Gas Transport
If 100mL of plasma is exposed to an atmosphere with a pO 2 of 100mm Hg, only 0.3mL of oxygen would be absorbed. However, if 100mL of blood is exposed to the same atmosphere, about 19mL of oxygen would be absorbed. This is due to the presence of haemoglobin, the main means of oxygen transport in the body. The respiratory pigment haemoglobin is made up of an iron-containing porphyron, haem, combined with the protein globin. Each iron atom in haem is attached to four pyrole groups by covalent bonds. A fifth covalent bond of the iron is attached to the globin part of the molecule and the sixth covalent bond is avaiable for combination with oxygen. There are four iron atoms in each heamoglobin molecule and therefore four heam groups.