thirst ship sheep sailor evolution

When the Topic is “Thirst”

A sailor’s wisdom is definitely on target

In my younger years I was a sailor. I worked on cargo ships to see the world and that’s how I know what life is like to be one of the crew. A sailor’s saying goes “Thirst is worse than homesickness” and although a seaman’s thirst is a very special kind of thirst, our seafaring folk sure know a thing or two about some basic drives. Thirst, without question, is one of them. But how do we know we need to drink and how do we know we’ve had enough? It’s not easy as we all know, but you will be able to understand from the following account why it would have been difficult to do the experiments (that I am about to describe) on a human being and why instead sheep were chosen.

When a thirsty sheep, minutes before it was allowed to drink, was given sufficient water through a tube directly into its fore-stomach (sheep are ruminants like cows) to make up for its body’s deficit in liquid, the sheep still guzzled up almost the same amount afterwards that it would have needed to kill its thirst. So, it ended up containing in its body twice the amount of liquid, it would have needed to quench its thirst in the first place. With all that extra fluid, I bet its blood pressure must have increased quite a bit, but that wasn’t part of the research.

In another experiment a sheep was operated in such a way that the water it was allowed to swallow never reached its stomach, but ran out of a hole in the throat before it would reach the stomach. That water was diverted and collected in a bucket instead. Such sheep drank from 2 to 9 times more over a 2 hour time period than what the body deficit would have required. If the two experiments were combined, i.e., water squirted into the fore-stomach before the operated animal was permitted to drink freely, it still drank much more than its body actually needed.

Obviously, there is a whole spectrum of stimuli that the animal uses: the sensation of water in the mouth, the number of gulps to swallow the water, the flow of the water down the throat and the stretching of the stomach wall caused by the weight and volume of the water in the stomach. Only when the combination of all of these sensations is correct in timing and sequence, the thirst centre of the brain will register satiation and instruct the animal to stop drinking. If the combination of the various sensations is not right, the desire to drink will not end. To what extent the temperature of the water influences the result was not tested, but it could still be an additional factor.

What is clear, however, is that the control of food uptake is quite different: there, two brain centres (one that excites and one that inhibits) are known to be involved, but an area of the brain which inhibits thirst still has yet to be located and I would bet our sailor friends would tell you from experience that no such centre will ever be found – at least not in a sailor’s brain.

jellyfish lighter cigarette fire

Dear sailor, wanna light a cigarette with a jellyfish?


© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

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