A respiratory pigment is any molecule that increases the oxygen carrying capacity of blood. The oxygen carrying capacity is the amount of oxygen the red blood cells in blood can carry to our tissues and organs. In humans and most other vertebrates, the most common respiratory pigment is a protein called haemoglobin. Haemoglobin is the protein molecule in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues back to the lungs.