BODY FLUID COMPARTMENTS
Approximately 60% of the total body weight of an adult male consists of water. Approximately 50% of the total body weight of an adult female is water. Because the water content of adipose tis-sue is relatively low, the fraction of the body’s weight composed of water decreases as the amount of adipose tissue increases. A smaller percentage of the body weight of an adult female consists of water because females generally have a greater percentage of body fat than do males. Water and the ions dissolved in it are distributed in two major compartments: the intracellular fluid compartment and the extracellular fluid compartment (table 18.2). Water and ions move between these compartments, but their movement is regulated.
The intracellular fluid compartment includes the fluid inside all the cells of the body. The cell membranes of the individual cells enclose the intracellular compartment, which actually consists of trillions of small compartments. Both the composition of the fluid in all these compartments and the regulation of fluid movement across all these cell membranes are similar. Approximately two-thirds of all the water in the body is in the intracellular fluid compartment.
The extracellular fluid compartment includes all the fluid outside the cells. It constitutes approximately one-third of the total body water. The extracellular fluid compartment includes the inter-stitial fluid, the plasma within blood vessels, and the fluid in the lymphatic vessels. A small portion of the extracellular fluid volume is separated by membranes into subcompartments. These special subcompartments contain fluid with a composition different from that of the other extracellular fluid. Fluids within the subcompart-ments include the aqueous humor and vitreous humor of the eye, cerebrospinal fluid, synovial fluid in the joint cavities, serous fluid in the body cavities, fluid secreted by glands, renal filtrate, and bladder urine.
Intracellular fluid has a similar composition from cell to cell. It contains a relatively high concentration of ions, such as K+, mag-nesium (Mg2+), phosphate (PO43−), and sulfate (SO42−), compared to the extracellular fluid. It has a lower concentration of Na+, Ca2+, Cl−, and HCO3− than does the extracellular fluid. The concentration of protein in the intracellular fluid is also greater than that in the extracellular fluid. Like intracellular fluid, the extracellular fluid has a fairly consistent composition from one area of the body to another.
The cell membranes that separate the body fluid compartments are selectively permeable. Water continually passes through them, but ions dissolved in the water do not readily pass through the cell membrane. Water movement is regulated mainly by hydrostatic pressure differences and osmotic differences between the compart-ments. For example, water moves across the wall of the capillary at the arterial end of the capillary because the blood pressure there is great enough to force fluid into the interstitial space. At the venous end of the capillary, the blood pressure is much lower, and fluid returns to the capillary because the osmotic pressure is higher inside the capillary than outside it .
The major influence controlling the movement of water between the intracellular and extracellular spaces is osmosis. For example, if the extracellular concentration of ions increases, water moves by osmosis from cells into the extracellular fluid.
The intracellular fluid can help maintain the extracellular fluid volume if it is depleted. When a person becomes dehydrated, the concentration of ions in the extracellular fluid increases. As a consequence, water moves from the intracellular fluid to the extracellular fluid, thus maintaining the extracellular fluid volume. Because blood is an important component of the extracellular fluid volume, this process helps maintain blood volume. Movement of water from the intracellular fluid compartment to the extracellular fluid compartment can help prolong the time a person can survive a condition such as dehydration or cardiovascular shock.
If the concentration of ions in the extracellular fluid decreases, water moves by osmosis from the extracellular fluid into the cells. This water movement can cause the cells to swell. Under mostconditions, the movement of water between the intracellular and extracellular fluid compartments is maintained within limits that are consistent with survival of the individual.
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