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The Kidneys and Homeostasis

The Kidneys and Homeostasis. Kidneys have four major functions: -excrete metabolic wastes -maintain water-salt balance -maintain pH -secrete hormones (ex. e rythropoietin, r enin ). Salt + Water Balance.

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The Kidneys and Homeostasis

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  1. The Kidneys and Homeostasis • Kidneys have four major functions: • -excrete metabolic wastes • -maintain water-salt balance • -maintain pH • -secrete hormones (ex. erythropoietin, renin)

  2. Salt + Water Balance • Water and salt reabsorption occurs through the wall of the proximal convoluted tubule. • This is important for excreting hypertonic urine • Loop of nephron • Collecting Duct • Water => Aquaporins

  3. Continued • Loop of the nephron • Renal medulla • The loop of the nephron is composed of a descending limb and an ascending limb. • The bottom of the ascending limb is thin, and salt passively diffuses out. • The upper part of the ascending limb is thicker and the salt is actively transported out. • No water can leave the ascending limb because it is impermeable to water.

  4. Continued • Osmotic gradient in the tissues of the renal medulla; • Salt > concentration in direction of the inner medulla. • This is because the further up the thick part of the ascending limb the fluid goes, less salt is available for transport. • The innermost medulla itself has the highest concentration of solutes; • Because urea leaks out of the collecting duct.

  5. Continued • Water diffuses out the entire length of the descending limb due to this osmotic gradient. • Countercurrent mechanism • Water leaves the descending limb, and as it diffuses out, the remaining fluid contains a greater osmotic concentration of solutes, and therefore can diffuse through the limb from top to bottom. • The collecting duct also has the osmotic gradient • Water defuses out into the renal medulla, and thus urine is hypertonic to blood plasma.

  6. ADH + Reabsorption • Antidiuretic hormone regulates urine formation and excretion. • This is released by the posterior lobe of the pituitary gland. • More ADH = More water reabsorption, less urine, raise in blood volume + pressure • Less ADH = Less water reabsorption, more urine • Ideally, dependent on how much water you drink • Diuretics (ex. caffeine and alcohol) interfere with ADH and cause an increased urine flow.

  7. Hormones + Salt Reabsorption • 99% of Na+ filtered at the glomerulus returns to blood; 67% reabsorbed at the proximal convoluted tubule; 25% extruded by the ascending limb. • Blood volume + pressure is partially regulated by salt reabsorptions. • When glomerular filtration can not be supported, renin is secreted.

  8. Continued • Renin (enzyme) • Angiotensinogenangiotensin I angiotensin II • This vasoconstrictor stimulates adrenal glands on the kidneys to release aldosterone. • Aldosterone is a hormone that promotes excreting K+ and absorbing Na+ at the distal convultedtubeule. • Water reabsorption follows and blood volume + pressure increase (and therefore glomerular filtration can now occur).

  9. ANH • AtrialNatriuretic Hormone does the exact opposite. • Secreted by the atria of the heart when cardiac cells get too stretched from higher blood volume and pressure. • Inhibits the secretion of renin (from the juxtaglomerular apparatus) and aldosterone (from the adrenal cortex) • As a result, natriuresis occurs (increased secretion of Na+ and water, lowering blood volume + pressure. • Other hormones secreted to regulate other ions as well ( ex. K+, HCO3-, Mg2+)

  10. Acid-Base Balance (pH) • Bicarbonate Buffer System • Respiratory, powerful, does most of the work regulating blood pH • Essentially, kidneys reabsorb bicarbonate ions and excrete hydrogen ions as needed. • pH high (acidic) = Hydrogen excreted and bicarbonate reabsorbed. • pH Low (basic) = Hydrogen excreted, but bicarbonate NOT reabsorbed. • Urine is usually acidic, showing excess hydrogen is usually excreted. • Ammonia = NH3 + H+ = NH4+

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