Oxygen is transported dissolved in the blood and also in combination with haemoglobin in the red blood cells. Although haemoglobin carries most of the oxygen, it is only the dissolved oxygen that passes into the tissues. Breathing high levels of oxygen under barometric conditions dissolves more oxygen in the body’s fluids and it can reach areas where the circulation is diminished or blocked, and so improves recovery. Extra oxygen has additional benefits because it greatly enhances the ability of white blood cells to kill bacteria. It also reduces swelling and allows new blood vessels to grow more rapidly into the affected areas.