The Respiratory System
The Structure of the Respiratory System
The respiratory system moves air into and out of the body for gas exchange — taking in oxygen for cellular respiration and expelling carbon dioxide as a waste product.
Air travels through: Nose/Mouth → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
| Structure | Function |
|---|---|
| Nose/mouth | Filters, warms, and moistens incoming air |
| Pharynx | Common passage for air and food (throat) |
| Larynx | Contains vocal cords; produces sound |
| Trachea | Main airway to lungs; C-shaped cartilage rings keep it open; lined with cilia and mucus |
| Bronchi | Two main branches, one leading to each lung |
| Bronchioles | Progressively smaller branches within each lung |
| Alveoli | Tiny air sacs (about 300 million per lung) where gas exchange occurs |
| Diaphragm | Dome-shaped muscle below the lungs; main muscle of breathing |
| Pleural membranes | Two membranes surrounding each lung; fluid between them reduces friction |
Breathing Mechanics
Breathing works by changing the volume of the chest cavity, which changes the air pressure inside:
Inhalation (breathing in):
- Diaphragm contracts and flattens (moves downward)
- External intercostal muscles contract, pulling ribs up and outward
- Chest volume increases
- Pressure inside chest falls below atmospheric pressure
- Air flows in (from high to low pressure)
Exhalation (breathing out):
- Diaphragm relaxes and curves upward
- Internal intercostal muscles pull ribs down and inward
- Chest volume decreases
- Pressure inside chest rises above atmospheric pressure
- Air flows out
During forced exhalation (e.g., blowing out hard), abdominal muscles contract to push the diaphragm up more forcefully.
Lung Volumes
| Volume | Description | Approximate value |
|---|---|---|
| Tidal volume | Air breathed in/out per normal breath | 0.5 L |
| Vital capacity | Maximum air exhaled after maximum inhalation | 4–5 L |
| Residual volume | Air remaining in lungs after maximum exhalation | 1–1.5 L |
| Total lung capacity | Total air lungs can hold | 5–6 L |
During exercise, tidal volume and breathing rate both increase to supply more oxygen.
Gas Exchange in Alveoli
Alveoli are highly specialised for rapid, efficient gas exchange by diffusion:
Adaptations:
- Enormous surface area — approximately 70 m squared total (the size of a tennis court)
- Extremely thin walls — one cell (epithelial cell) thick; very short diffusion distance
- Moist lining — gases dissolve in moisture before diffusing through the membrane
- Dense capillary network — each alveolus is surrounded by capillaries; blood constantly replenished to maintain steep concentration gradients
- Large number — approximately 300 million alveoli per lung
Gas exchange process:
- Oxygen: concentration high in alveolar air, low in deoxygenated blood → diffuses from alveolus into blood → binds to haemoglobin in red blood cells
- Carbon dioxide: concentration high in blood (from respiration), low in alveolar air → diffuses from blood into alveolus → exhaled
Both gases move by simple diffusion — from high to low concentration.
Composition of Inhaled vs Exhaled Air
| Gas | Inhaled | Exhaled |
|---|---|---|
| Nitrogen | 78% | 78% |
| Oxygen | 21% | 16% |
| Carbon dioxide | 0.04% | 4% |
| Water vapour | Variable | Saturated |
Effects of Smoking on the Respiratory System
Tobacco smoke contains thousands of harmful chemicals including nicotine, tar, and carbon monoxide. Effects include:
- Damage to cilia — cilia in airways are paralysed or destroyed; mucus and pathogens accumulate → chronic cough ("smoker's cough") as the only mechanism to clear airways
- Emphysema — tar and inflammation destroy alveolar walls; alveoli merge into larger sacs with reduced surface area → reduced gas exchange efficiency; breathlessness
- Chronic bronchitis — excess mucus production; repeated infections
- Lung cancer — carcinogens in tar cause uncontrolled cell division in lung tissue
- Carbon monoxide poisoning — CO binds to haemoglobin (more strongly than O2), reducing oxygen-carrying capacity
Key Terms
- Alveoli — tiny air sacs in the lungs where gas exchange occurs
- Diaphragm — dome-shaped muscle that contracts during inhalation
- Tidal volume — volume of air breathed in or out during a normal breath
- Diffusion — movement of molecules from high to low concentration
- Cilia — hair-like projections lining the trachea that move mucus upward
- Emphysema — lung disease where alveolar walls are destroyed, reducing gas exchange surface area
- Vital capacity — maximum volume of air that can be exhaled after maximum inhalation
Common Mistakes
- Saying the diaphragm pushes air out during exhalation — the diaphragm relaxes during exhalation; elastic recoil of the lungs helps push air out
- Confusing the respiratory system (breathing and gas exchange) with cellular respiration (energy release in cells)
- Saying oxygen is "made" in the alveoli — it simply diffuses from the air into the blood
- Forgetting that exhaled air still contains oxygen (~16%) — it is just less than inhaled air (~21%)
Tips and Tricks
- Inhalation: diaphragm contracts DOWN, ribs move UP and OUT, volume INCREASES, pressure DECREASES, air flows IN
- Exhalation: the opposite of all the above
- Remember alveoli adaptations: large surface area + thin walls + moist + good blood supply = FAST diffusion
- Gas exchange works purely by diffusion — no energy is needed; the concentration gradients do all the work
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