Gas transport and disorders

MDCAT Biology · Biology · Respiration. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Almost all oxygen travels bound to haemoglobin in the red blood cells. Each haemoglobin molecule has four haem groups, each with one iron ion (Fe²⁺), so it can carry four O₂ molecules. It loads oxygen in the lungs, where oxygen is plentiful, and unloads it in the tissues, where oxygen is scarce.

Active tissues make more CO₂ and acid. This lowers the pull of haemoglobin on oxygen, so more oxygen is released exactly where it is needed. This is the Bohr effect.

Most carbon dioxide is carried in the plasma as hydrogencarbonate (bicarbonate) ions. Smoking damages every stage of gas exchange and transport.

Rules to remember

Hb + 4O₂ ⇌ Hb(O₂)₄ (oxyhaemoglobin); the oxygen dissociation curve is S-shaped (sigmoid)
Bohr effect: more CO₂ or lower pH shifts the curve to the right (higher temperature does the same), so haemoglobin gives up more oxygen
CO₂ transport: about 70% as hydrogencarbonate ions, most of the rest as carbaminohaemoglobin, a small amount dissolved in plasma
In red cells: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, speeded up by carbonic anhydrase
Chloride shift: HCO₃⁻ diffuses out of the red cell and Cl⁻ moves in to keep the charges balanced
Smoke: tar causes cancer and damages cilia; nicotine is addictive and raises heart rate and blood pressure; carbon monoxide cuts oxygen carriage
Emphysema = alveolar walls break down, less surface; pneumonia = alveoli fill with fluid; tuberculosis = Mycobacterium tuberculosis, prevented by BCG vaccine

Common mistake

A shift of the dissociation curve to the right does not mean haemoglobin carries more oxygen. It means haemoglobin holds oxygen less tightly and releases more of it to the tissues.

Worked example

A man is found unconscious in a closed room with a faulty gas heater. The air in the room still contains plenty of oxygen. Why are his tissues short of oxygen?

A faulty heater burns its fuel incompletely and gives off carbon monoxide.
Carbon monoxide binds to the haem groups of haemoglobin far more strongly than oxygen does, forming stable carboxyhaemoglobin.
Haemoglobin that is holding carbon monoxide cannot carry oxygen, so little oxyhaemoglobin forms in the lungs.

Answer: Carbon monoxide occupies the haemoglobin, so the blood carries too little oxygen even though the air has enough.

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