Exchange of gases takes place between the alveoli of lungs and the surrounding blood capillaries.  The two main gases exchanged are oxygen and carbon-dioxide. Oxygen is absorbed by the blood capillaries from the lungs alveoli by diffusion while carbon dioxide is absorbed by the lungs alveoli from blood capillaries by diffusion. Transport of oxygen and carbon dioxide occurs with the help of respiratory pigment called hemoglobin. Haemoglobin, the iron-containing respiratory pigment is a red coloured pigment of blood, which has a very high affinity for oxygen. Haemoglobin though is purple coloured but oxyhemoglobin is of bright red colour. Oxygen is transported from lungs to the body cells in the form of oxyhemoglobin. Carbon dioxide is transported from the body cell to the lungs in the forms of carboxyhemoglobin, carbonic acid, bicarbonates of sodium and potassium.