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Understanding Cardiovascular Function: Sodium Control as an Irreducibly Complex System

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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.jpgAs we saw in the last article in this series, sodium (Na) is vital for life. When table salt (NaCl) dissolves in the body’s water, it naturally breaks up into Na+ and Cl ions. The sodium-potassium pumps in the plasma membrane of the cell use energy to push Na+ ions out of the cell so it can control its chemical content and volume. This activity results in the fluid outside the cells (extracellular) having more than 90 percent of the body’s sodium, and being ten times more concentrated with Na+ ions than the intracellular fluid.

Moreover, since water generally follows Na+ ions wherever they go in the body, this means that this relatively high concentration of sodium in the fluid outside the cells is responsible for its water content and blood volume as well. Furthermore, the laws of nature make the body constantly lose sodium from the gastrointestinal system, perspiration, and urine formation. Therefore, in following the rules, when it comes to having enough sodium to survive, the body must take control. Here’s one of the main ways it does so.

Recall, the first thing you need to take control in such a context is a sensor that can detect what needs to be controlled. Just like when air is pumped into a balloon or a tire, as blood flows through a blood vessel, or into a chamber, the force it applies against the walls causes them to stretch. Some of this wall movement is due to the amount of blood within the circulation, which is a reflection of its water content. However, since the presence of water in the blood is dependent on its sodium content, this means that the wall motion that takes place as blood flows into a blood vessel or chamber is also a reflection of the body’s sodium content. One set of sensors that detect this wall motion is located within the kidneys, where blood enters to be filtered. And another set of sensors is located in the walls of the upper chambers of the heart (atria).

The second thing you need to take control is something to integrate the data by comparison with a standard, decide what must be done, and then send out orders. The sensory cells in the kidneys release a hormone, called renin, in an amount that is inversely related to how much wall motion they detect. The more the walls stretch, indicating more blood volume, the less renin is sent out, and the less the walls stretch, indicating less volume, the more renin is sent out.

In contrast, the atrial cells send out a hormone, Atrial Natriuretic Peptide (ANP), in an amount that is directly related to how much wall motion they detect. The more the walls stretch, indicating more blood volume, the more ANP is sent out, and the less the walls stretch, indicating less blood volume, the less ANP is sent out. A natriuretic is a chemical that causes the excretion of sodium (in Latin, natrium means sodium).

The third thing you need to take control is an effector that can do something about the situation. Renin is an enzyme that acts on angiotensinogen, a protein made in the liver, to form another protein called angiotensin I. Angiotensin I is then acted upon by an enzyme in the lungs to produce angiotensin II. Angiotensin II not only stimulates the thirst center but also our appetite for salt.

Angiotensin II attaches to specific angiotensin II receptors on certain cells in the adrenal glands and tells them to release a hormone called aldosterone. Aldosterone attaches to specific aldosterone receptors on the cells lining some of the tubules in the kidneys and tells them to bring more sodium back into the body.

The final effect of renin is to make the body increase its sodium content by taking in more salt and taking back more Na+ ions from the urine in production. In contrast, ANP reduces our desire for salt and blocks the release of renin and aldosterone. It also attaches to specific ANP receptors on the tubules in the kidneys and tells them to release more sodium into the urine. The ANP released from the atria in the heart acts as a counterbalance to the renin that is released by the sensory cells in the kidney.

The way our body makes sure it has enough sodium is not as simple as just taking in more salt. Neither is it as simple as having properly working gastrointestinal and renal systems. To control its sodium content, the body needs (1) the sensory cells in the kidneys with stretch receptors that produce (2) renin, which converts (3) angiotensinogen into angiotensin I that is then converted by (4) a specific enzyme in the lungs into angiotensin II which attaches to (5) specific angiotensin II receptors on (6) certain adrenal cells that produce aldosterone which attaches to (7) specific aldosterone receptors on (8) certain tubule cells in the kidneys, and (9) atrial cells with stretch receptors that produce (10) ANP which attaches to (11) specific ANP receptors in the adrenals and kidneys.

If any one of these eleven parts were missing, the whole system would fail. In that case the body’s ability to control its sodium content would be lost, resulting in death. In Darwin’s Black Box, biochemist Michael Behe has characterized as “irreducibly complex” any system where the absence of a single part renders it useless. Explaining how such a system could arise via the unguided Darwinian mutation/selection mechanism, where each piece of the system must contribute an advantage even before the whole is complete, remains a major stumbling block for modern evolutionary theory. The system our body uses to control its sodium content would seem to demonstrate irreducible complexity.

Yet the challenge for Darwinian evolution goes deeper. For when it comes to life and death, real numbers have real consequences. By way of analogy, picture the driver of a car, who, to keep her car running properly, must do the right thing in response to what the fuel gauge tells her. So too, when it comes to the body’s sodium content, each of the components mentioned above must do its job almost unerringly.

Evolutionary biologists tell imaginative, theoretical stories about how control systems like this may have arisen. However, a person who daily practices applied human biology (medicine) can hardly help noticing something. These theories seems to assume that life occurs in a vacuum instead of being, as a physician knows well, subject to the highly demanding laws of nature. When it comes to sodium, how do real numbers impact life? That’s what we’ll look at next time.

Image: � Sebalos / Dollar Photo Club.