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  • 1. Overview of excretion in mammals The transport epithelium that lines the tubule is greatly permeable to water Materials from the blood are transferred to the but not to salt. nephrons where filtration, reabsorption and secretion will occur. Excretion will occur at the 4. The thin ascending loop of Henle moves urethra. Remember: substances do not move salt from the filtrate passively. The back to the lumen of the tubule from the thick ascending loop of Henle moves interstitial fluid because of small surface area in NaCl actively. the exterior side compared to interior (lumen part) Important: animals with very long loop of Henle or with juxtamedullary 1. Filtrate is produced when substances nephrons conserve water efficiently from the blood is filtered in the because of the mechanisms mentioned glomerulus and the Bowman’s capsule. in 3 and 4. The mechanism involve is The concentration of this filtrate is the the countercurrent exchange of same compared to the concentration of substances. At upper part of the loop of the interstitial fluid in other parts of the Henle concentration of solute is not as body. high as you descend down the loop. Water is reabsorb by the interstitial 2. The filtrate will move towards the fluid all the way down because of proximal tubule. Volume and varying change in osmolarity of the composition of the filtrate is changed interstitial fluid. The interstitial fluid here. Production of H+ ions and NH3 to becomes more hypersomotic compared balance the pH of the filtrate (produced to the filtrate as you descend because by the transport epithelium). Drugs and the ascending loop of Henle transports poison are transferred from the the NaCl in the filtrate. peritubular capillaries to the proximal tubule. 5. The distal tubule acts on the secretion and reabsorption of substances just like Remember: the P. tubule reabsorbs the p tubule. It also controls the pH of NaCl and H2O. The transport epithelium the filtrate by secretion of H+ and in p tubule transport Na+ (active) and Cl- reabsorption of HCO3- (passive) into the interstitial fluid. Water follows via osmosis. Important: reabsorbed: NaCL, HCO3- (active); H2O (passive) Important: transferred back to the capillaries: NaCl, Nutrients (active); Secreted: K+ and H+ (active) HCO3-, H2O, K+ (passively) 6. The collecting duct determines how + Secreted into the p. tubule: H (active); much salt is excreted in the urine. It is NH3 (passive) permeable to water but not to salts. 3. Water is reabsorbed greatly in the descending part of the loop of Henle.
  • 2. Important: reabsorbed: H2O, urea (due gradient that will move the salt from to high concentration in the urine) the filtrate back to the interstitial fluid. (passive) NaCl (active) A gradient is produced between the interstitial fluid and that of the filtrate. Water will always move out from any point in the descending limb because the surrounding interstitial fluid will always be hyperosmotic. The surrounding capillaries do not affect this gradient. It moves opposite that of the limb of the loop of Henle. Conservation of water In the Distal tubule: filtrate is hypoosmotic. Here filtrate concentration is always compared to normal concentration of In the collecting duct: because of interstitial fluid. permeability to water the filtrate becomes hyperosmotic along the way. High concentration of urea in the In the Bowman’s capsule: same filtrate allows its diffusion to maintain concentration because only filtration of the gradient. Even though the filtrate small substances occurred. (About 300 lost some solute along the way the mosm/L) filtrate produced is still hyperosmotic compared to interstitial fluid of the body. In the descending loop of Henle: increases from 300 to 1200 at the bottom part of the loop (water is Nervous and hormonal control greatly reabsorbed) In the ascending limb: filtrate concentration decreases The mammalian kidney has the ability to adjust the volume and osmolarity of Importance: lose of water in the urine through water and salt balance ascending limb produces a and rate of urea production. hyperosmotic filtrate. This hyperosmotic filtrate will produce the
  • 3. Water reabsorption through osmolarity of blood. Water reabsorption through blood pressure or low blood volume. Osmoreceptor in the hypothalamus detects osmolarity of blood. Decrease blood pressure or blood volume may be a result of dehydration or low salt intake. Hyperosmotic blood will trigger the release of antidiuretic hormone (ADH). JGA or the juxtaglomerular apparatus monitors the blood pressure in the ADH is produced by the hypothalamus afferent arteriole. Low blood pressure but is stored and released in the will stimulate the JGA to release renin pituitary gland. in the bloodstream. ADH targets the transport epithelium of Renin will convert angiotensinogen into the distal tubule and collecting duct. angiotensin II. (why not the descending loop of Henle?) Angiotensin II can increase blood pressure and volume in different ways. The transport epithelium becomes It can increase blood pressure by permeable to water. Water is constriction of arterioles. It can also reabsorbed and decreases the increase raise blood pressure and osmolarity of blood. volume by stimulating the proximal tubule to reabsorb more water and NaCl. It can also stimulate the release of Negative feedback mechanism will aldosterone found in the adrenal stimulate the osmoreceptors in the glands. hypothalamus to inhibit release of ADH. Aldosterone acts on the distal tubule Drinking water also decreases release of that stimulates reabsorption of Na+ and ADH. water. Alcohol disrupts the release of ADH. Thereby, producing dilute urine.
  • 4. The RAAS also function in a negative feedback mechanism. Decrease in blood pressure an volume stimulate production of rennin and aldosterone. Increase in blood pressure and volume inhibit the release of these hormones. ADH- through blood osmolarity RAAS- through blood volume and pressure This is important because an animal can reabsorb water even without a change in blood osmolarity. ANF or atrial natriuretic factor opposes action of rennin. It decreases blood volume and pressure. It inhibits release of renin and aldosterone.