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 CO
2. 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, CO
2 leaves the blood and enters the lungs to be exhaled as O
2 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