About 80% of the ECF is interstitial fluid, which occupies the microscopic spaces between tissue cells, and 20% of the ECF is plasma, the liquid portion of the blood. Other extracellular fluids that are grouped with interstitial fluid include lymph in lymphatic vessels; cerebrospinal fluid in the nervous system; synovial fluid in joints; aqueous humor and vitreous body in the eyes; endolymph and perilymph in the ears; and pleural, pericardial, and peritoneal fluids between serous membranes. The body is in fluid balance when the required amounts of water and solutes are present and are correctly proportioned among the various compartments. Water is by far the largest single component of the body, making up 45–75% of total body mass, depending on age and gender. | About 80% of the ECF is interstitial fluid, which occupies the microscopic spaces between tissue cells, and 20% of the ECF is plasma, the liquid portion of the blood. Other extracellular fluids that are grouped with interstitial fluid include lymph in lymphatic vessels; cerebrospinal fluid in the nervous system; synovial fluid in joints; aqueous humor and vitreous body in the eyes; endolymph and perilymph in the ears; and pleural, pericardial, and peritoneal fluids between serous membranes. The body is in fluid balance when the required amounts of water and solutes are present and are correctly proportioned among the various compartments. Water is by far the largest single component of the body, making up 45–75% of total body mass, depending on age and gender. |