Haemoglobin and the transport of oxygen

Why a carrier?

Try to dissolve oxygen in a glass of water. Only a very little goes in. Blood plasma is mostly water, so it can also hold only a little oxygen. The body needs far more than that, so the blood uses a carrier. The carrier is haemoglobin.

How the gases travel

Blood is the medium for the transport of both gases.

• Oxygen: about 97 per cent in the red blood cells, and about 3 per cent dissolved in plasma.
• Carbon dioxide: about 20 to 25 per cent in the red blood cells, about 7 per cent dissolved in plasma, and about 70 per cent as bicarbonate.

This lesson is about oxygen. The next course takes up carbon dioxide.

Haemoglobin

Haemoglobin is a red coloured pigment that contains iron. It is found in the red blood cells, not in the plasma.

Oxygen binds to haemoglobin in a reversible way. The product is oxyhaemoglobin.

Each haemoglobin molecule can carry a maximum of four oxygen molecules. Reversible means that the oxygen can also come off again. It does so when the blood reaches the tissues.

What decides the binding?

The binding of oxygen to haemoglobin depends mainly on the partial pressure of oxygen. Three other things interfere with it.

• The partial pressure of carbon dioxide.
• The concentration of hydrogen ions.
• The temperature.

The lungs and the tissues compared

In the alveoli, oxygen binds and oxyhaemoglobin forms. In the tissues, oxygen comes off oxyhaemoglobin. Every condition of the alveoli favours the formation of oxyhaemoglobin. Every condition of the tissues favours its dissociation. So the blood picks up oxygen at the lungs and gives it up at the tissues.

How much does the blood give?

Under normal conditions, every 100 mL of oxygenated blood delivers around 5 mL of oxygen to the tissues. The blood does not give up all its oxygen. The next lesson shows that blood leaving the tissues still holds most of it.

A note on the 97 and the 3 per cent

NCERT says that about 97 per cent of the oxygen travels in the red blood cells and about 3 per cent is dissolved in plasma. This is the share of the oxygen that is carried from the lungs to the tissues. Look at the whole content of arterial blood and the dissolved part is smaller, about 1.5 per cent. Plasma dissolves only about 0.003 mL of oxygen in each 100 mL for each mm Hg of pO$_2$. The blood falls by about 60 mm Hg between the lungs and the tissues, so it delivers only a little dissolved oxygen, compared with the 5 mL from haemoglobin. The answer is a few per cent, which agrees with NCERT.

You should find that a person at rest, with 5000 mL of blood passing the tissues each minute, receives about 250 mL of oxygen a minute. Hard exercise needs much more, so the flow and the oxygen taken from each 100 mL both rise.

Try it in Python

The program counts the oxygen that a sample of haemoglobin carries, and its saturation.

Practise with new numbers

Common mistakes

• Putting haemoglobin in the plasma. It is inside the red blood cells.
• Saying that a haemoglobin molecule carries two oxygen molecules. It carries up to four.
• Saying that all the oxygen travels dissolved in plasma. About 97 per cent travels on haemoglobin.
• Calling the binding of oxygen permanent. It is reversible, which is why the tissues get their oxygen.
• Thinking that blood gives up all its oxygen in the tissues. Each 100 mL gives up only about 5 mL.

Remember these

• About 97 per cent of oxygen is carried by the red blood cells, and about 3 per cent is dissolved in plasma.
• Haemoglobin is a red, iron-containing pigment of the red blood cells. It forms oxyhaemoglobin with oxygen, reversibly.
• One haemoglobin molecule carries at most four oxygen molecules.
• The binding depends mainly on pO$_2$. pCO$_2$, hydrogen ions and temperature also change it.
• Alveoli: high pO$_2$, low pCO$_2$, fewer hydrogen ions, lower temperature. The tissues are the opposite.
• Every 100 mL of oxygenated blood delivers about 5 mL of oxygen to the tissues.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What share of the oxygen is carried in the red blood cells, and what share is dissolved in the plasma?

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

3. Name four conditions that favour the binding of oxygen in the alveoli.

4. Why does oxygen leave haemoglobin in the tissues?

5. How much oxygen does 100 mL of oxygenated blood deliver to the tissues in normal conditions?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids.
• West J. B. and Luks A. M. (2021). West's Respiratory Physiology: The Essentials, 11th edition, Wolters Kluwer, chapter Gas Transport by the Blood.