Respiratory system: Gas exchange systems at alveoli and muscles.

The idea

Gas exchange moves oxygen into the blood and carbon dioxide out, driven by partial pressure gradients. At the alveoli, oxygen diffuses from alveolar air into pulmonary capillary blood, where partial pressure of oxygen is lower, while carbon dioxide diffuses from blood into the alveolar air. At the muscles, the gradients reverse: oxygen leaves the blood to enter active muscle fibres, and carbon dioxide produced by respiration enters the blood. Diffusion is rapid because alveoli and muscle capillaries have large surface areas, thin walls and short diffusion distances. Myoglobin in muscle helps store and transport oxygen, and the steep gradient created by continued oxygen use maintains uptake.

What earns the marks

• Oxygen diffuses from alveolar air into pulmonary capillary blood down a partial pressure gradient.
• Carbon dioxide diffuses from pulmonary capillary blood into alveolar air down its partial pressure gradient.
• At muscles, oxygen diffuses from blood into muscle fibres because muscle partial pressure of oxygen is lower.
• Carbon dioxide produced by muscle respiration diffuses from muscle fibres into the blood.
• Alveoli and muscle capillaries have large surface areas, thin walls and short diffusion distances that make exchange efficient.
• Myoglobin in muscle aids oxygen storage and transport, helping maintain the diffusion gradient.

Common mistakes and how to put them right

• Thinking oxygen and carbon dioxide diffuse in the same direction at both sites.

  Put it right: At alveoli oxygen enters blood and carbon dioxide leaves it, while at muscles oxygen leaves blood and carbon dioxide enters it.

• Believing diffusion needs active transport.

  Put it right: Gas exchange is passive diffusion down partial pressure gradients.

• Assuming the gradient stays constant.

  Put it right: Continued oxygen use in muscle and ventilation at alveoli maintain the gradients.

• Confusing partial pressure with concentration only.

  Put it right: Partial pressure differences drive diffusion of gases.

Exam tips

• State the direction of movement for each gas at each site using the terms alveoli, blood and muscle.
• Link each direction to the partial pressure gradient rather than saying gases simply move.
• Use surface area, thin walls and short diffusion distance to explain efficiency of exchange.