Why oxygen transport needs haemoglobin
Because oxygen dissolves poorly, haemoglobin (a protein in red blood cells) carries almost all of it, binding it reversibly so it can pick oxygen up and give it back.
Remember
Oxygen dissolves poorly, so at the normal oxygen pressure in the lungs about 98% of the oxygen in blood is carried bound to haemoglobin, and the binding is reversible so oxygen can be released in the tissues.
Memory aid
plasma carries a teaspoon of oxygen and haemoglobin carries the bucket
Think of the dissolved oxygen in plasma as a teaspoon and the oxygen bound to haemoglobin as a bucket.
Oxygen is not soluble enough in blood (it does not dissolve well) for dissolving to be the only way of carrying it around the body. The amount that does dissolve follows the partial pressure of oxygen, which is the pressure the dissolved gas exerts. At the normal partial pressure of oxygen in the lungs, 13.3 kPa (a unit of pressure), only about 0.13 mmol/L (a unit of concentration) is dissolved.
So blood needs a second transport system: haemoglobin, the iron-containing protein inside red blood cells. A normal haemoglobin concentration is about 2.2 mmol/L, and each molecule can bind four oxygen molecules, so about 8.8 mmol/L of oxygen is bound. The oxygen content of blood (the total amount carried in each litre) is the dissolved plus the bound oxygen, about 8.93 mmol/L, and about 98% of it is bound to haemoglobin.
The binding is reversible: haemoglobin can pick oxygen up and give it back. It loads oxygen in the lungs, where the partial pressure is high, and unloads it in the tissues, where the pressure is lower. This happens over a narrow range of partial pressures, which makes loading and unloading efficient. An irreversible bond would stop delivery to the tissues.
Haemoglobin adds to the amount of oxygen carried without raising the partial pressure, because the partial pressure reflects only the dissolved oxygen. Myoglobin, which is found in muscle, is a storage molecule and not the carrier in red blood cells.
Key facts
- Dissolved oxygen
- about 0.13 mmol/L at 13.3 kPa, only around 1.5% of the total
- Bound oxygen
- a normal haemoglobin concentration of about 2.2 mmol/L, with each haemoglobin molecule able to bind four oxygen molecules, can bind about 8.8 mmol/L
- Total content
- about 8.93 mmol/L, the dissolved oxygen plus the oxygen bound to haemoglobin
- Share carried bound
- about 98% of the oxygen in blood is bound to haemoglobin, at the normal oxygen pressure in the lungs of 13.3 kPa
- Loading and unloading
- haemoglobin loads oxygen in the lungs, where the partial pressure is high, and unloads it in the tissues, where the pressure is lower
- Anaemia
- someone with anaemia (too little haemoglobin) can have a normal oxygen pressure but a low oxygen content
Exam traps
- Dissolved oxygen does not contribute more than haemoglobin-bound oxygen, because only about 0.13 mmol/L is dissolved against roughly 8.8 mmol/L bound.
- Haemoglobin does not bind oxygen irreversibly, because reversible binding is what lets it release oxygen in the tissues, and an irreversible bond would stop delivery.
- Oxygen bound to haemoglobin does not exert a higher partial pressure, because the partial pressure comes from dissolved molecules only, so binding adds to the content without adding to the partial pressure.
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