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Understanding Cardiovascular Function: Beyond Irreducible Complexity

Howard Glicksman 

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." Dr. Glicksman practices palliative medicine for a hospice organization.

Our body comprises trillions of cells that must follow the laws of nature. These laws demand that to survive, our body must have enough water and that water must be properly distributed between the cells and the blood.

They also demand that water be lost from the body when it breathes (to obtain oxygen and get rid of carbon dioxide), perspires (to control its temperature), and forms urine (to get rid of toxic chemicals). Previously, as we've seen in this series, despite the imperative to lose water due to its metabolism, the body is able to follow the rules and take control of its water content by drinking fluids and conserving water loss from the kidneys.

This takes place through osmoreceptors, Anti-Diuretic Hormone (ADH), and the ability of ADH to stimulate the thirst center and specific ADH receptors in the kidneys. In my previous article I pointed out that this system must have had the capacity to provide the body of our ancient ancestors with the amount of water needed to live and reproduce. But what would have happened if this complex system failed to work properly?

Water makes up about 60 percent of the body by weight and the average adult has a total of about 42 liters. Our cells must keep their volume and chemical content within a narrow range. This ability is directly related to the body's total water content. A water loss of about 5 percent (two liters) results in a dry mouth, increased thirst, and fatigue. If this isn't corrected and progresses to 10 percent (four liters), then extreme thirst, headache, and moderate sluggishness and weakness take place. A loss of 15 percent (sxi liters) results in heart flutter, dizziness, and problems with concentration. With a water loss of 20 percent (eight liters) there comes confusion and lethargy, which can lead to coma. And at about 25 percent water loss (ten liters), death follows from dehydration.

At complete rest the body loses about one liter of water daily due to respiration, perspiration, and urine formation. That's why, when a debilitated person stops all intake, he usually dies from dehydration in about ten days.

The gastrointestinal system readily absorbs water, and the kidneys filter water out of the circulation at a rate of about 7.5 liters/hour (180 liters/day). If none of this filtered water were to be taken back into the body, we would die in about 90 minutes. The tubules in the first part of the kidney automatically take back about 90 percent of the filtered water. However, that still leaves the remaining 18 liters of water that is filtered out daily.

If our ancestors lacked osmoreceptors, ADH, or ADH receptors, just to stay alive at complete rest would have required them to take in 18 liters of water per day. This means that every hour of every day of every year of their existence (probably not very long), they would have had to empty their bladder while at the same time drinking 750 mL (26 oz) of water. Obviously, the absence of osmoreceptors, or ADH, or ADH receptors is incompatible with life.

When most people hear the word diabetes, they think of diabetes mellitus, the "sugar disease." The term diabetes refers to diuresis or excessive urination. The term mellitus refers to honey or sweetness. A person with diabetes mellitus passes excessive amounts of sweet urine.

However, there is another, less common, condition that is also called diabetes, in which the person passes excessive amounts of urine that is bland, tasteless, or insipid. Diabetes insipidus (DI) takes place when there is a deficiency in the effect of ADH within the body. This manifests as passing at least four liters of urine daily, but the more severe the deficiency the more urination takes place.

Diabetes insipidus occurs from a reduced production of ADH in the hypothalamus or posterior pituitary gland (central or neurogenic DI) or a reduced response by the ADH receptors in the kidneys (nephrogenic DI). Without treatment it is difficult to remain very active while at the same time being able to maintain adequate total water content. Today, there is a treatment for DI called desmopressin, which is inhaled through the nose.

However, DI is not the only medical condition that exists in relationship to ADH. Just as not having enough of a response to ADH can cause debility and death, so too can having too much. The Syndrome of Inappropriate ADH (SIADH) is a condition, often due to cancer or brain injury, in which, due to excessive ADH activity, the body holds on to too much water.

This leads to a significant drop in the body's total chemical concentration and makes water move into the brain cells by osmosis. This can lead to cerebral edema and brain malfunction resulting in lethargy, weakness, and confusion that can rapidly progress to seizures, coma, and death. Fortunately, once SIADH is diagnosed there is medical treatment available that can prevent debility and death.

Being able to control the body's water content is a matter of life and death. So, in explaining how life came into being, it's important to remember a few things.

Human life would be impossible without the presence of osmoreceptors, ADH, and ADH receptors. As Michael Behe might say, the system the body uses to control its water content is irreducibly complex. Evolutionary biology needs to explain how all of these parts came together as a functioning system.

Moreover, as I've often said in this series, when it comes to life and death, real numbers have real consequences. The presence of osmoreceptors, ADH, and ADH receptors is not good enough to explain how the body controls its water content. The osmoreceptors have to respond fast and well enough to give the body proper warning. The posterior pituitary gland has to send out the right amount of ADH, not too much and not too little. Finally, the ADH receptors have to respond in the right way as well.

So, while irreducible complexity is a good indicator of intelligent design, in maintaining life, an irreducibly complex system must also have natural survival capacity. By this I mean the system must know what is needed to survive within the laws of nature and then do that naturally.

Now that you understand how the body controls its water content to make sure it has enough blood in the circulation, it's time to move on. Life is a bit more complicated than evolutionary biologists would have us believe, isn't it? In addition to water, there's another very important chemical the body has to control to maintain its blood volume. What is it? For that we'll have to wait until next time.