Chapter 5: Fluids and Electrolytes, Acids and Bases

Movement of water between these compartments follows principles of osmotic and hydrostatic pressure gradients, described through Starling's law of net filtration. Pathological fluid accumulation or edema develops through multiple mechanisms including elevated capillary hydrostatic pressure, diminished plasma oncotic pressure, increased vascular permeability, and compromised lymphatic drainage, each producing distinct clinical presentations ranging from localized to generalized swelling. The chapter then addresses sodium, chloride, and potassium disturbances, recognizing these ions as primary determinants of osmotic balance in extracellular and intracellular spaces respectively. Regulatory systems including the renin-angiotensin-aldosterone system, antidiuretic hormone, and natriuretic peptides maintain sodium homeostasis, while disruptions cause isotonic, hypertonic, or hypotonic imbalances with specific clinical consequences. Sodium excess or deficit alters serum osmolality and produces neurological symptoms through cellular swelling or shrinkage, while potassium imbalances directly compromise cardiac conduction and muscle contractility, creating life-threatening arrhythmias. Additional coverage includes calcium, phosphate, and magnesium regulation through parathyroid hormone, vitamin D, and calcitonin, with discussion of how deficiencies or excesses affect neuromuscular function and skeletal integrity. The final section explores acid-base physiology, emphasizing that hydrogen ion concentration must remain within the narrow physiological range of 7.35 to 7.45 for enzymatic function. Multiple buffer systems including bicarbonate-carbonic acid, proteins, hemoglobin, and phosphate work alongside respiratory and renal compensation mechanisms to prevent dangerous pH shifts. Primary acid-base disorders—metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis—each carry distinct pathophysiological mechanisms and compensatory responses, illustrated through clinical examples such as diabetic ketoacidosis, vomiting-induced alkalosis, and chronic obstructive pulmonary disease. The chapter concludes by addressing age-related variations in fluid and electrolyte regulation, noting heightened vulnerability in infants due to immature renal systems and increased risk in older adults from diminished thirst sensation and reduced total body water.