Water Balance

What do we mean by "water balance?"

All day long a person gains and loses water. This is fine as long as the total amount of water in that person's body remains constant. Our bodies have to be able to control this water balance (of gain and loss of water), or water homeostasis. As long as a water balance exists, the osmotic concentrations of body fluids will remain as they should. Once there is a change in water balance, there will be a discrepancy in the osmotic concentration of solutions, and osmotic pressure will exist in certain regions. Remember, osmotic pressure can be quite large, and can even cause cells or vessels to explode! We need to prevent anything like that from occurring.

Ways we gain water:

	Through our mouths, by drinking water (60% of all gained water) and eating moist foods (30% of all gained water). This water is mainly absorbed through the large (and small) intestines.
	As a byproduct of cellular respiration (10% of all gained water). Did you remember that cellular respiration produces water? Here's a reminder of the cellular respiration chemical formula: oxygen + glucose -----> carbon dioxide + water (+ energy!)

Ways we lose water:

	In our sweat (6%) Note that we have to lose water through sweat in order to maintain our body temperatures... we cannot prevent this water loss.
	In our feces (6%) Note that we have to lose some water with our feces, otherwise we will not properly form the feces or the feces will not be removed from the body.
	By evaporation as we breathe and through our skin (28%) Note that we cannot prevent evaporative water loss... it is a fact that must occur because we live in environments that are less humid than our own bodies.
	In our urine (60%) Note that this is the only water loss that we can possibly control!

How do we know when to consume more water?

We have a "thirst center" in our hypothalamus. This thirst center contains osmoreceptors, which are simply sensory receptors that detect changes in fluid concentrations. If we lose too much water, our body fluids begin to become more concentrated. As some of these fluids change in their concentration, osmotic pressure pushes on the osmoreceptors and they become active. Once the osmoreceptors are activated within the hypothalamus of a person, that person gets the sensation of being thirsty. This sensation goes away if that person drinks enough fluid to distend their stomach.

All that is needed to activate the osmoreceptors is a 1% change in total body water. Considering that we have approximately 40 liters of body fluid (see Figure 21.1), 1% is a change of 0.4 liters, or 400 mL. 400 mL is the equivalent of 1.6 cups of water (250 mL = 1 cup). And, with a typical water intake of 2500 mL per day, 400 mL is only 16% of our total daily water intake.

How do we regulate the amount of water we lose from our bodies?

As I mentioned above, the only control we have over water loss is through our kidneys. A person typically loses 1.5 liters of water per day in urine. However, if we are unable to gain water, we have to prevent water loss to maintain water balance. So, we have to be able to decrease the 1.5 liters of water lost when we need to. (We also have to be able to lose more water if we consume more than we need.)

Where in the kidney can we have water reuptake if needed? In the distal convoluted tubules and the beginning of the collecting ducts. These regions are normally impermeable to water. However, in the presence of the posterior pituitary hormone, antidiuretic hormone (ADH), these tubules become permeable to water. Under conditions of body water loss, not only do the osmoreceptors evoke a thirst sensation, but they also trigger the release of ADH from the posterior pituitary. ADH then leads to water reuptake through the distal convoluted tubules and the collecting ducts via osmosis.

ADH has also been called vasopressin. That is because if ADH is administered to a person or animal, that individual's blood pressure increases. External administration of ADH is not part of the homeostatic mechanism, so if an individual receives it without needing to retain water, the extra water retention will lead to an increased volume of blood, which causes an increase in blood pressure.

How effective is ADH in providing for water retention? When needed, water loss through urine can decrease to only 500 mL per day! That is a 2/3 reduction!