Hypoxia is a condition in which tissues within the body are starved of oxygen, to the point where adequate homeostasis can no longer take place. It is broken down into two levels: localised hypoxia occurs when a specific region of the body is oxygen deprived whereas generalised hypoxia is when the entire body is affected. It can result from insufficient oxygen levels in the blood (termed hypoxemia) or insufficient blood delivery to a specific tissue. If hypoxia continues to a state where there is no oxygen present at all, it is termed anoxia. Oxygen passively diffuses into the blood at the lungs, where it binds to haemoglobin and is pumped to the tissues around the body. At peripheral tissues, oxygen again diffuses down the pressure gradient, leaving the blood and entering the tissue cells where it is used by mitochondria to produce energy. Issues delivering oxygen at any stage of this process can cause hypoxia, with problems associated with the lungs, haemoglobin and blood circulation increasing the risk. Altitude sickness is a form of generalised hypoxia, where the lower concentration of oxygen in the air prevents the body accessing adequate volumes. In contrast, localised hypoxia can occur due to poor circulation, with weaker blood flow to bodily extremities preventing adequate oxygen delivery.