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Regulation of Water Balance

It is important to realize that the amount of water that remains in the urine must be carefully controlled to ensure that we do not become over or underhydrated. As noted earlier, the amount of water present in the body is one of the major factors determining concentrations of all solutes, and its regulation is therefore vital to the maintenance of homeostasis. Additionally, as land creatures, we are in constant danger of losing water to the environment, and mechanisms of excreting urine that is hypertonic to our body fluids have thus evolved for purposes of water conservation. Depending on the circumstances, of course, we also may need to create urine that is hypotonic to body fluids for example, after we have consumed a large amount of water. How is the nephron able to regulate the amount of water that leaves in the urine?
Firstly, as the filtrate passes through the proximal convoluted tubule, its volume is drastically reduced as sodium ions, glucose, and amino acids are actively transported out, causing water to follow passively by osmosis. In addition, negatively charged ions return to the blood due to the electrical imbalance created by active transport of positive ions. All of these substances are readily picked up by the peritubular capillaries and returned to the blood. As the filtrate begins its passage through the loop of Henle, its composition is altered more dramatically. Both active transport and diffusion of both Na+ and urea make the area surrounding the loop extremely hypertonic to the filtrate; however, the walls of the ascending branch of the loop and the collecting duct itself are impermeable to water. So as salt is removed from the filtrate and water is not allowed to follow, the urine becomes very dilute (hypotonic). If nothing else was to happen, this would be the end of the process and urine would always be very dilute. Remember, however, that it is the lack of permeability of the collecting duct wall that prevents water from diffusing out of the duct. If the wall could be made permeable to water, it would rapidly move by osmosis out of the urine and back into the surrounding area to be ultimately returned to the blood, causing a concentrated, hypertonic urine to be excreted. As you’ve probably guessed, there is a way of controlling the permeability of the wall of the collecting duct to water, and thus of regulating the concentration of the urine. This mechanism depends on the action of the hormone ADH (antidiuretic hormone), which increases the permeability of the walls of the collecting duct to water, promoting the formation of a concentrated urine.
As we discussed in an earlier chapter, ADH is manufactured by the hypothalamus and stored and secreted by the pituitary gland in the brain. The hypothalamus monitors the osmotic composition of the blood and the blood pressure, and triggers a release of ADH when necessary. If the blood is dilute, as might occur after drinking a lot of fluids, the hypothalamus decreases ADH production, so more water is excreted in the urine. Conversely, if the body is dehydrated, the osmotic pressure of the blood increases, and the hypothalamus steps up ADH production, causing more water to be retained.
The hormone aldosterone, secreted by the adrenal cortex, triggers the nephron to increase reabsorption of sodium ions as well as increase secretion of potassium. Thus, the electrolyte balance of the body is also controlled by the nephron. In addition, the nephron participates in the regulation of pH by secreting hydrogen ions into the urine. All in all, the kidney really is an amazing organ, which performs and controls many processes leading to an incredibly fine-tuned regulation of body fluid composition.

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