Renal mechanism of urine formation and Permeability of the tubules
Renal mechanism of urine formation
Renal mechanism of concentrated and dilute urine formation-
Concentrated urine is formed by passive water reabsorption from the tubules while many solutes in the tubular fluid are absorbed by active process. Dilute urine is formed by absorption of solutes alone and excretion of water in the urine.
Dilution or concentration of urine depends on-
Hypertonicity/osmolality of the interstitial fluid in the renal medulla. Dilution of the tubular fluid by the thick ascending limb and distal tubules by solute reabsorption which allows formation of dilute urine.
Variation in the water permeability of collecting ducts due to ADH, which determines the final concentration of urine.
For concentration of urine, there is generation of hypertonic medullary fluid and increase water reabsorption in the distal tubule.
- For generating hypertonic medullary fluid, two factors are essential:
- Reabsorption of osmotically active substance by the tubules into the medulla.
- Removal of water from the interstitium by the vasa recta.
These two factors are produced by counter current mechanism.
Hyperosmolality of the medullary fluid
Normal osmolality of the GF as it enters the proximal tubule is 300 milli osm/L. But osmolality in the medullary interstitial fluid is higher reaching a maximum in the inner most regions of the medulla, it increases from 300 milli osm/L to 1200 milli osm/L. The cause of this increased osmolality is the active transport of both sodium and chloride out of the Loop of Henle’s ascending limb and slight reabsorption of sodium actively from the collecting tubule into the interstitial fluid.
Also the cause for accumulation of sodium chloride in medulla is-
- Sluggish blood flow through vasa recta which helps to prevent removal of sodium chloride from the interstitial fluid.
- Presence of counter current mechanism in the Loop of Henle and vasa recta.
Permeability of the tubules
- Descending limb of Loop of Henle: High permeability for water and no permeability for sodium, chloride and urea
- Ascending thin limb of Loop of Henle: No permeability for water, highly permeable to sodium, chloride and moderately permeable to urea.
- Ascending thick limb of Loop of Henle: Permeable to sodium, chloride, low permeability to water and urea.
- Distal tubule: Permeable to sodium, chloride and low permeability for water and urea.
- Cortical collecting tubule, outer medullary collecting duct and inner medullary collecting duct: Sodium reabsorption is stimulated by aldosterone and water and urea reabsorption by ADH.