Скачать или смотреть Fluid and electrolytes: Total Body Water

Total Body Water
Water constitutes approximately 50% to 60% of total body weight. The relationship between total body weight and total body water (TBW) is relatively constant for an individual and is primarily a reflection of body fat. Lean tissues such as muscle and solid organs have higher water content than fat and bone. As a result, young, lean males have a higher proportion of body weight as water than elderly or obese individuals. In an average young adult male, TBW accounts for 60% of total body weight, whereas in an average young adult female, it is 50%.1 The lower percent- age of TBW in females correlates with a higher percentage of adipose tissue and lower percentage of muscle mass in most. Estimates of percentage of TBW should be adjusted downward approximately 10% to 20% for obese individuals and upward by 10% for malnourished individuals. The highest percentage of TBW is found in newborns, with approximately 80% of their total body weight comprised of water. This decreases to approximately 65% by 1 year of age and thereafter remains fairly constant.
Fluid Compartments
TBW is divided into three functional fluid compartments: plasma, extravascular interstitial fluid, and intracellular fluid. The extracellular fluids (ECF), plasma, and interstitial fluid together compose about one-third of the TBW, and the intracellular compartment composes the remaining two thirds. The extracellular water composes 20% of the total body weight and is divided between plasma (5% of body weight) and interstitial fluid (15% of body weight). Intracellular water makes up approximately 40% of an individual's total body weight, with the largest proportion in the skeletal muscle mass. Composition of Fluid Compartments The normal chemical composition of the body fluid compartments. The ECF compartment is balanced between sodium, the principal cation, and chloride and bicarbonate, the principal anions. The intracellular fluid compartment is composed primarily of the cation's potassium and magnesium, the anions phosphate and sulfate, and proteins. The concentration gradient between compartments is maintained by adenosine triphosphate-driven sodium-potassium pumps located within the cell membranes. The composition of the plasma and interstitial fluid differs only slightly in ionic composition. The slightly higher protein content (organic anions) in plasma results in a higher plasma cation composition relative to the interstitial fluid, as explained by the Gibbs-Donnan equilibrium equation. Proteins add to the osmolality of the plasma and contribute to the balance of forces that determine fluid balance across the capillary endothelium. Although the movement of ions and proteins between the various fluid compartments is restricted, water is freely diffusible. Water is distributed evenly throughout all fluid compartments of the body so that a given volume of water increases the volume of any one compartment.