 Dear students, in this topic we shall discuss the urine concentrating mechanisms. You know that mammalian kidneys can produce hypertonic urine as compared to the blood plasma. This is because of the presence of loop of Henle that has central importance in producing concentrated urine. The efficiency of water reabsorption in the loop of Henle is due to a counter-current exchange system between the ascending and descending limbs of loop of Henle. This counter-current exchange happens because both limbs lie very close to each other and are separated by a peritubular fluid. There are two limbs parallel to each other and there is a fluid between the two, which is called peritubular fluid. The filtrate moves in the opposite direction between the two limbs. The first is the descending limb and the second is the ascending limb. In this way, the fluid moves in descending and ascending order in both the limbs. During this flow, exchange of water and salts occurs between the filtrate in the two limbs and the peritubular fluid. Both the limbs have water and solutes and the peritubular fluid has exchange of water and solutes. This exchange is referred to as counter-current exchange because it has a multiplication effect so we call it counter-current multiplication. Now we shall discuss the mechanism of counter-current multiplication. The two parallel limbs of the loop of Henle have different permeability characteristics. Descending limb is permeable to water but it is impermeable to solutes. Water moves through its wall out into the peritubular space. As it is permeable for water, water comes out of it and comes into the peritubular space. As a result, the filtrate in it becomes hypertonic. This filtrate, that is, hypertonic filtrate enters the ascending limb. The ascending limb is impermeable to water but it is permeable to solutes. That is why water does not come out of it. Instead of this, salts i.e. sodium, chloride and potassium ions are actively transported out of the filtrate and they also come into the peritubular fluid. This peritubular fluid or the peritubular space in which the fluid enters, it becomes hypertonic. When it is hypertonic, it creates an osmotic pressure. Due to this osmotic pressure, more water comes out of the descending limb into the peritubular space. As a result, filtrate in the descending limb becomes more hypertonic. This more hypertonic filtrate, when it enters the ascending limb, it speeds up pumping out of sodium and chloride ions into the peritubular fluid. The result is that it becomes a counter current flow which multiplies the osmotic gradient. In the first cycle, it became hypertonic but in the second cycle, it became more hypertonic. In this way, the further cycles go, the more hypertonic it becomes. Peritubular fluid or as a result, the descending limb removes more water from the fluid. This counter current flow multiplies the osmotic gradient between tubular fluid and interstitial space. As a result, the osmotic removal of more and more water out of the filtrate happens. Dear students, this water is coming into interstitial space. This water is absorbed back into the circulatory system through the capillaries, that is, vesarecta, which surround the loop of Henle. The amount of water reabsorbed and the concentration of urine is controlled by anti-diuretic hormone. The amount of water and the amount of solutes to be removed or absorbed is controlled by anti-diuretic hormone because it increases the water permeability of the renal tubules.