explain how urine is concentrated mammalian kidney
Answers
Explanation:
The urine concentrating mechanism of the mammalian kidney, which can produce a urine that is substantially more concentrated than blood plasma during periods of water deprivation, is one of the enduring mysteries in traditional physiology. Owing to the complex lateral and axial relationships of tubules and vessels, in both the outer and inner medulla, the urine concentrating mechanism may only be fully understood in terms of the kidney’s three-dimensional functional architecture and its implications for preferential interactions among tubules and vessels.
During periods of water deprivation, the urine concentrating mechanism of mammals stabilizes the osmolality of blood plasma by producing a urine that has an osmolality substantially exceeding that of blood plasma. Urine is concentrated in the final stages of its production: water is absorbed, in excess of solute, from the collecting ducts and into the vasculature of the medulla, thus increasing the osmolality of the collecting duct fluid and thus the osmolality of the urine that emerges from the collecting ducts.
When a mammal is producing concentrated urine, an increasing osmolality gradient is maintained in all tubules and vessels along the cortico-medullary axis of the outer medulla by means of active NaCl transport from specialized renal tubules (viz., thick ascending limbs). However, the thin ascending limbs found in the inner medulla have no significant active transepithelial transport of NaCl or any other solute (7, 8, 29, 30). Thus active solute transport coupled with countercurrent flow does not explain the concentrating process in the inner medulla, where the steepest osmotic gradient is generated.
The most influential theory for the generation of the inner medullary osmolality gradient is the “passive mechanism” hypothesis, proposed independently in 1972 by Kokko and Rector (15) and by Stephenson (41). The passive mechanism depends on the assumption that the interstitium has a much higher urea concentration than NaCl concentration and that fluid in the ascending limbs has a much higher NaCl concentration than urea concentration. If the ascending thin limb has a sufficiently high permeability to NaCl and a sufficiently low permeability to urea, then much NaCl will diffuse (passively) from the ascending thin limb lumen into the interstitium, whereas simultaneously little urea will diffuse from the interstitium into the thin limb lumen. If these transepithelial concentration differences are sustained, the interstitial fluid will be concentrated while the luminal fluid is being diluted. The passive mechanism hypothesis assumes that the concentrations are sustained by continuous diffusion of urea from the collecting duct lumen and by continuous delivery of tubular fluid having a high NaCl concentration to the ascending thin limb; this delivery depends on the descending thin limb having sufficiently low NaCl and urea permeabilities that transepithelial concentration gradients are not dissipated along the course of the descending thin limbs. Thus the passive mechanism is critically dependent on specific loop-of-Henle permeabilities to NaCl and urea.
However, mathematical models using measured values of urea permeability have generally been unable to predict a significant axial osmolality gradient (38). The inconsistency between measured urine osmolalities and the predictions of mathematical models has motivated the formulation of a number of alternative hypotheses, including the potential roles of anatomical complexity (47–49), of accumulation of an external osmolyte (6, 10, 44, 43), of muscular contractions of the pelvic wall (13, 40), and of solute secretion into the loops of Henle (24). The attempts to reconcile mathematical models with the formation of highly concentrated urine have been extensively reviewed (13, 25, 38).
In this review, we summarize new findings on the three-dimensional functional architecture of the renal medulla of the rat kidney, and we consider the significance and implications of these findings for the urine concentrating mechanism.
The mechanism of formation 1of concentrated urine in mammals
Explanation:-
- Mammals can produce concentrated urine. The loop of Henle and the vasa recta play an important role in the same.
- The transport of sodium chloride in the ascending limb of the loop of Henle results in an osmotic gradient.
- It is inactive in the thin region of the loop while active in the thick region.
- The hypertonic medulla is produced by the renal tubules and the vasa recta in the following way:-
- There is an active absorption of urea from the collecting duct.
- Passive absorption of water without NaCl occurs in the descending limb of the loop of Henle, which helps to concentrate NaCl entering the ascending limb.
- In the vasa recta, water is absorbed into the blood and the absorbed solute is returned to the interstitium. This is called a countercurrent exchange mechanism.
- Human kidneys can produce approximately four times more concentrated urine than the initial filtrate that is initially formed.