WATER HOMEOSTASIS
Total body water (TBW) contributes to ~78% of body weight at birth, but declines rapidly due to physiological diuresis in newborns and growth of adipose tissue in late infancy, to reach the adult level of ~55-60% by one year of age.
Post-pubertal females and obese persons have relatively less body water due to excess fat accumulation. In non-obese children, TBW may be calculated as:TBW (Liters) = 0.61 Body weight in kg + 0.251.
TBW is distributed into four body compartments:
a. Extracellular compartment or ECF (20-25%) as plasma (5%) and interstitial fluid (15%).
b. Intracellular compartment or ICF (30-40%).
c. Transcellular water (2%) as urine, gut secretions, CSF and body cavity fluids.
d. Slowly exchangeable fluid compartment in bones, cartilages and connective tissue.
At birth, ECF is more than ICF, but this ratio is reversed by the end of infancy due to postnatal diuresis and cellular growth. Transcellular water and water in slowly exchangeable fluid compartments, though important metabolically, does not play significant role in water homeostasis.
7.1.1