Gas Exchange Between Internal and External Environments
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.
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Your lungs contain millions of tiny air sacs. Before reading on, what do you think makes them so effective at getting oxygen into your blood? Think about the features that help a gas cross from air into a liquid — what would matter most?
Know
- Define partial pressure and explain how it drives gas exchange
- Trace O₂ and CO₂ movements from external air to mitochondria
- Explain the role of the cardiovascular system in maintaining gradients
- Compare gas exchange efficiency across different respiratory structures
- Explain the four universal features of gas exchange surfaces
Understand
- Explain gas exchange between the internal and external environments
- Relate surface area, membrane thickness, and concentration gradient to exchange rate
- Connect: L10 (respiratory structures), L13 (blood), L14 (cardiovascular circuit)
- IQ3: how transport medium composition changes
Can Do
- State the partial pressure of O₂ and CO₂ at five locations in the gas exchange pathway
- Explain why blood flow direction matters for maintaining concentration gradients
- Apply Fick's law to predict how changes affect gas exchange rate
- Explain why the alveolar surface area must be ~250m² and what happens if it falls
- Write a Band 6 response linking structure to function in alveolar gas exchange
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
Misconceptions to Fix
Wrong: Oxygen diffuses into blood because blood has low oxygen content.
Right: Diffusion is driven by partial pressure gradients, not absolute amounts. Oxygen diffuses from alveoli (PO₂ ≈ 100 mmHg) into arterial blood (PO₂ ≈ 95 mmHg) because of the small gradient. Even oxygenated blood has a lower PO₂ than alveolar air, so diffusion continues. The key is the gradient, not the total oxygen content.
The Data — Before the Theory
Read the numbers. Spot the patterns. Then we'll explain what you're seeing.
Where does oxygen go — and what drives it there?
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 |
Figure: Partial pressure of O₂ (red bars, decreasing) and CO₂ (blue bars, increasing) at each location. Bar length is proportional to partial pressure; O₂ scale: 159 mmHg = full bar; CO₂ scale: 50 mmHg = full bar.
Your prediction — before reading on: What pattern do you see? What direction is each gas moving at each step, and why?
The Mechanism
Reading the Data — Partial Pressure and Fick's Law
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.
The oxygen journey — each step has a lower partial pressure, driving diffusion forward
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
The Complete Gas Exchange Pathway — O₂ from Air to Mitochondrion
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).
Figure: Six-step gas exchange pathway. Steps 1 and 4 = bulk flow (long distances); Steps 2 and 5 = diffusion (short distances at exchange surfaces).
Ventilation — Bulk Flow to Alveoli (Not diffusion)
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.
Alveolar → Blood Diffusion (External gas exchange)
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 Binding to Haemoglobin in Red Blood Cells
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.
Cardiovascular Transport — Bulk Flow Through Vessels
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.
Blood → Tissue Diffusion (Internal gas exchange)
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 → Mitochondria: Cellular Respiration
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.
Maintaining the Gradients — Why Breathing and Blood Flow Never Stop
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.
Surface area: ↓↓ dramatically (fewer alveoli, less total surface area — may fall from 250m² to as low as 30m²)
Membrane thickness: ↑ (inflammation, mucus, thickening of remaining walls)
Concentration gradient: also impaired (reduced ventilation in damaged regions)
All three Fick variables move in the wrong direction simultaneously. Gas exchange rate collapses. Patients breathe room air but cannot get enough O₂ — they need supplemental O₂ therapy. This is why smoking is so directly damaging: it destroys the structural basis of Fick's law in your lungs.
Four Universal Features of All Gas Exchange Surfaces
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.
Large surface area
Thin membrane
Moist surface
Maintained concentration gradient
Copy into your books
▼Partial Pressure Values to Know
- Atmospheric O₂: 159 mmHg · CO₂: 0.3 mmHg.
- Alveolar O₂: 100 mmHg · CO₂: 40 mmHg.
- Arterial blood: O₂ ~95 mmHg · CO₂ ~40 mmHg.
- Venous blood (rest): O₂ ~40 mmHg · CO₂ ~45 mmHg.
- Tissue cells: O₂ ~20–30 mmHg · CO₂ ~50+ mmHg.
Fick's Law
- Rate ∝ (SA × concentration gradient) / membrane thickness.
- SA ↑ → more simultaneous diffusion → rate ↑.
- Gradient ↑ → steeper driving force → rate ↑.
- Thickness ↓ → shorter distance → rate ↑.
4 Universal Gas Exchange Features
- Large SA (SA ↑ in Fick's law).
- Thin membrane (thickness ↓ in Fick's law).
- Moist surface (gases dissolve before diffusing).
- Maintained gradient (ventilation + blood flow).
Bulk Flow vs Diffusion
- Diffusion: passive, short distances, gas exchange surfaces.
- Bulk flow: active/mechanical, long distances, breathing + circulation.
- Both required: diffusion can't cover metres; bulk flow can't exchange.
Activities
Interpreting Partial Pressure Data — Returning to the Table
Use the partial pressure data table from Card 1 to answer the following questions.
- At which location in the pathway is the driving force for O₂ diffusion greatest — alveoli to blood, or blood to tissue cells? Use specific values to justify your answer.
- The data shows venous blood at rest has a pO₂ of 40 mmHg. During intense exercise, this value drops to approximately 15 mmHg. Explain why this change occurs and predict the effect on O₂ delivery rate to active muscle cells.
- Atmospheric air has a pO₂ of 159 mmHg, but alveolar air has only 100 mmHg. Explain why alveolar O₂ is always lower than atmospheric O₂, and why this matters for the gas exchange gradient.
- A mountain climber at altitude experiences atmospheric pO₂ of approximately 75 mmHg (compared to 159 mmHg at sea level). Using Fick's law, explain how this would affect gas exchange across the alveolar membrane, and predict the physiological response.
Fick's Law Application — Clinical Scenarios
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 |
Extended Response — Alveolar Structure and Gas Exchange
"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.