The Regulation of Water Balance

At first glance, the regulation of water distribution in the body does not seem particularly important. In fact, the control of the amount of water contained by the body and the distribution of the water within the body are critically important to a wide variety of processes.

Water moves easily through the various compartments of our body, passing quickly through the membrane surrounding each cell, through the spaces between cells, and through the connective tissues and structures that provide the scaffolding that supports us. And of course this is advantageous because many of the essential nutrients required by the body are soluble in water, and the movement of water assures the effective distribution of these nutrients. The movement of water is so important that we have a particularly sophisticated system, the heart and blood vessels, to assure that delivery is fast and that every cell is well served.

At the cellular level, there is a central problem that must be managed. There is a fundamental law that says that substances always move from a region of higher concentration to a region of lower concentration. This principle is so common that we hardly are surprised when we see it in operation, for example we expect that heat will always flow from a warm area to a cold area.

Cell membranes act as a kind of filter, only allowing small molecules to pass through them. As a result, the large enzyme and structural protein molecules described earlier cannot leave the cell. Once they are synthesized inside the cell, they remain there. The interior concentration of protein and other substances that cannot cross the membrane is quite high.

Since cells contain a high

An electron micrograph of human liver cells. Water moves easily through the components of the body, passing through cell membranes and through the space between cells; note the porous appearance of the cell structure.

concentration of protein and other complex molecules they also must contain a relatively lower concentration of water. Said differently, all molecules take up space. The more non-water molecule are present, the fewer water molecules there are in that space. Because water can pass easily through the membranes of cells and tissues, it will always flow from an area where the water concentration is high to one where it is lower. This means that if you place a cell in water it will rapidly swell as water enters the cell, and the swelling will continue until the cell bursts.

Having our cells burst is a very bad idea! The only way this can be avoided in animal cells it to have the circulating fluids of the body contain a concentration of molecules that is equal to the concentration of molecules inside the cell. And, of course, these circulating molecules must not be able to pass through the cell membrane. Said differently, the water concentration of the blood, lymph, plasma, and fluids in the spaces between cells, must be the same as the water concentration of the cells.

To maintain water balance, the

blood contains a high level of protein that never enters the cellular spaces. Some of these proteins, the various kinds of antibodies produced in response to invasion by foreign substances or organisms, are associated with immunity. Others are involved in repair processes, like blood clotting. But the bulk of the circulating protein is composed of a single protein known as Albumin.

Albumin is produced by only one organ, and of course that organ is the liver. Throughout your life, the liver is sensing the level of complex molecules in the blood and, if the concentration of those molecules is too low, synthesizing and releasing albumin to the blood.

Liver disease and failure is often signaled by decreased albumin synthesis and release resulting in progressive entry of water into cells and tissues. This produces characteristic swelling and a state known as "edema." Obviously, this kind of water retention by cells and tissues can easily become a serious problem, interfering with cellular processes, causing disruption of cell structure and, if water movement into cells is too great, death.