Gas exchange in the alveoli is the process that moves oxygen from inhaled air into the blood and removes carbon dioxide from the blood to be exhaled. It matters because every cell needs oxygen for aerobic respiration, while carbon dioxide must be cleared to help maintain blood pH. The alveoli provide a huge surface area, a very thin barrier, and a moist surface that allow rapid diffusion.
Their close contact with capillaries makes the lungs an efficient exchange organ.
Key Facts
- Oxygen diffuses from alveolar air into blood because PO2 is higher in the alveolus than in the deoxygenated capillary blood.
- Carbon dioxide diffuses from blood into alveolar air because PCO2 is higher in the deoxygenated capillary blood than in the alveolus.
- Diffusion rate increases with larger surface area, steeper concentration gradient, and thinner membrane: rate is proportional to A × gradient / thickness.
- The respiratory membrane includes alveolar epithelium, fused basement membrane, and capillary endothelium, and is often less than 1 micrometer thick.
- Hemoglobin binds oxygen in red blood cells: Hb + O2 ⇌ HbO2.
- The oxygen dissociation curve shows that hemoglobin loads oxygen at high PO2 in the lungs and unloads oxygen at lower PO2 in body tissues.
Vocabulary
- Alveolus
- A tiny air sac in the lung where oxygen and carbon dioxide are exchanged with the blood.
- Partial pressure
- The pressure contributed by one gas in a mixture, which helps determine the direction of gas diffusion.
- Diffusion
- The net movement of particles from an area of higher concentration or partial pressure to an area of lower concentration or partial pressure.
- Hemoglobin
- An iron-containing protein in red blood cells that binds and transports oxygen.
- Oxygen dissociation curve
- A graph showing how hemoglobin oxygen saturation changes as the partial pressure of oxygen changes.
Common Mistakes to Avoid
- Saying oxygen is actively pumped into the blood, which is wrong because oxygen crosses the alveolar membrane by diffusion down its partial pressure gradient.
- Treating oxygen and carbon dioxide as moving in the same direction, which is wrong because oxygen moves into the blood while carbon dioxide moves into the alveolus.
- Ignoring membrane thickness, which is wrong because a thicker respiratory membrane slows diffusion and can reduce gas exchange efficiency.
- Thinking hemoglobin is only for storage, which is wrong because hemoglobin keeps dissolved oxygen levels low enough to maintain diffusion into the blood and then releases oxygen in tissues.
Practice Questions
- 1 In an alveolus, PO2 is 100 mmHg and the incoming capillary blood PO2 is 40 mmHg. What is the oxygen partial pressure gradient, and which direction will oxygen move?
- 2 A student compares two respiratory membranes. Membrane A has thickness 0.5 micrometers, and Membrane B has thickness 1.0 micrometers, with the same surface area and gradient. Using rate proportional to 1 / thickness, how does the diffusion rate of A compare with B?
- 3 Explain why exercise can increase oxygen unloading from hemoglobin in active muscles even though the lungs are still loading hemoglobin with oxygen.