Breathing (respiration) is the process of taking oxygen into the body, delivering it to the cells and removing the carbon dioxide they produce. Cells need oxygen to oxidise organic substances, releasing energy as ATP (this is cellular respiration, studied under energy metabolism). In humans respiration has three parts: external respiration (gas exchange between air and blood in the lungs), transport of gases by the blood and tissue respiration. Exams ask about the structure of the airways, gas exchange, the mechanism of breathing, lung volumes, the control of breathing and hygiene.
§1. The airways
Air reaches the lungs by this route:
The nasal cavity — divided by a septum and lined with mucous membrane. Here air is warmed (rich blood supply), moistened and cleaned of dust and microbes by hairs, mucus and ciliated epithelium; the smell receptors lie in its upper part. Breathing through the mouth skips this treatment, which is why it is harmful.
The nasopharynx and pharynx — where the air and food passages cross.
The larynx — built of cartilages (the largest is the thyroid cartilage). The epiglottis closes the entrance to the larynx during swallowing so food does not enter the airway. The larynx holds the vocal cords: exhaled air makes them vibrate and produce the voice. Men’s cords are longer, so their voices are lower.
The trachea (windpipe) — about 10–12 cm long, with 16–20 C-shaped cartilage rings in its wall. The back of each ring is soft: it lies against the oesophagus and does not hinder swallowing, while the cartilage stops the trachea collapsing. It is lined with ciliated epithelium, whose cilia sweep dust and mucus upwards.
The bronchi — the trachea splits into two main bronchi, which branch ever finer in the lungs into bronchioles and then alveoli (the “bronchial tree”).
The lungs are a paired organ in the chest cavity; the right lung has 3 lobes, the left has 2. Each lung is wrapped in pleura: between its two layers is the pleural cavity with a thin film of fluid that reduces friction.
The alveoli are air sacs whose walls are a single layer of flat epithelium, wrapped in a dense capillary network. The two lungs hold about 700 million alveoli with a total surface of about 100 m² — a huge area for gas exchange.
§2. Gas exchange
Gases move by diffusion — from where their concentration (partial pressure) is higher to where it is lower; no energy is used.
In the lungs: alveolar air is rich in O₂ and deoxygenated blood is poor in it, so O₂ passes from the alveoli into the blood; CO₂ is high in the blood, so it passes into the alveoli. As a result deoxygenated blood becomes oxygenated.
In the tissues: O₂ from oxygenated blood enters the cells (where it is constantly used up), and CO₂ made in the cells passes into the blood, which becomes deoxygenated.
Transport of gases:
O₂ is carried mainly as a loose compound with the haemoglobin of red cells — oxyhaemoglobin; in the tissues it easily breaks down and releases O₂.
CO₂ is carried mainly in plasma as hydrogencarbonate, partly bound to haemoglobin.
Carbon monoxide (CO) forms a very stable compound with haemoglobin — carboxyhaemoglobin; the haemoglobin can no longer carry O₂, causing poisoning. Faulty stoves and gas appliances are dangerous; the victim must be taken into fresh air at once.
Inhaled air: O₂ about 21%, CO₂ 0.03%, nitrogen about 79%. Exhaled air: O₂ about 16%, CO₂ about 4%.
Nitrogen is unchanged in the body; exhaled air is also rich in water vapour.
§3. The mechanism of breathing
The lungs themselves have no muscle; they follow the movements of the chest. This is because the pressure in the pleural cavity is below atmospheric (negative), so the lungs stay stretched and “cling” to the chest wall.
Inhalation (inspiration) — the external intercostal muscles contract and raise the ribs; the diaphragm contracts, its dome flattens and moves down. The chest volume increases, the lungs stretch, the pressure inside falls below atmospheric — air flows in. This is an active process.
Exhalation (expiration) — the muscles relax, the ribs fall, the diaphragm domes up again, the chest volume decreases, lung pressure rises — air flows out. At rest it is passive; in forced breathing the internal intercostal and abdominal muscles help.
If the chest wall is injured and air enters the pleural cavity (pneumothorax), the lung collapses and stops working.
§4. Lung volumes
Tidal volume — the air moved in quiet breathing: about 500 ml.
Inspiratory reserve volume — extra air that can be breathed in after a quiet breath in: about 1500 ml.
Expiratory reserve volume — extra air that can be forced out after a quiet breath out: about 1500 ml.
Residual volume — air left in the lungs even after the deepest breath out: about 1000–1500 ml; it cannot be exhaled.
Vital capacity is measured with a spirometer; in athletes it can reach 5000–6000 ml. At rest an adult breathes about 16–18 times a minute.
§5. Control of breathing
The respiratory centre lies in the medulla oblongata; nerve impulses from it travel to the breathing muscles and make them contract rhythmically.
Chemical (humoral) control: a rise in blood CO₂ stimulates the respiratory centre — breathing becomes deeper and faster. That is why you cannot hold your breath for long: CO₂ builds up and the centre forces a breath.
Nervous control: stretch receptors in the lungs limit the depth of inhalation; the cerebral cortex allows voluntary changes (speaking, singing, holding the breath).
Protective reflexes: sneezing — when the nasal lining is irritated; coughing — when the larynx, trachea or bronchi are irritated.
During exercise the muscles produce a lot of CO₂, so breathing speeds up automatically.
§6. Respiratory hygiene and disease
Airborne diseases: influenza, common respiratory viral infections, tuberculosis (Koch’s bacillus), diphtheria, tonsillitis, COVID-19. Prevention: ventilating rooms, vaccination, avoiding close contact with the sick, washing hands.
Smoking: tar and harmful substances in tobacco smoke damage the ciliated epithelium and lead to chronic bronchitis and lung cancer; carbon monoxide ties up haemoglobin.
Dusty air and industrial emissions harm the lungs; planting trees cleans the air.
First aid when breathing stops: clear the airway and give rescue breaths (mouth-to-mouth or mouth-to-nose), combined with chest compressions if the heart has stopped.
Worked example
At rest a person breathes 16 times a minute, 500 ml each time. How much air passes through the lungs in an hour, and how much O₂ is absorbed from it?
Per minute: 16 × 500 = 8000 ml = 8 l.
Per hour: 8 × 60 = 480 l.
Share of O₂ absorbed: 21% − 16% = 5%.
O₂: 480 × 0.05 = 24 l.
Answer: 480 l of air, from which about 24 l of O₂ is absorbed.
Worked example
Tidal volume is 500 ml, inspiratory reserve 1600 ml, expiratory reserve 1400 ml, residual volume 1200 ml. Find the vital capacity and the total lung capacity.