Transport of Oxygen and Carbon Dioxide

• Haemoglobin is a globular, conjugated protein (quaternary structure: 2α chains and 2β chains) with 4x haem prosthetic group; each contains an iron atom (Fe (III)), respiratory pigment found in erythrocytes and transports of O_2 and CO_2
• Haemoglobin is found in all vertebrates except a few fish; haemocyanin is another respiratory pigment found in animals such as shellfish and insects, but contains copper rather than iron
• In the lungs, oxygen concentration is very high; the partial pressure of oxygen (ppO_2) is very high, so oxygen diffuses across the alveolar wall and into the erythrocytes because Hb has a high affinity for oxygen and binds readily
• One oxygen molecule (O_2) binds loosely to the iron atom in each haem group (no oxidation of Fe occurs; Hb (Haemoglobin)+ 4O_2 -> HbO_8 (Oxyhaemoglobin)
• When the erythrocytes reach respiring tissues/cells, oxygen concentration is very low; partial pressure of oxygen (ppO_2) is very low so the oxygen dissociates from Hb and diffuses out into the tissue fluid and then respiring cells
• Therefore, in respiring tissues, where the ppO_2 is low, Hb has a lower affinity for oxygen and so oxygen readily dissociates