人体的气体交换

Gas exchange 气体交换 is how oxygen 氧气 gets into the blood and carbon dioxide 二氧化碳 gets out, by diffusion 扩散. A good gas exchange surface has four features:

• a large surface area 表面积 — so more gas can cross at once.
• a thin surface — so gases have only a short distance to diffuse.
• a good blood supply — to keep a steep difference in concentration.
• good ventilation 通气 with air — fresh air keeps the difference steep.

Air travels in and out through these parts:

The alveoli make excellent gas exchange surfaces: there are millions of them (a huge surface area), each has a wall only one cell thick (a short distance), and each is surrounded by capillaries (a good blood supply).

The lungs sit in the chest, protected by the ribs 肋骨. Below them is a sheet of muscle, the diaphragm 膈肌. Between the ribs are the intercostal muscles 肋间肌. (Supplement) The trachea is held open by rings of cartilage 软骨, so it cannot collapse when you breathe in.

Breathing changes the volume and pressure 压力 inside the chest (the thorax 胸腔).

Breathing in (inhaling 吸气):

• the external intercostal muscles contract, pulling the ribs up and out.
• the diaphragm contracts and flattens (moves down).
• the thorax becomes bigger, so the pressure inside drops below the outside air pressure.
• air is pushed in.

Breathing out (exhaling 呼气) is the opposite: the muscles relax (and the internal intercostal muscles may contract), the thorax becomes smaller, the pressure rises, and air is pushed out.

You can test for carbon dioxide with limewater 石灰水, which turns cloudy. Expired air turns limewater cloudy much faster than inspired air, showing it contains more carbon dioxide.

During exercise your muscles respire faster and make more carbon dioxide. This raises the concentration 浓度 of carbon dioxide in the blood. (Supplement) Your brain 大脑 detects the rise and makes you breathe at a faster rate 速率 and a greater depth. This brings in more oxygen and removes the extra carbon dioxide quickly.

Worked example. At rest a person takes 12 breaths per minute, moving 500 cm³ of air per breath. During exercise this becomes 20 breaths per minute at 900 cm³ per breath. How much air passes through the lungs each minute in each case? Multiply the rate by the depth. At rest: 12 × 500 = 6000 cm³ per minute. During exercise: 20 × 900 = 18 000 cm³ per minute, three times as much. Exercise raises both the rate and the depth of breathing - an answer that changes only the rate throws away half the marks.

The airways are lined with two kinds of cell that trap and remove dirt:

• goblet cells 杯状细胞 make mucus 黏液, which traps pathogens 病原体 and dust particles 粒子.
• ciliated cells 纤毛细胞 have tiny hairs that sweep the mucus, with the trapped dirt, up to the throat, where it is swallowed.

Common mistakes.

• Mixing up breathing and respiration. Breathing moves air in and out of the lungs; respiration is the chemical reaction in every cell that releases energy from glucose.
• Saying expired air contains no oxygen. It still has about 16% oxygen; only about a quarter of the oxygen breathed in is taken up.
• (Supplement) Getting the order wrong: "air comes in, so the chest gets bigger". The muscles make the chest bigger first; this lowers the pressure inside, and then air is pushed in.
• (Supplement) Saying the diaphragm moves up when you breathe in. It contracts and flattens (moves down); it moves up when it relaxes as you breathe out.
• (Supplement) Saying the brain detects a lack of oxygen during exercise. The brain detects the rise in carbon dioxide in the blood.

• A gas exchange surface is large, thin, with a good blood supply and good ventilation.
• Air path: larynx → trachea → bronchi → bronchioles → alveoli.
• Breathing in: external intercostal muscles and diaphragm contract → thorax bigger → pressure lower → air in. Breathing out is the opposite.
• Expired air has less oxygen, more carbon dioxide and more water vapour. Limewater tests for carbon dioxide.
• Exercise → more carbon dioxide in the blood → brain → faster, deeper breathing.