Physiology: Homeostasis and System Integration
Physiology: what is tested
Physiology tests how normal systems maintain homeostasis: membrane potentials, cardiac output, renal handling, acid-base balance, respiratory gas exchange, endocrine feedback and gastrointestinal secretion. The exam favours calculations and cause-effect reasoning over pure recall.
Core rules
- Resting membrane potential is set mainly by K+ permeability; the Nernst equation gives equilibrium potentials, and the Goldman equation adds Na+ permeability.
- Cardiac output = heart rate x stroke volume. The Fick principle gives CO = O2 consumption divided by the arteriovenous O2 difference.
- Renal clearance Cx = (Ux x V) / Px. Inulin clearance measures GFR; para-aminohippurate clearance measures renal plasma flow.
- Acid-base: Henderson-Hasselbalch applies. Metabolic acidosis triggers hyperventilation, and Winter's formula predicts compensation: expected PaCO2 = 1.5 x HCO3- + 8 +/- 2.
- Hormones act by negative feedback: insulin lowers glucose, glucagon raises it, and aldosterone expands extracellular volume.
Worked example. A patient has pH 7.22, HCO3- 12 mmol/L and PaCO2 26 mmHg. Reasoning: low bicarbonate with acidaemia indicates metabolic acidosis. Winter's formula gives expected PaCO2 = 1.5 x 12 + 8 = 26 +/- 2 mmHg, so the measured 26 is appropriate respiratory compensation, not a mixed disorder. If PaCO2 were 45 mmHg, a respiratory acidosis would also be present.
Traps that cost marks
- Calling it compensation when PaCO2 lies outside the Winter range.
- Using serum HCO3- without checking the anion gap; hypoalbuminaemia lowers the measured anion gap.
- Confusing clearance with excretion rate.
- Placing aldosterone action in the proximal tubule rather than the distal nephron.