Why is it, that when I’m cool and relaxed, drinking several glasses of my favorite beverage, soon after I have to pass lots of dilute urine, whereas, when I’m working or playing hard in the heat and humidity (without drinking water) I can go hours before I pass only small amounts of very concentrated urine?
As noted in the previous two articles, water is vital for life. By weight, it makes up three-fifths of your body. The water in your body is located either within your cells, or between and surrounding them, or in your blood. If your cells don't have enough water, they malfunction and die. And if there's not enough water in your blood, your circulation can't provide your cells with what they need, and your body dies. In general, a 25% loss of total body water results in death. The fluid that’s between and surrounding your cells acts as a bridge for water to go back-and-forth between the cells and the circulation. Your body is always losing water by breathing out water vapor, evaporating water from your skin (especially when you perspire), and, through the gastrointestinal system and the kidneys which make urine. Unlike oxygen, which the body can't store and must breathe in every few seconds, it is able to store water within its cells. As your body loses water, your cells transfer some to the circulation to make up for its losses so you can keep on going. As this water loss takes place it causes all of the cells in your body to shrink a little and certain "shrink sensitive" cells in the hypothalamus (called osmoreceptors) signal your thirst center telling you to drink water. When you drink water, it comes into the body through the gastrointestinal system and is placed into the blood within your circulation. From there the water is sent to your cells to replenish them, plumping them back up to normal size. The osmoreceptors detect this return to normal, stop signaling your thirst center, and the urge disappears but returns in a while because your body is always losing water.
Although the body has to breathe every few seconds because it needs oxygen for energy and can't store it, at least it's readily available in the surrounding air. The same cannot be said for water. By necessity, the bodies of our earliest ancestors would have had to have been able to store water in their cells to support their circulation because, by experience, we know that their bodies were always losing water but it wasn't always available for them to drink. So, being able to conserve water would have been another very important body function for survival, but how? As noted above, your body loses water through the lungs, the skin, the gastrointestinal system and the kidneys. You can't stop breathing, and you can't stop losing water through your skin or the gastrointestinal system either. That just leaves the kidneys.
Every day, the water your kidneys filter out of the blood equals almost five times the water content of your body. No matter what your water content is, about 90% of this daily filtered water is automatically brought back into your body. But, that still leaves the remaining 10%, which represents about one-half of the body's water content. Since the body dies if it loses 25% of its water, this means that if none of this remaining 10% were brought back into the body, it would die in about 12 hrs.
When you work or play hard in the heat and humidity, your body loses a lot of water and, as noted above, the osmoreceptors in your hypothalamus signal your thirst center, telling you to drink. But they also do something else. They signal the release of a chemical called anti-diuretic hormone (ADH). ADH travels in the blood, attaches to specific ADH receptors in some of the cells within your kidneys, and tells them to bring more water back into the body from the urine presently in production. So, the harder you work and play in the humidity, the more water your body loses and the more your cells shrink. This causes the osmoreceptors in your hypothalamus to not only send out stronger messages to your thirst center but also signal the release of more ADH which goes to your kidneys and tells them to bring back more and more water. That's why you can go hours before passing just small amounts of concentrated urine. However, if you're cool and relaxed, drinking several glasses of your favorite beverage, your cells tend to plump up. This causes the osmoreceptors in your hypothalamus to turn off their messages to your thirst center and signal a reduction in the release of ADH, which tells your kidneys to release more water. That’s why you have to pass lots of dilute urine soon afterward.
Three Questions for Mr. Darwin
1) How could such a control system, that requires osmoreceptors, ADH, and ADH receptors, have developed within intermediate organisms which lacked one or more parts?
2). How do my osmoreceptors know how much ADH needs to be sent out for a given amount of water loss, or gain, to keep my body working properly and prevent death?
3) Where is the information that tells my body how much water it needs to survive, what to do when things go wrong, and where did it come from?
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Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.
Copyright 2017 Dr. Howard Glicksman. All rights reserved. International copyright secured.
