Thirsting for Life

Why do we get thirsty?

By weight, three fifths of your body is made up of water.  This water is located either inside your cells, or between and surrounding your cells, or in the blood that circulates throughout your body.  If your cells don’t have enough water, they malfunction and die.  And, if there’s not enough blood in your circulation (usually due to water loss), your body malfunctions and dies.  So, you can see why water is so important for you to be alive. 

Life is a dynamic process where you are constantly losing water by breathing out water vapor, evaporating water from your skin (especially when you perspire), and, from the gastrointestinal system and the kidneys which make urine.  Although your cells can restore some of this lost water to support the blood in your circulation, there is a limit to how much water loss your body can withstand.   In general, a 25% loss of its total water content causes the body to die.  Although your body has ways of limiting water loss, it has only one way to replenish itself, and that’s by drinking.  When you drink water, it enters the gastrointestinal system where it’s readily brought into the blood and from there is sent to your cells to replenish their losses.  But, how does your body know when it needs water and how much is enough?    

As noted above, water is important not only for cell function but also for the circulation.  So, your body has two ways, through the cells, and through the circulation, of knowing about its water content.  When you lose water, since your cells send some over to support your circulation, they shrink a little.  This stimulates certain "shrink-sensitive" cells in your hypothalamus, called osmoreceptors.  With as little as a one percent decrease, in your body water, the osmoreceptors signal your thirst center, giving you more than fair warning about your body’s need to bring in new supplies.  Also, the blood flow into the kidneys is closely monitored by sensory cells.  When this renal blood flow decreases (often due to water loss) these sensory cells send out a hormone (called renin) that eventually causes signaling of your thirst center for you to drink.          

Experience tells us that we can usually ignore these messages of thirst.  But, as time goes by, and our body loses more water, they become stronger and more urgent until we finally give in and drink some water.  The water we drink goes into our blood where it eventually passes into our cells, making them plump up again, bringing them back to normal size.  It also fills up the blood with water so that the blood flow through the kidneys is restored to normal.  The osmoreceptors in the hypothalamus and the sensory cells in the kidneys detect this return to normal and turn off their messages that signal your thirst center and all is calm again.  So, that’s why we get thirsty.

Three Questions for Mr. Darwin

    1. How did my body know that it was going to have to control its water content to survive?

    2. Where did my body get the information to make the osmoreceptors in my hypothalamus and sensory cells in my kidneys, place them exactly where they were needed, and tell them what they’re supposed to do?

    3. Where’s the information in my body that tells it how much to stimulate my thirst center when it decides it needs more water and how does it know when to turn itself off?

Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"

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.

Comments and questions are welcome.

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