Chapter 25: Fluid, Electrolyte, and Acid-Base Balance

Body composition consists of approximately 50-60 percent water distributed between the intracellular fluid found within cells and the extracellular fluid comprising interstitial fluid and blood plasma, with these compartments maintained in osmotic equilibrium despite having distinct ionic compositions. Fluid balance occurs when daily water gains from dietary sources and metabolic production equal losses through urination, perspiration, and respiration, while fluid shifts allow rapid water movement between compartments in response to osmotic gradients, such as during dehydration when the extracellular fluid becomes hypertonic and draws water from cells to restore equilibrium. Mineral balance requires that inorganic ion absorption matches excretion, particularly through renal mechanisms, with sodium serving as the predominant extracellular cation whose regulation is tightly coupled to water balance and blood volume control through hormonal mediators including antidiuretic hormone, aldosterone, and natriuretic peptides, while potassium as the major intracellular cation is regulated primarily through aldosterone-sensitive tubular exchange that reabsorbs sodium while secreting potassium. Acid-base balance maintains extracellular pH within the narrow range of 7.35-7.45 through equilibration of hydrogen ion production with elimination, categorizing acids as fixed, metabolic, or volatile depending on their origin and elimination pathway. The chapter emphasizes three major buffer systems that provide immediate pH stabilization: the phosphate buffer system prominent in intracellular and urinary environments, protein-based buffers including the hemoglobin system with direct effects on blood pH, and the carbonic acid-bicarbonate buffer system as the most critical extracellular buffer that converts excess hydrogen ions into water and carbon dioxide for respiratory elimination. Compensation mechanisms triggered by acid-base disturbances include respiratory adjustments that modify carbon dioxide elimination rates and renal adjustments that alter hydrogen ion secretion or bicarbonate reabsorption, with respiratory acidosis representing the most frequently encountered acid-base imbalance requiring increased ventilation for correction.