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 cells that make up the organs and tissues of the body require the blood in the circulation to give them what they need so they can do what they need to do. The heart must pump enough blood with enough pressure behind it to maintain sufficient blood flow. Blood flow can be defined as the volume of blood that passes a given point in the circulation system within a given amount of time, and is usually measured in "milliliters per minute" (mL/min).
As I've shown in prior articles in this series, the laws of nature state that blood flow (Q) to a given organ is directly related to the pressure (P) of the blood as it enters its capillaries and inversely related to the vascular resistance (R) applied by its arterioles. This natural relationship can be expressed as Q = P/R. The higher the pressure, the more blood flow and the lower the pressure, the less blood flow. And the higher the vascular resistance, the less blood flow, and the lower the vascular resistance, the more blood flow.
Common sense tells us that the wider the passageway the easier the flow. Just consider rush hour traffic moving along a highway. The more lanes there are available, the more cars can move through in a given amount of time. Now think about what happens when the cars leave the highway. Compared to a single-lane exit ramp, a double-lane one provides much less resistance and allows easier flow off the highway. The blood hurtling through the smaller arteries, trying to enter the arterioles on the way to the tissues, is like cars in a crush of rush hour traffic trying to enter the various exit ramps to reach their destinations. The wider the opening in the arterioles, the more blood can flow through, and the narrower the opening, the less blood can flow through.
The arterioles can increase or decrease the amount of resistance they apply to the blood trying to enter an organ by increasing or decreasing the contraction of the muscle surrounding them. An increase in muscle contraction closes down the opening in the arteriole, making the passageway (lumen) smaller. This increases the resistance and lowers the blood flow. And a decrease in muscle contraction opens up the lumen, decreasing the resistance and increasing the blood flow. In fact, the laws of nature state that the change in blood flow is directly related to the fourth power of the change in the luminal diameter. This means that if the luminal diameter of the blood vessel doubles, the blood flow increases by a factor of sixteen, and if it halves it decreases by a factor of sixteen.
At rest, total blood flow within the systemic circulation (cardiac output) is about 5,000 mL/min (5 L/min). With high levels of activity, something our earliest ancestors would have had to do often, it rises to about 25L/min. However, just because the body can generate enough cardiac output to meet its metabolic needs doesn't mean that the increase in blood flow will automatically go to the organs and tissues that really need it. This requires the body to take control in order to follow the rules that nature throws at it. Real numbers have real consequences, and if with increased activity the body can't get enough blood flow to the heart and skeletal muscle while preserving it to the brain, the body cannot function. This is what the body of our earliest ancestors would have had to have been able to do to survive within the laws of nature, something that evolutionary biology has yet to even mention, never mind explain.
The blood flow to a given organ or tissue is dependent, not only on its mass, but also its energy needs, in other words, what it's doing. The brain of a 70 Kg man has a mass of only 1500 gm, about 2 percent of his total mass. But at rest, the brain receives 750 mL/min, or 15 percent of the cardiac output. The brain needs a high amount of blood flow, over and above what one would expect for its size, because even though the body may be at rest, the brain is always working hard. In fact, no matter how little or how much the body exerts itself, the amount of blood flow to the brain must stay at 750 mL/min for it to work properly. The heart, with a mass of only about 300 gm, less than 1 percent of the body's total, is another organ that must constantly work, even when the body is at rest. At rest, the heart receives about 250 mL/min of the cardiac output, or about 5 percent of the total blood flow.
In contrast, the skeletal muscle, with a mass of about 30 Kg, or 40 percent of the body's total, at rest receives only 15 percent of the cardiac output, or 750 mL/min. At rest, the remaining blood flow mostly goes to the liver and gastrointestinal system (25 percent), the kidneys (20 percent), the fat (5 percent), the bones (5 percent), the skin (5 percent), and the lungs (2.5 percent).
The muscles surrounding the arterioles respond to several different factors. Some of these are intrinsic to what is going on inside and around the arterioles. This includes the pressure the blood applies as it enters and stretches the arteriolar wall and the presence of certain chemicals related to the metabolism of the tissues. Other factors are extrinsic to the arterioles, which include various hormones released by glands and neurohormones released by nerve cells. At rest, the main extrinsic factor that affects local blood flow is the sympathetic nervous system.
Except for in the brain, the sympathetic neurohormone, norepinephrine, attaches to specific receptors on the muscles surrounding most of the arterioles in the body and tells them to stay contracted. The resulting vasoconstriction causes limited blood flow through most of the organs and tissues. At rest, particularly in the brain and the heart, the main intrinsic factor that affects blood flow is autoregulation, in which the arteriolar resistance is constantly adjusted to match the pressure of the entering blood to maintain a relatively constant blood flow.
When the body is very active, such as when our ancient ancestors were running to find food or trying to avoid becoming food, the cardiac output is about 25L/min. The majority of this quintupling of blood flow must go to the skeletal and heart muscle so the body can do what it needs to do to survive. In fact, compared to what it receives at rest, during extreme physical exertion, the amount of blood flow to the skeletal muscle increases 28-fold to about 21 L/min and the blood flow to the heart muscle more than quadruples, going from 250 mL/min at rest, to over 1,000 mL/min. The brain is able to maintain its usual blood flow of 750 mL/min, but most of the other organs and tissues of the body see a decrease in blood flow.
For example, the blood flow to the liver and gastrointestinal system drops about 60 percent, from 1.25 L/min to 500 mL/min and the blood flow to the kidneys drops 75 percent, from 1,000 mL/min to only 250 mL/min. Since the change in blood flow is directly related to the fourth power of the change in the luminal diameter this means that the luminal diameter of the arterioles supplying blood to the skeletal muscle must increase by 130 percent and those to the heart muscle by 40 percent. Those to the liver, the gastrointestinal system and the kidneys decrease by about 10 percent. So, how does the body know when to make these changes?
When the body becomes active the main intrinsic factor that affects local blood flow is something called metabolic or functional hyperemia. Increased muscle activity causes the local buildup of several different chemicals that make the muscles surrounding the local arterioles relax. This vasodilation reduces the vascular resistance and increases local blood flow. This is one of the main reasons why the blood flow to the skeletal and heart muscle increases with activity.
In addition, the main extrinsic factor that affects local blood flow with increased activity is an increase in the sympathetic response as well, but with an added twist. Except for the brain, an increase in norepinephrine usually makes the muscles surrounding the arterioles everywhere else in the body contract. This causes an increased vascular resistance and less blood flow. This explains why, with increased physical activity, the blood flow to most of the other organs and tissues is reduced. But with increased activity, more epinephrine is released as well. The muscles surrounding the arterioles that supply blood to the skeletal and heart muscle are unique in that they have specific receptors for epinephrine. Epinephrine stimulates these muscles to relax, reversing the effects of norepinephrine, which reduces the vascular resistance and increases the blood flow to the skeletal and heart muscle.
We have seen that to survive under the laws of nature, the body must follow the rules and take control by making sure that, when it comes to blood flow to specific organs and tissues, the numbers follow the Goldilocks principle and be "just right." Having all the parts in place to maintain this type of control requires, not only that the system be irreducibly complex, but also have anatural survival capacity to be able to do exactly what has to be done and at the right time. In fact, to do all of this the body must inherently know that Q = P/R and the change in Q is directly related to the fourth power of the change in the luminal diameter.
This completes our discussion of cardiovascular function and how the body makes sure that its trillions of cells get what they need to live, grow, and work properly. However, life is a dynamic process and our earliest ancestors would have had to have remained very active to win the battle for survival. The body does not live within the imaginations of evolutionary biologists but within the laws of nature in which battles involves injuries. Injury to blood vessels leads to bleeding, which if serious enough can be fatal. That's what we'll start to look at next time.
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