Hunger and thirst are sensations arising from diffuse internal stimuli that occur when the body needs food or water. They are among the homeostatic mechanisms that maintain constancy in the internal environment.
Hunger sensations coincide with strong peristaltic contractions of the stomach. However, hunger is more than a stomach sensation, since removal of the stomach or severance of sensory nerves supplying it will not abolish other signs of hunger which include increased body movements, weakness, fatigue, headache, and irritability. An animal soon learns that these symptoms are relieved by the ingestion of food. After this is learned, a more complex conditioned behavior called appetite developsÑa desire for food that is not necessarily associated with true hunger. In contrast to hunger, appetite is pleasant and can be stimulated by the smell, sight, or even the thought of food. Obesity can result if appetite is excessively dissociated from the body's actual needs.
Normally, the ingestion of food and the expenditure of energy derived from it are balanced, so that body weight remains fairly constant. This regulation involves the hypothalamus region of the brain. Animal experiments show that destruction of the ventromedial hypothalamus (VMH), or the satiety center, results in excessive eating (hyperphagia) and obesity, while destruction of the lateral hypothalamus (LH), or the hunger or feeding center, causes complete anorexia (loss of appetite). There is evidence that the hypothalamus contains receptors sensitive to changes in the blood glucose level and brain temperature.
The VMH responds to high blood glucose levels and increased brain temperature, and the LH responds to the opposite situations.
The sensation of thirst is experienced mainly in the mouth as a dry feeling. This is due to a reduction in the secretion of saliva. When there is severe water deprivation, through sweating, diarrhea, excessive urination, or hemorrhage, the secretion of saliva may even stop. The dryness of the mouth gives rise to the impulse to drink, so that the discomfort can be relieved. However, the body has a regulatory system that monitors the amount of water ingested, so that an animal will not drink any more than is needed to supply the body's needs.
The hypothalamus also plays a role in the regulation of the body's water content. Certain hypothalamic cells, called osmoreceptors, are sensitive to osmotic changes in the blood. When the water content of the blood is diminished, either through water deprivation or through excessive water loss, these specialized cells are stimulated to release antidiuretic hormone, which conserves body water by reducing the production of urine. When thirst has been satiated and the water content of the blood has been restored to normal levels, the hypothalamic cells reduce their production of antidiuretic hormone.