The water and solute content of body fluids is regulated by the kidney. This function of regulation of osmotic concentration of blood by the kidney is called osmoregulation.
When an animal takes in large amount of water, as in freshwater fishes, the urine passed out of the body is hypotonic (more dilute than the blood plasma).
In animals like reptiles and birds that live in areas with scarce water condition, urine passed out of the body is hypertonic (more concentrated than the blood plasma). This reduces the water loss from the body.
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In human beings, the kidney is extremely flexible in its working. Generally in summer when most of water is lost from the body by perspiration, the urine passed out is hypertonic. However, in winter as there is no perspiration, the urine passed out is hypotonic.
This flexibility in the working of human kidney and renal tubules is brought about by the movement of water and Na+ in and out of nephron under the influence of certain hormones, thus osmolarity is regulated.
When there is Excess of Water in Body Fluids:
In such a condition, the urine passed out of the body is more dilute (hypotonic) than the body fluids. It is achieved by two processes:
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i. Excess of water increases blood volume which increases the hydrostatic pressure in glomerulus, hence, more nephric or glomerular filtrates are formed.
ii. The sensation of excess of water in the body fluid is received by osmoregulator cells in the hypothalamus part of the brain. These cells in turn influence the posterior lobe of the pituitary gland to inhibit or reduce the release of antidiuretic hormone (ADH).
Deficiency of this hormone lowers the permeability of cells in the distal convoluted tubule and collecting duct, thereby decreasing the reabsorption of water. More filtration and less reabsorption of water produces excessively dilute urine. This brings down the volume of the body fluids to normal.
If the Body Fluids Fall Below Normal:
During summer, due to excessive sweating the body fluids reduce and urine turns thick and yellow. To maintain water balance of the body, the following changes take place in the body.
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i. The glomerular filtration slows down due to decreased filtration pressure in the glomerular capillaries.
ii. The osmoreceptor cells of the hypothalamus send impulses to the posterior pituitary lobe to release ADH. This hormone increases the reabsorption of water in the distal convoluted tubule and the collecting duct by making these regions of the nephron more permeable to water.
Less filtration and more reabsorption of water leads to the discharge of small amounts of hypertonic urine. This raises the volume of body fluids to normal.