Loop of Henle

Two parts of the loop of henle

The loop of henle has two parts known as the descending and ascending limbs.

Direction of flow

The thick ascending limb of nephron loop connects with the distal convoluted tubule, which connects with the urine connecting duct.

What is happening to the Na

The loop of henle dips down into the medulla, which is highly salty because of the ion absorption, Sodium is constantly being pumped out of the ascending limb into the medulla.

Water leaves by osmosis

The ascending limb is impermeable to water. The descending limb is permeable to water, but has low permeability to Sodium. As the filtrate travels down the descending limb the water leaves by osmosis because of the salty environment into the medulla.

Bottom of the loop reaches its highest conc.

By the time the filtrate reaches the bottom of the loop it is highly concentrated and significant amounts of water has left.

NaCl is pumped out

As the filtrate makes it way up the ascending limb the water cannot escape this area. Sodium chloride leaves passively from the ascending limb, then is actively pumped out of the thicker part of the limb.

Countercurrent Mechanisms

But, lets dig a little dipper into the meaning of countercurrent mechanisms and what it means as the descending and ascending limbs both go into opposite directions.

Countercurrent multiplier

The multiplier means that it reabsorbs ions in the ascending limb which makes the medulla salty & by not absorbing water it drives water to be reabsorbed passively in the descending limb. No energy is used to reabsorb water and this is because we have used energy here in the descending limb to reabsorb these ions.

Medulla becomes hyperosmotic / hypoosmotic

So active transport is used. By actively pumping ions into the medulla and no water in the descending limb to make it salty, this will multiply the amount of water that is being reabsorb passively because it is driven into this space around the tubule or the interstitium. As a result, interstitial fluid throughout the whole medulla becomes hyperosmotic and the fluid leaving the ascending thick limb and entering the distal tubule is hypoosmotic. All the ions in the interstitium space drives the absorption of the water.