A level Biology exam revision resources written by A level Examiners


It is simplistic to think about the transport function of blood only in terms of respiratory gases. Blood is a transport medium, it is the fluid that circulates through the cardiovascular system bringing metabolites to tissues and removing waste materials. Many substances are dissolved in the plasma and transported around the body in the blood, these include

  • sugars
  • amino acids
  • fatty acids and glycerol
  • hormones
  • nitrogenous waste
  • mineral ions

The two main respiratory gases oxygen and carbon dioxide are both transported by the blood but in different ways. In order to examine how these gases are transported in the blood it is essential to understand the units that their concentration is measured in. Gas concentrations are often defined by their partial pressure. Essentially it is a measure of the concentration of the gas in a mixture of gases or solution. It is written in shorthand as P and is measured in kilopascals (kPa).

The transport of carbon dioxide in the blood

Carbon dioxide is transported in the blood in 3 ways:

dissolved in solution in the plasma

bound to haemoglobin

as hydrogen carbonate ions in plasma

The greater solubility of carbon dioxide than oxygen means that up to 5% can be transported in simple solution. When carbon dioxide binds to haemoglobin it forms carbamino compounds forms (the carbon dioxide combines with the globin portion of haemoglobin). Both these methods of transport are minor and learning should be concentrated on the next and most important method.

Most carbon dioxide is transported in the blood as hydrogencarbonate ions. These are produced by a reaction which proceeds in two stages. The reaction is shown below

carbon dioxide + water carbonic acid hydrogen ion + hydrogencarbonate ion

CO2 + H2O H2CO3 H+ + HCO3-

The first stage of this reaction is catalysed by the enzyme carbonic anhydrase which is present in high concentration inside red blood cells so this reaction normally occurs inside red blood cells.

The second stage (disassociation) occurs immediately. The hydrogencarbonate diffuses out of the red blood cells into the plasma, the hydrogen ions are taken up by the haemoglobin within red blood cells (i.e. they are buffered). Chloride ions diffuse into red blood cells to equalise the charge when the hydrogencarbonate leaves, this is known as the chloride shift.

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