Why do two different parts of the body indicate an unmet need for the same system?
In: Biology
Put simply, both thirst and hunger sensations are primarily mediated by the brain. However, they are both controlled by separate hormonal circuits.
Thirst is mostly a consequence of activity by antidiuretic hormone (ADH or vasopressin), which is released from the posterior pituitary gland. ADH is produced when the blood becomes excessively concentrated (i.e., there isn’t enough water to dilute out the electrolytes and solute components of the blood) or, to a lesser extent, when your blood volume is low. ADH can also be produced under some states of stress. While ADH works on many receptors, it also helps stimulate a thirst response in the brain, which motivates you to seek and consume water/liquid.
Hunger is a bit more of a complex hormonal pathway. Leptin and ghrelin are the hormones mentioned most frequently. Leptin (among other hormones) mediates a sensation of satiety and is found in higher concentrations within ~20-30 minutes after you eat. Leptin tells your hypothalamus that you’re full, motivating you to stop eating. Ghrelin, on the other hand, is the “hunger hormone” produced by the stomach and found at higher levels after periods of fasting. It tells your hypothalamus to go find food and eat, as a prolonged period of fasting may indicate that your body is about to run low on energy or may have to rely on its backup stores.
You feel hungry when your stomach is empty, which is why *it* tells you when it’s time to eat. You feel thirsty when your mouth is dry, which is why *it* tells you when it’s time to drink something.
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