the kidneys! - THE KIDNEYS: The most important function of...

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THE KIDNEYS: The most important function of the kidneys is the homeostatic regulation of water and ion content of the blood. (fluid/electrolyte balance) SIX General Functions: 1) Regulation of ECF volume and BP. [When ECF volume decreases, blood pressure decreases also] 2) Regulation of Osmolarity [kidney functioning with behavioral drives like thirst help to maintain the blood osmolarity at a value close to 290 mosm.] The 180 L of fluid that filters into Bowman’s capsule each day are almost identical in composition to plasma and nearly isosmotic- about 300 mosm. As this filtrate flows through the proximal tubule, about 70% of its volume is reabsorbed, leaving 54 L in the lumen. Reabsorption occurs when proximal tubule cells transport solutes out of the lumen, and water follows by osmosis. Filtrate leaving the proximal tubule has the same osmolarity as filtrate that entered. For this reason, we say that the primary function of the proximal tubule is the bulk reabsorption of isoosmotic fluid. After leaving the proximal tubule, filtrate passes into the loop of Henle, the primary site for creating dilute urine. As the filtrate passes through the loop, proportionately more solute is reabsorbed than water, and the filtrate becomes hyposmotic relative to the plasma. By the time the filtrate flows out of the loop, it averages 100 mosm, and its volume has fallen from 54 L/day to about 18 L/day. Now 90% of the volume originally filtered into Bowman’s capsule has been reabsorbed into the capillaries. From the loop of henle, filtrate passes into the distal tubule and the collecting duct. 3) Maintenance of ion balance [NA is the major ion involved in the regulation of ECF volume and osmolarity. K and CA concentrations are closely regulated.] In the distal tubule and collecting duct, the fine regulation of salt and water balance takes place under the control of several hormones. 4) Homeostatic regulation of pH [If ECF fluid becomes to acidic, the kidneys remove h and conserve bicarbonate ions, which act as a buffer. When ECF becomes too alkaline, the kidneys remove bicarbonate and conserve H.] 5) Excretion of wastes. [The kidneys remove metabolic waste products and foreign substance such as drugs and environmental toxins. This can include cratinine, urea, uric acid, and urobilinogin, a metabolite of hemoglobin that gives urine its yellow color. The kidneys also clear hormones from the blood.] 6) Production of hormones. [1) Erythropoetin: regulates RBC synthesis, 2) Renin: Regulates the production of hormones involved in sodium balance and blood pressure homeostasis, 3) Renal enzymes help convert vitamin D3 into a hormone that regulates Ca balance] Anatomy of the Urinary System: o Kidneys: Site of urine formation. Renal arteries which branch off of the abdominal aorta supply blood to the kidneys. Renal veins carry blood from the kidneys to the inferior vena cava. o
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the kidneys! - THE KIDNEYS: The most important function of...

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