Hemoglobin is a tetrameric globular protein consisting of two alpha and two beta polypeptide chains.

Each of these chains has an iron-containing heme pigment molecule capable of reversibly binding oxygen.

As an oxygen molecule binds the first heme group, it induces conformational changes in hemoglobin, allowing the remaining heme groups to bind oxygen easily.

After all four heme groups bind oxygen, the oxygen-saturated hemoglobin is called oxyhemoglobin, which gives the erythrocytes in arterial blood a bright red hue.

Conversely, while passing through the tissue capillaries, hemoglobin loses bound oxygen to become deoxyhemoglobin. That's why the oxygen-poor venous blood appears dark red.

Due to high carbon dioxide concentration in the tissues, hemoglobin readily binds carbon dioxide upon releasing oxygen, forming carbaminohemoglobin, which accounts for about 20% of the carbon dioxide transport in the blood.

Once this blood returns to the lungs with high oxygen concentration, hemoglobin readily releases carbon dioxide and binds to oxygen.

Apart from respiratory gasses, deoxyhemoglobin binds to nitric oxide, causing vasodilation and improved blood flow.