Gas Exchange in Humans

Mouth and nose.

• Air entering is WARMED (by capillaries lining the nasal passages), MOISTENED (by mucus), and FILTERED (by hairs and mucus).
• Why warm? Cold air would slow gas exchange and irritate alveoli.
• Why moisten? Dry air would dry out alveolar walls.
• Why filter? Removes dust and pathogens.

Trachea (windpipe).

• ~12 cm long tube from throat to chest.
• Walls reinforced with C-SHAPED RINGS of CARTILAGE → keep it OPEN (don't collapse when you breathe in).
• Lined with goblet cells (make mucus) and ciliated cells (sweep mucus up).

Bronchi.

• Trachea splits at the top of the chest into TWO bronchi — one to each lung.
• Same cartilage support, mucus, and cilia as trachea.

Bronchioles.

• Bronchi divide into smaller and smaller branches.
• No cartilage in the smallest ones — held open by surrounding tissue.
• End in clusters of alveoli.

Alveoli.

• Tiny air sacs (~0.2 mm wide).
• Each lung has ~300 million.
• Total surface area ~70 m² (size of a tennis court).

Worked qualitative. Why does smoking damage the lungs?

• Tar coats the airways → paralyses cilia.
• Mucus + pathogens build up → 'smoker's cough'.
• Tar damages the alveoli walls → emphysema (alveoli merge into bigger sacs with less surface area).
• Carbon monoxide blocks haemoglobin → less O₂ carried.
• Nicotine narrows blood vessels → high BP.

Cambridge tip. Memorise the order: trachea → bronchi → bronchioles → alveoli. Cambridge often gives a flow chart and asks you to fill in the blanks.

Adaptations (each paired with reason):

Feature	Why
MILLIONS of alveoli	Massive total SURFACE AREA.
Walls ONE CELL thick	SHORT diffusion distance.
Capillary network around each	Steep CONCENTRATION GRADIENT (O₂ taken away; CO₂ supplied).
MOIST inner surface	Gases dissolve before diffusing.
Capillary walls also one cell thick	Total diffusion path: just 2 cells thick.
VENTILATION (breathing)	Refreshes air in alveoli → keeps gradient steep.

The exchange:

• O₂ in alveolar air > O₂ in blood → O₂ DIFFUSES INTO BLOOD.
• CO₂ in blood > CO₂ in alveolar air → CO₂ DIFFUSES OUT into the alveoli.

Where the blood comes from and goes to:

• Pulmonary artery → capillaries around alveoli (DEOXYGENATED).
• Capillaries → pulmonary vein (OXYGENATED, after gas exchange).

Worked qualitative. Why is breathing rate higher when you exercise?

• Muscles respire more → use more O₂ + make more CO₂.
• Brain detects rising blood CO₂ → signals breathing muscles to work faster.
• More breaths/min → faster ventilation → maintained gradient → more O₂ in, more CO₂ out.

Cambridge tip. When asked HOW alveoli are adapted, list 4 features WITH reasons. 'Many alveoli for surface area' = 1; 'walls one cell thick for short diffusion' = 1; etc.

Lungs themselves have NO muscles. Breathing happens because the muscles around them change the volume of the chest cavity.

Inspiration (breathing in).

1. Diaphragm CONTRACTS → flattens (moves DOWN).
2. External intercostal muscles CONTRACT → ribs lift UP and OUT.
3. Chest cavity VOLUME INCREASES.
4. Pressure inside chest DECREASES (Boyle's law).
5. Pressure outside > inside → AIR rushes IN through trachea, fills alveoli.

Expiration (breathing out, at rest).

1. Diaphragm RELAXES → curves UP.
2. External intercostal muscles RELAX → ribs fall DOWN and IN.
3. Chest cavity VOLUME DECREASES.
4. Pressure inside INCREASES.
5. Air pushed OUT.

Forced expiration (e.g. coughing, hard exhale).

• Internal intercostals CONTRACT → pull ribs further down + in.
• Abdominal muscles push diaphragm up.
• Faster, more forceful exhalation.

Why "passive" vs "active":

• Inspiration is always ACTIVE (uses energy).
• Expiration at rest is PASSIVE (just elastic recoil of stretched lung).
• Forced expiration is ACTIVE.

Worked qualitative. Why do you 'forget to breathe' when nervous and then sigh?

• Anxious tension causes shallow chest breathing → low gas exchange.
• Eventually CO₂ builds up → triggers a deep breath (sigh) to reset.
• Yoga breathing exercises retrain this — slow deep diaphragmatic breaths reduce stress.

Cambridge tip. Always describe BOTH the diaphragm AND the intercostal muscles. Cambridge marks both. Just 'lungs expand' = 0.

Approximate composition:

Component	Inspired	Expired	Why?
Nitrogen (N₂)	78%	78%	Inert; not absorbed.
Oxygen (O₂)	21%	16%	Some absorbed into blood.
Carbon dioxide (CO₂)	0.04%	4%	Released from blood.
Water vapour	Variable	Saturated (~6%)	Lungs warm + moist.
Other gases	~1%	~1%	Argon etc.

Temperature.

• Inspired: room temp.
• Expired: ~37°C (warmed in lungs).

Why O₂ doesn't drop more.

• Even after exchange, plenty of O₂ left in alveoli.
• That's why CPR (mouth-to-mouth) works — your exhaled breath has enough O₂ to revive someone.

Demonstrating the difference (limewater test).

• Limewater (calcium hydroxide solution) goes CLOUDY in the presence of CO₂.
• Bubble inspired air through it → stays clear (very little CO₂).
• Bubble expired air through it → goes cloudy quickly (lots of CO₂).
• Common Paper 4 question — describe the result and explain.

Worked qualitative. Why does your breath fog up a cold window?

• Expired air is WARM and SATURATED with water vapour.
• Cold window cools the air → water vapour CONDENSES → tiny droplets form on the glass.
• That's why you can write your name on a cold mirror with your breath.

Cambridge tip. Memorise approximate values: 21 → 16 for O₂; 0.04 → 4 for CO₂. Numbers may be asked.