Carbon Dioxide Transport
As mentioned earlier, nearly all oxygen transported through blood is bound to hemoglobin of red blood cells and only a few percent is dissolved in the plasma. Hemoglobin consists of four subunits (blue, purple, green and yellow in the 3D-model below), each containing a heme group (red). Each heme in turn contains an iron atom which can combine with oxygen. Hemoglobin is an allosteric protein in that oxygen binds cooperatively (binding of O 2 enhances the binding of additional O 2 ) and metabolic factors (for example pH, PCO 2 and temperature) influence the affinity for oxygen.
Out of the carbon dioxide released from respiring cells, 7% dissolves into the plasma, 23% binds to the multiple amino groups of haemoglobin (Caroxyhaemoglobin), and 70% is carried as bicarbonate ions. Carbon dioxide created by respiring cells diffuses into the blood plasma and then into the red blood cells, where most of it is converted to bicarbonate ions. It first reacts with water forming carbonic acid, which then breaks down into H + and CO 3 - . Most of the hydrogen ions that are produced attach to haemoglobin or other proteins. In this