The Roles of the Nephron of the 120 ml of blood that is filtered by the kidneys each minute, only I ml (that's less than I%) turns into urine that will eventually leave the body (after approximately 300 - 400 mis of it accumulates to fill the bladder!) That leaves 119 ml of fluid called filtrate to be returned back to the blood stream. Good thing, otherwise you would have to micturate (pee, urinate) once every 3 minutes and drink 1 L of fluid every 10 minutes in order to maintain Homeostasis!!! The one million nephrons in each human kidney are amazingly efficient at selectively removing wastes from the blood while at the same time conserving water, salt ions, glucose and other needed materials. The nephrons accomplish this task in 3 main steps; these 3 steps are also called the 3 main roles of the nephron: Filtration, Reabsorption and Secretion.
Fiftratffln Filtration is aecomplished by the movement of fluids from the blood into the Bowman's capsule.
Beabsorpttort Reatuorptias wolves the selective transfer of essential solutes and water back into the blood. Secretion Secretion Involves the movement of wastes from the blood into the mphron.
1. Filtration The renal artery carries blood into the kidney (approximately 600 mis of blood enters a kidney each minute). The renal artery then branches into arterioles which then branch intoaspecialized capillaries called the glomerulus. Because of the great difference in diameter between the renal artery and the glomerul us, blood entering the glomerulus is under very high pressure. This pressure forces about 20% of the blood plasma (about 120m1 of the 600 ml) out of the glomerulus and across the membrane of Bowman's
capsule. Bowman's capsule acts to "filter" or separate some of the substances that are located in blood plasma from others. This is because some substances are small enough to fit through the pores of the membrane of Bowman's capsule and some are too large and thus do not enter Bowman's capsule with the rest of the blood plasma. Water, salt ions (sodium, potassium and choride), glucose molecules, amino acids and urea molecules are all small enough to go through the membrane pores into Bowman's capsule. Blood cells (rbc, wbc and platelets) and proteins on the other hand are too large to leave the capillaries or enter Bowman's capsule. The fluid inside of Bowman's capsule gets a name change; it is now called "filtrate" because it is blood plasma that has been filtered, This filtrate is identical to blood plasma minus the blood cells and proteins. Filtrate is said to be isotonic to blood plasma with respect to its concentration of water, salt ions, glucose, amino acids and urea. The filtrate will then proceed from Bowman's capsule through the rest of the nephron in the following order: proximal convoluted tubule, loop of henle, distal convoluted tubule and finally the collecting tubule. From the collecting tubule, the filtrate will enter the pelvis of the kidney and be called urine. 2. Reabsorption Useful materials such as sugars and salt ions are reabsorbed back into the blood stream. That is, materials that could still be used by the body are sent back to the blood. Reabsorption happens as filtrate passes sequentially through the nephron. Materials re-entering the blood stream do so through the capillary network surrounding the nephrons. In short, "good" stuff is sent from the nephron back into the blood. A. Proximal Convoluted Tubule (PCn: As the filtrate enters the PCT approximately 80% of the salt ions (sodium and potassium), glucose and amino acids are ACTIVELY TRANSPORTED out of the PCT and back into the blood stream by special "pumping" cells located in the walls of the PCT. The process of active transport requires energy. Energy in the form of ATP is supplied by the numerous mitochondria that are embedded in the walls of the PCT. Because of ionic attraction, negatively charged chloride ions (CI-) will flow passively out of the PCT as they are attracted...
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