 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-crunt exchange system between the ascending and descending limbs of loop of Henle. This counter-crunt 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 in both the limbs. The first is the descending limb and the second is the ascending limb. Similarly, the fluid moves in the 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-crunt exchange because it has a lot of multiplication effects, so we call it counter-crunt multiplication. Now we shall discuss the mechanism of counter-crunt multiplication. The two parallel limbs of the loop of Henle have different permeability characteristics. The descending limb is permeable to water, but it is impermeable to solutes. Water moves through its wall out 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. They are actively transported out of the filtrate and they also come in the flow of the fluid. They 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 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 on, it becomes more hypertonic. The peritubular fluid and as a result, more water is removed from the descending limb 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 antidiuretic hormone. The amount of water and the amount of solutes to be removed or absorbed is controlled by antidiuretic hormone because it increases the water permeability of the renal tubules.