone Ranger; 08-23-2007 at 11:47 PM.
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Old 08-24-2007, 12:48 AM
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Originally Posted by The Lone Ranger View Post
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.
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Old 08-24-2007, 02:34 AM
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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 ;)
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Old 08-25-2007, 11:00 AM
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Default Re: A Question For The Lone Ranger

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?
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Old 08-26-2007, 01:40 AM
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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 ([I]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.



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
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Crumb (08-26-2007), Dragar (08-26-2007), Ensign Steve (08-26-2007), irukandji (02-03-2010), JoeP (08-26-2007)
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