Haemoglobin present in RBC is respiratory pigment in Human being which transport Oxygen molecule. When this oxygen is used then blood becomes deoxygenated as it now contains CO2. As CO2 is dissolved in water so it is transported in dissolved form in blood.

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. Hemoglobin, the iron containing respiratory pigment is a red colored pigment of blood, which has a very high affinity for oxygen. Hemoglobin though is purple colored but oxy-hemoglobin is of bright red color. Oxygen is transported from lungs to the body cells in the form of oxy-hemoglobin. Carbon-dioxide is transported from the body cell to the lungs in the forms of carboxy-hemoglobin, carbonic acid, bicarbonates of sodium and potassium.

In human being, oxygen is carried from the lungsby the respiratory pigment haemoglobin whivh is present in red blood cells.Haemoglobin has a very high affinity for oxygen . Carbon dioxide is more soluble in waterthan oxygen. so,most of the carbon dioxide produced during respiration in human body is transported in the dissolved form in our blood.