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. These differences are greatest in premature babies, when ECF exceeds ICF.Approximately 60% of the total body weight of an adult male consists of water. The ICF volume is obtained by subtracting the ECF volume from the TBW.ĮCF exceeds 30% and ICF is <40%. The interstitial fluid volume is determined as the difference between ECF volume and plasma volume. These substances neither leave the vascular system nor penetrate the erythrocytes. Plasma volume can be measured either by radioactive albumin or by Evans blue. ECF volume is measured using inulin, which is proportionally distributed between plasma volume and interstitial volume. TBW is measured using deuterium oxide (heavy water). Vol1 = volume of indicator, Vol2 = volume to be measured.Īgents used for measurement of fluid compartments Relationship between the volumes of major fluid compartmentsĪ known amount of tracer is introduced into the space to be measured, and its concentration measured after mixing.Ĭon1 = initial concentration of indicator, Con2 = final concentration of indicator
Total body water (TBW) constitutes 55-60% of the body weight in men and 45-50% of the body weight in young women.
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