SPEC 3.3.2 - EXCHANGE & TRANSPORT
Human Gas Exchange
Structure of the lungs, ventilation mechanism, lung diseases — and how every feature links to Fick's Law of Diffusion
Gross Structure of the Lungs
Air travels along a branching airway tree before reaching the gas exchange surface:
T
Trachea C-shaped cartilage rings keep lumen open; ciliated epithelium and goblet cells produce mucus to trap pathogens — cilia beat it upward to the throat.
Br
Bronchi (left & right) Same structure as trachea — cartilage plus smooth muscle. Airways to the left and right lungs respectively.
Bl
Bronchioles No cartilage — smooth muscle only. Contract (bronchoconstriction) or dilate to regulate airflow. Branch repeatedly; no cartilage — smooth muscle only allows bronchoconstriction or dilation to regulate airflow.
Al
Alveoli ~300 million air sacs; total SA ~70 m². Walls: one cell thick (simple squamous epithelium). Dense capillary network. Site of all gas exchange.
Examiner Alert
Trachea & bronchi = cartilage. Bronchioles = smooth muscle, NO cartilage. This is tested regularly in structure-function questions and in RP5 dissection questions.
Alveolar Adaptations & Fick's Law
Every structural feature of the alveoli can be explained using Fick's Law:
Fick's Law
Rate of diffusion ∝ (Surface area × Concentration gradient) ÷ Diffusion distance
Alveolar adaptationFick's Law link
~300 million alveoli → SA ~70 m²Large surface area ↑ → rate ↑
Walls one cell thick (squamous epithelium)Short diffusion distance → denominator ↓ → rate ↑
Dense capillary network; continuous blood flowO₂ removed rapidly → gradient maintained on blood side
Ventilation continuously replaces airFresh O₂ supplied → gradient maintained on air side
Moist surface liningGases dissolve before diffusing through epithelium
Exam Tip
Never just name an adaptation. Always complete the chain: "large surface area → increases SA in Fick's Law → rate of diffusion increases." Incomplete chains lose marks even if the adaptation is correct.
Ventilation: Inspiration
Inspiration is an active process requiring muscle contraction to increase thoracic volume.
1
Diaphragm contracts — flattens from dome shape → increases vertical dimension of thorax.
2
External intercostal muscles contract — ribs move upward and outward → increases lateral and anterior-posterior dimensions of thorax.
3
Thoracic volume ↑ → pressure ↓ below atmospheric (Boyle's Law: P ∝ 1/V at constant temperature).
4
Air flows in down the pressure gradient (atmosphere → lungs) until pressures equalise.
Exam Tip
State the full causal chain: volume ↑ → pressure ↓ → pressure gradient created → air flows in. Omitting any step loses marks. For 6-mark answers, state Boyle's Law explicitly.
Ventilation: Expiration
Quiet Expiration — PASSIVE
Diaphragm and external intercostals relax
Elastic recoil of lung tissue reduces thoracic volume
Pressure rises above atmospheric → air flows out
No muscles contract
Forced Expiration — ACTIVE
Internal intercostal muscles contract
Pulls ribs down and inward more forcefully
Abdominal muscles may also contract
Used during exercise or coughing
Most Common Mistake
Quiet expiration is passive — no muscles contract. Internal intercostals are only used in forced expiration. Writing "internal intercostals contract during normal breathing out" is wrong and loses marks.
Pulmonary Ventilation Rate
Pulmonary ventilation rate (PVR) is the total volume of air moved per minute:
Formula (MS 2.2)
PVR = Tidal Volume × Breathing Rate
units: dm³ min⁻¹ = dm³ × min⁻¹
TermDefinitionTypical resting value
Tidal volume (TV)Volume of air moved per breath~0.5 dm³
Breathing rate (BR)Number of breaths per minute~15 min⁻¹
PVRTotal air moved per minute~7.5 dm³ min⁻¹
Rearrangements
TV = PVR ÷ BR | BR = PVR ÷ TV
Always check units: if TV is in dm³ and BR is in min⁻¹, PVR must be in dm³ min⁻¹.
Extension: Dead Space & Alveolar Ventilation Rate
Extension beyond core spec — may appear in applied/data questions. Not all inhaled air reaches the alveoli. The volume remaining in the trachea, bronchi and bronchioles — where no gas exchange occurs — is anatomical dead space (~0.15 dm³).
Alveolar Ventilation Rate (AVR)
AVR = (TV − Dead Space) × Breathing Rate
Worked Example
TV = 0.5 dm³, BR = 15 min⁻¹, dead space = 0.15 dm³
AVR = (0.5 − 0.15) × 15 = 0.35 × 15 = 5.25 dm³ min⁻¹
PVR = 0.5 × 15 = 7.5 dm³ min⁻¹ — overestimates effective ventilation by 2.25 dm³ min⁻¹.
PVR ≠ AVR
If a question asks for the volume of air that reaches the alveoli, or mentions dead space, you must subtract dead space from TV before multiplying. PVR = total air moved; AVR = effective alveolar ventilation.
Practice: Ventilation Calculations
A student at rest has a tidal volume of 0.48 dm³ and a breathing rate of 14 breaths min⁻¹. Dead space = 0.15 dm³.
a) Calculate the pulmonary ventilation rate (PVR). [2]
b) Calculate the alveolar ventilation rate (AVR). [2]
c) After exercise, the student's PVR doubles to 20 dm³ min⁻¹ and breathing rate rises to 20 breaths min⁻¹. Calculate the new tidal volume. [2]
a)
PVR = TV × BR = 0.48 × 14 = 6.72 dm³ min⁻¹ [1 correct method; 1 correct answer with units]
b)
AVR = (TV − dead space) × BR = (0.48 − 0.15) × 14 = 0.33 × 14 = 4.62 dm³ min⁻¹ [1 dead space subtracted; 1 correct answer]
c)
TV = PVR ÷ BR = 20 ÷ 20 = 1.0 dm³ [1 correct rearrangement; 1 correct answer]
Lung Disease: Fibrosis & Emphysema
Pulmonary Fibrosis
Inflammation → scar (fibrous) tissue replaces healthy alveolar tissue
Diffusion distance ↑ → Fick denominator ↑ → rate ↓
Loss of elasticity → lungs harder to expand → reduced tidal volume
Symptoms: persistent dry cough, breathlessness on exertion
Emphysema
Alveolar walls break down → alveoli merge into larger spaces
SA ↓ → Fick numerator ↓ → rate ↓
Elastic tissue lost → air trapped on expiration → gradient ↓ (stale air)
Progressive breathlessness on exertion
Emphysema — Two Mechanisms Required
Two separate Fick's Law effects: (1) reduced surface area AND (2) reduced concentration gradient (trapped stale air reduces O₂ gradient). One-mechanism answers score at most half marks.
Lung Disease: Asthma & Lung Cancer
Asthma
Allergic/inflammatory response to trigger (pollen, dust mites, exercise)
Bronchiole smooth muscle contracts → bronchoconstriction
Mucus secretion ↑ → airway narrows further
Alveoli structurally intact — problem is restricted airflow, not alveolar damage
Reversible with bronchodilator inhaler
Lung Cancer
Uncontrolled cell division → tumour formation
Tumour blocks airways → reduces ventilation and exchange
Carcinogens in tobacco smoke cause DNA mutations
Strong positive correlation between smoking and incidence; biological mechanism supports causation
Asthma vs Emphysema
In asthma, alveoli are intact. In emphysema, alveoli are destroyed. Confusing these in a comparison question is one of the most penalised errors on lung disease mark schemes.
Correlation vs Causation
Data linking smoking to lung cancer shows a strong positive correlation — but correlation alone does not prove causation.
Key Distinctions
TermMeaningCorrelationTwo variables change together; a third (confounding) variable may be responsibleCausationOne variable directly causes the change; requires mechanism + dose-response + confounders ruled out
Evidence needed to establish causation:
Mechanism — known biological pathway (carcinogens → DNA mutations → uncontrolled division)
Dose-response — greater exposure → greater risk
Confounding variables removed — alternative explanations ruled out
Exam Tip
The expected answer to "does this graph prove smoking causes lung cancer?" is always: "No — it shows a positive correlation; correlation does not prove causation because there may be confounding variables." Always use the phrase confounding variables.
Required Practical 5 & Spirometry
Required Practical 5: Dissection of the gas exchange system of a mammal (typically a sheep's pluck — lungs, trachea and heart intact). Students identify gross structures, cut to reveal internal anatomy, and inflate the lungs via the trachea to observe elastic recoil.
Spirometry — Lung Volumes
A spirometer records changes in lung volume over time.
VolumeDefinition
Tidal volume (TV)Volume per resting breath (~0.5 dm³)
Inspiratory reserve volume (IRV)Extra volume inhaled beyond TV during a deep breath
Expiratory reserve volume (ERV)Extra volume forcibly exhaled beyond TV
Residual volume (RV)Air remaining after maximum exhalation; cannot be measured by spirometer alone (~1.5 dm³)
Vital capacity (VC)TV + IRV + ERV — maximum exchangeable volume per breath
Reading a Spirometer Trace
TV = vertical distance of a resting cycle. BR = number of complete cycles per minute. The trace baseline drifts downward as soda lime absorbs CO₂ — this is expected and does not indicate tidal volume change.
Practice: Lung Disease & Correlation
a) Explain why emphysema reduces the rate of gas exchange. Use Fick's Law in your answer. [4]
b) A student states: "Asthma and emphysema both reduce gas exchange because both damage the alveoli." Evaluate this statement. [3]
c) A study reports a positive correlation between air pollution levels and asthma incidence. Explain why this does not prove that air pollution causes asthma. [2]
a)
Mechanism 1 — SA: Alveolar walls break down → alveoli merge → total SA decreases → Fick: SA ↓ → rate ↓ [2]. Mechanism 2 — gradient: Elastic tissue destroyed → air trapped on expiration → stale CO₂-rich air remains → O₂ gradient reduced → rate ↓ [2]. (Both mechanisms needed for full marks.)
b)
Partly correct for emphysema: alveolar walls are destroyed → SA reduced → rate ↓ [1]. Incorrect for asthma: alveoli are structurally intact [1]; it is bronchoconstriction and excess mucus that restrict airflow to otherwise healthy alveoli — not alveolar damage [1].
c)
A positive correlation shows two variables change together [1]; a confounding variable could be responsible (e.g. people in polluted areas may also be exposed to other allergens or have different lifestyles) — without ruling out confounders, causation cannot be established [1].
Summary
Key Points — Lesson 3.03
TopicCore fact
Airway structureTrachea + bronchi = cartilage; bronchioles = smooth muscle, no cartilage
Alveoli~300 million; ~70 m² SA; 1 cell thick; moist; capillary network
InspirationActive — diaphragm + ext. intercostals contract → volume ↑ → pressure ↓ → air in
Quiet expirationPassive — elastic recoil only; no muscles contract
Forced expirationActive — internal intercostals (+ abdominals) contract
PVRTV × BR; AVR = (TV − dead space) × BR
FibrosisDiffusion distance ↑ → Fick denominator ↑ → rate ↓
EmphysemaSA ↓ AND gradient ↓ (trapped air) — two mechanisms always required
AsthmaBronchoconstriction; alveoli structurally intact
Correlation/causationPositive correlation ≠ causation; cite confounding variables
Exam Tip
Every lung disease question is answered by identifying the Fick's Law term affected: fibrosis = diffusion distance; emphysema = SA and gradient; asthma = airflow restricted (alveoli intact). Tie each answer explicitly to Fick's Law for maximum marks.
Next: Gross Structure of the Lungs