There is 21% oxygen in the air that you breathe.

Air is breathed into the lungs by the action of the diaphragm and other respiratory muscles drawing in air through the upper airways.

In the alveoli (air sacs of the lung) oxygen passes into the blood and combines with haemoglobin.

Arterial blood is pumped to the tissues by the heart. The tissues receive oxygen from haemoglobin in the capillary networks in each organ. The cells burn the oxygen and create carbon dioxide which is returned to the lungs in venous blood and excreted in the exhaled gas.