Your kidneys have the ability to conserve or waste water. For example, if you drink a large glass of water, you'll find that you will have the urge to urinate within an hour or so. In contrast, if you don't drink for a while, such as overnight, you will not produce much urine and it will usually be very concentrated (i.e. darker). How does your kidney know the difference? The answer to this question involves two mechanisms:
- The structure and transport properties of the loop of Henle in the nephron.
- The anti-diuretic hormone (ADH), also called vasopressin, secreted by the pituitary gland.
The loop of Henle has a descending limb and an ascending limb. As filtrate moves down the loop of Henle, water is reabsorbed, but ions (Na,Cl) aren't. The removal of water serves to concentrate the Na and Cl in the lumen. Now, as the filtrate moves up the other side (ascending limb), Na and Cl are reabsorbed, but water isn't. What these two transport properties do is set up a concentration difference in NaCl along the length of the loop, with the highest concentration at the bottom and lowest concentration at the top. The loop of Henle can then concentrate NaCl in the medulla. The longer the loop, the bigger the concentration gradient. This also means that the medulla tissue tends to be saltier than the cortex tissue.
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. Water moves from an area of low Na concentration (high water concentration) in the collecting ducts to an area of high Na concentration (low water concentration) in the medullary tissue. If you remove water from the filtrate at this final stage, you can concentrate the urine.
ADH, which is secreted by the pituitary gland, controls the ability of water to pass through the cells in the walls of the collecting ducts. If no ADH is present, then no water can pass through the walls of the ducts. The more ADH present, the more water can pass through.
Specialized nerve cells, called osmoreceptors, in the hypothalamus of the brain sense the Na concentration of the blood. The nerve endings of these osmoreceptors are located in the posterior pituitary gland and secrete ADH. If the Na concentration of the blood is high, the osmoreceptors secrete ADH. If the Na concentration of the blood is low, they don't secrete ADH. In reality, there is always some very low level of ADH secreted from the osmoreceptors.
Now let's look at how your kidneys maintain water volume.