Which of the following statements about gas exchange between the air in the alveoli and the blood in the pulmonary capillaries is correct?
- AThe oxygen concentration in the capillaries leaving the pulmonary artery is higher than the oxygen concentration in the alveoli.
- BGases must diffuse across the endothelium of the pulmonary capillaries and the endothelium of the alveoli.
- CThe elastic fibres in the alveoli walls allow the alveoli to expand to increase the surface area available for diffusion into the pulmonary capillaries.
- DBreathing out reduces the carbon dioxide concentration gradient between the blood in the pulmonary capillaries and the air in the alveoli.