This is the 2007 version. Click here for the 2017 chapter 09 table of contents.

Thirst

Walter B. Cannon speculated that thirst was triggered by the sensation of a dry mouth. Hunger, he thought, was due to stomach contractions. But we have seen that hunger involves much more than stomach contractions. Similarly, thirst involves much more than a dry mouth. Gilman (1937) showed that injecting dogs with a salt solution caused them to drink more than normal. When salt levels in the blood are high, this draws water out of the body's cells. So Gilman's experiment suggested that loss of water from the cell interior due to elevated salt levels in the bloodstream was a stimulus for thirst.

What is the "double-depletion hypothesis"?

However, excessive loss of blood can also cause thirst. This led to the double-depletion hypothesis : that there are two variables the body attempts to optimize or protect through thirst. One is the volume of water inside cells; the other is the volume of water in the whole body.

Experiments with rats confirm that several different factors contribute to thirst-quenching:

1. The amount of water passing through the mouth has some effect. Even if water is siphoned away from the mouth so it does not reach the stomach, thirsty rats will eventually stop drinking.

What factors contribute to thirst-quenching in rats?

2. The fluid capacity of the stomach sets an upper limit on the amount drunk. When the passage from the stomach to the large intestine is pinched off, thirsty rats keep drinking until their stomachs fill up, then they stop.

3. The amount of water that the bloodstream can absorb sets a limit on drinking. If rats are given salt water, so water enters the bloodstream but not the cells, they drink abnormally high amounts. They become bloated, but they do not burst—they eventually stop.

4. Fourth and most importantly, water enters the cells of the body. This is the stimulus that most directly satisfies thirst and prevents bloating.

Salt water cannot enter cells. That is why sailors can die of thirst when stranded on the ocean. Drinking the salt water will do them no good, because it cannot replenish the water levels inside their cells.


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