Composition & Compartment Distribution of Body Fluids
← Body fluids are distributed between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments. The ICF compartment consists of fluid contained within all of the billions of cells in the body. It is the larger of the two compartments, containing approximately two thirds of the body water in healthy adults.
← The remaining one third of body water is in the ECF compartment, which contains all the fluids outside the cells, including that in the interstitial or tissue spaces and blood vessels. The ECF, including the plasma and interstitial fluids, contain large amounts of sodium and chloride, moderate amounts of bicarbonate, but only small quantities of potassium. In contrast to the ECF fluid, the ICF contains small amounts of sodium, chloride, and bicarbonate and large amounts of potassium
← The cell membrane serves as the primary barrier to the movement of substances between the ECF and ICF compartments. Lipid-soluble substances such as gases (i.e., oxygen and carbon dioxide), which dissolve in the lipid bilayer of the cell membrane, pass directly through the membrane. Many ions, such as sodium (Na+) and potassium (K+), rely on transport mechanisms such as the Na+/K+-ATPase pump that is located in the cell membrane for movement across the membrane
Diffusion and Osmosis
Diffusion is the movement of charged or uncharged particles along a concentration gradient. All molecules and ions, including water and dissolved molecules, are in constant random motion. particles move from an area of higher concentration to one of lower concentration.
Osmosis is the movement of water across a semipermeable membrane (i.e., one that is permeable to water but impermeable to most solutes). As with solute particles, water diffuses down its concentration gradient, moving from the side of the membrane with the lesser number of particles and greater concentration of water to the side with the greater number of particles and lesser concentration of water
← Laboratory measurements of electrolytes in body fluids are expressed as a concentration or amount of solute in a given volume of fluid, such as milligrams per deciliter (mg/dL), milliequivalents per liter (mEq/L), or millimoles per liter (mmol/L).
← The use of milligrams (mg) per deciliter expresses the weight of the solute in one tenth of a liter (dL). The concentration of electrolytes, such as calcium, phosphate, and magnesium, is often expressed in mg/dL.
← The milliequivalent is used to express the charge equivalency for a given weight of an electrolyte: 1 mEq of sodium has the same number of charges as 1 mEq of chloride, regardless of molecular weight.
Compartmental Distribution of Body Fluids
← Body water is distributed between the ICF and ECF compartments.In the adult, the fluid in the ICF compartment constitutes approximately 40% of body weight.1 The fluid in the ECF compartment is further divided into two major subdivisions: the plasma compartment, which constitutes approximately 4% of body weight, and the interstitial fluid compartment, which constitutes approximately 15% of body weight
← It includes the cerebrospinal fluid and fluid contained in the various body spaces, such as the peritoneal, pleural, and pericardial cavities; the joint spaces; and the gastrointestinal tract.When the transcellular fluid compartment becomes considerably enlarged, it is referred to as a third space, because this fluid is not readily available for exchange with the rest of the ECF.
Intracellular Fluid Volume
← The intracellular fluid volume is regulated by proteins and organic compounds in the ICF and by solutes that move between the ECF and ICF.
← Water entry into the cell is regulated by these osmotically active substances as well as by solutes such as sodium and...