Transport of oxygen and carbon dioxide

Biology AS · Transport in mammals. 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

Red blood cells carry oxygen bound to haemoglobin. One haemoglobin molecule can carry four oxygen molecules. The oxygen dissociation curve shows the percentage saturation of haemoglobin at each partial pressure of oxygen. It is S-shaped: haemoglobin is almost fully saturated in the lungs and gives up much of its oxygen in respiring tissues.

Most carbon dioxide travels as hydrogencarbonate ions. Inside the red cell, carbonic anhydrase joins carbon dioxide and water to form carbonic acid, which splits into H⁺ and HCO₃⁻. Haemoglobin takes up the H⁺ and releases oxygen.

So more carbon dioxide means more oxygen released. This is the Bohr shift.

Rules to remember

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (first step catalysed by carbonic anhydrase).
H⁺ + haemoglobin → haemoglobinic acid, which keeps the pH steady.
Chloride shift: HCO₃⁻ diffuses out of the red cell into the plasma and Cl⁻ moves in to keep the charge balanced.
Carbon dioxide is carried mostly as HCO₃⁻ in the plasma, with a smaller part as carbaminohaemoglobin and a little dissolved in the plasma.
Carbaminohaemoglobin: CO₂ bound to the amine groups of haemoglobin, not to the haem.
Bohr shift: at high CO₂ the curve moves to the right, so haemoglobin has a lower affinity for oxygen.

Common mistake

Saying carbon dioxide binds to the same site as oxygen. Oxygen binds to the haem groups; carbon dioxide binds to amine groups of the polypeptide chains, and most of it is not carried on haemoglobin at all.

Worked example

100 cm³ of blood contains 15 g of haemoglobin. Each gram can carry 1.3 cm³ of oxygen when fully saturated. How much oxygen does 100 cm³ of blood carry when the haemoglobin is 75% saturated?

Oxygen carried at full saturation = 15 × 1.3 = 19.5 cm³.
At 75% saturation: 19.5 × 0.75 = 14.625 cm³.
Round to three significant figures: 14.6 cm³.

Answer: About 14.6 cm³ of oxygen per 100 cm³ of blood.

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