Explain the Transport properties of Loop of Henle

The loop of Henle, as shown in Figure, has a descending limb and an ascending limb. As the filtrate moves down the loop of Henle, water is reabsorbed, but the ions (Na, CI) are not. The removal of water serves to concentrate the Na and Cl ions in the lumen. Now, since the filtrate moves up the other side (ascending limb), Na and Cl ions are reabsorbed but the water is not. What these two type of transport properties do is set up a concentration difference in NaCl along the length of the loop along with the highest concentration at the bottom and lowest concentration at the top. The loop of Henle can as compared to the concentrate NaCl in the medulla.

The longer the loop, the bigger the concentration gradient. This as well means that the medulla tissue tends to be saltier than the cortex tissue. This property of loop of Henle to enhance osmolarity of filtrate in a counter current flow - is known as Counter current Multiplier System - where Na is multiplied every time it passes through loop of Henle. That is the reason why filtrate is isotonic in PCT, becomes hypertonic as it passes through loop 5f Henle, hypo as it goes to DCT and finally hypertonic as it leaves collecting duct. So normally urine is highly concentrated and hypertonic as compared to blood.

Now, as the filtrate flows through the collecting ducts, which go back down through the medulla, water can be reabsorbed from the filtrate by osmosis. Have a look at the Figure. Here you will notice that water moves from an area of low Na concentration (high water concentration) in the collecting ducts to an area of high Na conceritration (low water concentration) in the medullary tissue. If you remove water from the filtrate at this final stage, you can concentrate the urine.