HUNGER AND THIRST
Hunger and thirst may be called visceral sensations, in that they are triggered by internal changes. Hunger is a sensation that seems to be far more complex than was first thought, but thirst seems to be somewhat simpler. The receptors for both senses are specialized cells in the hypothalamus. Receptors for hunger are believed to detect changes in blood nutrient levels, the blood levels of hormones from the stomach and small intestine, and a hormone released by adipose tissue; all of these chemical signals are collected by the hypo-thalamus. The receptors for thirst detect changes in the body water content, which is actually the water-to-salt proportion.
Naturally we do not feel these sensations in the hypothalamus: They are projected. Hunger is pro-jected to the stomach, which contracts. Thirst is pro-jected to the mouth and pharynx, and less saliva is produced.
If not satisfied by eating, the sensation of hunger gradually diminishes, that is, adaptation occurs. The reason is that after blood nutrient levels decrease, they become stable as fat in adipose tissue is used for energy. With little or no digestive activity in the gas-trointestinal tract, secretion of hormones diminishes. With no sharp fluctuations of the chemical signals, the receptors in the hypothalamus have few changes to detect, and hunger becomes much less intense.
In contrast, the sensation of thirst, if not satisfied by drinking, continues to worsen. There is no adaptation. As body water is lost, the amount keeps decreasing and does not stabilize. Therefore, there are constant changes for the receptors to detect, and prolonged thirst may be painful.