What follows is more than you could possibly want to know about thirst; please read with caution! :)
can be divided into four major categories. Intracellular fluid, the fluid portion of cytoplasm
, makes up about 67% of all bodily fluids. The remaining fluid type
s are extracellular, and include intravascular fluid (a.k.a. blood plasma
), cerebrospinal fluid
, and interstitial fluid (the fluid found around the outside
of your cells). Thirst
is the result of either the loss of one of these fluids, or from an increase in the concentration of salt
s in one of the fluids. Depending on the cause
of the thirst, most fluid-seek
ing behavior can be categorized as either osmometric thirst or volumetric thirst.
Osmometric thirst -- Osmometric thirst occurs when the concentration of salts in the interstitial fluid is greater than that inside the cells, resulting in the movement of intracellular water outside of the cell by osmosis. An example of something that would cause osmometric thirst is eating salty food; the sodium from the meal is absorbed into the bloodstream, which causes water to move from the interstitial fluid into the blood plasma, putting an osmotic strain on the cells. The loss of intracellular water causes the cells to shrink. In turn, this shrinkage changes the firing rate of specialized neurons, resulting in the secretion of vasopressin, which reduces water excretion (way to go, kidneys!). The body must have water to excrete in order to rid itself of nitrogenous wastes, so the reduction in water excretion causes fluid-seeking behavior.
Volumetric thirst -- Volumetric thirst happens when the volume of blood plasma decreases. This can be the result of blood loss (of course), vomiting, diarrhea and other similar things. Said fluid losses result in hypovolemia, a state in which your heart is unable to pump blood effectively because of loss of blood pressure. Groups of receptor cells in the kidneys detect decreases in blood flow, and stimulate the secretion of renin. Renin converts a protein in the blood into the hormone angiotensin, which stimulates the secretion of vasopressin (see above) and aldosterone, which prevents the kidneys from excreting salt. This results in a salt appetite. Volumetric thirst causes fluid- and salt-seeking behaviors. A second set of receptors in the heart can detect loss of blood volume, and causes fluid-seeking behavior directly, circumventing the whole kidney system.
There is one other circumstance in which fluid-seeking behavior commonly occurs that is not associated with physiological thirst. Food-related, or prandial drinking is fluid consumption that occurs before thirst occurs, in anticipation of the need for fluids. This type of fluid-seeking behavior seems to involve angiotensin and histamine.
More than you could possibly want to know about thirst has been brought to you by: Physiology of Behavior, 6th edition, 1998. Neil R. Carlson, the letter L, and the number 12.