Extracellular water (20%
Total body water (60% body weight; 42 L)
|Water distribution in the body. This dia-"gram is for an average young adult man weighing 70 kg. In an average young adult woman, total body water is 50% of body weight, intracellular water is 30% of body weight, and extracellular water is 20% of body weight.
ter is in the ICF, and one third is in the ECF (Fig. 24.1). These two fluids differ strikingly in terms of their electrolyte composition. However, their total solute concentrations (osmolalities) are normally equal, because of the high water permeability of most cell membranes, so that an osmotic difference between cells and ECF rapidly disappears.
The ECF can be further subdivided into two major sub-compartments, which are separated from each other by the endothelium of blood vessels. The blood plasma is the ECF found within the vascular system, it is the fluid portion of the blood in which blood cells and platelets are suspended. The blood plasma water comprises about one fourth of the ECF or about 3.5 L for an average 70-kg man (see Fig. 24.1). The interstitial fluid and lymph are considered together because they cannot be easily separated. The water of the interstitial fluid and lymph comprises three fourths of the ECF. The interstitial fluid directly bathes most body cells, and the lymph is the fluid within lymphatic vessels. The blood plasma, interstitial fluid, and lymph are nearly identical in composition, except for the higher protein concentration in the plasma.
An additional ECF compartment (not shown in Fig. 24.1), the transcellular fluid, is small but physiologically important. Transcellular fluid amounts to about 1 to 3% of body weight. Transcellular fluids include cerebrospinal fluid, aqueous humor of the eye, secretions of the digestive tract and associated organs (saliva, bile, pancreatic juice), renal tubular fluid and bladder urine, synovial fluid, and sweat. In these cases, the fluid is separated from the blood plasma by an epithelial cell layer in addition to a capillary endothelium. The epithelial layer modifies the electrolyte composition of the fluid, so that transcellular fluids are not plasma ultrafiltrates (as is interstitial fluid and lymph),- they have a distinct ionic composition. There is a constant turnover of transcellular fluids, they are continuously formed and absorbed or removed. Impaired for mation, abnormal loss from the body, or blockage of fluid removal can have serious consequences.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.