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Blood Pressure: Standing Up to Gravity


Editor's note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that's because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News & Views is delighted to present this series, "The Designed Body." For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

the-designed-body4.jpgThe cardiovascular system's job is to provide the cells of the body with what they need to live, grow, and work properly by providing enough blood flow to the tissues. To do this, the heart must pump well enough, there must be enough blood in the arteries, and the arterioles must limit how much blood enters the tissues by applying peripheral vascular resistance. It is these three factors that mainly determine the arterial blood pressure.

But life is a dynamic process, and changes in fluid balance, body position, and physical activity require that each of these factors be constantly adjusted to keep the blood pressure where it needs to be to survive. Just as a car's ability to perform well enough goes above and beyond its mere blueprints, so too, the ability of the body to survive within the laws of nature goes above and beyond its mere DNA.

Anyone who has ever felt dizzy upon standing up knows this to be true. When it comes to being able to stand up to gravity, the numbers regarding blood pressure have to be just right or else we'll find ourselves lying on the ground. Just imagine our ancient ancestors trying to survive if their body couldn't control their blood pressure so as to remain in the upright position for an extended period. Evolutionary biologists seem to forget that explaining how life came about requires showing, not just how it looks, but also how it must work within the laws of nature to survive.

It's important to remember that, barring a local obstruction from a clot, or a hemorrhage from an injury, the blood pressure provides the force resulting in adequate blood flow to the tissues of the body. The blood pressure fluctuates between the diastolic (DP) and the systolic pressure (SP) but it is the mean arterial blood pressure (MAP) that is used when considering blood flow. As noted previously, since systole takes up one third of the cardiac cycle and diastole, two thirds, the MAP can be calculated as 1/3 SP + 2/3 DP. An MAP of 100 mmHg could come from a BP of 150/75 or 120/90 and a BP of 90/60 would give an MAP of 70 mmHg.

Since gravity pulls blood down, when the body is upright the MAP in the brachial artery is higher than the one in the arteries leading to the brain. Because the gravitational constant and the distance between the heart and the head are constant, the MAP in the brain, compared to the one in the aorta and in the brachial artery, can be determined.

At rest, the MAP in the aorta, as the blood exits the left ventricle, is usually about 80-110 mmHg. By the time the blood travels to the arms it has lost some energy and the MAP has dropped to 70-100 mmHg. Due the additional effect of gravity, when the body is in the upright position, the MAP in the brain drops to about 60-75 mmHg. Since it is the blood pressure that generates blood flow, this means that, in general, the lower the MAP in a given organ the lower the blood flow.

When you are sitting or standing, the force moving blood to your hands is higher than the one moving blood to your brain. However, despite this relatively low pressure, the brain has a mechanism in place, called autoregulation, to help ensure adequate blood flow so its metabolic needs can be met.

Autoregulation within a given organ usually takes place independent of nervous or hormonal control and allows for adequate blood flow. It is important to understand that the blood flow (Q) within a given organ is directly related to its MAP (P) and inversely related to the vascular resistance in its arterioles (R). The higher the pressure, the more blood flow and the lower the pressure, the less blood flow. And the higher the arteriolar resistance within a given organ, the less blood flow, and the lower the arteriolar resistance, the more blood flow.

This natural law can be expressed as Q = P/R. One can therefore see that if the MAP drops in the brain, it can try to compensate by reducing its vascular resistance at the same time so it can maintain adequate blood flow to meet its metabolic needs. This is particularly important for brain function because of its high metabolic rate and extreme sensitivity to changes in blood flow.

Vascular resistance is directly related to the degree of muscle contraction around the arterioles. More contraction causes more resistance and less contraction causes less resistance. When the MAP drops in the brain, autoregulation makes the vascular resistance drop almost immediately by relaxing the arteriolar muscles, increasing blood flow back to where it is supposed to be.

But, like everything else in life, autoregulation and its ability to control the blood flow in the brain does have objective limits. Regarding significant drops in blood pressure, there is only so much relaxation of the arteriolar muscles and reduction in vascular resistance that can take place.

Wince life is a dynamic process where the blood pressure is always fluctuating due to various circumstances, the body has to always keep it under control to prevent brain malfunction.

When you stand up, not only does gravity prevent blood from traveling to the brain, it also makes about 500 mL move from the veins in your chest and abdomen into your legs. This sudden drop in the volume of blood returning to the heart through the veins usually causes a

40 percent drop in the cardiac output and with it, a further drop in the MAP in the brain. So much so that autoregulation is incapable of adequately compensating for this sudden drop in pressure causing reduced blood flow. When this happens, we feel light-headed. If our body doesn't correct the situation or we don't get our head below our heart fast enough, we may pass out.

To fix this problem quickly, so we can remain standing, the body must know that if Q = P/R, then P = Q x R. This expresses what has already been explained above. The blood pressure (P) is directly related to the total blood flow in the circulation (Q) (which is determined by how well the heart pumps and how much blood is in the arteries) and the peripheral vascular resistance (R). To prevent passing out on suddenly standing up, the body must immediately increase the cardiac output and the peripheral vascular resistance as well.

When it comes to making quick adjustments to blood pressure, it is the sympathetic nervous system, in response to the data it receives from the baroreceptors in main arteries carrying blood to the brain, that acts to solve the problem. When the MAP in the major arteries is in the normal range (70-100 mmHg), the frequency of impulses from the baroreceptors to the brain remains relatively stable. However, as the MAP deviates up or down from normal, the increase or decrease in the stretching of the vessel wall makes the baroreceptors increase or decrease the frequency of their messages. In response, the brain stem stimulates the sympathetic nervous system in an inverse pattern.

An increase in the MAP causes an increase in the frequency of impulses from the baroreceptors and a decrease in sympathetic activity. And a decrease in the MAP (like from standing up too quickly) causes a decrease in the frequency of impulses from the baroreceptors and an increase in sympathetic activity.

When we stand up too quickly, causing the MAP in our brachial artery to drop below 70 mmHg and the MAP in our brain to drop below 50 mmHg, we feel dizzy. By attaching to specific receptors, the increased output of sympathetic neurohormones quickly makes the systemic veins send more blood back into the arteries and the heart pump harder and faster. These quick actions combine to immediately increase the cardiac output (Q). And, increased sympathetic activity makes the muscles surrounding the arterioles in most of the tissues of the body contract more, quickly increasing the peripheral vascular resistance (R) as well.

As noted above, P = Q x R, so this rapid increase in Q and R, brings the MAP in the brain back to a level that allows its autoregulatory mechanism to maintain adequate blood flow. We experience this by our dizziness resolving within a second or two and being able to carry on. So it looks like the system the body uses to control its blood pressure to stand up to gravity seems to know what it's doing.

What has been described here is the real situation that faces human life. Not only must it know how the laws of nature work and have the mechanisms in place to deal with them, it must also do the right thing at exactly the right moment and do it well enough to survive.

By only describing how life looks, and not how it actually works, evolutionary biologists claim to have shown how life came into being by chance and the laws of nature alone. This means that all of the control mechanisms needed for all the chemical and physiological parameters to support life must have come about the same way.

Moreover, it is interesting to consider that in the last century, by using their intellectual powers to measure these parameters and sometimes do something about them, medical scientists have only recently been able to do what the body has had to do, by necessity, since the very beginning of its existence. For, as the next two articles in this series will demonstrate, when circumstances arise to overwhelm the body's ability to keep its blood pressure where it needs to be, real numbers matter.

Image credit: Toni Frissell [Public domain], via Wikimedia Commons.