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.

Since they are made up of atoms and molecules, our bodies must follow the laws of nature. For the cells to live and work properly, maintaining a sufficient blood volume to feed them, the body needs to have enough water. In the previous article in this series, I showed that the water content of the body is mainly controlled by Anti-Diuretic Hormone (ADH). The osmoreceptors in the hypothalamus are shrink-sensitive and they send out nerve messages reflecting their volume. The more they shrink, the more frequent the messages, and the less they shrink, the less frequent the messages.

The amount of ADH released into the blood from the posterior pituitary gland is based on the frequency of these signals. The higher the frequency, the more ADH is sent out, and the lower the frequency, the less ADH is sent out. ADH stimulates the thirst center and also attaches to specific receptors in the kidneys, telling them to conserve water by reducing how much urine they make. The more ADH is sent out, the thirstier you are and the less urine you make, while the less ADH is sent out, the less thirsty you are and the more urine you make.

It is the presence of the osmoreceptors, the storage of ADH in the posterior pituitary gland, and the ADH receptors that, together, allow the body to take control so it can follow the rules to stay alive.

Michael Behe defines an irreducibly complex system as one that consists of several interrelated parts where the absence of any one of them renders the system non-functional. Assembling such a system requires planning and foresight since function isn’t achieved until everything is in place. As an example, he gives the common mousetrap, which consists of a platform, a hammer, a spring, a catch, and a holding bar. Take away any one of these parts and the mousetrap can’t trap mice.

So too, one sees that to control its water content, the body needs osmoreceptors in the hypothalamus, posterior pituitary gland cells to send out ADH, and ADH receptors in the kidneys.

However, it’s important to note that a system such as a mousetrap doesn’t operate in a vacuum. When it comes to the job of trapping mice, real numbers have real consequences. The mouse has a certain size, power, and speed. Therefore the parts of the trap must meet particular specifications. The platform has to be sturdy enough, the hammer has to be firm enough, and the spring has to have enough recoil power to move the hammer with enough speed and force to trap the mouse.

Similarly, when it comes to controlling the body’s water content, the osmoreceptors must be sensitive enough and send the right information to the right place. The posterior pituitary gland cells must send out the right amount of ADH. And the thirst center and the kidneys must respond to ADH well enough.

Experience tells us that when we’re working or playing outside in the heat, and we don’t drink water, we tend to produce small amounts of concentrated urine. Conversely, if we’re relaxing in an air-conditioned room, and we drink a lot of our favorite beverage, we soon pass large amounts of dilute urine.

In the first scenario, the continued loss of water makes our cells shrink and stimulates our osmoreceptors, causing the release of more ADH. Besides stimulating the thirst center to tell us to drink, the ADH travels in the blood to the specific tubules in our kidneys and tells them to bring more water back into our body. This results in the release of small amounts of concentrated urine.

In the second scenario, the increase in water content causes an increase in cell volume, inhibiting the messages from our osmoreceptors which causes less ADH to be released into the blood. Besides reducing our desire to drink, the lower amount of ADH tells the specific tubules in our kidneys to bring less water back into our body. This results in the release of large amounts of dilute urine. This is the way our body normally makes sure that, no matter what we’re doing or how much water we drink, we always have the right amount of water.

Water makes up about 60 percent of the body by weight and the average adult has a total of about 42 liters. With normal activity the body loses about two to three liters of water per day. However, high levels of activity, especially in a hot and humid environment, can result in losses of one to two liters of water per hour.

Remember, the cells must control their volume and chemical content and if there’s not enough water in the body both of these will be adversely affected. And there has to be enough water in the body to make sure there’s sufficient blood in the circulation. A water loss of about 5 percent (two liters) results in a dry mouth, increased thirst, and fatigue. If this is not corrected and it progresses to 10 percent (four liters), then extreme thirst, headache, and moderate sluggishness and weakness take place. A loss of 15 percent (six liters) results in heart flutter, dizziness, and problems with concentration. At 20 percent water loss (eight liters) there is increased confusion and lethargy, which can lead to coma. At about 25 percent (ten liters) water loss, death from dehydration follows.

The gastrointestinal system readily absorbs water and the kidneys filter water out of the circulation at a rate of about 7.5 liters per hour (180 liters per day). If none of this filtered water were to be taken back into the body, we would die in about 90 minutes. Thus 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. This represents about twice the amount of water loss needed to cause death (ten liters). In other words, if the remaining 10 percent of water that the kidneys don’t automatically take back is lost, the body can live for just over 12 hours.

The osmoreceptors adjust the release of ADH with every 1 percent change in the body’s water content (400 ml, or14 oz). This represents only 4 percent of the water loss needed to cause death. So, just like the fuel gauge on the dashboard of a car, the control system for water content gives the body fair warning (thirst) about impending problems to which it must attend. Also, the effects of ADH on kidney tubule function normally results in the recovery of water rising to above 99 percent, leading to the excretion of only 1-1.5 liters of urine per day. It certainly looks like the system that controls water in our body knows what it’s doing and, with the help of the gastrointestinal system and the kidneys, keeps the numbers where they need to be.

It’s difficult to explain how the components for water control came together as a functional system in the presumed evolutionary past. For when it comes to maintaining life, real numbers have real consequences.

Our very existence means that this system works within the laws of nature. One wonders how the posterior pituitary gland knows how much ADH should be sent out in response to a given signal from the osmoreceptors and how the thirst center and the kidneys know how to respond to achieve adequate control of the body’s total water content. Evolutionary biology must explain how life actually works, not just how it looks.

Image: beeboys / Dollar Photo Club.