A man has a total lung capacity of 6000 mL and a residual volume of 1200 mL. What is his vital capacity?
- Total lung capacity = vital capacity + residual volume.
- Vital capacity = 6000 − 1200.
- Vital capacity = 4800 mL.
Answer: 4800 mL (4.8 L).
Human respiratory system
Biology · Gaseous exchange. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
Air passes through the nasal cavity, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli. The alveoli are thin-walled, moist sacs wrapped in capillaries, and they are the only place where gases are exchanged with the blood.
Breathing is mechanical. The lungs have no muscle of their own. The diaphragm and intercostal muscles change the volume of the chest, and the pressure change moves the air. The rate is set by the respiratory centre in the medulla, which responds mainly to the CO2 concentration of the blood.
Nearly all oxygen is carried by haemoglobin as oxyhaemoglobin. Most carbon dioxide travels as bicarbonate ions in the plasma.
Rules to remember
- Inspiration (active): diaphragm contracts and flattens, external intercostals raise the ribs, chest volume rises, lung pressure falls, air enters.
- Expiration at rest (passive): muscles relax, volume falls, pressure rises, air leaves.
- One haemoglobin molecule carries up to 4 O2. The dissociation curve is S-shaped because binding one O2 makes the next easier.
- Bohr effect: more CO2 (lower pH) makes haemoglobin release more oxygen.
- CO2 transport: about 70% as bicarbonate ions, roughly 20 to 23% joined to haemoglobin (carbaminohaemoglobin), and the small remainder dissolved in plasma.
- Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume. Total lung capacity = vital capacity + residual volume. Tidal volume at rest is about 500 mL.
- Tuberculosis: Mycobacterium tuberculosis. Asthma: bronchioles narrow. Emphysema: alveolar walls break down. Lung cancer: mainly caused by smoking.
Common mistake
Students say the lungs suck in air by expanding on their own. The chest cavity is enlarged first by the diaphragm and intercostal muscles; the lungs follow and air flows in because the pressure inside falls.
Worked example
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