Thirst

Thirst is the craving for fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids and/or an increase in the concentration of certain osmolites, such as salt. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, the brain signals thirst.

Continuous dehydration can cause many problems, but is most often associated with neurological problems such as seizures and renal problems. Excessive thirst, known as polydipsia, along with excessive urination, known as polyuria, may be an indication of diabetes.

There are receptors and other systems in the body that detect a decreased volume or an increased osmolite concentration. They signal to the central nervous system, where central processing succeeds. Some sources, therefore, distinguish "extracellular thirst" from "intracellular thirst", where extracellular thirst is thirst generated by decreased volume and intracellular thirst is thirst generated by increased osmolite concentration. Nevertheless, the craving itself is something generated from central processing in the brain, no matter how it is detected.

Detection
There are many different receptors for sensing decreased volume or an increased osmolite concentration.

Decreased volume

 * Further reading: Hypovolemia

Hypovolemia leads to activation of the renin angiotensin system (RAS) and a decrease in atrial natriuretic peptide. These mechanisms, along their other functions, contribute to elicit thirst, by affecting the subfornical organ. For instance, angiotensin II, activated in RAS, is a powerful dipsogen (ie it stimulates thirst) which acts via the subfornical organ.
 * Renin-angiotensin system


 * Other
 * Arterial baroreceptors sense a decreased arterial pressure, and signals to the central nervous system in the area postrema and nucleus tractus solitarius.
 * Cardiopulmonary receptors sense a decreased blood volume, and signal to area postrema and nucleus tractus solitarius as well.

Cellular dehydration and osmoreceptor stimulation
Osmosis is the movement of liquid from one side of a semipermeable membrane with lower concentrations of dissolved substances, to the other side of the membrane with higher concentrations, resulting in dilution of the higher concentration solution. In living cells the transport of water from low concentration to high concentration solutions through the cell membrane tends to continue until equilibrium is achieved. The pressure needed to halt the movement through the membrane is the "osmotic pressure."

As the volume of extracellular fluids such as blood plasma and cerebrospinal fluid drops due to loss of water through perspiration, respiration, urination and defecation, the concentration of sodium (the major substance affecting osmosis in the extracellular fluid) and hence the osmotic pressure, rises; water migrates from the cells of the body, through their membranes, to the extracellular compartment, and cellular dehydration occurs. (Infusion of hypertonic saline solution into the blood stream has the same cellular dehydration effect.)

In the mammalian brain, the posterior surface of the hypothalamus forms the front wall of the third ventricle (a cerebrospinal fluid-filled cavity) and clusters of cells (osmoreceptors) on this surface, notably in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), signal this cellular dehydration to other parts of the brain, and thirst is experienced. Destruction of this part of the hypothalamus in humans and other animals results in partial or total loss of desire to drink even with extremely high salt concentration in the extracellular fluids.

In addition, there are visceral osmoreceptors. These project to the area postrema and nucleus tractus solitarius in the brain.

Salt craving
Because sodium is also lost from the plasma in hypovolemia, the body's need for salt proportionately increases in addition to thirst in such cases. This is also a result of the renin-angiotensin system activation.

Elderly
For adults over age 50, the body’s thirst sensation diminishes and continues diminishing with age, causing many to suffer symptoms of dehydration.

Central processing
The area postrema and nucleus tractus solitarius signal, by 5-HT, to lateral parabrachial nucleus, which in turn signal to median preoptic nucleus. In addition, the area postrema and nucleus tractus solitarius also signal directly to subfornical organ.

Thus, the median preoptic nucleus and subfornical organ receive signals of both decreased volume and increased osmolite concentration. They signal to higher integrative centers, where ultimately the conscious craving arises. However, the true neuroscience of this conscious craving is not fully clear. In general, the end-result is towards behavior of drinking for hydration, but this can to some degree be resisted, such as in voluntary fluid restriction.

In addition to thirst, the organum vasculosum of the lamina terminalis and the subfornical organ contribute to fluid balance by vasopressin release.