The Physiology of Motivation
Your survival is directly dependent on two fundamental operations: body temperature and sufficient water supply. Homeostatic processes ensure these operations run smoothly and when necessary activate emergency systems in an effort to restore balance to the body. When these systems fail — temperatures are too high or too low for too long, or dehydration is present for several days — death is inevitable.
Since protein is one of the major components that propel a cell's activity, when it becomes dormant in temperatures above 113°F, cells cannot perform their responsibilities. Similarly, when water inside cells freezes, ice crystals develop and ultimately shut them down.
When temperature is not at the desired level, physiological adjustments in addition to possible behavioral adjustments are automatically induced in a variety of ways, including sweating, panting, and shivering. The skin's capillaries expand and contract to control blood by either producing excessive amounts of it beneath the surface of the skin (expansion) or reserving the depleting supply of it for organs (contraction). Excessive heat is then released through sweat glands located on the body in the form of sweating, as seen in humans, horses, and cows, or through sweat glands located on the tongue in the form of panting, as seen in dogs, cats, and rats. The act of shivering resolves heat shortages by creating warmth.
Internal Water Supply
Water is essential to human existence because it transports nutrients and oxygen to tissues and helps purge wastes from the body. If you become dehydrated, your basic thirst motivation is triggered and replenishing liquids in your body becomes a primary goal. Because constant water loss is a natural function of the body, it is kept in check by maintaining intracellular and extracellular fluid.
Depletion of intracellular fluid occurs when water outside a cell contains higher quantities of sodium than water inside a cell. The cell then becomes dehydrated and shrinks, sending the message that an adjustment needs to be made.
Kidneys play an important role in this adjustment. Before water is released through urination, the antidiuretic hormone (ADH) is unleashed, allowing water instead to be retrieved and directed to the bloodstream.
Extracellular fluid loss is caused by excessive amounts of blood loss as well as loss of sodium through sweating. The kidneys become key players in regulating fluids by restoring blood volume to normal levels. They do this by constricting blood vessels, which allow blood loss to take place, through the release of an enzyme into the bloodstream.
Since it is impossible to decide when these fluids are fully restored without being monitored by machines, satiety sensors (found in the intestines) do all the work by telling the body it has enough water to regulate dehydrated cells.