Gas Exchange

Air enters through the nose or mouth → trachea (windpipe) → splits into two bronchi → branches into bronchioles → end in tiny sacs called alveoli.

The whole system looks like an upside-down tree:

• Trachea = trunk.
• Bronchi = main branches.
• Bronchioles = smaller branches.
• Alveoli = grapelike clusters of leaves at the tip.

Part	Notable feature
Nose	Warms, moistens, filters air
Trachea	C-shaped cartilage rings hold it open
Bronchi (2)	Cartilage rings; mucus + cilia trap dust
Bronchioles	No cartilage; have smooth muscle
Alveoli	~500 million in human lungs; total surface area ~70 m²

The trachea has rings of cartilage that prevent it collapsing when you breathe in. The lining is covered with CILIA — tiny hair-like projections that sweep mucus + trapped dust UP towards the throat where it's swallowed.

An alveolus is a tiny air sac, about 0.2 mm across. Each is surrounded by a network of capillaries. Oxygen from the air diffuses across the alveolar wall into the blood; CO₂ goes the other way.

Adaptations of an alveolus (all maximise diffusion):

• Huge surface area — millions of alveoli together = ~70 m² (tennis-court area).
• Thin wall — only ONE CELL thick → short diffusion distance.
• Moist surface — gases dissolve in the moisture before diffusing.
• Rich blood supply — capillaries on the outside maintain a concentration gradient (steady supply of low-O₂ blood arriving, high-O₂ blood leaving).

A useful contrast: an ELEPHANT has the same alveolus design as a MOUSE — just many more. Each individual alveolus does roughly the same thing.

Lungs themselves have no muscles. Air is moved in and out by:

Breathing in (INHALATION):

• DIAPHRAGM contracts → flattens downwards.
• EXTERNAL INTERCOSTAL MUSCLES contract → ribs lift up and out.
• Chest cavity ENLARGES → pressure inside drops below atmospheric → air rushes in.

Breathing out (EXHALATION):

• DIAPHRAGM relaxes → moves up.
• INTERCOSTAL muscles relax → ribs drop down and in.
• Chest cavity SHRINKS → pressure rises above atmospheric → air pushed out.

It's the same principle as a bellows or a syringe — change volume to change pressure to move air.

A typical adult breathes ~12-20 times per minute at rest, taking in ~500 mL of air per breath. During exercise: up to 40 breaths/min, ~3 L per breath.

Don't confuse:

• VENTILATION = breathing — the mechanical movement of air.
• GAS EXCHANGE = the diffusion of O₂ and CO₂ at the alveoli.
• RESPIRATION = the chemical reaction in cells using O₂ to release energy.

All three are different processes.

Cigarette smoke contains 7,000+ chemicals, including:

Tar. Sticky brown coating that:

• Damages cilia in airways → no clearance of mucus + dust → bacteria grow → 'smoker's cough' and bronchitis.
• Contains carcinogens → lung cancer.

Nicotine. Highly addictive drug. Raises heart rate, narrows blood vessels → high blood pressure, heart disease.

Carbon monoxide. Binds to haemoglobin instead of O₂ → less O₂ delivered to tissues. Smokers' babies tend to be smaller because of reduced O₂ supply during pregnancy.

Long-term effects of smoking:

• Emphysema: alveoli walls break down → larger sacs, less surface area → breathlessness.
• Bronchitis: chronic inflammation of bronchi, narrowed airways.
• Lung cancer: tar carcinogens cause mutations.
• Heart disease: from nicotine and CO.

Smoking causes ~8 million deaths globally per year. Quitting at any age improves health outcomes; lung cilia even grow back over a few months.