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Blood Pressure and the Goldilocks Principle


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.jpgMy previous article in this series showed that having enough blood pressure is required to have sufficient blood flow to the organs so they can work properly. However, having too high of a blood pressure can be problematic as well. When it comes to blood pressure, and most of the other chemical and physiological parameters of life, the body must follow the Goldilocks principle. The numbers have to be “just right,” not too high and not too low, for the body to survive within the laws of nature, a vital aspect of life that evolutionary biologists never seem to address.

Using electrical devices without destroying them depends on having the right amount of electricity going through them. The same applies to the organs and tissues of the body as blood flows into the capillaries to supply them with the water and nutrients they need to live and work properly.

To get energy to your home, generating stations send out electricity along transmission lines that carry hundreds of thousands of volts. This electrical energy is scaled down for consumer use by passing through substations and transformers, finally entering the home through wires carrying a safe voltage. Similarly, the mean arterial pressure (MAP) as the blood leaves the left ventricle and enters the aorta is about 100 mmHg, and by the time it travels through the arteries and arterioles to the capillaries, has been scaled down to about 35 mmHg. This reduction in pressure is accomplished by the vascular resistance applied to the blood as it flows from the larger arteries through the smaller branching arteries and in particular, the microscopic arterioles which lead into the capillaries.

As the blood enters the capillaries under pressure, this causes some fluid to be pushed out of the circulation into the tissues, just like how water is squeezed out of a sponge. As noted previously, when the pressure on entering the capillary is about 35 mmHg, most of this filtered fluid is brought back into the circulation by the osmotic pull of albumin. This usually keeps the amount of fluid between the cells where it should be for proper organ function. Also, as noted a couple of articles back, if the MAP rises above 100 mmHg, the brain is at risk of having too high of a pressure as blood enters its capillaries, so the body protects itself by using autoregulation to contract the muscles surrounding its arterioles.

But, there are definite limits to how much vascular resistance the arterioles in the brain can apply to protect itself. In fact, if the MAP persistently stays above 170 mmHg (BP of 250/130) this can result in a hypertensive crisis. When this happens, too much fluid from the capillaries can enter the tissues and cause swelling of the brain. This usually manifests as headache and dizziness followed soon after by confusion and lethargy. If not quickly relieved, it can progress to coma and can be fatal.

Another way that too high of a blood pressure can quickly cause severe debility and even death, is related to the physical limitations of the blood vessels. After all, life doesn’t take place within a vacuum or within the imaginations of evolutionary biologists. Just like in pipelines carrying water or oil, real numbers have real consequences, and when the pressure inside them is higher than what their walls can withstand, an acute rupture can take place. Probably the commonest location for this type of hemorrhage is within the brain. The brain is located in the skull, which protects it from physical injury. However, since bone is harder and less compliant than soft tissue, bleeding within the brain can cause a quick buildup of pressure, since the blood has nowhere to go.

Nerve cells do not like being put under pressure. Think of what happens when you hit your funny bone and feel tingling in your pinky for several seconds. The tingling takes place because the nerve traveling inside the bony channel of the elbow has suffered a pressure injury and malfunctions temporarily. When pressure like this builds up in the brain, it often causes headache and dizziness which, depending on the severity and location of the bleed, can quickly progress to confusion, lethargy, and coma.

For our earliest ancestors to survive long enough to reproduce, their blood pressure would have had to have stayed within a specific range. The brain generally doesn’t have to use autoregulation when the MAP is between 70 and 100 mmHg (BP of 90/60 – 140/80). However, chronic elevations of MAP above 110 mmHg (BP of 140/90) can often lead to heart disease, stroke and kidney disease. A chronically elevated MAP causes an increased strain on the inner lining of the arteries, eventually resulting in endothelial damage and hardening of the arteries.

Over 95 percent of people with high blood pressure have primary idiopathic essential hypertension, which means we really don’t know what causes it. It is a silent and serious condition that is very prevalent in our modern society, even in teenagers as the incidence of obesity rises within this age group. The remaining five percent have secondary hypertension, usually related to kidney disease. The higher the chronic elevation in blood pressure, the more likely and faster these complications of heart and kidney disease and stroke can occur. People with hypertension are also at increased risk of having episodes of hypertensive urgency and crisis, which, as noted above, can be fatal.

Rembember, the three main factors that affect blood pressure are how strong and fast the heart pumps, how much blood is in the arteries, and the peripheral vascular resistance applied by the downstream arterioles. It is interesting to note that the treatment of hypertension usually consists of medication that reduces the rate and force of heart contraction (beta blockers), pushes sodium and water out of the body through the kidneys (diuretics), and relaxes the muscles surrounding the arterioles to reduce the peripheral vascular resistance (vasodilators). It would appear that it is only in this modern age that medical science been finally been able to figure out what the human body has already inherently known since it first came on the scene.

Since the various functions of the body must follow the Goldilocks principle, being “just right” when it comes to the numbers, evolutionary biology’s explanation for how life came into being by just looking at its various parts without considering what it takes for it to survive within the laws of nature should be viewed as, not only inadequate but also unscientific.

Now that you understand how the body makes sure its cells get what they need through the circulation of blood by way of the cardiovascular system with the right amount of blood pressure, it’s time to consider another dilemma. Remember, life is a dynamic process that must come up with innovations to survive within the laws of nature. When our earliest ancestors were literally fighting to win the battle for survival, how did their bodies make sure that the necessary increase in cardiac output would result in it going where it was needed, to organs like the heart and tissues like the muscles, while at the same time preserving it within the brain?

Next time we’ll answer this question and you can ask yourself why students aren’t being taught these “facts of life.”

Image credit: Arthur Mee and Holland Thompson, eds. The Book of Knowledge (New York, NY: The Grolier Society, 1912) [Public domain], via Wikimedia Commons.