Your lungs have the surface area of a tennis court folded into your chest. Your small intestine, uncoiled, would stretch to 6 metres. Both are solving the same fundamental problem: moving substances across a barrier fast enough to keep you alive.
Before reading on, make a prediction:
A single-celled organism like Amoeba can exchange all its gases and nutrients directly through its cell membrane. A human cannot. Predict: what changes as an organism gets larger that makes direct surface exchange impossible, and what structural solutions might evolve to solve this problem?
Come back to this at the end of the lesson.
Core Content
A single cell exchanges substances by diffusion across its surface membrane. This works because the cell is small — the distance from surface to centre is tiny, and the surface area relative to the cell's volume is large enough to supply all its needs.
As an organism grows larger, volume increases faster than surface area. Double the linear dimensions of a cube and its surface area increases by a factor of four — but its volume increases by a factor of eight. The SA:V ratio halves. A large organism relying on surface diffusion alone would have far too little surface relative to the volume it needs to supply — cells in the interior would be starved of oxygen and nutrients.
The evolutionary solution: fold, subdivide, and specialise. Create internal exchange surfaces with enormous total area, positioned close to transport systems that distribute substances to and from every cell.
The rate of diffusion across an exchange surface is described by Fick's Law:
Every adaptation of a biological exchange surface can be traced back to this equation — maximising surface area, maximising concentration gradient, or minimising diffusion distance.
More area = more molecules can diffuse simultaneously. Achieved by folding (villi, microvilli, alveoli), branching, or flattening. The human lung has ~70 m² of alveolar surface.
Alveolar walls are ~0.2 µm thick — one epithelial cell. Capillary walls are one endothelial cell. Total diffusion distance across both: ~0.5 µm. Minimised at every step.
Maintained by continuous blood flow removing diffused molecules from one side, and ventilation delivering fresh molecules to the other. Without this, gradient would equilibrate and diffusion would stop.
Gases must dissolve in water before diffusing through cell membranes. All gas exchange surfaces are kept moist — alveoli are lined with a thin film of fluid containing surfactant to reduce surface tension.
Human lungs contain approximately 300–500 million alveoli — tiny air sacs clustered at the end of bronchioles. Each alveolus is surrounded by a dense capillary network. The combination produces ~70 m² of gas exchange surface — packed into a chest cavity.
| Alveolar Feature | How it maximises exchange | Fick's Law variable |
|---|---|---|
| Huge number of alveoli (300–500 million) | Massive total surface area for simultaneous diffusion across entire lung | Surface area ↑ |
| Thin alveolar epithelium (one cell, ~0.2 µm) | Minimises diffusion distance from air to blood | Diffusion distance ↓ |
| Dense capillary network surrounding each alveolus | Blood continuously removes O₂ and delivers CO₂, maintaining gradient; short distance between alveolar air and blood | Gradient ↑, distance ↓ |
| Moist lining with pulmonary surfactant | Gases dissolve before crossing membrane; surfactant prevents alveolar collapse by reducing surface tension | Enables diffusion |
| Continuous ventilation (breathing) | Refreshes air in alveoli, maintaining high O₂ and low CO₂ on the air side — steepens concentration gradient | Gradient ↑ |
The small intestine must absorb digested nutrients — glucose, amino acids, fatty acids, vitamins, minerals — across its wall into the bloodstream. The same Fick's Law constraints apply: maximise surface area, minimise distance.
| Villus Feature | How it maximises absorption |
|---|---|
| Villi (finger-like projections, ~1 mm tall) | Increase internal surface area of small intestine ~10-fold compared to a smooth tube |
| Microvilli (brush border) on each epithelial cell | Further increase surface area ~20-fold — total increase ~200-fold over a smooth tube |
| Single epithelial cell layer | Minimises diffusion distance from intestinal lumen to capillary blood |
| Dense capillary network inside each villus | Removes absorbed glucose and amino acids immediately — maintains steep concentration gradient |
| Lacteals (lymph capillaries) inside each villus | Absorb fatty acids and glycerol (as chylomicrons) that cannot enter blood capillaries directly |
| Carrier proteins and active transport pumps | Allow absorption against concentration gradient — glucose and amino acids are actively transported even when blood concentration is already high |
Misconception: Larger organisms have more surface area, so they don't need specialised exchange surfaces.
While larger organisms have more absolute surface area, their SA:V ratio is lower. The volume needing to be supplied grows faster than the surface through which exchange can occur. A blue whale has far more surface area than a bacterium — but a far worse SA:V ratio. Specialised exchange organs exist precisely because the body surface is insufficient.
Misconception: O₂ is actively pumped across the alveolar membrane.
Gas exchange in the lungs occurs entirely by passive diffusion — no energy (ATP) is required. O₂ diffuses from high partial pressure in the alveolus to low partial pressure in the blood; CO₂ diffuses in the opposite direction. The steep concentration gradient is maintained by ventilation and blood flow, not by active transport.
Misconception: The villi absorb all nutrients by diffusion.
Small lipid-soluble molecules (fatty acids, glycerol) cross by simple diffusion. However, glucose and amino acids are absorbed by active transport — they are moved against their concentration gradient using ATP-powered carrier proteins. This allows the intestine to absorb nutrients even when blood concentration is already high.
Rate ∝ (SA × concentration gradient) / diffusion distance
Activities
A student models organisms as cubes of different sizes and measures the time for a coloured dye (representing oxygen) to diffuse to the centre. Results are below.
| Cube side length (mm) | SA (mm²) | Volume (mm³) | SA:V ratio | Time for dye to reach centre (min) |
|---|---|---|---|---|
| 2 | 24 | 8 | 3.0 | 4 |
| 4 | 96 | 64 | 1.5 | 16 |
| 6 | 216 | 216 | 1.0 | 36 |
| 8 | 384 | 512 | 0.75 | 64 |
Write your responses here or in your book.
Fish gills extract dissolved oxygen from water as water flows over them. A gill consists of gill arches, each bearing rows of gill filaments. Each filament is covered in thin, plate-like lamellae — the actual gas exchange surface. Blood flows through the lamellae in the opposite direction to water flow (countercurrent exchange).
Write your responses here or in your book.
Assessment
1. According to Fick's Law, which of the following changes would DECREASE the rate of diffusion across an exchange surface?
2. A cube has a side length of 3 cm. What is its SA:V ratio?
3. Pulmonary surfactant lines the alveoli. Its primary function is to:
4. Emphysema destroys alveolar walls, merging small alveoli into larger spaces. The primary consequence for gas exchange is:
5. Glucose absorption in the small intestine requires active transport rather than simple diffusion. The most likely reason is that:
1. Explain why large, multicellular animals require specialised exchange surfaces. In your answer, refer to surface area to volume ratio. (3 marks)
1 mark SA:V concept explained; 1 mark how SA:V changes with size; 1 mark why specialised surfaces are needed as a consequence
2. Describe how the structure of the alveolus maximises the rate of gas exchange. Refer to Fick's Law in your answer. (3 marks)
1 mark per Fick's Law variable (SA, gradient, distance) correctly linked to an alveolar structural feature
3. Compare the effect of asthma and emphysema on gas exchange efficiency. In your answer, use Fick's Law to explain why each condition reduces O₂ uptake, and explain why emphysema is considered more serious in the long term. (3 marks)
1 mark asthma mechanism (gradient); 1 mark emphysema mechanism (SA); 1 mark comparison of reversibility/severity
Answers
SA1: The SA:V ratio describes how much surface area is available relative to the volume of an organism — essentially, how much exchange surface exists per unit of living tissue. As an organism increases in size, its volume increases proportionally faster than its surface area (volume ∝ length³ while surface area ∝ length²). This means the SA:V ratio decreases as organisms get larger. A large multicellular animal therefore has insufficient body surface area to exchange gases and nutrients for all its internal cells by surface diffusion alone — cells in the interior would receive no oxygen. Specialised exchange organs (lungs, gills, intestines) solve this by creating an internal surface with a very large total area, positioned close to transport systems (blood) that carry substances to and from every cell.
SA2: Surface area is maximised by the presence of 300–500 million alveoli, creating approximately 70 m² of total exchange surface in the lung — a large SA term in Fick's Law increases the rate of simultaneous diffusion. The concentration gradient is maintained by continuous ventilation (replacing O₂-depleted air with fresh air on the alveolar side) and by constant blood flow through surrounding capillaries (removing O₂ as soon as it diffuses in, and delivering fresh CO₂) — keeping both gradients steep. Diffusion distance is minimised by the extremely thin alveolar epithelium (~0.2 µm, one cell thick) and the thin capillary wall immediately adjacent, giving a total diffusion distance of approximately 0.5 µm — the smallest possible distance across which exchange can occur.
SA3: Asthma reduces gas exchange efficiency primarily by narrowing the airways through inflammation and bronchoconstriction. Less fresh air reaches the alveoli per breath, reducing the O₂ partial pressure on the alveolar side. This decreases the concentration gradient across the alveolar membrane (the gradient term in Fick's Law), reducing the rate of O₂ diffusion into the blood. The alveolar surface itself remains structurally intact. Emphysema reduces gas exchange differently — progressive destruction of alveolar walls merges small alveoli into fewer, larger spaces, dramatically reducing total surface area (the SA term in Fick's Law). A person with severe emphysema may have only ~30 m² of exchange surface remaining. Emphysema is more serious in the long term because the structural damage is irreversible — lost alveoli cannot be regenerated. Asthma, while debilitating, can be managed with bronchodilators and anti-inflammatory medications that restore airway diameter and therefore restore the concentration gradient. Emphysema's lost surface area cannot be restored by any current treatment.
You predicted what changes as organisms get larger that makes direct surface exchange impossible, and what solutions might evolve.
The answer: as size increases, SA:V ratio decreases — the surface becomes proportionally too small to supply the volume. The evolutionary solutions are precisely what you encounter in every large organism: folded internal surfaces (alveoli, villi), close to a transport system (blood) that distributes materials, with thin membranes minimising diffusion distance and flow maintaining gradients. Every large organism independently evolved some version of this same solution.
If you predicted "the centre becomes too far from the surface for diffusion to reach" — that's exactly right. If you predicted "some kind of internal distribution system" — also right, that's the circulatory system working in conjunction with exchange surfaces.