Breathing and Exchange of Gases · Lesson 4 of 9
Gas Exchange at Alveoli and Body Tissues
“Analyse how partial-pressure gradients, gas solubility and the alveolar membrane produce directional diffusion of oxygen and carbon dioxide.”
• Define partial pressure and compare values in alveoli, blood and tissues. • Predict diffusion directions for oxygen and carbon dioxide. • Explain how barrier structure, solubility and membrane thickness influence exchange. • Interpret gas-pressure values given in mm Hg.
Why air does not need to be pushed across alveolar walls
Ventilation brings fresh air into microscopic alveolar spaces, but transfer of oxygen into blood does not need a pump in the wall of each alveolus. Individual molecules cross the alveolar–capillary barrier by diffusion. Net diffusion occurs from a region of higher partial pressure of a given gas towards a region of lower partial pressure of that gas, provided the barrier allows passage. Carbon dioxide diffuses in the opposite direction because its pressure difference is reversed. Oxygen and carbon dioxide therefore follow their own gradients simultaneously across the same surface.
Partial pressure describes each gas in a mixture
Air contains nitrogen, oxygen, carbon dioxide, water vapour and other gases. The partial pressure of one constituent is the pressure it would contribute to the mixture under the same conditions. It is written pO₂ for oxygen and pCO₂ for carbon dioxide. The total pressure is the sum of constituent partial pressures for an ideal gas mixture. In physiological tables the unit mm Hg is often used. Partial pressure is the useful quantity for predicting direction of gas movement, not merely the percentage by mass of gas dissolved in blood.
Consider the source values for an illustrative sea-level condition. Atmospheric pO₂ is approximately 159 mm Hg, whereas alveolar pO₂ is 104 mm Hg; the alveoli contain humidified and mixed air, not precisely the same gas composition as the outer atmosphere. Deoxygenated blood arriving at the lungs has pO₂ near 40 mm Hg, so oxygen diffuses from alveoli to blood. Blood leaving the pulmonary exchange region is shown at about 95 mm Hg, reflecting realistic mixing rather than a claim that arterial pO₂ must equal alveolar pO₂ exactly.
For carbon dioxide, alveolar pCO₂ is around 40 mm Hg while incoming deoxygenated blood has about 45 mm Hg. CO₂ therefore diffuses into the alveolus. At systemic tissues, blood arriving with pO₂ near 95 mm Hg releases oxygen to tissues around 40 mm Hg; tissue CO₂ near 45 mm Hg moves into blood around 40 mm Hg. The direction of gas movement must be derived separately for each gas; a general statement that all gases move from the same compartment to the next is incorrect.
| Location | pO₂ (mm Hg) | pCO₂ (mm Hg) |
|---|---|---|
| Atmospheric air | 159 | 0.3 |
| Alveolar air | 104 | 40 |
| Deoxygenated blood reaching lungs | 40 | 45 |
| Oxygenated systemic arterial blood | 95 | 40 |
| Body tissues (representative) | 40 | 45 |
Why the alveolar diffusion barrier is effective
The alveolar wall is principally a very thin layer of squamous epithelial cells, associated basement-membrane material and an equally thin capillary endothelial layer. These surfaces lie closely together and separate an enormous combined air-exposed area from a dense capillary network. The diffusion distance is normally only micrometres, far less than a millimetre. An increased surface area, a thin barrier and continuing blood flow help keep the required gradients and support efficient exchange.
A subtle point is that the capillary blood itself is not a static pool. As oxygen enters blood, haemoglobin binding helps keep the amount of free dissolved oxygen relatively low until more oxygen can enter, and circulation carries the newly oxygenated blood away. Fresh deoxygenated blood replaces it. Likewise ventilation removes CO₂-rich gas from alveoli and brings in air with low CO₂. Gas exchange depends on both diffusion through the thin membrane and bulk transport of gases on either side.
Gas solubility also matters
Carbon dioxide is much more soluble in aqueous biological fluids than oxygen; the textbook gives a comparison of roughly twenty to twenty-five times. Therefore a smaller CO₂ partial-pressure gradient can support substantial CO₂ movement. The steepness of a pressure difference alone does not tell the entire diffusion rate. Surface area, the permeability and thickness of the membrane, gas solubility and the amount of time blood remains in contact with alveoli also influence exchange.
A thickened or damaged alveolar barrier can make gas exchange less efficient even when breathing movements are normal. Reduced surface area, for example after destruction of alveolar walls, can similarly decrease overall transfer capacity. This explains why airway airflow, gas diffusion and blood transport should be analysed as separate stages that must all operate together. The body cannot compensate indefinitely for poor diffusion by simply breathing more frequently.
A method for solving partial-pressure questions
Write the two compartments and the pO₂ or pCO₂ values for a single gas. Draw an arrow from higher to lower partial pressure. Then repeat for the second gas without assuming it follows the first arrow. At the lungs, oxygen goes alveolus → blood while CO₂ goes blood → alveolus. At the tissues, oxygen goes blood → tissues while CO₂ goes tissues → blood. Finish by explaining why the thin membrane, solubility and circulation make the movement possible.
Partial pressures shown in the source are typical illustrative values, not fixed constants for every altitude or physiological condition. Diffusion direction is determined from the relative values at the actual exchange site.
Quick Quiz
Quiz
If alveolar pO₂ is 104 mm Hg and incoming venous pO₂ is 40 mm Hg, where does oxygen move?
Why does CO₂ move from systemic tissues into blood?
Which structures together form the principal alveolar diffusion barrier?
Why can CO₂ diffuse effectively with a relatively small pressure gradient?
How does a greatly thickened alveolar barrier usually affect exchange?
Practice Problems
- Explain the directions of oxygen and carbon dioxide diffusion at the lungs using source partial pressures.
- Explain gas diffusion at systemic tissues using oxygenated blood and tissue values.
- Why can alveolar pO₂ differ from atmospheric pO₂?
- Describe the three main layers of the alveolar-capillary diffusion barrier.
- Why is oxygen exchange affected by blood flow as well as alveolar membrane thickness?
Model Answers
1. Alveolar O₂ 104 > incoming blood O₂ 40, so O₂ moves into blood. Incoming blood CO₂ 45 > alveolar CO₂ 40, so CO₂ moves into alveoli.
2. Blood pO₂ around 95 exceeds tissue pO₂ around 40, so O₂ moves to tissues. Tissue pCO₂ about 45 exceeds incoming arterial pCO₂ about 40, so CO₂ moves into blood.
3. Inspired air is humidified and mixes with residual alveolar gas while O₂ is absorbed and CO₂ is released, so alveolar composition is different from dry atmospheric air.
4. Thin squamous alveolar epithelium, associated basement-membrane region and capillary endothelium, with a very small diffusion distance.
5. Circulation brings deoxygenated blood and removes oxygenated blood, helping maintain gradients; a thin barrier alone would not sustain exchange without continuing perfusion.
Key Takeaways
• O₂ moves alveoli → blood → tissues; CO₂ diffuses in the opposite direction at both exchange sites. • A gas moves down its own partial-pressure gradient, not necessarily in the same direction as another gas. • The alveolar wall is thin, extensive and richly supplied with capillaries. • CO₂ is substantially more soluble than O₂, supporting its effective exchange with smaller gradients. • Partial pressures in mm Hg describe representative conditions and change with environment and physiology.