Distribution of Body Fluids
1. Fluid compartments
a. The intracellular fluid compartment includes the fluids and electrolytes cell membranes enclose.
b. The extracellular fluid compartment includes all the fluids and electrolytes outside cell membranes.
2. Body fluid composition
a. Extracellular fluids have high concentrations of sodium, chloride, calcium, and bicarbonate ions, with less potassium, magnesium, phosphate, and sulfate ions. Plasma contains more protein than does either interstitial fluid or lymph.
b. Intracellular fluid contains high concentrations of potassium, magnesium, and phosphate ions. It also has a greater concentration of sulfate ions and lesser concentrations of sodium, chloride, calcium, and bicarbonate ions than does extracellular fluid.
3. Movement of fluid between compartments
a. Hydrostatic and osmotic pressure regulate fluid movements.
(1) Hydrostatic pressure forces fluid out of plasma, and colloid osmotic pressure returns fluid to plasma.
(2) Hydrostatic pressure drives fluid into lymph vessels.
(3) Osmotic pressure regulates fluid movement in and out of blood vessels and cells.
b. Sodium ion concentrations are especially important in regulating fluid movement.