13-Thirst - Neuroendocrinology of water balance and thirst...

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Neuroendocrinology of water balance and thirst. “Dogs that drink from the toilet bowl – right after this message”
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Obviously, we get water from drinking, but where does it go? Urine and feces. Sweat (both water and salt). Insensible water loss through evaporation from mouth and skin (just water). (Don’t give your dog Gatorade.)
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Thirst (water balance) is more tightly regulated than other motivated behaviors such as mating and eating. Why? You’re much more likely to die from dehydration than from not having sex. Unlike calories, it is impractical to carry large stores of water around with you.
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Physiological controls of thirst: what is regulated: Body water is found in two compartments. Interstitial fluid – in tissue surrounding cells Intravascular fluid – in the bloodstream * Cerebrospinal fluid The rest is found outside cells – extracellular fluid About two-thirds of the body’s water is found inside cells – intracellular fluid * *critical regulated compartments
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The body carefully monitors and controls both the intracellular fluid and the blood volume Too little water in cells disrupts chemical reactions; too much can cause the cell to burst Too little water in the blood stream interferes with circulation – low blood pressure The amount of fluid in the two compartments is regulated separately. Too much water in blood stream – high blood pressure Interstitial fluid volume is not tightly regulated, e.g ., edema
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Intracellular volume is controlled mainly by the concentration of dissolved substances (solutes) in the interstitial fluid When the concentration of solutes dissolved in the interstitial fluid is the same as inside the cell, it is said to be isotonic . (iso = same as) When the concentration of solutes dissolved in the interstitial fluid is higher than inside the cell, it is said to be hypertonic . (hyper = greater than) When the concentration of solutes dissolved in the interstitial fluid is lower than inside the cell, it is said to be hypotonic . (hypo = less than)
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The laws of physics and chemistry require that the tonicity of the interstitial fluid and the intracellular fluid be the same – isotonic.
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