Chapter 28 - Water and acid-base balance

Abstract

Water represents ~65% of the weight of an adult human. Daily water intake and elimination is ~2500mL. Water is distributed in the body with ~50% in the intracellular fluid (ICF), ~15% in the extracellular fluid (ECF); and within this last compartment ~10% in the interstitial fluid (IF) and ~5% in the intravascular space (IV). The main cation of the ECF is Na+, while the major cation in the ICF is K+. Among the anions, Cl− predominates in the ECF; phosphates, sulfates, and proteins are found in the ICF. The presence of proteins contributes to the uneven distribution of diffusible ions between the cells and ECF (Gibbs-Donnan equilibrium). The oncotic pressure exerted by proteins helps maintain fluid exchange across the capillaries (Starling equilibrium). Dehydration and hyper hydration are alterations in the body water balance. They can present without changes in osmolarity (isotonic with respect to normal plasma) or with increased or reduced osmolarity (hypotonic or hypertonic, respectively). Regulation of ECF osmolarity and volume mainly depends on renal activity and the mechanism of thirst. The Acid-base balance of body fluids is essential, and the respiratory system and kidneys play a major role in body pH control. Disorders of acid-base balance lead to four types of acid-base alterations: (1) Respiratory acidosis consists of a decrease in body pH originated by alterations in pulmonary ventilation. (2) Metabolic acidosis is the drop in body pH caused by excessive loss of bicarbonate or retention of acids. (3) Respiratory alkalosis is characterized by the increase in pH due to pulmonary hyperventilation. (4) Metabolic alkalosis is the state caused by excessive intake of alkali or the excessive removal of acids from the body. Respiratory and metabolic acidosis and alkalosis may be uncompensated or compensated.