Respiration and Circulation
Respiration and Circulation tests gas exchange, transport and the heart as a pump. Expect lung volumes, oxygen-haemoglobin behaviour, cardiac cycle, ECG, blood pressure and blood groups.
Core rules
- Breathing: inspiration is active, using diaphragm and external intercostals; expiration is passive at rest.
- Alveolar exchange occurs by diffusion along partial-pressure gradients.
- Oxygen is carried by haemoglobin. The Bohr effect means low pH or high CO2 shifts the curve right, unloading oxygen.
- Carbon dioxide is mainly transported as bicarbonate; carbonic anhydrase speeds this.
- Heart: SA node is the pacemaker; AV node delays the impulse; cardiac output = heart rate × stroke volume.
- ABO and Rh blood groups depend on antigens on red cells and antibodies in plasma.
Worked example
Heart rate is 72 beats per minute and stroke volume is 70 mL. Calculate cardiac output.
Reasoning: cardiac output = 72 × 70 = 5040 mL/min, about 5 L/min. If exercise raises heart rate to 100 with stroke volume unchanged, output becomes 7000 mL/min. In reality stroke volume also rises. Answer: about 5 L/min at rest.
Traps that cost marks
- Pulmonary artery carries deoxygenated blood; pulmonary vein carries oxygenated blood.
- SA node, not AV node, initiates the impulse.
- ECG P wave is atrial depolarisation, not ventricular.
- Systemic veins carry deoxygenated blood, except pulmonary veins.