Breathing mechanics and alveolar gas exchange

MCAT content library / Biology & biochemistry / Breathing mechanics and alveolar gas exchange

Use pressure differences, curve slopes and controlled gas gradients to explain how lungs expand and gases equilibrate.

About 17 minutes, including the worked examples and checks.

Useful foundations

• Membrane transport and osmosis

On this page

• Ask what is moving, and across which boundary
• Concept: distinguish the two pressure differences
• Structure explains the relevant forces and barriers
• Worked application: measure expansion and exchange separately
• Rule: compare the gradient with the endpoint
• Application: convert partial pressure into dissolved concentration
• Check your reasoning
• Continue learning
• Sources

Ask what is moving, and across which boundary

Breathing brings a gas mixture into contact with an exchange surface. Ventilation moves air through airways; diffusion transfers individual gases across the alveolar-capillary barrier. A change in one process does not establish a change in the other. The MCAT connects these mechanisms through pressure, lung structure, surface tension and gas solubility.¹

Conducting airways carry air toward the respiratory zone, where alveoli provide the exchange surface. Their branching arrangement also matters experimentally: a change in total air movement can arise upstream of an intact exchange membrane. Conversely, a membrane can transfer gas slowly even when its gas-facing side receives an adequate supply.²

Concept: distinguish the two pressure differences

With an open airway, air enters when atmospheric pressure exceeds alveolar pressure and leaves when the inequality reverses. During ordinary inspiration, diaphragm contraction moves the diaphragm downward and external intercostal activity expands the rib cage. Expansion lowers alveolar pressure enough to draw air inward. Quiet expiration mainly uses elastic recoil as these muscles relax; forced expiration can recruit additional muscles.³

The pressure that distends lung tissue is a different comparison. Here define P_dist = P_alv − P_pl, where P_pl is pleural pressure. This alveolar-minus-pleural definition is often called transpulmonary pressure. At a no-flow pause, alveolar pressure may equal atmospheric pressure while positive P_dist still holds the lung expanded. Use one common pressure reference before subtracting; a negative gauge pressure means below atmosphere, not negative absolute pressure.⁶

Airway resistance limits flow for a given airway pressure difference. Static compliance instead describes how much lung volume changes per change in distending pressure: C = ΔV/ΔP_dist. A steep volume-versus-pressure slope indicates greater compliance over that interval. It does not, by itself, measure how quickly air can traverse the airways.³⁶

Structure explains the relevant forces and barriers

Thin type I alveolar epithelial cells and nearby capillary endothelium form a short diffusion path. Type II cells secrete surfactant, which reduces surface tension at the air-liquid interface. Surface tension otherwise contributes an inward recoil; reducing it makes expansion easier and helps stabilize alveoli. Surfactant does not actively pump oxygen across the membrane.²³

The conducting airways also filter, warm and humidify incoming air. Mucus traps material, and cilia move it toward the throat. Alveolar macrophages can remove material reaching the exchange region. Warming and adding water transfer heat and moisture to inspired air; the nasal passages recover some water during expiration. These protective and conditioning functions are distinct from diffusion of oxygen or carbon dioxide.²

Worked application: measure expansion and exchange separately

The following original data describe two simplified lung models, A and B, at a fixed temperature. First, volume is measured after each no-flow pause. The same measurement protocol is used, and the displayed pressure-volume segments are stipulated to be linear only over the tested interval.

Distending pressure (cm H₂O)	A volume (L)	B volume (L)
5	2.0	2.0
10	3.0	2.5
15	4.0	3.0

For A, C = (4.0 − 2.0)/(15 − 5) = 0.20 L/cm H₂O. For B, C = (3.0 − 2.0)/10 = 0.10 L/cm H₂O. B gains half as much volume for the same pressure increment. These static results establish lower compliance in B over this range, but do not identify an airway obstruction or a particular tissue alteration. A single volume at one pressure would not supply the slope.

A separate exchange assay uses a nonreactive tracer gas G that neither binds a carrier nor undergoes metabolism. Its gas-side partial pressure is held at 100 mmHg. A liquid stream enters with a G partial pressure of 40 mmHg. Temperature, liquid composition, flow and contact time are matched. The experiment independently varies effective membrane exchange, rather than assuming that the measured compliance change causes it.

The supplied illustrative curves are P_liquid(t) = 100 − 60e^−kt, with t in seconds, k = 4 s⁻¹ for A and 1 s⁻¹ for B. This defined model generates the table; the lesson does not require deriving it or treating it as a universal oxygen-transport equation.

Contact time (s)	A: G pressure (mmHg)	B: G pressure (mmHg)
0	40.0	40.0
0.25	77.9	53.3
0.5	91.9	63.6
0.75	97.0	71.7

At 0.75 s, A reaches 97.0 mmHg and B reaches 71.7 mmHg. Their remaining gas-to-liquid differences are about 3.0 and 28.3 mmHg. B has not approached the maintained gas-side value as closely during the available time. Since gas supply, flow and solubility are controlled, this supports slower exchange under the defined conditions. It does not follow merely from B’s shallower pressure-volume slope.

Rule: compare the gradient with the endpoint

Each gas diffuses according to its own partial-pressure difference. Greater exchange area and a shorter diffusion path favor transfer when other conditions are fixed. For a given membrane and gas, a smaller driving difference reduces net transfer. Total gas pressure alone does not tell you whether oxygen will move into or out of a liquid.⁴

Suppose an endpoint is low. Check the gas-side pressure before assigning a membrane problem. A low endpoint that nearly equals a low gas-side pressure differs from an endpoint that remains far below a maintained high gas-side pressure. The first observation may reflect the supplied gas environment; the second demonstrates incomplete equilibration during the measured interval. Neither pattern alone identifies a specific biological lesion.

Application: convert partial pressure into dissolved concentration

For a gas obeying Henry’s law at a specified temperature and in a specified liquid, C_dissolved = k_HP_gas. The coefficient’s definition and units matter: some tables use its reciprocal. For an original example with k_H = 0.020 mmol L⁻¹ kPa⁻¹, equilibrium with 12 kPa gives 0.24 mmol/L; 24 kPa gives 0.48 mmol/L. This describes dissolved gas under the stated convention, not the total amount carried by reactive or binding species.⁵

Use the same sequence for a new passage: identify the compartment and pressure reference, select the relevant slope or gradient, then state what the controls allow you to infer. A change in airflow, tissue expansion and gas equilibration can be tested separately even when all three occur in one respiratory system.

Check your reasoning

Choose an answer before opening its feedback. Each option explains the reasoning behind that choice.

Continue learning

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Sources

1. AAMC — B/B Content Category 3B

2. OpenStax Anatomy and Physiology 2e — 22.1 Organs and Structures of the Respiratory System

3. OpenStax Anatomy and Physiology 2e — 22.3 The Process of Breathing

4. OpenStax Anatomy and Physiology 2e — 22.4 Gas Exchange

5. OpenStax Chemistry 2e — 11.3 Solubility

6. Amarillo College — RSPT 1307 Respiratory Physiology: Ventilation