Respiratory system and breathing

MDCAT Biology · Biology · Respiration. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Air passes through the nasal cavity, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli, where gas exchange takes place. An alveolus has a wall one cell thick, a moist lining and a dense capillary network, and together the alveoli give a very large surface. Oxygen and carbon dioxide cross by diffusion only.

Breathing moves air by changing the volume of the thorax. The lungs have no muscle of their own. They follow the chest wall and the diaphragm because of the pleural membranes.

When the volume of the thorax increases, the pressure inside the lungs falls below atmospheric pressure and air enters. Rising CO₂ in the blood is the main signal that makes the medulla increase breathing.

Rules to remember

Inspiration (active): diaphragm contracts and flattens; external intercostal muscles contract, ribs move up and out; volume rises, pressure falls, air enters
Quiet expiration (passive): diaphragm and external intercostals relax; elastic recoil of the lungs; volume falls, pressure rises, air leaves
Forced expiration is active: internal intercostal muscles and abdominal muscles contract
Trachea and bronchi have cartilage; bronchioles have none, only smooth muscle. Goblet cells make mucus and cilia sweep it up to the pharynx
Tidal volume = air breathed in or out in one quiet breath, about 500 cm³ (0.5 dm³)
Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume
Total lung capacity = vital capacity + residual volume; minute volume = tidal volume × breathing rate

Common mistake

Students write that air enters and so the lungs expand. It is the other way round: the muscles enlarge the thorax first, the pressure in the lungs falls, and then air flows in.

Worked example

A woman has a tidal volume of 0.5 dm³, an inspiratory reserve volume of 2.4 dm³, an expiratory reserve volume of 0.9 dm³ and a residual volume of 1.1 dm³. Find her vital capacity and total lung capacity.

Vital capacity = tidal volume + inspiratory reserve + expiratory reserve.
Vital capacity = 0.5 + 2.4 + 0.9 = 3.8 dm³.
Total lung capacity = vital capacity + residual volume = 3.8 + 1.1 = 4.9 dm³.

Answer: Vital capacity = 3.8 dm³; total lung capacity = 4.9 dm³.

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