Gas Exchange and the Respiratory System
The Respiratory System
The respiratory system is responsible for bringing oxygen into the body and removing carbon dioxide — a process called gas exchange.
Structure of the Respiratory System
Air travels through the following pathway:
Nose/mouth → trachea → bronchi → bronchioles → alveoli
| Structure | Description |
|---|---|
| Nose/mouth | Air enters; nose hairs and mucus filter particles; air is warmed and moistened |
| Trachea (windpipe) | Tube reinforced with C-shaped cartilage rings to keep it open; lined with ciliated epithelial cells and goblet cells |
| Bronchi (singular: bronchus) | Two branches from the trachea, one to each lung; also have cartilage and cilia |
| Bronchioles | Smaller branches of the bronchi; smooth muscle walls can constrict or relax to control airflow |
| Alveoli | Tiny air sacs at the ends of bronchioles (approximately 500 million in both lungs); site of gas exchange |
Defence Against Pathogens
The respiratory system has built-in defences:
- Goblet cells secrete mucus that traps dust, bacteria, and particles
- Ciliated cells have tiny hair-like projections (cilia) that sweep the mucus and trapped pathogens up towards the throat, where it is swallowed and destroyed by stomach acid
- Smoking damages cilia and increases mucus production, leading to a persistent cough (smoker's cough) and increased infection risk
Gas Exchange in the Alveoli
The alveoli are adapted for efficient gas exchange:
- Huge surface area — approximately 70 m² total (about the size of half a tennis court)
- Walls are one cell thick — very short diffusion distance
- Rich blood supply — dense capillary network maintains a steep concentration gradient
- Moist lining — gases dissolve before diffusing across the membrane
- Good ventilation — breathing constantly refreshes air in the alveoli
Gas exchange at the alveoli:
- Oxygen diffuses from the alveoli into the blood (high O₂ in alveoli, low O₂ in blood arriving from the body)
- Carbon dioxide diffuses from the blood into the alveoli (high CO₂ in blood, low CO₂ in alveoli)
- Both move by diffusion down their concentration gradients — no energy needed
In the blood, oxygen binds to haemoglobin in red blood cells to form oxyhaemoglobin, which is transported to body cells.
Ventilation — Breathing
Ventilation (breathing) maintains a steep concentration gradient in the alveoli by constantly replacing air.
Inhalation (breathing in):
1. Intercostal muscles contract → ribs move up and out
2. Diaphragm contracts → moves down (flattens)
3. Volume of the thorax (chest cavity) increases
4. Pressure decreases
5. Air is drawn into the lungs
Exhalation (breathing out):
1. Intercostal muscles relax → ribs move down and in
2. Diaphragm relaxes → moves up (domes)
3. Volume of the thorax decreases
4. Pressure increases
5. Air is pushed out of the lungs
Normal exhalation is passive — no energy is used. Forced exhalation uses the internal intercostal muscles.
Lung Diseases
Asthma:
- Airways become inflamed and constricted (bronchioles narrow)
- Excess mucus is produced
- Symptoms: wheezing, difficulty breathing, tight chest
- Treated with inhalers — relievers relax the smooth muscle; preventers reduce inflammation
Lung cancer:
- Uncontrolled cell division in lung tissue, often caused by smoking (tar is a carcinogen)
- Tumours reduce the surface area for gas exchange
COPD (Chronic Obstructive Pulmonary Disease):
- Long-term damage to lung tissue, usually from smoking
- Includes chronic bronchitis (inflamed bronchi, excess mucus) and emphysema (alveoli walls break down, reducing surface area)
- Leads to severe breathlessness
Exam Tips
- Always explain gas exchange in terms of diffusion down a concentration gradient
- Remember the alveoli adaptations — large surface area, thin walls, good blood supply, moist — and explain why each helps
- Inhalation is an active process (muscles contract); normal exhalation is passive (muscles relax)