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Thirst Detection and Regulation
– Organisms maintain fluid levels within narrow ranges to balance interstitial and intracellular fluid concentrations.
– Thirst is triggered to replenish water lost from cells.
– Thirst receptors detect interstitial fluid concentration and blood volume.
– Different conditions like isotonic, hypertonic, and hypovolemia affect thirst regulation.
– The renin-angiotensin system is activated in response to hypovolemia to retain water and sodium.
– Osmoreceptors detect changes in blood plasma concentration and angiotensin II presence.
– Brain areas like the hypothalamus play a key role in regulating thirst.
– Neurotransmitters like serotonin are involved in thirst regulation.

Causes and Effects of Decreased Volume
– Loss of blood volume from conditions like blood loss, vomiting, or diarrhea can lead to hypovolemic shock.
– Kidney cells detect blood volume loss and trigger thirst for water and salt.
– Thirst for water and salt is initiated via the renin-angiotensin system.
Dehydration and osmoreceptor stimulation can lead to osmometric thirst.
– High sodium intake or loss of extracellular fluids can cause cellular dehydration.

Salt Craving
– Loss of sodium in hypovolemia increases the body’s need for salt.
– Activation of the renin-angiotensin system can lead to increased salt craving.

Thirst in the Elderly
– Thirst sensation decreases in adults over 50 years, putting them at risk of dehydration.
– Elderly individuals have lower total water intake compared to younger adults.
– Women are particularly at risk of inadequate water intake.
– Recommended water intake for the elderly is the same as younger adults due to increased water requirement.
– Reduction in renal concentrating capacity leads to an increased water requirement in the elderly.

Neurophysiology of Thirst
– Brain areas like the midbrain and hindbrain contribute to thirst regulation.
– The signaling pathway involves areas like the area postrema and nucleus tractus solitarii.
– Certain brain regions initiate water-seeking behavior.
– Damage to the hypothalamus can result in a loss of desire to drink.

Thirst (Wikipedia)

Thirst is the craving for potable 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 or an increase in the concentration of certain osmolites, such as sodium. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst.

Thirst (1886), by William-Adolphe Bouguereau

Continuous dehydration can cause acute and chronic diseases, but is most often associated with renal and neurological disorders. Excessive thirst, called polydipsia, along with excessive urination, known as polyuria, may be an indication of diabetes mellitus or diabetes insipidus.

There are receptors and other systems in the body that detect a decreased volume or an increased osmolite concentration. Some sources 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.

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