Cardiovascular Function: Sodium's Real Life Consequences
Howard Glicksman June 16, 2015 12:51 PM
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.
My last two articles in this series showed that when it comes to sodium and the laws of nature, the body has to take control to follow the rules. It uses sensors to detect wall motion in the blood vessels of the kidneys and the atria of the heart, which reflect the blood volume and sodium content of the body. In response to what they detect, these sensory cells send out hormones called renin and ANP (Atrial Natriuretic Peptide) respectively. The more wall motion, indicating more blood volume, the less renin and the more ANP is released. This, in turn, decreases wall motion, indicating less blood volume, releasing more renin and less ANP.
Renin eventually causes the formation of angiotensin II, which tells the body to take in salt and, through an adrenal hormone called aldosterone, tells the kidneys to hold onto more sodium. In contrast, ANP reduces the desire for salt, blocks the release of renin and aldosterone, and tells the kidneys to release more sodium into the urine. The effects of renin and ANP counter each other to help the body control its sodium content. How does all of this work in real life?
Evolutionary biologists hypothesize about how such irreducibly complex systems could have come about. But anyone who deals with keeping people alive must consider in very practical terms how the laws of nature affect the functional capacity of these systems. It's like trying to imagine how the Saturn V rocket may have come into being without accounting for the energy its propellants would have needed to generate to overcome the gravitational pull of the earth and launch Apollo 11 into space. When it comes to a functional capacity that allows for life, real numbers have real consequences.
Since the body is always losing sodium through the gastrointestinal system, perspiration, and urine formation, one way for it to try to control its sodium content is to take in sodium (table salt). The gastrointestinal system readily absorbs all of the sodium it receives independent of the body's actual needs. But how much sodium is enough and what happens if you don't take in enough or you take in too much? Do you think our earliest ancestors were able to make these determinations and do what was needed to stay alive? Did they perhaps read the sodium content off the nutrition facts label of their favorite packaged foods?
The minimum daily amount of sodium needed is about 500 mg but most people take in about 3,000 to 4,000 mg per day. So how the does the body deal with the excess?
In reality, it is the kidneys that control the body's sodium content. Every hour, they filter 7.5 liters of fluid out of the bloodstream, which represents about one-sixth of the body's total water content. But because the plasma has such a high Na+ ion concentration, in that hour it also filters out about one-half of the body's total sodium content. I noted in a previous article that if the kidneys couldn't take back any of the water they filter, the body would die in about 90 minutes. But if they couldn't take back any of the sodium they filter, it would die in just 30 minutes.
The tubules in the kidneys automatically take back about 85 percent of the sodium they filter. But that still leaves the remaining 15 percent to deal with. Without aldosterone to tell them to bring most of it back into the body, it would only take four hours for sodium to reach critical levels.
In fact, clinical experience shows that the total absence of aldosterone is incompatible with life. The combined effects of aldosterone and ANP tell the kidneys to bring back about 99.5 percent of the sodium they filter so life can continue. The kidneys' control of the water and sodium content of the body results in a normal Na+ ion level in the blood of about 135-145 units. It appears that the system in the body that uses sensors and hormones with their specific receptors to control its sodium really knows what it's doing.
Na+ ion level in the blood above 170 units or below 100 units is fatal. However, it's important to keep in mind that, since water follows Na+ ions in the body, the Na+ ion concentration is directly related to the total water content. Independent of the body's total sodium, if the total water content drops too much, then the Na+ ion concentration will rise. If the total water content rises too much, the Na+ ion concentration will fall.
This means that the body could have too much sodium, but if it also has too much water, the Na+ ion concentration will often drop. Likewise, the body could have too little sodium, but if the body has also lost too much water, the Na+ ion concentration can actually rise. Although rare conditions of too little or too much aldosterone can lead to life-threatening chemical imbalances, the more common causes of serious changes in the Na+ ion concentration are usually related to the total water content of the body, not the sodium content.
Life-threateningly high Na+ ion levels, usually above 170 units, are often due to dehydration. When dehydration occurs, the body has lost too much water and the cells don't have enough to maintain their proper chemical concentration and volume. There is usually a serious drop in blood volume and blood pressure which leads to poor blood flow to the tissues and cell death.
Conversely, life-threateningly low Na+ ion levels, usually below 100 units, are due to the body having too much water. As I have noted before, this not uncommon condition is usually due to excessive ADH activity, which makes the body hold on to too much water. In this setting, the excess water will naturally move, by osmosis, from the extracellular fluid into the cells. If this takes place too rapidly in the brain, it causes confusion, coma, and even death.
When it comes to explaining how human life came into being, it's important to remember that it would have been impossible without the presence of all of the parts needed to control sodium. As Michael Behe would say, the system the body uses to control its sodium is irreducibly complex. So, to begin with, evolutionary biology first needs to explain how all of these parts came together as a functioning system.
However, as we've seen, when it comes to life and death, real numbers have real consequences. The mere presence of sensory receptors in the kidneys and atria, and the production of hormones like angiotensin II, aldosterone, and ADH, with their specific receptors, is not enough to explain how the body controls its sodium. Each component has to do the right thing well enough to keep the Na+ ion concentration in the blood within the right range.
Irreducible complexity is a good indicator of intelligent design, but when it comes to life and death and the laws of nature, there must also be natural survival capacity. By this I mean that the system must know what is needed to survive, given the laws of nature, and then do it naturally. Any theory that tries to explain how life came into being must address not only how it looks, which is all that evolutionary biology tries to do, but also how it works to stay alive.
Now that you understand how the body controls its water and sodium content, it's time to look at another very important chemical for cardiovascular function. When the blood level of potassium rises too high or drops too low it can cause serious neuromuscular and heart problems. That's what we'll discuss next time.
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