The Thirst Motive
This is next to hunger in strength. In some instances thirst may be more vital and impelling as organism can survive for shorter period with thirst than hunger. Thirst and drinking are controlled by processes within the body. How does the body conserve its water and maintain the concentrations of bodily fluids?
These concentrations are regulated by elaborate physiological mechanisms in which a hormone plays a vital role. The hormone is ADH (Anti Diuretic Hormone) that controls the loss of water through the kidneys. These mechanisms are not directly involved in the thirst drive and the resultant drinking. The same factors, however, involved in conservation of water also trigger the thirst drive. While water is being conserved physiologically, more water is being taken in by drinking to make up water deficits.
Two conditions of the body, in fact, trigger the thirst drive and drinking water. The two conditions are (i) loss of water from cells and (i) reduction of blood volume. When water is lost from bodily fluids, water leaves the interior of cells, thus dehydrating them. In the interior, or front, of the hypothalamus are nerve cells called osmoreceptors which generate more impulses when they are dehydrating.
These nerve impulses act as a signal for thirst and drinking. This is called cellular dehydration, or thirst triggered by the loss of water from the osmoreceptors. Decrease in water quantity in the body also results in hypovolemia or fall in the volume of the blood. Blood pressure also goes ‘down with the going down of the blood volume. The drop in blood pressure stimulates the kidneys to release an enzyme called renin. Renin again is involved in formation of angiotensin II that circulates in the blood and can trigger drinking’.
We can observe both mechanisms at work when we are back from a tennis court after a sweaty game. Our body has lost water (by sweating), the osmoreceptors have, been dehydrated and the blood volume has gone down. Thirst is, now, triggered and we drink to rehydrate the cells and to bring blood volume back to its normal level.
Now, how does the drinking stop? The cessation of drinking is probably under the control of stimuli from mouth and stomach. Mouth is perhaps the ‘meter’ which inhibits further drinking on satiety. Drinking, obviously, stops because enough water has been taken in to rehydrate the osmoreceptors plasma and to restore the blood to normal.
As we see hunger and thirst are the motives and food and drinking are the means to satisfy them. Similarly parental or offspring is the motive while sexual behaviour is the means to accomplish the goal. Every organism has been predisposed to an innate desire to have a young one (offspring) in one’s own place in whose form to continue one’s entity or survival. This sublime and legitimate motive has been channelized through reproduction which can only take place by union of two mates of opposite sex. Had there not been that much attraction in sex, none of the organisms would have ever been prepared for that colossal under taking of reproduction and bringing up a young one.
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