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Kidney Function.

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Kidney Function Introduction and definition of terms: The kidneys are the main organs in the urinary system. They filter waste products out of blood from the renal artery. These are then excreted. Useful solutes are reabsorbed into the blood. They also have a major homeostatic role in the body, and help to control the water content (osmoregulation) and pH of the blood. Homeostasis is the maintenance of a constant internal environment within a living organism. Excretion is the removal from the body of waste products made in the cells during metabolism. Osmoregulation is the homeostatic control of body water. Water intake needs to balance with water loss. Urea is instantly converted from ammonia, as it is a less soluble and less toxic compound. It is the main nitrogenous excretory product of humans. Each kidney receives its blood supply from the renal artery. The kidney consists of millions of filtering units called nephrons. Blood comes to the kidney under high pressure to make filtration efficient. The filtered blood leaves the kidney along the renal veins. The filtered waste products are excreted by the kidney as urine. A narrow tube called the ureter carries urine from the kidney to the bladder. ...read more.


Its mesh-like structure acts as the filter during ultrafiltration. * The second layer makes up the wall of the Bowman's capsule. These cells have many tiny finger-like projections, with gaps in between them and are called podocytes. The gaps in the capillary endothelium and the Bowman's capsule wall allow most molecules through. But the basement membrane prevents large molecules (proteins and blood cells) from passing through and so acts as the filter. Only small, soluble molecules can pass through the basement membrane. The rate at which fluid seeps from the blood in the glomerular capillaries into the Bowman's capsule depends on the difference in water potential between the contents of the two areas. The water potential of the blood plasma in the glomerulus is higher than that of the liquid in the Bowman's capsule. Overall the effect of differences in pressure outweighs the effect of differences in solute concentration. This filtration is a passive process, because molecules are not actively transported across the membranes, they diffuse down a concentration gradient as well as being pushed through due to the pressure build up in the glomerulus. The membranes are non selective. Reabsorption in the proximal convoluted tubule and secretion into the distal tubule: Many of the substances in the filtrate need to be kept by the body, so they are reabsorbed into the blood as the fluid passes along the nephron. ...read more.


So water is drawn out of the collecting ducts by osmosis into surrounding capillaries' blood, resulting in a far more concentrated urine. Control of water reabsorption: Osmoregulation is the homeostatic control of body water, which is operated on a principle of negative feedback. The receptors responsible for detecting changes are located in the hypothalamus of the brain. These osmoreceptors react to change in the solute concentration of the blood as it flows through the hypothalamus. It detects if blood has a low water potential (more concentrated) and stimulates the pituitary to release antidiuretic hormone (ADH). The release of ADH into the bloodstream brings about the following: * ADH make the distil convoluted tubule and the collecting duct more permeable to water. * This allows more water to be reabsorbed from the distal convoluted tubule and the collecting duct into the region of high solute concentration in the medulla. * This produces a smaller volume of more concentrated urine. If the blood has a high water potential (less concentrated), it is detected and less ADH is secreted by the pituitary. This decrease in the amount of ADH in the bloodstream result in the following: * The distil convoluted tubule and the collecting duct becomes less permeable to water. * Less water is reabsorbed into the medulla. * Larger quantities of dilute urine are produced. ...read more.

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