Controlling Blood Pressure Requires an Irreducibly Complex System
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 body is a multi-cellular organism that requires the circulation of blood within its cardiovascular system to give its cells what they need to live, grow, and work properly.
In the last article in this series, I explained what blood pressure is -- the force that blood exerts against the walls of the large systemic arteries as it flows through them.
Since blood has mass, its flow within the body is prevented by natural forces such as inertia, vascular resistance and gravity. The heart pumps the blood throughout the circulatory system and it is the blood pressure that represents the driving force for blood flow. When the left ventricle contracts, it pumps more blood into the systemic arteries, which causes the blood pressure to rise to a maximum, called the systolic pressure.
During diastole, while the heart is relaxed, the blood in the large systemic arteries rebounds back and forth between the arterioles and the closed aortic valve as some of it makes its way into the capillaries within the tissues. This causes the blood pressure to slowly drop, reaching its nadir just before systole and is called the diastolic pressure.
The three main factors that affect the blood pressure are the cardiac output, the blood volume and its distribution within the cardiovascular system, and the peripheral vascular resistance of the arterioles. In general, the more cardiac output, blood in the arteries, and peripheral vascular resistance, the higher the blood pressure and the less cardiac output, blood in the arteries and peripheral vascular resistance, the lower the blood pressure.
Life is a dynamic process in which the physiological functions of the body are always in a state of flux. Evolutionary biologists claim to have explained how human life has come about, but they only describe how it looks. What about how it actually works within the laws of nature to survive? Think about it. You are always on the move: going from lying down to sitting and standing up, from walking to running and jumping, from crouching and crawling to kneeling. All of these changes in position affect the blood pressure and how effective the cardiovascular system is in giving the tissues what they need to live and work properly. That's what the bodies of our earliest ancestors would need to have been able to do to survive. I will now look at some of the ways that the body takes control to maintain an adequate blood pressure.
Three of the most important chemicals involved in blood pressure control have already been mentioned within another context in previous articles. Norepinephrine and epinephrine, the neurohormones of the sympathetic nervous system, act quickly, within a split second. Angiotensin II, a hormone that comes about from the action of renin, secreted by the kidneys, and Anti-Diuretic Hormone, sent out by the posterior pituitary gland, are slower and usually act within a few minutes. It is important to realize that the effects of these chemicals is limited to only several minutes which allows the body to maintain moment to moment control of its blood pressure. The sensors, integrators and effectors that make up each of the systems for these chemicals to affect blood pressure will be looked at one at a time below.
There are sensors located in the main arteries directly supplying blood to the brain, which can detect wall distension. These are the baroreceptors, which by sensing the stretching within the arterial walls are able to detect the arterial blood pressure. They are a type of mechanoreceptor that senses movement, in contrast to the chemoreceptors which detect chemicals like oxygen, carbon dioxide and hydrogen ion. The baroreceptors send their data on the blood pressure by way of nerves to the brain. The brain integrates this information, and if the blood pressure is too low, it causes the release of more norepinephrine and epinephrine from the sympathetic nerves. By attaching to specific receptors, increased sympathetic stimulation affects all three of the factors mentioned above, which makes the blood pressure rise.
As noted previously, it makes the heart pump harder and faster, which increases the cardiac output. In addition, it causes the kidneys to absorb more Na+ ions and more water by the release of more ADH, which increases the blood volume. It also stimulates the systemic veins to send more blood back into the systemic arteries. Finally, it tells the muscles surrounding the arterioles to contract more which increases the peripheral vascular resistance. All of these actions combine to increase the blood pressure. However, if the blood pressure is where it should be or higher than normal, the sympathetic system releases less of these neurohormones, usually at only a basal rate.
As we've already seen, there are wall motion sensors located within specialized cells within the kidneys where the blood enters to be filtered. These sensory cells release a hormone, called renin, in an amount that is inversely related to how much wall motion they detect. The more the walls stretch, the less renin is sent out, and the less the walls stretch, the more renin is sent out. Renin is an enzyme that starts a chemical reaction which results in the formation of a hormone called angiotensin II. By attaching to specific receptors, angiotensin II affects two of the three factors that make blood pressure rise.
First, it causes the body to take in more salt and water and the kidneys to hold onto more as well. All of these actions increase the blood volume. And second, as its name denotes, angiotensin II makes the muscles surrounding the arterioles constrict, causing a rise in the peripheral vascular resistance. In fact, it is the most powerful vasconstrictor in the body, even more than norepinephine. Both of these actions make the blood pressure rise.
The osmoreceptors in the hypothalamus, which detect the water content of the body, are shrink sensitive and affect the release of ADH. The less water in the body, the more they shrink, and the more ADH they cause to be sent out by the posterior pituitary gland. And the more water in the body, the less they shrink and the less ADH is sent out. By attaching to specific receptors ADH affects two of the three factors that impact the blood pressure. More ADH causes the body to take in more water and the kidneys to bring back more from the urine in production, all of which increases blood volume. Another name for ADH is, vasopressin, which like norepinephrine and angiotensin II increases the peripheral vascular resistance by making the muscles surrounding the arterioles contract more as well. Both of these actions increase the blood pressure.
Each of the three systems mentioned above have their own sensors, integrators, and specific receptors, while using the same effectors to affect blood pressure. Dr. Michael Behe would call each of these systems irreducibly complex because without any one component, each system would fail and life would be impossible. But to anyone who has ever had a momentary feeling of dizziness on standing up, this experience tells us that just trying to explain the simultaneous development of each of these systems, or all of them at once, as difficult as that may be, should not be enough. For, when it comes to life, and being able to stand up to gravity, real numbers have real consequences.
Evolutionary biologists seek to tell us how life came into being. Yet they only even purport to explain how the different parts of the body supposedly came together -- without considering how biological function must also meet specific numerical benchmarks to work within the laws of nature. Maybe that's why the famous British journalist Malcolm Muggeridge was quoted as saying, "I myself am convinced that the theory of evolution, especially to the extent to which it has been applied, will be one of the greatest jokes in the history books of the future. Posterity will marvel that so very flimsy and dubious an hypothesis could be accepted with the incredible credulity it has."
With that in mind, next time we'll look at how, where blood pressure is concerned, the numbers must be just right for us to stay standing.
Image: � Monkey Business / Dollar Photo Club.