Transport of oxygen and carbon dioxide

The red blood cell

A red blood cell is adapted for one job:

• Biconcave disc shape, which gives a larger surface area to volume ratio than a sphere and shortens the diffusion distance to the centre.
• No nucleus, mitochondria or endoplasmic reticulum, leaving the maximum room for haemoglobin. Around 250 million molecules fit in each cell.
• Flexible, so it can squeeze through capillaries narrower than itself.

Having no mitochondria means the cell respires anaerobically and does not consume any of the oxygen it is carrying, which would otherwise be a waste.

Oxygen transport

Haemoglobin has four polypeptide chains, each with a haem group containing an iron ion, and each haem group binds one oxygen molecule. So one haemoglobin molecule carries four oxygen molecules, that is eight oxygen atoms.

The product is oxyhaemoglobin. The reaction is reversible, and which way it goes depends on the partial pressure of oxygen.

Loading happens where the partial pressure of oxygen is high, in the lung capillaries. Unloading happens where it is low, in respiring tissue.

The dissociation curve

The graph plots percentage saturation of haemoglobin against partial pressure of oxygen. It is S-shaped, and the shape is the whole point.

Why it is S-shaped: cooperative binding

At low partial pressures the curve is shallow. Haemoglobin's four chains are packed closely and the haem groups are hard to reach, so the first oxygen molecule binds with difficulty.

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Once that first molecule binds, it changes the tertiary and quaternary structure of the haemoglobin, making the remaining haem groups more accessible. The second and third oxygen molecules therefore bind much more readily, which is the steep middle section.

The fourth is harder again, simply because most sites are already occupied, so the curve levels off at the top.

This is cooperative binding, and it is the answer to any question asking why the curve is not a straight line.

Why the shape is useful

The steep middle section is positioned exactly where the partial pressure of respiring tissue lies. That means a small fall in partial pressure produces a large fall in saturation, so a large amount of oxygen is released for a small change in conditions.

The flat top means that haemoglobin is still close to fully saturated in the lungs even if the partial pressure there falls somewhat, for example at moderate altitude.

Carbon dioxide transport

Carbon dioxide travels in three ways:

• about 85 per cent as hydrogencarbonate ions in the plasma
• about 10 per cent as carbaminohaemoglobin, bound to the amino groups of haemoglobin
• about 5 per cent dissolved in the plasma

The hydrogencarbonate route runs through the red blood cell:

1. Carbon dioxide diffuses from the respiring tissue into the red blood cell.
2. The enzyme carbonic anhydrase catalyses its reaction with water to form carbonic acid.
3. Carbonic acid dissociates into hydrogen ions and hydrogencarbonate ions.

1. The hydrogencarbonate ions diffuse out of the red cell into the plasma. To balance the charge, chloride ions move in. This is the chloride shift.
2. The hydrogen ions are taken up by haemoglobin, which acts as a buffer and prevents the cell becoming acidic. Haemoglobin doing this is called haemoglobinic acid.

Carbonic anhydrase is inside the red blood cell rather than in the plasma, which is why the reaction happens there.

The Bohr effect

A higher partial pressure of carbon dioxide causes haemoglobin to release oxygen more readily. On a graph, the dissociation curve shifts to the right.

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The mechanism runs straight out of the section above. More carbon dioxide means more carbonic acid, so more hydrogen ions. Those hydrogen ions bind to haemoglobin and change its tertiary structure, lowering its affinity for oxygen. Oxygen is released.

So it is the hydrogen ions, not the carbon dioxide directly, that do the work. An answer naming them is a better answer.

The value of this is that it is self-regulating. A tissue respiring hard produces more carbon dioxide, and that extra carbon dioxide is exactly what causes haemoglobin to release more oxygen there. The oxygen goes where it is most needed, with no control system required.

Reading a shifted curve. A curve to the right means lower affinity, so oxygen is released more easily. A curve to the left means higher affinity, so oxygen is held more tightly and loaded more readily. Getting this the wrong way round is the commonest error in the topic.

Fetal haemoglobin

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its curve lies to the left.

It has to. The fetus obtains oxygen from the mother's blood at the placenta, and at the partial pressure found there, adult haemoglobin is releasing oxygen. If fetal haemoglobin had the same affinity, no net transfer would happen. Because its affinity is higher, it takes up oxygen the mother's haemoglobin is releasing at that same partial pressure.

Fetal haemoglobin is replaced by the adult form in the months after birth, since holding oxygen tightly is a disadvantage once the infant is breathing air.

Myoglobin

Myoglobin is found in muscle. It has a single polypeptide chain and one haem group, so it carries one oxygen molecule and shows no cooperative binding. Its curve is therefore not S-shaped but a steep hyperbola, and it lies far to the left.

Its very high affinity means it only releases oxygen at extremely low partial pressures, which makes it an oxygen store in muscle rather than a transporter. It hands over its oxygen when the muscle is working hard enough to have nearly exhausted the supply from the blood.

Diving mammals such as seals carry very high concentrations of myoglobin, which is what lets them stay submerged.

Altitude

At high altitude the partial pressure of oxygen in the air is lower, so haemoglobin is less fully saturated in the lungs.

Over weeks, the body responds by producing more red blood cells, so total oxygen-carrying capacity rises. Some populations native to high altitudes also have haemoglobin with a slightly higher affinity, shifting the curve left so that loading is more complete in thin air.

Common mistakes

• Saying haemoglobin carries four oxygen atoms. It carries four molecules, so eight atoms.
• Reading a right shift as higher affinity. Right means lower affinity and easier release.
• Saying carbon dioxide binds to the haem group. Oxygen binds to haem; carbon dioxide binds to the amino groups of the protein.
• Saying the Bohr effect is caused by carbon dioxide directly. It works through the hydrogen ions produced from it.
• Saying fetal haemoglobin has a lower affinity so oxygen passes to it. It is higher.

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