The osmotic concentration of the plasma is about 300 mOsm. One very crucial function of the kidney is to keep the solute load of the fluids of the body at equal value to that of the normal osmotic concentration of the plasma. This is done by regulating the concentration of the urine and its volume.
In order for the kidneys to maintain normal osmotic concentration, it uses a process known as countercurrent mechanisms. This mechanism entails that the fluids are flowing through adjacent tubes in opposing directions. The countercurrent mechanism happens as the filtrate flows through the ascending and descending limbs of the loop of Henle. Another venue that this countercurrent mechanism happens is when the blood flows through the ascending and descending portions of the vasa recta blood vessels.
As this happens, the kidneys are able to maintain an osmotic gradient, which extends to the medulla, thereby allowing the ability of the kidney to vary the concentration of the urine dramatically.
As the filtrate enters the proximal convoluted tubules, its osmolality is similar to the plasma, which is 300 mOsm. As the filtrate travels to the deeper parts of the medulla, the osmolality greatly increase to 1200 mOsm. This happens because of the unique working of the loop of Henle of the juxtamedullary nephrons together with the vasa recta.
The Countercurrent Multiplier
The countercurrent multiplier helps in the establishing of the all important osmotic gradient within the kidney. Three factors are responsible for the countercurrent multiplier to function. One of this is that the descending loop of Henle is permeable to water but impermeable to solutes. This phenomenon allows the water to pass through osmotically out of the filtrate. The remaining filtrate, containing less amount of water, then reaches its highest osmolality point at 1200 mOsm.
As the filtrate is heading towards the ascending loop, the permeability of the tubules changes drastically, it becomes selectively permeable to salt but is impermeable to water. Because of this, the sodium and chloride concentration found in the filtrate that enters the ascending limb is relatively high. Because the filtrate is losing salt but retaining water, the filtrate then becomes significantly dilute to a point where it reaches 100 mOsm at the distal convoluted tubule.
Urea enters the filtrate at the loop of Henle by facilitated diffusion. As the filtrate moves, it passes through a point where water is reabsorbed and a membrane that is not permeable to urea exists. As the filtrate reaches the collecting ducts, the urea present is already highly concentrated. It is then transported out of the collecting ducts to the interstitial fluid of the medulla, thereby influencing to the medullary osmotic gradient.
Formation of Concentrated Urine
The antidiuretic hormone is more like an explanation of what it does rather than being a name. It prevents diuresis or what we call urine output. The concentration of the urine depends on the amount of ADH released, which may cause the concentration to rise as high as 1200 mOsm. The ability of the kidneys to make concentrated urine is a reflection of our ability to survive without water.