Physiology and Pathophysiology
What the exam tests
Physiology and Pathophysiology tests organ-system function and the mechanisms that explain clinical signs and laboratory abnormalities.
Core rules
- Mean arterial pressure is approximately DBP + 1/3(SBP − DBP). At 120/80 mmHg, MAP is about 80 + 13.3 = 93.3 mmHg. MAP, not systolic pressure alone, drives organ perfusion.
- Acid–base: Winter formula for expected PaCO2 in metabolic acidosis = 1.5 × HCO3 + 8 ± 2. For metabolic alkalosis, PaCO2 rises about 0.7 mmHg per 1 mEq/L HCO3.
- Renal clearance Cx = (Ux × V)/Px. Clearance of inulin estimates GFR; PAH clearance estimates renal plasma flow.
- Oxygen delivery DO2 = cardiac output × arterial oxygen content × 10; anemia and low output both reduce delivery.
Worked example
A patient has pH 7.28, HCO3 12 mEq/L, and PaCO2 26 mmHg. Winter expected PaCO2 = 1.5 × 12 + 8 ± 2 = 26 ± 2 mmHg. The measured 26 mmHg is appropriate respiratory compensation, so this is a simple metabolic acidosis. If the PaCO2 were 40 mmHg, there would be an additional respiratory acidosis; if it were 18 mmHg, an additional respiratory alkalosis. The anion gap then determines whether the acidosis is high-gap or normal-gap.
Traps
Do not label any low pH as respiratory without checking PaCO2; do not ignore a mixed disorder when compensation is not in the expected range; do not use serum creatinine alone to estimate GFR in low-muscle-mass patients; and do not confuse oncotic pressure with hydrostatic pressure.