Thirst
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Thirst is the basic instinct of humans or animals to drink. 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. The osmolite concentration in the blood is measured with specialized sensors in the hypothalamus, notably in two circumventricular organs that lack an effective blood-brain barrier, the organum vasculosum of the lamina terminalis and the subfornical organ. These areas project to the supraoptic nucleus and paraventricular nucleus, which contain neurons that secrete the antidiuretic hormone, vasopressin, from their nerve endings in the posterior pituitary, but also project to other hypothalamic areas, including especially the median preoptic nucleus to trigger thirst. Continuous dehydration can cause myriad 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. Angiotensin II is a hormone which is a powerful dipsogen (ie it stimulates thirst) which acts via the subfornical organ.
Extracellular Thirst
Extracellular thirst can be caused by a reduction in the volume of fluid found between cells. This type of thirst can be referred to as volumetric thirst or hypovolemia. Volumetric thirst can be caused by a number of things including bleeding, vomiting, diarrhea, sweating, and alcohol consumption. Volumetric thirst arises when the volume of blood plasma, i.e. intravascular fluid, decreases.[1] As intravascular fluid decreases, blood pressure is reduced and the body attempts to compensate by moving fluid from other cellular compartments into the vasculature. Fluid is transferred from all of the fluid compartments in the body, including intracellular, interstitial and other extravascular compartments.[1] Pure volumetric thirst is caused by the loss of blood and because sodium is also lost the from the plasma, the body's need for salt proportionately increases.[1]
Characteristic of all ingestive behaviors, there must be a mechanism by which volumetric thirst is intiated. This function is accomplished by two sets of receptors; one in the kidneys and the other in the heart.
The kidneys have a specialized set of cells that enable the recognition of changes in blood flow to the kidneys.[1] Naturally, these cells detect the presence of hypovolemia and react accordingly to the loss of blood volume. These cells secrete a hormone called renin when there is a decrease in the flow of blood to the kidneys.[1] Renin flows into the blood and there, initiates the conversion of a protein called angiotensinogen to angiotensin.[1] In order to exert its effects on the body, angiotensin I must be converted by enzymes into its active form, angiotensin II. Physiologically, angiotensin II stimulates the release of hormones by the posterior pituitary gland and the adrenal cortex, causing the kidneys to conserve water and sodium and increase blood pressure by contracting arterial muscles.[1] Behaviorally, angiotensin II causes drinking and a salt appetite.[1] Seemingly, it is this action by the kidneys, by retaining both salt and water and constricting blood vessels, that helps to maintain homeostasis by encouraging the animal to both find and ingest salt and water.[1]
The next set of receptors responsible for detecting volumetric thirst are located in the heart atria. Commonly referred to as stretch receptors, these atrial baroreceptors detect the amount of blood that is being pumped back into the heart from the veins.[1] The body constantly returns blood to the heart through veins. Therefore, when the volume of blood being transported back to the heart is decreased, these receptors detect the change in the amount of blood thereby stimulating thirst.[1]
A liquid rich in electrolytes is needed to replenish the organism of lost sodium ions.
- Nomadic peoples of the Sahara drink and carry blood as a beverage for their extracellular thirst.
Intracellular Thirst
Thirst triggered when fluid is drawn out if cells due to an increase in the concentration of salts and minerals outside of the cell. Caused by salty snacks etc.
Intracellular thirst is commonly referred to by scholars as osmometric thirst. Osmometric thirst is produced by an increase in the osmotic pressure of the interstitial fluid relative to the intracellular fluid, also known as cellular dehydration.[1] Osmometric thirst also occurs when the tonicity or solute concentration of the interstitial fluid increases.[1] This increase in solute draws water outside of the cell, causing the cell to shrink. Similar to osmosis, osmometric thirst can be correlated with the movement of water through a semipermeable membrane from an area with a low concentration of solute to an area with a high solute concentration.[1] Neurons that respond to changes in osmotic pressure are known as osmoreceptors.[1] Osmoreceptors are neurons that adjust their firing rate in relation to their level of hydration or lack thereof.[1] Accordingly, as fluid surrounding the cell becomes more concentrated, water flow out of the cell causing it to shrink. The change in the size of the cell causes the osmoreceptors to change their rate of firing, sending signals to different areas of the brain.
As a person eats a salty meal, he or she sustains pure osmometric thirst. Salt is absorbed from the digestive system into the blood plasma causing it to become hypertonic. A hypertonic solution refers to a solution that contains enough solute so that it will draw water out of the cell through the process of osmosis.[1] It is this condition that draws water from the interstitial fluid causing it too to become hypertonic. Because the level of solute is greater outside the cell, water leaves the cell and the cell shrinks, causing a change in the firing of the osmoreceptors thereby initiating osmometric thirst.
Drinking water is best to rid this thirst.
Preventing Subtle Dehydration
For optimal health, experts recommend that humans get 8-10 servings of about 8-ounces of water (in total, approximately 2 litres) per day to maintain hydration. This figure does vary according to ambient temperature, movement and physical size. Being that water is essential to the general function of the human and all animal bodies, eight servings is widely regarded as the minimum for the body to function optimally. However, water can be obtained from many sources, such as foods and other beverages containing water. Getting enough water from your diet and staying hydrated is key to your overall health, including urinary tract and digestive tract health.
When getting your daily water intake, it's important to not rely heavily on caffeinated beverages, as they actually work as a diuretic. Further, moderate or excessive alcohol consumption can lead to dehydration, thus it's important to maintain hydration when drinking caffeinated and alcoholic beverages.
References
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- M.J. McKinley and A.K. Johnson (2004). "The Physiological Regulation of Thirst and Fluid Intake". News in Physiological Sciences. 19 (1): 1–6. Retrieved 2006-06-02.
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