气体交换

Your body needs to take in oxygen and get rid of carbon dioxide. This swap happens in the gas exchange 气体交换 system. Air follows this path into the body:

• down the trachea 气管 (the windpipe),
• into two bronchi 支气管 (one to each lung),
• into many smaller bronchioles 细支气管,
• and finally into tiny air sacs called alveoli 肺泡, deep in the lungs 肺.

Each alveolus is wrapped in a network of capillaries 毛细血管, so air and blood are brought very close together.

The cilia, goblet cells and mucous glands work together to keep the lungs clean and healthy: the mucus traps dirt and microbes, and the cilia carry it away to be swallowed.

The alveoli are excellent surfaces for exchanging gases, because they have:

• a very large total surface area (millions of tiny sacs),
• very thin walls — the squamous epithelium of the alveolus and the capillary wall are each only one cell thick, so the distance to cross is tiny,
• a rich blood supply from the capillary network,
• a moist lining, so gases dissolve before crossing.

Gases move by diffusion 扩散 down their concentration gradients 浓度梯度:

• oxygen 氧气 is at a high concentration in the alveolar air and a low concentration in the blood, so it diffuses from the air into the blood.
• carbon dioxide 二氧化碳 is at a high concentration in the blood and a low concentration in the alveolar air, so it diffuses from the blood into the air to be breathed out.

Under the microscope, real lung tissue looks like a fine pink lace. The many open spaces are the alveoli, and the thin pink lines between them are the walls where gas exchange happens:

Breathing keeps fresh air in the alveoli, and the flowing blood keeps carrying gases away. Both of these keep the concentration gradients steep, so gas exchange stays fast.

Worked example. Use Fick's law to explain why the alveoli allow such rapid gas exchange. Fick's law makes the rate of diffusion proportional to

so a fast rate needs all three terms working for it. Surface area: millions of alveoli give a huge total area, roughly $70\ \text{m}^2$. Diffusion distance: the alveolar epithelium and the capillary endothelium are each one flattened cell thick, so oxygen crosses in under $1\ \mu\text{m}$. Concentration difference: ventilation constantly refreshes the air while the circulation constantly carries oxygenated blood away, so a steep gradient is maintained. Tie each adaptation to the term of the equation it serves - listing "big surface area, thin walls, good blood supply" without linking them to Fick's law is the weaker answer.

Common mistakes.

• Writing "the alveoli have thin cell walls". Animal cells have no cell walls; write that the alveolar wall is one cell thick, or that the epithelium is thin.
• Saying elastic fibres contract to push air out. Only muscle contracts. Elastic fibres are stretched when you breathe in and recoil when you breathe out.
• Saying cilia kill or filter bacteria. The cilia beat to move the mucus, with the trapped dust and bacteria, up to the throat.
• Putting cartilage in the small bronchioles. Cartilage is in the trachea and bronchi; the bronchioles have smooth muscle and elastic fibres but no cartilage.
• Saying gas exchange happens in the bronchioles. It happens across the walls of the alveoli and their capillaries.

• Link each alveolar feature to fast diffusion: large surface area, thin (one-cell) walls, moist surface, good blood supply — a "surface area, short distance, steep gradient" answer.
• Match airway tissue to function: cartilage (holds airways open), ciliated + goblet cells (trap and sweep mucus), smooth muscle and elastic fibres.
• Frame answers with Fick's law ideas: rate $\propto$ surface area $\times$ concentration difference $\div$ distance.