The water content of the body is divided into two compartments. Approximately 67% of the total body water is contained within cells, in the intracellular compartment. The remaining 33% of the total body water comprises the extracellular compartment.
About 20% of this extracellular fluid is contained within the vessels of the cardiovascular system, where it comprises the fluid portion of the blood, or blood plasma.
The blood transports oxygen from the lungs to the body cells, and carbon dioxide from the body cells to the lungs. It also transports nutrients derived from food in the intestine to the body cells; other nutrients between organs (such as glucose from the liver to the brain, or lactic acid from muscles to the liver); metabolic wastes from the body cells to the liver and kidneys for elimination in the bile and urine, respectively; and regulatory molecules called hormones from endoctrine glands to the cells of their target organs.
The remaining 80% of the extracellular fluid is located outside of the vascular system, and comprises tissue fluid, also called interstitial fluid. This fluid is contained in the gel-like extracellular matrix, as described in the next section. Body fluid distribution is illustrated in figure 14.8, p. 413, in conjunction with a discussion of the cardiovascular system. This is because the interstitial fluid is formed continuously from blood plasma, and it continuously returns to the blood plasma through mechanisms described in chapter 14 (see fig. 14.9). Oxygen, nutrients, and regulatory molecules traveling in the blood must first pass into the interstitial fluid before reaching the body cells; waste products and hormone secretions from the cells must first pass into the interstitial fluid before reaching the blood plasma (fig. 6.1).
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