However, rapid changes in fluid volume without changes in ionic components cause dilation or concentration of those components. Cellular osmotic concentration gradients are maintained largely through the active pumping of transmembrane ionic transport proteins. Less than this is hypoosmotic, and greater is hyperosmotic. Physiologic blood plasma osmolarity is approximately 286 mOsmoles/L. Osmolarity is defined as the number of particles per liter of fluid. Mechanismįluid moves throughout cellular environments in the body by passively crossing semipermeable membranes. Intracellular fluids tend to be inversed with high levels of phosphate, magnesium, potassium, and proteins but lower sodium, chloride, and bicarbonate. Interstitial fluids physiologically tend to have a low concentration of proteins. Extracellular spaces contain high concentrations of sodium, chloride, bicarbonate, and proteins but are relatively lower in potassium, magnesium, and phosphate. Extracellular fluid and interstitial fluid are similar in composition. The exact chemical composition of body fluid is highly variable. This is dependent on which portion of the body, as well as which organ of the body, contains the fluid. Additional fluid spaces are possible in pathological scenarios and are categorized as a transudate or exudate based on location and etiology. The extracellular fluid comprises approximately 20% of total body weight and further subcategorizes as plasma at approximately 5% of body weight and interstitial space, which is approximately 12% of body weight. As such, it is important to maintain an appropriate osmolality. This space is where many chemical reactions occur. In general, intracellular fluids are stable and do not readily adjust to rapid changes. It is the total space within cells primarily defined as the cytoplasm of cells. Intracellular fluid is approximately 40% of the total body weight. The distribution of fluid throughout the body can be broken down into two general categories: intracellular fluid and extracellular fluid. Fluid is largely regulated through passive diffusion following the concentration gradients of osmotically active solutes however, hydrostatic pressures can influence fluid movement between spaces. Furthermore, fluid is always in flux through a variety of regulatory mechanisms to maintain appropriate concentrations throughout the various compartments of the body. Humans are approximately 75% water by mass as infants and 50% to 60% water by mass as adults. It is the essence of life and the aqueous base solution in which all essential biochemical processes occur that produce life. Human beings are creatures that are primarily composed of water.
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