Identify the predominant extracellular anion.Name the disorders associated with abnormally high and low levels of the six electrolytes.List the role of the six most important electrolytes in the body.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.By the end of this section, you will be able to: This water movement can cause the cells to swell. If the concentration of ions in the extracellular fluid decreases, water moves by osmosis from the extracellular fluid into the cells. 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. Because blood is an important component of the extracellular fluid volume, this process helps maintain blood volume. As a consequence, water moves from the intracellular fluid to the extracellular fluid, thus maintaining the extracellular fluid volume. When a person becomes dehydrated, the concentration of ions in the extracellular fluid increases. The intracellular fluid can help maintain the extracellular fluid volume if it is depleted. For example, if the extracellular concentration of ions increases, water moves by osmosis from cells into the extracellular fluid. The major influence controlling the movement of water between the intracellular and extracellular spaces is osmosis. 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. 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. Water movement is regulated mainly by hydrostatic pressure differences and osmotic differences between the compart-ments. Water continually passes through them, but ions dissolved in the water do not readily pass through the cell membrane. The cell membranes that separate the body fluid compartments are selectively permeable. Like intracellular fluid, the extracellular fluid has a fairly consistent composition from one area of the body to another. The concentration of protein in the intracellular fluid is also greater than that in the extracellular fluid. It has a lower concentration of Na +, Ca 2+, Cl −, and HCO 3 − than does the extracellular fluid. It contains a relatively high concentration of ions, such as K +, mag-nesium (Mg 2+), phosphate (PO 4 3−), and sulfate (SO 4 2−), compared to the extracellular fluid. Intracellular fluid has a similar composition from cell to cell. 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.Ĭomposition of the fluid in the Body fluid Compartments These special subcompartments contain fluid with a composition different from that of the other extracellular fluid. A small portion of the extracellular fluid volume is separated by membranes into subcompartments. The extracellular fluid compartment includes the inter-stitial fluid, the plasma within blood vessels, and the fluid in the lymphatic vessels. It constitutes approximately one-third of the total body water. The extracellular fluid compartment includes all the fluid outside the cells. Approximately two-thirds of all the water in the body is in the intracellular fluid compartment. Both the composition of the fluid in all these compartments and the regulation of fluid movement across all these cell membranes are similar. The cell membranes of the individual cells enclose the intracellular compartment, which actually consists of trillions of small compartments. The intracellular fluid compartment includes the fluid inside all the cells of the body. Water and ions move between these compartments, but their movement is regulated. 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). 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. 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. Approximately 50% of the total body weight of an adult female is water. Approximately 60% of the total body weight of an adult male consists of water.
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