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.

As we saw in the last preceding two articles in this series (here and here), for cell life generally, including the blood to feed cells, there must be the right amount of water inside and outside the cells. I explained how it’s mainly the sodium-potassium pumps in the plasma membrane of the cells and the albumin in the blood that, together, make sure the water in the body is distributed properly.

However, what about the body’s total water content? After all, even if the water in your body is distributed properly, if you don’t have enough of it you’re as good as dead. With respect to water, we need to look at how the body takes control to follow the rules of nature.

Experience tells us that, in contrast to breathing, which we must do several times a minute, we only have to take in water several times a day. In dealing with the laws of nature, our body is constantly losing water, mainly through respiration, perspiration, and the formation of urine.

When the cells use glucose to release energy in the presence of oxygen, carbon dioxide and water are formed. Every time we breathe air out of our lungs, we also release water. The amount of water we lose by respiration depends on how hard and fast we’re breathing, which depends on what we’re doing.

The body must also maintain its temperature within a narrow range so its enzymes can work properly. Just like a car engine, our metabolism gives off heat that affects our temperature. The more active we are, the more heat our cells release. One of the main ways the body controls its temperature is by releasing heat through perspiration. The more active we are and the hotter and more humid our surroundings, the more water we lose through this process.

Finally, protein metabolism produces ammonia which is converted by the liver into a more soluble chemical called urea. Just like carbon dioxide, the build-up of ammonia and urea in the body can be toxic. The kidneys continuously filter some of the water in the blood through specialized capillaries. This fluid moves through millions of tubules and becomes more concentrated with urea as it becomes urine.

There is a minimum amount of urine the body must excrete to rid itself of the toxins (like urea) it makes on a daily basis. At total rest, respiration, perspiration, and urine formation together result in the body losing about one liter of water per day. With increased levels of activity or symptoms of fever, vomiting, or diarrhea, even more water is lost.

Though we are constantly losing water from the circulation, our body is able to compensate for a while thanks to osmosis. With this continuous loss of water, the circulation’s total chemical concentration rises above the interstitial fluid that surrounds the capillaries. This rise in total chemical concentration makes water naturally move, by osmosis, from the interstitial fluid into the circulation.

This loss of water from the interstitial fluid, in turn, raises its total chemical concentration with respect to the intracellular fluid. The higher total chemical concentration in the interstitial fluid now makes water naturally move, by osmosis, from the cells into the interstitial fluid.

The shift of water from the cells into the interstitial fluid and the plasma allows the body to maintain the vital 2/3:1/3 relationship of the intracellular and extracellular fluid while at the same time shoring up losses from the circulation. The end result is a drop in the body’s total water content and an increase in its total chemical concentration.

When we drink water, it enters the blood through the gastrointestinal system and then moves, by osmosis, back into the interstitial fluid and the cells in the opposite direction to replenish them. The total chemical concentration thus returns toward normal.

One can see that the cells act as a reservoir for the water needs of the circulation, and the interstitial fluid is the bridge that connects the two. However, there is a limit to how much water the cells can give up to the extracellular fluid before they begin to malfunction and die from dehydration. A cell’s survival depends on being able to keep its volume and chemical content relatively constant. Here’s how the body makes sure it has enough water to survive.

As I’ve noted before in other contexts, the first thing you need to take control is a sensor that can detect what needs to be controlled. When the cells lose water by osmosis, to shore up the circulation, they shrink. In contrast, after drinking water, when the circulation sends water back into the cells, again by osmosis, they expand.

There are sensory nerve cells in the hypothalamus that are shrink-sensitive. They are called osmoreceptors because they can detect these changes in cell volume that take place due to osmosis. By monitoring cell volume, the osmoreceptors sense the total water content of the body. The frequency of nerve messages the osmoreceptors send out is directly related to their cell volume. The more the osmoreceptors shrink, the more frequent the nerve messages, and the less they shrink, the less frequent the nerve messages.

Recall that the second thing you need to take control is something to integrate the data, comparing it with a standard and then deciding what must be done. The osmoreceptors send nerve impulses that result in the posterior pituitary gland releasing the hormone called Anti-Diuretic Hormone (ADH). A diuretic is a chemical that makes the kidneys pass more water, so an anti-diuretic is a chemical that makes them pass less water. The more frequent the signal from the osmoreceptors, the more ADH is released and the less frequent the signal, the less ADH is released.

Recall, the third thing you need to take control is an effector that can do something about the situation. One thing that ADH does is to stimulate the thirst center in the hypothalamus. A high level of ADH will make you very thirsty whereas a low level of ADH will tend not to stimulate your thirst at all.

ADH also travels in the blood to the kidneys and attaches to specific ADH receptors on some of its tubules, telling them to bring water back into the body from the urine that is presently in production. The more ADH sent out by the posterior pituitary gland, the more water is taken back into the body by the kidneys and the less urine is produced. And the less ADH sent out by the posterior pituitary gland, the less water is taken back into the body and the more urine is produced.

Nobody knows how the development of the osmoreceptors, ADH production, and the ADH receptors took place or how any one of them could have been used to control the water content of anything without the other two being present.

But just trying to explain the simultaneous development of these three components, as difficult as that may be, isn’t enough. For even though we know what parts are needed to control the body’s water content, we still need to keep in mind that in dealing with the laws of nature, real numbers have real consequences. Next time we’ll look at how all of this works in real life.

Image: Warren Goldswain / Dollar Photo Club.