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| Excretion and Osmoregulation in Man - contd. |
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| Excretion |
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 Removes nitrogenous metabolic wastes like urea, uric acid, creatinine, ammonia and hippuric acid |
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Removes excess salts like sodium, potassium and vitamins |
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Removes bile pigments |
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Removes excess water |
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| This process regulates the water content in the blood by producing hypotonic or hypertonic urine. It regulates pH (acid - base balance) of the blood. The rate of secretion of acid or alkaline phosphates is regulated when the blood becomes too acidic or alkaline. |
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| Branching from the dorsal aorta is the renal artery that supplies oxygenated blood to the kidneys. The blood contains large amounts of water and nitrogenous metabolic wastes. On entering the kidneys the renal artery divides into numerous minute vessels called afferent arterioles. On entering the Bowman's capsule the afferent arteriole branches further into fine capillaries and forms a knot called the glomerulus. |
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| The lumen of the glomerular capillaries is smaller than the lumen of the afferent arteriole. Similarly the lumen of the outgoing efferent arteriole is twice as small as the afferent arteriole. Consequently, blood is forced to flow through the glomerular capillaries under pressure. This high hydrostatic pressure helps in filtration and causes the fluid components of the blood to be squeezed out of the glomerulus into the Bowman's capsule. The filtrate in the capsule is a fluid that is similar in composition to the plasma of the blood except for the plasma proteins, because capillary wells are impermeable to macromolecules like proteins. The fluid that enters the renal tubule after filtration is called glomerular filtrate or primary urine or nephric filtrate and the process of filtration is called ultrafiltration. The glomerular filtrate contains large amounts of water along with glucose, urea, amino acids, uric acids and salts. The blood flowing out from the glomerulus into the efferent arteriole is very thick and contains plasma proteins, blood corpuscles and very little water. |
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| The filtrate draining into the renal tubule contains several substances that are required by the body, such as glucose, amino acids, certain salts (especially sodium and potassium) and water. These substances need to be reabsorbed into the blood so that the normal constituents in the right concentration are restored and fluidity of the blood is maintained. In this process 80% of the water is reabsorbed. This selective reabsorption takes place by the capillary network surrounding the proximal convoluted tubule, loop of Henle and distal convoluted tubule. Absorption takes place both by osmosis and by active transport. Certain constituents like sodium and chloride ions are reabsorbed as they pass through the semi-permeable epithelial tissue into the capillaries. This is passive absorption and does not require energy. Active transport takes place by the use of energy. Substances like glucose and amino acids are returned to the blood by this process. |
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| As the tubular filtrate drains through the distal convoluted tubule toxic substances that could not be filtered in the glomerulus are secreted from the blood through the tubular wall into the filtrate. |
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| As a result of this mechanism homeostasis of the blood is maintained and all the waste products filtered out in the tubular fluid constitute urine which is ready for excretion from the body. |
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| The distal convoluted tubule drains the urine into the collecting tubule. Several collecting tubules join together and drain their content into the collecting duct which in turn flow into the Ducts of Bellini. Urine finally reaches the renal pelvis. Urine is drained from here into the ureter. Movement of urine into the bladder is assisted by gravity and ureteral peristalsis. |
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| Back flow of urine from the bladder into the ureter is prevented as the ureters enter the bladder obliquely forming a sort of a valve. |
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| Located at the exit of the bladder are two sphincter muscles |
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Bladder Sphincter |
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Urethral Sphincter |
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| The urinary bladder acts as a temporary storage sac for the urine. Urine is expelled out from the bladder through the urethra by the simultaneous contraction of the bladder and the relaxation of the sphincter muscles. |
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| The kidney performs the function of both excretion and osmoregulation. It regulates the osmoconcentration of the blood. |
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| In aquatic animals or animals that consume a lot of water, the blood needs to get rid of excess of water and so the urine excreted is more dilute that the plasma of blood. This dilute urine is called hypotonic urine. On the contrary terrestrial animals need to conserve water. The urine excreted is hypertonic. Minimum water is expelled and osmotic concentration of the blood is maintained. |
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| Man excretes hypertonic urine which is more concentrated than their blood plasma to ensure that excess water is not flushed out of the body. The filtered fluid in the Bowman's capsule is isotonic (same osmoconcentration as the blood). As the filtrate passes through the tubules of the nephron a large quantity of water and some useful solutes are reabsorbed. The urine that finally flows into the collecting duct is hypertonic. |
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| In summer a lot of water is lost through sweat and so the urine expelled out is hypertonic. On the contrary there is no water loss due to sweating in winter. So the urine excreted is hypotonic. |
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