Maintaining Water Volume
Your kidneys have the ability to conserve or waste water. For example, if you drink a large glass of water, you will find that you will have the urge to urinate within an hour or so. In contrast, if you do not 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.
Loop of Henle
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) are not. 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 is not. 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.
Anti-Diuretic Hormone (ADH)
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 do not secrete ADH. In reality, there is always some very low level of ADH secreted from the osmoreceptors.
(snip)
When you drink a large glass of water, the water gets absorbed into the blood and the following happens:
The absorbed water increases the amount of water filtered in the glomerulus.
The absorbed water in the blood reduces the Na concentration a little.
The reduced Na concentration lowers the amount of Na filtered in the glomerulus.
The nephron reabsorbs all of the reduced Na load and some of the accompanying water, leaving excess water in the filtrate.
The reduced Na concentration is sensed by the osmoreceptors.
The osmoreceptors do not secrete as much ADH.
Because the collecting ducts do not see as much ADH, they do not allow much water to be reabsorbed in response to the Na concentration gradient set up by the loop of Henle.
The excess water gets excreted in the urine.
When the excess water is excreted, the Na concentration of the blood returns to normal.
(snip)
Typically, we do not drink water overnight when we sleep. So, our intestines are not absorbing water:
Decreased water absorption by the intestine reduces the amount of water in the blood.
Decreased water in the blood reduces the amount of water filtered in the glomerulus.
Decreased water in the blood increases the Na concentration in the blood.
Increased Na concentration in the blood increases the amount of Na filtered in the glomerulus.
The nephron does not reabsorb all of the filtered Na, and some water remains with it in the filtrate.
The increased Na concentration in the blood is sensed by the osmoreceptors.
The osmoreceptors secrete ADH.
The collecting ducts see more ADH and allow water to be reabsorbed in response to the Na concentration gradient set up by the loop of Henle.
More water gets reabsorbed from the collecting ducts, producing a concentrated urine. A little water is lost in the urine because of the Na; we cannot excrete solid urine.
The removal of Na and increased reabsorption of water help return the blood concentration of Na to normal.
So, the loop of Henle sets up the Na concentration gradient across the medulla, allowing for water to be reabsorbed from the collecting ducts, and ADH allows the water to pass through those collecting ducts.
Hope that helps.
BigTrancer