Exchange of gases by diffusion

A drop of ink

Put a drop of ink into a glass of still water. The colour spreads until the whole glass is the same. Nobody stirred it. The ink moved from the place where it was crowded to the places where it was thin.

Gases do the same. They move from where their pressure is higher to where it is lower. This is simple diffusion. It needs no energy. The oxygen that you breathe in reaches your blood in this way, and so does the carbon dioxide that goes the other way.

Where does the exchange happen?

Gases are exchanged at two places.

• In the alveoli. Oxygen goes from the alveolar air into the blood. Carbon dioxide goes from the blood into the alveolar air.
• Between the blood and the tissues. Oxygen goes from the blood into the cells. Carbon dioxide goes from the cells into the blood.

The alveoli are the primary sites of exchange between the air and the blood.

Partial pressure

Air is a mixture of gases. Each gas in a mixture presses on its own. The pressure that one gas contributes to the total is its partial pressure. We write $p_{O_2}$ and $p_{CO_2}$, or pO$_2$ and pCO$_2$.

The partial pressure of a gas is its share of the total pressure. Air at sea level has a pressure of 760 mm Hg. About 20.95 per cent of the air is oxygen, so

Carbon dioxide is only about 0.04 per cent of the air, so its partial pressure is about 0.3 mm Hg.

The partial pressures in the body

The table gives the values that the textbook uses, in mm Hg.

Gases always move from the higher partial pressure to the lower one. We can now read the table.

• Oxygen in the alveoli. The alveoli have 104 mm Hg, and the deoxygenated blood has 40 mm Hg. The gradient is $104 - 40 = 64$ mm Hg, from the alveoli into the blood.
• Carbon dioxide in the alveoli. The blood has 45 mm Hg, and the alveoli have 40 mm Hg. The gradient is 5 mm Hg, from the blood into the alveoli.
• Oxygen in the tissues. The oxygenated blood has 95 mm Hg, and the tissues have 40 mm Hg. The gradient is 55 mm Hg, from the blood into the tissues.
• Carbon dioxide in the tissues. The tissues have 45 mm Hg, and the blood has 40 mm Hg. The gradient is 5 mm Hg, from the tissues into the blood.

Notice that the pO$_2$ falls at every step from the air to the tissues: 159, 104, 95 and 40. The pCO$_2$ rises from the air to the tissues.

What decides the rate of diffusion?

Three things matter.

1. The gradient of partial pressure. A steeper gradient moves more gas.
2. The solubility of the gas. A gas that dissolves more easily crosses the membrane more easily.
3. The thickness of the membrane. A thinner membrane lets more gas through.

Carbon dioxide is 20 to 25 times more soluble than oxygen. Its gradient is only 5 mm Hg, but much more carbon dioxide than oxygen crosses the membrane for each mm Hg of difference. So both gases get across.

The diffusion membrane

The gases cross three thin layers, which together are called the diffusion membrane.

1. The thin squamous epithelium of the alveoli.
2. The endothelium of the alveolar capillaries.
3. The basement substance between the two.

The whole membrane is much thinner than a millimetre. Measurements on human lungs give a mean thickness of less than one micrometre. A large surface, a thin membrane and steep gradients all favour the diffusion of oxygen from the alveoli to the tissues and of carbon dioxide from the tissues to the alveoli.

You should find that the partial pressure of oxygen falls in step with the total pressure. At the top of a high mountain the air still holds 20.95 per cent oxygen, but its partial pressure is much lower.

Try it in Python

The program works out the partial pressures of the gases of the air from their percentages.

Practise with new numbers

Common mistakes

• Saying that gases move from low pressure to high pressure. Simple diffusion goes from high partial pressure to low.
• Saying that oxygen and carbon dioxide have the same gradient. Oxygen has the steep one, 64 and 55 mm Hg. Carbon dioxide has 5 mm Hg.
• Thinking that carbon dioxide moves slowly because its gradient is small. It is far more soluble, so it moves well.
• Leaving out a layer of the membrane. There are three: the epithelium, the basement substance and the endothelium.
• Calling the partial pressure of oxygen in the air 21 mm Hg. The 21 is a per cent. The partial pressure is about 159 mm Hg.

Remember these

• Exchange is by simple diffusion, from higher to lower partial pressure.
• pO$_2$ in mm Hg: air 159, alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40.
• pCO$_2$ in mm Hg: air 0.3, alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45.
• The rate depends on the gradient, the solubility of the gas and the thickness of the membrane.
• Carbon dioxide is 20 to 25 times more soluble than oxygen.
• Diffusion membrane: squamous epithelium, basement substance, capillary endothelium. It is much thinner than a millimetre.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the partial pressure of a gas in a mixture?

2. In which direction does oxygen move between the alveoli and the deoxygenated blood, and what is the gradient?

3. Carbon dioxide has a gradient of only 5 mm Hg. Why does enough of it still cross the membrane?

4. Name the three layers of the diffusion membrane.

5. What three things change the rate of diffusion of a gas?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
• OpenStax (2022). Anatomy and Physiology 2e, chapter 22, The Respiratory System, section on Gas Exchange.
• West J. B. and Luks A. M. (2021). West's Respiratory Physiology: The Essentials, 11th edition, Wolters Kluwer, chapter Diffusion.
• Knudsen L., Hummel B., Wrede C., Zimmermann R., Perlman C. E. and Smith B. J. (2023). Acinar micromechanics in health and lung injury: what we have learned from quantitative morphology. Frontiers in Physiology, 14, 1142221. DOI 10.3389/fphys.2023.1142221.