Before we explain the mechanism, look at the data. The numbers tell a story about gradients, distances, and surfaces — and if you read them right, you'll understand the mechanism before you've been taught it.
Content from this lesson that appears directly in HSC Biology exams
Explaining how alveolar structure enables efficient gas exchange is tested in nearly every HSC paper — 4–6 marks in Section II. Must link: large SA, thin membrane, moist surface, maintained gradient (blood flow + ventilation) to each structural feature of the alveolus.
Graph or table showing partial pressure of O₂ and CO₂ across locations (atmospheric → alveoli → blood → tissues → mitochondria). Tested as 3–4 mark data interpretation in Section I or Section II — must use "partial pressure gradient" language and explain direction of diffusion.
Rate of diffusion is proportional to SA × concentration gradient / membrane thickness. Tested as 2–3 mark application question — "explain how condition X affects gas exchange rate." Must name the Fick variable affected and describe the directional effect.
Explaining why continuous ventilation (refreshing alveolar air) and blood flow (removing O₂ and delivering CO₂) are essential to maintain the partial pressure gradients that drive diffusion. Tested as 2–3 mark mechanism questions.
Start with the Data
Read the numbers. Spot the patterns. Then we'll explain what you're seeing.
The table below shows the partial pressure of O₂ and CO₂ at five locations along the gas exchange pathway — from the air you breathe to the mitochondria inside your cells. Partial pressure is the pressure exerted by one gas in a mixture; it is directly proportional to concentration and determines the direction gases move by diffusion.
| Location | O₂ partial pressure (mmHg) | CO₂ partial pressure (mmHg) | Where in the body |
|---|---|---|---|
| Atmospheric air | 159 | 0.3 | Outside the body — inhaled air |
| Alveolar air | 100 | 40 | Inside the lung air sacs — after mixing with dead-space air |
| Arterial blood (pulmonary vein) | 95 | 40 | Blood leaving lungs — just loaded with O₂ |
| Venous blood (at rest) | 40 | 45 | Blood returning from body tissues |
| Tissue cells / mitochondria | 20–30 | 50+ | Inside metabolically active cells |
Your prediction — before reading on: What pattern do you see? What direction is each gas moving at each step, and why?
The Mechanism
The numbers reveal the mechanism. Here is what they mean.
If you noticed that O₂ partial pressure consistently decreases from atmosphere to mitochondria, and CO₂ consistently increases in the opposite direction — you have already identified the fundamental mechanism of gas exchange. Gases always diffuse from high to low partial pressure. The size of the difference at any step determines how fast the gas moves.
Atmosphere (159) → Alveoli (100): O₂ drops significantly because inhaled air mixes with residual air already in the lungs (dead space air) that hasn't been exhaled yet — this dilutes the incoming O₂. Alveolar O₂ is never as high as atmospheric O₂ for this reason.
Alveoli (100) → Arterial blood (95): A small 5 mmHg drop drives O₂ across the alveolar membrane into blood. This gradient is small but the alveolar surface is enormous (~250m²) and the membrane is extremely thin (~0.5 μm) — so diffusion rate is high despite the modest gradient.
Arterial blood (95) → Venous blood at rest (40): A dramatic drop of 55 mmHg as blood delivers O₂ to body tissues over the systemic circuit. Tissues consuming O₂ maintain a low partial pressure in cells — this keeps the gradient between blood and tissues that drives O₂ into cells.
Venous blood (40) → Tissue cells (20–30): Blood still contains significant O₂ even after the systemic circuit — haemoglobin is never fully depleted at rest. During exercise, this venous O₂ drops much lower as demand increases.
This is Fick's Law of Diffusion in action — the rate of diffusion is proportional to surface area and concentration gradient, and inversely proportional to membrane thickness:
Rate of diffusion ∝ (Surface Area × Concentration Gradient) / Membrane Thickness
Every adaptation of the alveolus can be mapped onto one of these three variables
| Fick Variable | Alveolar Adaptation | How It Maximises Gas Exchange Rate |
|---|---|---|
| Surface Area ↑ | ~500 million alveoli · total SA ~250m² | More surface available simultaneously — rate increases proportionally |
| Concentration Gradient ↑ | Continuous ventilation refreshes alveolar air · continuous blood flow removes loaded O₂ | Gradient maintained by never letting alveolar O₂ fall or blood O₂ rise — diffusion continues at maximum rate |
| Membrane Thickness ↓ | Alveolar wall 1 cell thick · capillary wall 1 cell thick · combined ~0.5 μm total | Minimum possible diffusion distance — gases cross in milliseconds |
Six steps, six concentration gradients, one continuous downhill journey
Gas exchange is not a single event — it is a series of diffusion steps, each driven by a concentration gradient maintained by the previous step. Break one step and the whole chain fails. The cardiovascular system's role is to be the active transporter between the two passive diffusion zones (lungs and tissues).
Air moves into lungs by bulk flow driven by pressure differences created by the diaphragm and intercostal muscles expanding the thorax — not by diffusion. Breathing is essential because diffusion alone is too slow to move gases over the distances from mouth to alveolus (~30 cm). Ventilation continuously delivers fresh high-O₂ / low-CO₂ air to the alveoli and removes CO₂-enriched expired air, maintaining alveolar partial pressures.
O₂ diffuses from alveolar air (pO₂ 100 mmHg) across the alveolar epithelium and capillary endothelium (combined ~0.5 μm) into pulmonary capillary blood (pO₂ ~40 mmHg arriving, leaves at ~95 mmHg). CO₂ moves in the opposite direction simultaneously. This is "external gas exchange" — between the external environment (alveolar air) and the blood. The ~60 mmHg O₂ gradient is maintained by ventilation on one side and blood flow on the other.
O₂ that has diffused into plasma rapidly binds haemoglobin inside red blood cells — forming oxyhaemoglobin. This binding removes free O₂ from solution, keeping plasma pO₂ low, which maintains the diffusion gradient from alveoli into blood. Without haemoglobin, blood could carry only ~3 mL O₂ per 100 mL; with haemoglobin, it carries ~20 mL O₂ per 100 mL — a 70-fold increase in capacity.
Oxygenated blood is pumped by the left ventricle through arteries to systemic capillaries — again bulk flow, not diffusion. The cardiovascular system delivers fully loaded haemoglobin to within diffusion distance of every cell in seconds. Without circulation, O₂ would exhaust the tissue-adjacent blood and the gradient would collapse. The heart is gradient maintenance.
At systemic capillaries, O₂ diffuses from capillary blood (pO₂ ~95 mmHg arriving) into tissue cells (pO₂ ~20–30 mmHg) down the gradient. CO₂ diffuses from tissue cells (pCO₂ ~50+ mmHg) into blood (pCO₂ ~40 mmHg). This is "internal gas exchange" — between blood and the internal tissues. The tissue pO₂ is kept low by continuous cellular respiration consuming O₂; without metabolism, the gradient would disappear.
O₂ diffuses from cytoplasm into mitochondria where it is used as the terminal electron acceptor in oxidative phosphorylation, producing ATP. CO₂ is produced as a byproduct, diffusing from mitochondria into cytoplasm, then into capillary blood. The mitochondria maintain the lowest pO₂ in the entire pathway — ensuring the gradient from air to mitochondria is always downhill for O₂, and from mitochondria to air is always downhill for CO₂.
If either stops, gradients collapse — and so does gas exchange
The partial pressure gradients that drive gas exchange are not passive permanent features — they must be actively maintained every second. Ventilation and blood flow are the two systems that do this. Understanding what happens when either fails reveals why they are both indispensable.
| If You Stop Breathing (no ventilation) | If Your Heart Stops (no blood flow) |
|---|---|
| Alveolar O₂ falls (cells and blood continue consuming it but no new air delivered) · Alveolar CO₂ rises (blood delivers CO₂ but none is removed by expiration) | Pulmonary blood becomes fully saturated with O₂ but no longer cycles — alveolar O₂ can't diffuse in because blood is already loaded · O₂ gradient collapses immediately |
| Alveolar pO₂ gradient (alveoli → blood) disappears → O₂ loading stops | Tissue blood becomes depleted of O₂ → tissue pO₂ rises until it equals blood pO₂ → gradient collapses → O₂ delivery stops |
| Result: blood O₂ falls, CO₂ rises → acidosis → loss of consciousness within ~4 minutes (brain O₂ deprivation) | Result: same outcome, same time frame — cardiac arrest causes immediate collapse of both circuits simultaneously |
Every respiratory surface — alveoli, gills, tracheoles — shares these four properties
Whether in a fish gill, an insect tracheal system, or a human alveolus, every effective gas exchange surface shares four structural properties. These are not arbitrary — each maps directly onto a variable in Fick's law or a requirement for effective diffusion. In HSC exam language, these are the features you must explain.
| Feature | Why It's Required | What Happens Without It | Alveolus Example |
|---|---|---|---|
| Large surface area | More area = more simultaneous diffusion events = higher total exchange rate (Fick: SA ↑) | Rate too slow to meet O₂ demand — cells starve of O₂ despite adequate breathing | ~500 million alveoli · ~250m² total · three levels of folding in small intestine (same principle) |
| Thin membrane | Short diffusion distance = fast exchange (Fick: membrane thickness ↓ = rate ↑) | Gases too slow to cross — inflammation (e.g. pneumonia) thickens the membrane, slowing exchange even when SA is normal | Alveolar epithelium (type I cells, ~0.2 μm) + capillary endothelium = combined ~0.5 μm total diffusion distance |
| Moist surface | Gases dissolve in the water layer before diffusing through the membrane — O₂ and CO₂ are not directly membrane-permeable in gaseous form | Dry surfaces → gas exchange impossible (insects that dry out suffocate; fish removed from water suffocate partly for this reason) | Alveolar lining fluid (surfactant + water) · surfactant also reduces surface tension preventing alveolar collapse |
| Maintained concentration gradient | Diffusion rate proportional to gradient (Fick: concentration gradient ↑). Without maintenance, gradient equilibrates and diffusion stops | Without ventilation: alveolar O₂ falls, CO₂ rises → gradient collapses. Without circulation: tissue-side gradient collapses | Ventilation refreshes alveolar air · Blood flow continuously removes O₂ from blood (delivers to tissues) and delivers CO₂ to alveoli |
Activities
Use the partial pressure data table from Card 1 to answer the following questions.
For each condition, identify which Fick variable is affected, state the direction of change, and predict the overall effect on gas exchange rate.
| Condition | Fick Variable Affected | Direction of Change | Effect on Gas Exchange Rate |
|---|---|---|---|
| Emphysema — alveolar walls break down, many alveoli merge into fewer, larger spaces | |||
| Pulmonary oedema — fluid accumulates in the space between alveoli and capillaries | |||
| Anaemia — reduced haemoglobin concentration in blood | |||
| Exercise training — lungs develop increased alveolar density and capillary network |
"Explain how the structure of the alveolus is adapted to enable efficient gas exchange. In your answer, refer to at least three structural features and explain how each contributes to gas exchange efficiency." (6 marks)
Band 6 structure: Feature → structural description → Fick variable → functional consequence. Three features × 2 marks each. Ventilation/blood flow as gradient maintenance is your fourth point for full marks.
Assessment
Select the best answer — feedback shown immediately
1. The partial pressure of O₂ in alveolar air is approximately 100 mmHg, but in arterial blood leaving the lungs it is approximately 95 mmHg. Which of the following correctly explains the 5 mmHg difference?
2. In emphysema, alveolar walls break down and many alveoli merge into fewer, larger air spaces. Which of the following best predicts the consequence for gas exchange, using Fick's law?
3. Which of the following correctly explains why continuous ventilation is essential for maintaining gas exchange, even when the body is at rest?
4. During intense exercise, the partial pressure of O₂ in venous blood drops from ~40 mmHg (at rest) to ~15 mmHg. What is the most likely explanation for this change?
5. Which combination of properties best explains why the alveolus can exchange large amounts of gas very quickly?
6. Explain the role of blood flow in maintaining efficient gas exchange across the alveolar membrane. In your answer, describe what would happen to gas exchange if blood flow stopped. 3 MARKS
7. Use Fick's law to explain how pulmonary oedema (fluid accumulation between alveoli and capillaries) impairs gas exchange. 3 MARKS
8. Distinguish between external gas exchange and internal gas exchange. In your answer, state where each occurs, which gases move in which direction, and what drives each exchange. 4 MARKS
Two exchange zones × two marks each: location + direction + driving gradient.
1. C — This is the ventilation-perfusion mismatch phenomenon. Not every alveolus receives perfectly matched ventilation and blood flow. Some areas of the lung have slightly impaired ventilation or slight anatomical variations — blood from these areas is slightly less oxygenated. When all pulmonary blood mixes in the pulmonary veins, the average O₂ is fractionally below the ideal alveolar level.
2. B — Emphysema destroys alveolar walls, merging many small alveoli into fewer large spaces. Total surface area falls dramatically (potentially from ~250m² to ~30m²). By Fick's law, rate ∝ SA — a massive reduction in SA causes a proportional drop in diffusion rate. The remaining individual alveoli may function normally, but there are far fewer of them.
3. D — Ventilation's role is gradient maintenance. Without ventilation, O₂ would be consumed from alveolar air by diffusion into blood but never replaced, so alveolar pO₂ would fall. CO₂ from blood would accumulate in alveolar air, raising alveolar pCO₂. Both changes collapse the partial pressure gradients that drive diffusion. Ventilation does not mechanically push gases across membranes — diffusion is passive and driven by the gradient ventilation maintains.
4. A — During intense exercise, muscles dramatically increase cellular respiration rate, consuming O₂ rapidly from capillary blood. Tissue pO₂ drops to ~15 mmHg. This actually increases the gradient from blood to tissue (~95 − 15 = 80 mmHg vs ~95 − 30 = 65 mmHg at rest), accelerating O₂ delivery — a positive feedback loop that meets increased demand. The low venous pO₂ is a consequence of greater O₂ extraction, not reduced loading at the lungs.
5. B — Fick's law: rate ∝ (SA × concentration gradient) / membrane thickness. Large SA ↑ rate, thin membrane ↑ rate, maintained gradient ↑ rate. All three must be simultaneously maximised for highest exchange rate — which is exactly what the alveolus achieves.
Blood flow maintains the concentration gradient driving O₂ from alveolar air into blood by continuously removing O₂-loaded blood from the pulmonary capillaries and replacing it with deoxygenated blood from the systemic circuit. This keeps the pO₂ of blood entering pulmonary capillaries low (~40 mmHg), maintaining a large gradient relative to alveolar air (~100 mmHg). Simultaneously, blood flow delivers CO₂-rich venous blood to the alveoli, maintaining the CO₂ gradient driving CO₂ from blood into alveolar air for exhalation.
If blood flow stopped, blood in the pulmonary capillaries would rapidly equilibrate with alveolar air — pO₂ in blood would rise until it equalled alveolar pO₂ (~100 mmHg) and pCO₂ would equilibrate at ~40 mmHg. With no partial pressure gradient remaining, diffusion would cease entirely. No further O₂ loading or CO₂ unloading would occur. Cardiac arrest produces exactly this outcome — stopping blood flow collapses both alveolar and tissue gas exchange gradients simultaneously.
Fick's law states that rate of diffusion is inversely proportional to membrane thickness: Rate ∝ (SA × concentration gradient) / membrane thickness. In pulmonary oedema, fluid accumulates in the interstitial space between the alveolar epithelium and pulmonary capillary endothelium. This effectively increases the diffusion distance (membrane thickness) that O₂ and CO₂ must cross — adding a layer of fluid several micrometres thick to the normal ~0.5 μm barrier.
By Fick's law, an increase in membrane thickness causes a proportional decrease in diffusion rate. Less O₂ crosses per unit time despite normal alveolar O₂ levels and normal blood flow. The patient experiences hypoxaemia (low blood O₂) and breathlessness — their lungs contain normal air but cannot transfer it efficiently into the blood.
External gas exchange occurs at the alveolar surface — the interface between the internal environment (blood in pulmonary capillaries) and the external environment (alveolar air). O₂ diffuses from alveolar air (pO₂ ~100 mmHg) into pulmonary capillary blood (pO₂ ~40 mmHg arriving), driven by a ~60 mmHg partial pressure gradient. CO₂ simultaneously diffuses from blood (pCO₂ ~45 mmHg) into alveolar air (pCO₂ ~40 mmHg), driven by a ~5 mmHg gradient. Both gradients are maintained by ventilation refreshing alveolar air and blood flow cycling venous blood through the capillaries.
Internal gas exchange occurs at systemic capillaries — the interface between blood and body tissues. O₂ diffuses from capillary blood (pO₂ ~95 mmHg) into tissue cells (pO₂ ~20–30 mmHg), driven by a ~65–75 mmHg gradient maintained by continuous cellular respiration consuming O₂. CO₂ diffuses from tissue cells (pCO₂ ~50+ mmHg) into blood (pCO₂ ~40 mmHg), driven by a ~10 mmHg gradient. Both gradients are maintained by tissue metabolism and continuous blood flow through the systemic circuit.
The key distinction is location and what maintains each gradient: external exchange is maintained by ventilation (refreshing alveolar air) and pulmonary blood flow; internal exchange is maintained by cellular metabolism (consuming O₂, producing CO₂) and systemic blood flow.
Tick when you've finished all activities and checked your answers.