Overview of excretion in mammals

Materials from the blood are transferred to the nephrons where filtration, reabsorption and secretion will occur. Excretion will occur at the urethra. Remember: substances do not move back to the lumen of the tubule from the interstitial fluid because of small surface area in the exterior side compared to interior (lumen part)

1. Filtrate is produced when substances from the blood is filtered in the glomerulus and the Bowman’s capsule. The concentration of this filtrate is the same compared to the concentration of the interstitial fluid in other parts of the body.

2. The filtrate will move towards the proximal tubule. Volume and composition of the filtrate is changed here. Production of H+ ions and NH3 to balance the pH of the filtrate (produced by the transport epithelium). Drugs and poison are transferred from the peritubular capillaries to the proximal tubule.Remember: the P. tubule reabsorbs NaCl and H2O. The transport epithelium in p tubule transport Na+ (active) and Cl- (passive) into the interstitial fluid. Water follows via osmosis. Important: transferred back to the capillaries: NaCl, Nutrients (active); HCO3

-, H2O, K+ (passively)Secreted into the p. tubule: H+ (active); NH3 (passive)

3. Water is reabsorbed greatly in the descending part of the loop of Henle. The transport epithelium that lines the tubule is greatly permeable to water but not to salt.

4. The thin ascending loop of Henle moves salt from the filtrate passively. The thick ascending loop of Henle moves NaCl actively.Important: animals with very long loop of Henle or with juxtamedullary nephrons conserve water efficiently because of the mechanisms mentioned in 3 and 4. The mechanism involve is the countercurrent exchange of substances. At upper part of the loop of Henle concentration of solute is not as high as you descend down the loop. Water is reabsorbed by the interstitial fluid all the way down because of varying change in osmolarity of the interstitial fluid. The interstitial fluid becomes more hypersomotic compared to the filtrate as you descend because the ascending loop of Henle transports the NaCl in the filtrate.

5. The distal tubule acts on the secretion and reabsorption of substances just like the p tubule. It also controls the pH of the filtrate by secretion of H+ and reabsorption of HCO3

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Important: reabsorbed: NaCL, HCO3- (active); H2O

(passive)Secreted: K+ and H+ (active)

6. The collecting duct determines how much salt is excreted in the urine. It is permeable to water but not to salts. Important: reabsorbed: H2O, urea (due to high concentration in the urine) (passive) NaCl (active) Conservation of water

Here filtrate concentration is always compared to normal concentration of interstitial fluid.

In the Bowman’s capsule: same concentration because only filtration of small substances occurred. (About 300 mosm/L)

In the descending loop of Henle: increases from 300 to 1200 at the bottom part of the loop (water is greatly reabsorbed) In the ascending limb: filtrate concentration decreasesImportance: lose of water in the ascending limb produces a hyperosmotic filtrate. This hyperosmotic filtrate will produce the gradient that will move the salt from the filtrate back to the interstitial fluid. 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.

In the Distal tubule: filtrate is hypoosmotic.

In the collecting duct: because of permeability to water the filtrate becomes hyperosmotic along the way. High concentration of urea in the filtrate allows its diffusion to maintain the gradient. Even though the filtrate lost some solute along the way the filtrate produced is still hyperosmotic compared to interstitial fluid of the body.

Nervous and hormonal control

The mammalian kidney has the ability to adjust the volume and osmolarity of urine through water and salt balance and rate of urea production.

Water reabsorption through osmolarity of blood.

Osmoreceptor in the hypothalamus detects osmolarity of blood.

Hyperosmotic blood will trigger the release of antidiuretic hormone (ADH).

ADH is produced by the hypothalamus but is stored and released in the pituitary gland.

ADH targets the transport epithelium of the distal tubule and collecting duct. (why not the descending loop of Henle?)

The transport epithelium becomes permeable to water. Water is reabsorbed and decreases the osmolarity of blood.

Negative feedback mechanism will stimulate the osmoreceptors in the hypothalamus to inhibit release of ADH.

Drinking water also decreases release of ADH.Alcohol disrupts the release of ADH. Thereby, producing dilute urine.

Water reabsorption through blood pressure or low blood volume.

Decrease blood pressure or blood volume may be a result of dehydration or low salt intake.

JGA or the juxtaglomerular apparatus monitors the blood pressure in the afferent arteriole. Low blood pressure will stimulate the JGA to release renin in the bloodstream.

Renin will convert angiotensinogen into angiotensin II.

Angiotensin II can increase blood pressure and volume in different ways. It can increase blood pressure by constriction of arterioles. It can also increase raised blood pressure and volume by stimulating the proximal tubule to reabsorb more

water and NaCl. It can also stimulate the release of aldosterone found in the adrenal glands.

Aldosterone acts on the distal tubule that stimulates reabsorption of Na+ and water. 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 osmolarityRAAS- 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 renin. It decreases blood volume and pressure. It inhibits release of renin and aldosterone.