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. įluid 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. For a complete diagram of body fluid compartments, see body fluid compartments of a 70-kg man and body fluid compartments of a 55-kg woman.The distribution of fluid throughout the body can be broken down into two general categories: intracellular fluid and extracellular fluid. Note that this diagram places focus only on these three major fluid compartments. Plasma is the smallest fluid compartment (~8% of total body water). Interstitial fluid contains ~25% of the total body water. The intracellular fluid compartment contains most of the water in the body (~67% of total). The right diagram shows the three major fluid compartments drawn to scale. The left diagram allows for a better demonstration of the relationship between the intracellular fluid, interstitial fluid, and plasma, however, the relative size of each of the compartment is not drawn to scale. Waste products produced by cells follow the reverse path from the cytoplasmic compartment to plasma. They then must cross the plasma membrane to enter the cytoplasmic compartment of cells. Nutrient molecules traveling in the blood must first cross the capillary endothelium to enter the interstitial fluid. The capillary endothelium is the physical barrier that separates the interstitial fluid from plasma. The physical barrier separating the intracellular fluid compartment (i.e., cytoplasm) and the interstitial fluid is the cell plasma membrane. Fluid, molecules, and ions flow across physical barriers between the fluid compartments. These are the (1) intracellular fluid compartment, (2) interstitial fluid, and (3) plasma. In the human body plan, there are three major fluid compartments that are functionally interconnected.
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