Kidney Notes - 7.5 FORMATION OF URINE There are three steps...

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7.5: FORMATION OF URINE There are three steps to the formation of urine: Filtration: movement of fluids from the blood into the Bowman’s capsule Reabsorption: the transfer of essential solutes (like glucose, amino acids) and water from the nephron and back into the blood Secretion: movement of materials from the blood like urea back into the distal tubule of the nephron Each nephron has an independent blood supply and is surrounded by a network of blood vessels called the glomerulus (a high-pressure filter) surrounded by Bowman’s capsule , a two layer membrane that opens into the convoluted tubule Blood from the heart travels to the renal artery thru small branches called afferent arterioles that supply the nephron with blood The afferent arterioles branch even further into a capillary bed, called the glomerulus The first step of urine formation, filtration begins here Filtration:
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Blood is propelled through the glomerulus into the Bowman’s capsule with high pressure Due to the small size of glomerulus, only materials large enough to pass through the wall actually make it through the kidneys by osmosis and diffusion and these include: water, glucose, NaCl, H + , amino acids, Cl - , wastes like urea/uric acid, vitamins but large materials like plasma proteins, platelets and red blood cells that are too large will not make it through the Bowman’s capsule Reabsorption: The transfer of essential solutes and water from the nephron and back into the peritubular capillaries (blood) is called reabsorption The materials that pass through the Bowman’s capsule are called glomerular filtrate and it enters the proximal convoluted tubule Proximal Tubule: Here, selective absorption occurs by osmosis, passive and active transport This tubule contains many microvilli which increases the surface area for the reabsorption and mitochondria supplying the energy needed for active transport Carrier molecules lining the proximal tubule move Na+ ions across the cell membranes that line the nephron and in doing so, negatively charged ions like Cl - and HCO 3 - -follow the positive Na+ ions by attraction of opposite charges These substances are reabsorbed back into the bloodstream through pertibular capillaries that surround the nephron The movement of these ions out of the nephron and back into the blood is done through active transport and the energy is supplied by ATP produced by the mitochondria in the cells of the proximal tubule Reabsorption occurs until the maximum amount of material can be moved across the nephron and into the blood and this is called the threshold level Excess NaCl remains in the nephron which is later reabsorbed or excreted in urine The flow of these ions cause a greater solute concentration in the intercellular spaces and therefore, most of the water will go back into the bloodstream by osmosis away from the nephron
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Other molecules like glucose and amino acids are actively transported from the proximal
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Kidney Notes - 7.5 FORMATION OF URINE There are three steps...

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