Trace the path of urea from the time it reaches the kidney by renal artery
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student-name Kshitij Pal asked in Science
trace the path of urea from the time it reaches the kidney by the renal arteries and is expelled out through the urethra
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student-name Vishes Rao answered this
424 helpful votes in Science, Class XII-Science
Urine is formed in the kidneys as a result of three processes: filtration, reabsorption, and secretion. Filtration takes place in the renal corpuscles; reabsorption and secretion take place in the renal tubules.
FILTRATION. Filtration is the movement of water and dissolved materials through a membrane from an area of higher pressure to an area of lower
Pressure. In the body, the pressure of blood in the capillaries is higher than the pressure of the interstitial fluid, or the fluid surrounding the body's cells. Thus, through filtration, blood plasma (fluid portion of blood) and nutrients such as amino acids, glucose, and vitamins are forced through the capillary walls into the surrounding interstitial fluid to be used by the cells.
The pressure of the blood in the glomeruli is higher than in other types of capillaries in the body. This high pressure forces plasma, dissolved waste substances, and small proteins out of the glomeruli and into the Bowman's capsules. The process is called glomerular filtration. Blood cells and larger proteins are too large to be forced out of the glomeruli, so they remain in the blood. The pressure in a Bowman's capsule is low and its inner membrane is permeable, so the material that filters out of a glomerulus passes into the capsule. The fluid and material in a Bowman's capsule is referred to as renal filtrate, which is very much like blood plasma, except it contains very little protein and no blood cells.
REABSORPTION. In an average twenty-four-hour period, the kidneys form 160 to 190 quarts (150 to 180 liters) of renal filtrate. Normal urinary output in that same time frame is only about 1.1 to 2.1 quarts (1 to 2 liters). Many factors (such as increased water intake or increased sweating) can significantly alter that output amount. Nonetheless, it is quite obvious that most of the renal filtrate does not become urine, but is reabsorbed or taken back into the blood. This is important because renal filtrate contains many useful substances—water, glucose, amino acids, and mineral ions—that are needed by the body.
Reabsorption is the return of water and other substances from the filtrate to the blood. The process begins as soon as the filtrate enters the renal tubule. Cells lining the tubule actively take up useful materials (such as glucose, amino acids, vitamins, proteins, and certain ions), move them through their cell bodies, then release them into the interstitial fluid outside the tubule.
As these materials collect in the interstitial fluid, water in the tubules is drawn out through the process of osmosis. Osmosis is the diffusion of water through a semipermeable membrane from an area where it is abundant to an area where it is scarce or less abundant. Once in the interstitial fluid, the materials and water then diffuse into or enter nearby capillaries, which empty into the renal vein.
The reabsorption process is selective. The cells of the renal tubules have been "programmed" to retain substances that are useful to the body, not those substances that are of no use (such as urea and uric acid). Also, the amount of a substance that is reabsorbed is dependent on its concentration in the blood. If it exists in a low concentration in the blood, a large amount of it will be reabsorbed from the renal tubules. Conversely, if it already exists in a high concentration, very little of it will be reabsorbed into the blood.
SECRETION. Secretion is the transport of materials from the interstitial fluid into the renal filtrate. It is essentially reabsorption in reverse. The process is important for getting rid of substances not already in the filtrate. Waste products such as ammonia, some creatinine, and the end products of medications move from the blood in the capillaries around the renal tubules into the interstitial fluid. They are then taken in by the cells of the tubules and deposited into the renal filtrate to be eliminated in the urine.