please explain the counter current mechanism in our nephrons..??? please....
- Higher vertebrates have evolved a counter current mechanism to create hypertonic urine for conserving body water, so necessary for land life.
- Henle’s loop and capillary loop play an important role in this mechanism. Glomerular fluid in the Henle’s loop and the blood in the vasa recta flow in opposite directions in the two descending limb and toward the renal cortex in the ascending limb. These two counter current systems help in concentrating urine by maintaining high salt concentration around the nephrons and collecting ducts.
- Kidneys also play a role in osmoregulation, i.e., maintenance of water and Na+ contents of blood. This is achieved by countercurrent mechanism in the loop of Henle and by the action of antidiuretic hormone from posterior pituitary gland of brain and another hormone aldosterone from the adrenal gland.
- The vasa recta bring blood supply at a very slow rate. It consists of a descending limb and an ascending limb. Vasa recta are also known as the straight arteries of the kidney.
- Counter-current mechanism first takes place in the region of the nephron called Henle's loop. The two limbs of Henle's loop are counter-current and these tubes are parallel and adjacent.
- Then it occurs in a region of the peritubular capillary bed called the 'vasa recta'. The two limbs of the vasa recta are counter-current and these tubes are parallel and adjacent.
- Both are involved in establishing an osmotic gradient throughout this region.
In the kidney, the loop of Henle is the portion of the nephron that leads from the proximal convoluted tubule (PCT) to the distal convoluted tubule. The loop has a hairpin bend in the renal medulla. The main function of this structure is to reabsorb water and ions from the urine. To do this, it uses a countercurrent multiplier mechanism in the medulla. It is named after its discoverer, F. G. J. Henle.
COUNTERCURRENT SYSTEM and the LOOP OF HENLE
1. The loop of henle establishes medullary hyperosmolarity
The ascending limb of the loop of Henle transports solutes (NaCl) out of the tubule lumen with little or no water, generating an hyperosmotic medullary interstitium and delivering an hyposmotic tubule fluid to the distal tubule. This is called the "single effect".
The osmolarity of the interstitium rises progressively from cortex to medulla and papilla through multiplication of the "single effect" by countercurrent flow in the branches of the loop: The single effect in fluid processed by loop segments located near the tip of the papilla occurs in fluid already subject to the single effect when the fluid was in loop segments located closer to the cortex.
Countercurrent exchange of solutes between ascending and descending vasa recta (the renal medullary capillaries) minimizes solute washout from the medullary interstitium.
2. The countercurrent system permits forming a concentrated urine
In the presence of ADH, which increases water permeability, the hyposmotic fluid that enters the distal tubule (DT) from the thick ascending limb (TAL) looses most of its water by osmotic equilibration with the surrounding cortical interstitium along the CNT and cortical collecting duct (CCD). It also continues loosing NaCl through reabsorptive transport along DT, CNT and CCD, until the tubule fluid becomes isoosmotic with plasma, by the end of the CCD.
The relatively small amount of isoosmotic fluid that flows into the medullary collecting ducts losses progressively more and more water to the hyperosmotic medullary and papillary interstitia and is finally excreted as hyperosmotic, highly concentrated urine.
3. The countercurrent system permits forming a dilute urine
In the absence of ADH, the hyposmotic fluid that enters the DT from the loop of Henle, continues to be diluted by transport of NaCl via NaCl (thiazide sensitive) cotransporters into DT cells and via Na channels (amiloride sensitive) along the CD. Water reabsorption is limited so that the tubule fluid becomes more and more dilute along DT, CNT and collecting ducts (CCD, OMCD and IMCD), until it is excreted as a large volume of hyposmotic urine.