Exchange of Gases and Transport of Oxygen: Quick Revision

A brief note on Exchange of Gases and Transport of Oxygen. Read it before a test. Try each question at the end before you open its answer.

Exchange of gases

• Gases cross by simple diffusion, from the higher partial pressure to the lower one. The alveoli are the primary site.
• Partial pressure of a gas is its share of the total pressure. Air at 760 mm Hg is 20.95 per cent oxygen, so pO$_2$ is about 159 mm Hg.
• 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: thin squamous epithelium of the alveoli, basement substance, endothelium of the capillary. Much thinner than a millimetre.

The table, in mm Hg

• Oxygen gradients: 64 (alveoli to blood) and 55 (blood to tissues). Carbon dioxide gradients: 5 each.

Transport of oxygen

• About 97 per cent in the red blood cells and about 3 per cent dissolved in plasma.
• Haemoglobin: red, iron-containing pigment of the red blood cells. Hb binds oxygen reversibly to form oxyhaemoglobin. At most 4 oxygen molecules for each haemoglobin.
• Binding depends mainly on pO$_2$. Higher pCO$_2$, more hydrogen ions and a higher temperature favour release.
• Every 100 mL of oxygenated blood delivers about 5 mL of oxygen to the tissues.

The oxygen dissociation curve

• A sigmoid graph of per cent saturation of haemoglobin against pO$_2$.
• Nearly full at 104 mm Hg, about 75 per cent at 40 mm Hg.
• Shift to the right: higher pCO$_2$, more hydrogen ions, higher temperature, so oxygen is released more easily. Shift to the left: the opposite.

A worked example

At the top of a mountain the pressure is 380 mm Hg. What is the pO$_2$ of the air?

1. Oxygen is 20.95 per cent of the air.
2. $380 \times 20.95 / 100 = 79.6$ mm Hg.

Practice with new numbers

A program to try

The program finds the partial pressure of oxygen at two pressures.

Mistakes to avoid

• Saying that gases move from low to high pressure. They move from high to low.
• Putting haemoglobin in the plasma. It is in the red blood cells.
• Saying that a shift to the right holds oxygen tighter. It releases oxygen more easily.
• Forgetting that carbon dioxide is far more soluble. That is why a small gradient is enough.

Check yourself

1. How many oxygen molecules can one haemoglobin carry at most?

2. What are the conditions of the alveoli that favour oxyhaemoglobin?

3. What does a shift of the curve to the right do?

4. Name the three layers of the diffusion membrane.

Sources

• NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
• West J. B. and Luks A. M. (2021). West's Respiratory Physiology: The Essentials, 11th edition, Wolters Kluwer, chapters Diffusion and Gas Transport by the Blood.