DSE Biology | Human Gas Exchange Lesson Prep (for tutors) | Ventilation, diffusion, transport as three layers

2026-11-04

Tutor lesson-prep guides · Secondary Biology

Students stop at large alveolar surface area, then stall on ventilation rate, dead space and saturation. This lesson separates gas exchange into three layers: air reaching the alveoli, gas crossing the membrane, and oxygen carried away by haemoglobin. Each layer has numbers to check.

Objectives — Students can: (1) compute total ventilation from rate and tidal volume, then subtract dead space for alveolar ventilation; (2) explain alveolar efficiency with the four factors of the diffusion law and predict what happens when one fails; (3) compare dissolved oxygen with haemoglobin capacity and distinguish anaemia, carbon monoxide poisoning and high altitude.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Respiration and burning	Where does cellular respiration happen?	Mainly the mitochondria
Diffusion	How does rate depend on distance?	Shorter distance, faster diffusion
Blood components	Which protein carries oxygen in red cells?	Haemoglobin

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three-question check	Site of respiration, diffusion direction, haemoglobin role	Answer orally
8-22 min	Ventilation mechanics	Diaphragm, intercostals, thoracic pressure	Draw pressure arrows
22-38 min	Alveolar design	Area, thickness, moisture, blood flow	Change one factor and predict
38-50 min	Oxygen and carbon dioxide transport	Capacity, curve, bicarbonate	Compare dissolved oxygen
50-57 min	Timed practice	Ventilation and curve questions	Work alone
57-60 min	Exit check	Three checkpoints, one question each	Self-mark and circle slips

03 Teaching script (read it out)

Scene 1: Ventilation: pressure difference first, muscles second

Inhalation is active: the diaphragm contracts and flattens, external intercostals lift and swing the ribs out, thoracic volume rises, intrapulmonary pressure drops below atmospheric, and air flows in. Quiet exhalation is mostly elastic recoil. Compute: 15 breaths per minute times 500 mL tidal volume gives 7500 mL/min total ventilation, about 7.5 L/min. The conducting airways hold about 150 mL that never exchange, so alveolar ventilation is only 5250 mL/min, about 5.25 L/min. One line: slow deep breathing beats fast shallow breathing at the same total ventilation.

Ask them to redo it as 10 breaths of 750 mL: alveolar ventilation rises to 6 L/min with the same total.

Scene 2: Alveolar design: four factors pulling the same way

Roughly 480 hundred million alveoli give about 70 square metres of surface, walls about 1 micrometre thick, a moist lining, and a dense capillary net. Diffusion rate depends on surface area, concentration gradient, diffusion distance, and the solubility and molecular mass of the gas. Emphysema destroys alveolar walls and cuts area; pulmonary oedema thickens the barrier and raises distance; altitude lowers oxygen partial pressure and shrinks the gradient. All three slow diffusion, but through different factors, and the answer must name the right one.

Draw a four-cell table, one factor per cell, and have students place each condition in a cell.

Scene 3: Oxygen and carbon dioxide transport: haemoglobin does the work

Each gram of haemoglobin binds about 1.34 mL of oxygen, blood holds 150 g/L, so capacity is about 201 mL/L. Dissolved oxygen at 100 mmHg is only about 3 mL/L, a factor of 67, so dissolved oxygen alone is nowhere near enough. The dissociation curve is sigmoid: near-saturation in the lungs, rapid unloading in tissues, efficient at both ends. Carbon dioxide travels mainly as bicarbonate, with small fractions bound to haemoglobin and dissolved. Carbon monoxide binds haemoglobin far more strongly than oxygen and does not let go, so partial pressure looks normal while carrying capacity collapses.

Close on the clinic: a blood report reads haemoglobin concentration, not oxygen partial pressure.

04 Question types and worked steps

Type	Example	Steps	Answer
Ventilation	Rate 15, tidal volume 500 mL, dead space 150 mL	Total first, then subtract dead space	5.25 L/min
Diffusion factors	Why emphysema causes breathlessness	Name the surface area fall	Alveolar walls destroyed
Curve question	More oxygen released in exercising muscle	Low partial pressure plus carbon dioxide and acid shifts right	Bohr effect
Comparison	Anaemia versus carbon monoxide poisoning	Fewer carriers versus occupied carriers	Both can show normal partial pressure

Sanity check (teach this): The numbers must agree: 15 times 500 gives 7500 mL/min and 5250 mL/min after dead space, roughly seventy per cent of total ventilation, matching textbook figures. On transport, 150 g/L times 1.34 gives 201 mL/L against 3 mL/L dissolved, over sixty times, so any answer resting on dissolved oxygen alone is wrong. 70 square metres spread over 480 hundred million alveoli gives about 0.15 square millimetres each, which is the right order.

05 Independent practice (3 questions)

Question	Answer	What to watch
Alveolar ventilation with rate 20, tidal volume 400 mL, dead space 150 mL	5.0 L/min	Dead space subtracted first
Explain the benefit of a right-shifted curve during exercise	More oxygen released to muscle	Mentions carbon dioxide and temperature
Compare partial pressure in anaemia and carbon monoxide poisoning	Both can be normal	Separates partial pressure from content

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reporting total ventilation as alveolar ventilation	Uses 7.5 L/min as the exchange figure	Airway air never reaches alveoli, so subtract 150 mL of dead space first.
Reducing diffusion to surface area	No mention of distance or gradient	Four factors must appear, or the answer is one step short.
Equating normal partial pressure with normal oxygen delivery	Says carbon monoxide poisoning is fine	Partial pressure can be normal while haemoglobin is occupied and content is low.
Confusing breathing with cellular respiration	Says the lungs make ATP	Lungs only ventilate; mitochondria make the ATP.

07 Homework (paper, section and question numbers)

Biology chapter index — All chapters
Biology practice papers — Gas exchange: ventilation and curve questions

08 Exit check (last 5 minutes)

Question	Answer	Decision
Alveolar ventilation	Why fast shallow breathing is inefficient	Cannot say: reteach dead space
Four factors	Which factor does pulmonary oedema change	Says surface area: return to act two
Transport	Why carbon monoxide is dangerous	Only says lack of oxygen: add occupied haemoglobin

09 Tutor FAQ

Why does breathing quicken during exercise?

Rising carbon dioxide and falling pH are detected by chemoreceptors, which drive ventilation up as negative feedback.

Why do alveoli not collapse?

Type II alveolar cells secrete surfactant, lowering surface tension and preventing collapse at end expiration.

Can dead space be reduced?

Anatomical dead space is fixed, but slow deep breathing raises the alveolar share, which is why trained athletes breathe that way.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.