Osmometric Thirst

Topics: Hypothalamus, Thirst, Vasopressin Pages: 9 (3132 words) Published: October 30, 2012
Thirst is the craving for liquids, resulting in the basic instinct of humans or 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 a myriad of 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 ofdiabetes. Thirst produced by an increase in the osmotic pressure of the interstitial fluid relative to the intracellular fluid thus producing cellular dehydration fluid, Intracellular fluid, fluid contained within cells. Osmometric thirst occurs when the osmotic balance between the amount of water in the cells & the water outside the cells becomes disturbed means 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. This is what happens when we eat salty pretzels. The Na is absorbed into the blood plasma, which disrupts the osmotic balance between the blood plasma & the interstitial fluid. This draws water out of the interstitial fluid and into the plasma, now upsetting the balance between the cells and the interstitial fluid. The result is water leaving the cells to restore the balance. The disruption in the interstitial solution is recognized by neurons called osmoreceptors. These osmoreceptors are located in the region of the anterior hypothalamus. These osmoreceptors send a signal that causes us to drink more water, in order to restore the osmotic balance between the cells and surrounding fluid. In the case of pretzel eating, if we do not drink more water, eventually the excess Na is simply excreted by the 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. OSMOMETRIC THIRST is stimulated by cellular dehydration. It occurs when the tonicity of the interstitial fluid increases, which draws water out of the cells (think of water seeking to be balanced), cells then shrink in volume. The word "osmosis" means movement of water, through semi permeable membrane, from low solute concentration to high solute concentration. 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. There are some RECEPTORS FOR OSMOMETRIC THIRST (already in the central nervous system more specifically in hypothalamus notably in two circumventrivular organs that lack an effective brain-barrier the organumvasculosum of the lamina terminalis (OVLT) and the subfornical organ (SFO). However, although located in the same parts of the brain, these osmoreceptors that evoke thirst are distinct from the neighboring osmoreceptors in the OVLT and SFO that evoke arginine vasopressin release to decrease fluid output. In addition, there are visceral osmoreceptors. These project to the area postrema and nucleus tractussolitarius in the brain), the neurons that respond to changes in the solute concentration of the interstitial fluid - start firing when water is drawn out of them due to hyper tonicity; most likely located in the anteroventral tip of the third ventricle (AV3V); if activated, they send signals to neurons that control rate of vasopressin secretion So, the question will be raised such as do we want more or less vasopressin? We want more vasopressin; remember high levels of vasopressin cause kidneys to retain water, sweating causes loss of water through skin, which increases tonicity of interstitial fluid, which then draws water out...
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