Physiology
Physiology tests integration from cell membrane to organ system, especially how cardiovascular, respiratory, renal, and neuromuscular function interact during exercise and rehabilitation.
Core rules and concepts
- Resting membrane potential depends on the Na+/K+ ATPase and selective permeability; the action potential follows depolarisation, threshold, and repolarisation.
- Excitation–contraction coupling: motor endplate acetylcholine, sarcolemma depolarisation, calcium release from sarcoplasmic reticulum, actin–myosin cross-bridge cycling.
- Cardiac output = heart rate × stroke volume; stroke volume depends on preload, afterload, and contractility.
- Lung volumes: tidal volume, inspiratory reserve, expiratory reserve, residual volume; vital capacity excludes residual volume.
- Renal filtration, reabsorption, and secretion; endocrine negative feedback; acid–base compensation.
Worked example
A patient has a heart rate of 70 beats/min and stroke volume of 70 mL. Cardiac output is 70 × 70 = 4,900 mL/min, or 4.9 L/min. During exercise, heart rate rises to 120 beats/min and stroke volume to 90 mL, giving 10.8 L/min. The reasoning is that cardiac output must rise to meet oxygen demand, and both variables contribute. If stroke volume is limited by disease, heart rate must compensate; this explains exercise intolerance and the need to monitor rate, blood pressure, and symptoms.
Traps that cost marks
- Confusing minute ventilation with alveolar ventilation; dead space is not involved in gas exchange.
- Shifting the oxyhaemoglobin curve in the wrong direction: increased CO2, H+, temperature, and 2,3-DPG shift it right.
- Reporting cardiac output in litres per minute when the question asks for millilitres, or omitting units.
- Treating plasma and interstitial fluid as identical compartments.