Physiology
Systems and integration
Physiology questions test normal function and compensation. Cardiovascular physiology includes cardiac output, preload, afterload, contractility, Starling forces, and baroreceptor reflexes. Respiratory physiology includes compliance, resistance, ventilation-perfusion matching, oxygen-hemoglobin dissociation, and acid-base compensation. Renal physiology includes glomerular filtration, clearance, tubular reabsorption and secretion, countercurrent multiplication, and hormone actions. Endocrine and gastrointestinal physiology reward understanding feedback loops.
Traps and worked rules
A frequent trap is mixing respiratory and metabolic compensation. Metabolic acidosis produces compensatory hyperventilation and a fall in PaCO2; respiratory acidosis produces renal bicarbonate retention over time. Another trap is assuming oxygen content depends only on PaO2. Oxygen content also depends on hemoglobin concentration and saturation. The anion gap is Na − (Cl + HCO3). A normal-gap acidosis suggests bicarbonate loss or hyperchloremia; a high-gap acidosis suggests unmeasured acids.
Worked example
In diabetic ketoacidosis, the anion gap is elevated because beta-hydroxybutyrate and acetoacetate are unmeasured anions. The respiratory compensation is deep, rapid breathing. The rule of thumb: calculate the anion gap before attributing acidosis to a single cause.