39. Explain the counter-current system in the nephron. 

Counter Current Mechanism

Mammals and birds secrete a hypertonic urine. This is the main adaptation  for the conservation of water. There are 2 counter-current mechanisms operating inside the kidneys.

a) Vasa rectae

Inside the renal medulla, the efferent arteriole forms a capillary network around the tubules. Some of these blood capillaries are parallel, wide, thin walled, straight and loop like and are called vasa rectae. The blood flows in opposite directions in the 2 limbs of the vasa rectae. Blood entering the renal medulla in the descending limb comes very close to the outgoing blood in the ascending limb.

When the blood flows towards the renal medulla, Na+ and Cl- diffuse in the blood from interstitial fluid, but as the blood flows towards the renal cortex Na+ and Cl- diffuse into the interstitial fluid. This checks the loss of Na+ and Cl- from the renal medulla and maintains a high concentration of these in the interstitial fluid.
Henle's loopBack to Top
The glomerular filtrate flows in opposite directions in the two limbs of Henle's loop.

Mechanism of henles loop

As the filtrate passes through the ascending limb of the loop of Henle. NaCl is lost to the interstitial fluid in the renal medulla by diffusion.

The increased concentration of the solutes in the interstitial fluid draws out water by osmosis from the narrow region of the descending limb and also from the collecting duct, both being permeable to water.
The water so drawn quickly enters the vasa recta and is being carried away. This maintains a high concentration of solutes in the interstitial fluid and helps turn the isotonic glomerular filtrate into a hypertonic urine.

There is no water reabsorption from the ascending limb because its walls are impermeable to water. As large amounts of Na+ is lost due to active transport, the filtrate again becomes isotonic.
The water reabsorption mainly occurs through the wall of the collecting tubules, the permeability of which is controlled by vasopressin or antidiuretic hormone (ADH) of the posterior lobe of pituitary gland. The secretion of ADH is controlled by the osmotic pressure of the blood.

Effect of ADH in Henle's loop

Effect of ADH on the permeabilities of the distal convoluted tubule and collecting duct to water and urea
So counter current mechanism is that process which changes isotonic filtrate to hypertonic urine. The urine is about 4 times as concentrated as blood plasma in humans. The longer is the loop of Henle in the nephron, the greater is the ability of an animal to form hypertonic urine.