Alveolus adaptations — five features
Large surface area (~700 million alveoli, ~70 m²) lets many molecules cross at once. Walls one cell thick give a diffusion path of only ~0.3 µm. A dense capillary network surrounding each alveolus carries away and brings , so the gradient never flattens. A moist lining lets gases dissolve before diffusing. Ventilation renews the air inside. Every feature exists to keep the diffusion gradient steep.
Inspired vs expired air — the numbers
Oxygen falls from ~21% to ~16%; carbon dioxide rises from ~0.04% to ~4%, a hundredfold jump; water vapour rises from low/variable to nearly saturated; nitrogen stays ~79% because it is metabolically inert; expired air leaves warmed to about 37 °C. Both gases are present in both samples — only the proportions differ, and only about 5 percentage points of oxygen are taken up per breath.
Ventilation — muscles, volume, pressure, flow
Inspiration: external intercostal muscles and the diaphragm contract, ribs swing up and out, the diaphragm flattens, thorax volume rises, pressure falls below atmospheric and air flows in. Expiration at rest is passive — those same muscles relax, the rib cage drops, the diaphragm domes upward, volume falls, pressure rises above atmospheric and air is pushed out. Forced expiration adds contraction of the internal intercostals.
Drawn from real examiner reports.
Volume change is not the cause
"Air enters because the volume of the thorax increases" is circular reasoning and earns nothing. The chain examiners want is: muscles contract → thorax volume increases → pressure inside falls below atmospheric → air flows in down the pressure difference. It is the pressure difference that moves the air. Avoid "a vacuum is created" as well.
Flagged s23 P22 Q20; s22 P42 Q5b — shared digest
External vs internal intercostals
External intercostals contract during inspiration to raise the ribs up and out. Internal intercostals contract only in forced expiration, pulling the ribs down and in. Quiet expiration is passive: the external intercostals must RELAX before the rib cage can move down and inwards — writing that they contract during expiration loses the mark.
Flagged s22 P23 Q20 — shared digest
Expired air still holds ~16% oxygen
"Expired air contains only carbon dioxide" and "inspired air contains only oxygen" both score zero. Only about 5 percentage points of oxygen are absorbed per breath, so ~16% is still breathed out — which is why expired air works for mouth-to-mouth resuscitation. Inspired air already carries ~0.04% . Never write 0% or "none" in a composition table.
Flagged w22 P11/12/13/21/22 Q20/Q22 · s23 P22/31/33 — shared digest
Alveoli are surrounded, not filled
Capillaries form a mesh around the OUTSIDE of each alveolus; blood never enters the air space. "The alveoli are filled with blood capillaries" reverses the anatomy and loses the mark. Use "surrounds" or "adjacent to". The diffusion route is air space → alveolus wall → capillary wall → blood, with each wall one cell thick.
Flagged w22 P41 Q6a ii — shared digest
(Extended) Cilia move mucus, not bacteria
Cilia beat to sweep the mucus layer toward the pharynx; bacteria and dust travel only because they are trapped in that mucus. "Cilia move bacteria out of the lungs" is marked wrong. Write: cilia move the mucus containing trapped pathogens and dust up to the pharynx, where it is swallowed and the pathogens are destroyed by stomach acid.
Flagged s23 P41 Q3a ii; w22 P41 Q6 — shared digest
(Extended) Goblet cells secrete, not sweep
Goblet cells only secrete mucus onto the airway lining — they carry no cilia and move nothing. Ciliated epithelial cells are a separate cell type that moves the mucus. Extended papers ask for the two in consecutive part-questions, so swapping or merging their roles costs both marks. Keep the answer in the trachea, not a digestive, context.
Flagged s23 P41 Q3a i — shared digest
Trachea cartilage often goes unnamed
C-shaped rings of cartilage hold the trachea open and stop it collapsing when pressure inside the thorax falls during inspiration. Many candidates describe the trachea without ever naming this function. Bronchi also contain cartilage; bronchioles do not — a useful way to tell the two apart in a labelling question.
Flagged s22 P42 Q5a ii — shared digest
Answer breathing questions as a chain
Write one clause per mark: muscles contract → ribs and diaphragm move → thorax volume changes → pressure changes → air flows. Five links comfortably cover a 4–5 mark question, and each link scores on its own even if a later one goes wrong.
Give a value for every cell
For "state the differences between inspired and expired air", quote both figures for each component: 21% → ~16%, 0.04% → ~4%, water vapour low → nearly saturated, nitrogen ~79% unchanged. Never use the word "only" about either gas.
(Extended) Exercise: rate AND depth
"Describe the effect of exercise on breathing" needs both changes — breathing rate increases and depth of breathing increases. Naming one earns 1 of 2. If asked where the rise in is detected, say chemoreceptors in the brain (medulla), not the lungs.
Percentage change: divide by the start
Breathing-rate and gas-composition calculations all use (new − old) / old × 100. Divide by the resting or inspired value, never the final one, and state the direction: oxygen falling from 21% to 16% is a change of about −24%, i.e. a decrease.
Gas exchange is the process by which oxygen () diffuses from the lungs into the blood, and carbon dioxide () diffuses from the blood into the lungs, through the alveolar walls.
Ventilation (breathing) is the mechanical process of moving air into and out of the lungs to maintain concentration gradients for gas exchange. Ventilation is NOT the same as respiration (which is the chemical release of energy from glucose inside cells).
State FIVE features of an efficient gas exchange surface.
State four features that make the alveoli an efficient gas exchange surface. [4]
