<P> The extracellular fluid is constantly "stirred" by the circulatory system, which ensures that the watery environment which bathes the body's cells is virtually identical throughout the body . This means that nutrients can be secreted into the ECF in one place (e.g. the gut, liver, or fat cells) and will, within about a minute, be evenly distributed throughout the body . Hormones are similarly rapidly and evenly spread to every cell in the body, regardless of where they are secreted into the blood . Oxygen taken up by the lungs from the alveolar air is also evenly distributed at the correct partial pressure to all the cells of the body . Waste products are also uniformly spread to the whole of the ECF, and are removed from this general circulation at specific points (or organs), once again ensuring that there is generally no localized accumulation of unwanted compounds or excesses of otherwise essential substances (e.g. sodium ions, or any of the other constituents of the ECF). The only significant exception to this general principle is the plasma in the veins, where the concentrations of dissolved substances in individual veins differs, to varying degrees, from those in the rest of the ECF . However this plasma is confined within the waterproof walls of the venous tubes, and therefore does not affect the interstitial fluid in which the body's cell live . When the blood from all the veins in body mixes in the heart and lungs, the differing compositions cancel out (e.g. acidic blood from active muscles is neutralized by the alkaline blood homeostatically produced by the kidneys). From the left atrium onward, to every organ in the body, the normal, homeostatically regulated values of all of the ECF's components are therefore restored . </P> <P> The arterial blood plasma, interstitial fluid and lymph interact at the level of the blood capillaries . The capillaries are permeable and water can move freely in and out . At the arteriolar end of the capillary the blood pressure is greater than the hydrostatic pressure in the tissues . Water will therefore seep out of the capillary into the interstitial fluid . The pores through which this water moves are large enough to allow all the smaller molecules (up to the size of small proteins such as insulin) to move freely through the capillary wall as well . This means that their concentrations across the capillary wall equalize, and therefore have no osmotic effect (because the osmotic pressure caused by these small molecules and ions--called the crystalloid osmotic pressure to distinguish it from the osmotic effect of the larger molecules than cannot move across the capillary membrane--is the same on both sides of capillary wall). </P> <P> The movement of water out of the capillary at the arteriolar end causes the concentration of the substances that cannot cross the capillary wall to increase as the blood moves to the venular end of the capillary . The most important substances that are confined to the capillary tube are plasma albumin, the plasma globulins and fibrinogen . They, and particularly the plasma albumin, because of its molecular abundance in the plasma, are responsible for the so called" oncotic" or "colloid" osmotic pressure which draws water back into the capillary, especially at the venular end . </P> <P> The net effect of all of these processes is that water moves out of and back into the capillary, while the crystalloid substances in the capillary and interstitial fluids equilibrate . Since the capillary fluid is constantly and rapidly renewed by the flow of the blood, its composition dominates the equilibrium concentration that is achieved in the capillary bed . This ensures that the watery environment of the body's cells is always close to their ideal environment (set by the body's homeostats). </P>

The chief ion (electrolyte) used to maintain the volume of fluid outside of the cell is