Urea, along with salt, water and glucose, etc., is extracted from the blood in the kidney by a process calledultrafiltration. Blood passing the top of the nephron is under high pressure, so fluid is forced through the sieve-like capillaries and into the capsule. This fluid is called the filtrate. It does not contain any blood cells or larger proteins, as they are too big to pass out of the capillaries and into the capsule.
Much of what has been filtered out needs to be returned to the blood - they are too precious to lose - so the next process is called selective reabsorption.
When the filtrate reaches the proximal convoluted tubule, sodium (Na+) and chloride (Cl−) ions, glucose, amino acids and vitamins move back into the blood. Generally they diffuse from the filtrate into the cells lining the proximal convoluted tubule. They are then actively transported out of these cells and into the blood capillaries. Some water follows by osmosis. Surprisingly, some unwanted urea also gets reabsorbed here.
Re-absorption of water
The basic idea is to create a strong salt concentration in the next part of the tubule. This will draw water out of the tube by osmosis, and then the water can be taken away by the blood. This conserves water levels in the body.
How the body does it:
It is easier to understand if you start with the ascending limb (the second part of the loop of Henle).
Na+ and Cl− are actively pumped out of the filtrate in the tube and into the tissue fluid around it. No water follows, however, as this part of the loop is impermeable to water. This has two consequences.
- Water flows out of the descending limb into the tissue fluid by osmosis.
- Na+ and Cl−, being at a very high concentration in the tissue fluid, diffuse down the concentration gradient into the descending limb.
By the time the filtrate reaches the bottom of the descending limb the fluid in the loop has lost a lot of water and is very concentrated. The fluid surrounding the bottom of the loop - in area of the kidney called the medulla - is also very concentrated because of the accumulation of Na+ and Cl− ions.
As the fluid then goes up the ascending limb, Na+ and Cl− ions are actively pumped out (as was mentioned a few lines ago) so it gets more and more dilute.
Filtrate passing down the descending limb of the loop of Henle is flowing in the opposite direction to fluid in the ascending limb. The fluid is increasingly concentrated as it moves down and increasingly dilute as it moves up. Thiscountercurrent flow (or countercurrent multiplier) allows concentrated urine to be produced.
This is important because yet more water is drawn out of the tube (at this point called the collecting duct) when it passes through the medulla again. This allows you to make concentrated urine. Any filtrate not reabsorbed - most of the urea, some water and some salt - is drained into the bladder.
Obviously the amount of water reabsorbed is controlled by the quantity of water in the blood. The less water in the blood, the more that it must be reabsorbed. The hormone ADH (anti-diuretic hormone) controls the extent to which water is reabsorbed. If the blood is concentrated, more ADH is released; making the walls of the collecting duct more permeable to water so more is reabsorbed back into the blood. If the blood is less concentrated, less ADH is released so less water is reabsorbed.
Control of ADH
The concentration of the blood (water potential) is monitored by osmoreceptors in the hypothalamus. The higher the concentration of the blood the less water there is in the blood.
If the concentration is too high impulses are sent to the pituitary gland which then releases more ADH. The water levels will be brought back to normal and the impulses stop.