List the physiological factors affecting the diffusion of oxygen across the alveolar membrane (70% of marks). Describe how this changes with exercise (30% of marks).
The first section of this question was generally well done, but few really good marks,
whereas the second section was poorly understood. The diffusing capacity is defined as the
volume of gas that will diffuse through the membrane each minute for a partial pressure
difference of 1mmHg. Factors affecting the rate of gas diffusion through the respiratory
membrane include those related to the Fick equation (e.g. thickness of the membrane,
surface area of the membrane, diffusion coefficient of the gas in the substance of the
membrane, partial pressure difference of the gas between the two sides of the membrane,
gas’s solubility), rate of combination of oxygen with reduced haemoglobin and factors
affecting surface area (e.g. lung volume, age, posture).
During strenuous exercise the oxygen diffusion is increased as pulmonary blood flow is
greatly increased and there is opening up of previously dormant pulmonary capillaries
increasing the surface area of blood into which oxygen can diffuse. Also, alveolar ventilation
increases and there is better matching of ventilation and perfusion increases from
21ml/min/mmHg up to 65ml/min/mmHg.