Briefly describe the part played by the kidneys in the regulating the salt concentration of the plasma in order to avoid the shrinkageof the red blood cells.
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The volume of fluid in various fluid compartments in the body depends on the balance of fluid intake and fluid output. Fluid intake in response to thirst is a source of fluid, and receptor cells found in the hypothalamus activate thirst when exposed to hypertonic conditions - such as when water loss has been excessive or salt intake high.
These receptor cells will only cease to give a thirst message when the hypertonic status has been corrected.
A large intake of fluid will also be corrected by the specialized receptors in the nervous system, since they monitor the changes in blood volume. Should you for instance take in a liter of fluid, the kidneys will excrete the excess, simply by increasing its urine production eight fold within thirty minutes.
When blood volume increases the pressure is increased within the atria of the heart which then activates stretch receptors, which in turn send a signal for reduction of ADH release in the posterior pituitary, leading to less fluid to be reabsorbed by the kidneys. It secondly reduces the vasomotor tone of the blood vessels, which leads to dilation of the blood vessels, causing an increase of the glomerular blood pressure and increased filtration is achieved, with less water being reabsorbed by osmosis.
The reserve happens when blood volume decreases and less urine is formed with more fluid being retained and so correcting blood volume.
Anti-diuretic hormone (ADH) and kidney function
The primary effect of ADH is to limit the amount of water being lost in urine, by increasing the amount of water being reabsorbed into the blood. The ADH targets the cells of the tubules and collecting ducts, which causes an increase of permeability of the cell surfaces, where the water then leaves the renal tubules by means of osmosis.
With more fluid being reabsorbed, the blood volume increases while the solutes concentration becomes more diluted.
Feedback control of ADH release and kidney function
As soon as the osmolarity of the blood and body fluids is reduced, with more fluid being reabsorbed by the tubules in the kidneys, the receptors in the hypothalamus are no longer stimulated and the level of ADH stimulation is reduced, which then in turn signals to the kidneys to start excreting more water in the urine production until the blood osmolarity increases enough for the cycle to be started again.