Generalized hypoxia is a medical condition in which the tissues of the body are deprived of the necessary levels of oxygen due to an insufficient supply of oxygen, which may be due to the composition or pressure of the breathing gas, decreased lung ventilation, or respiratory disease, any of which may cause a lower than normal oxygen content in the arterial blood, and consequently a reduced supply of oxygen to all tissues perfused by the arterial blood. This usage is in contradistinction to localized hypoxia, in which only an associated group of tissues, usually with a common blood supply, are affected, usually due to an insufficient or reduced blood supply to those tissues. Genralized hypoxia is also used as a synonym for hypoxic hypoxia This is not to be confused with hypoxemia, which refers to low levels of oxygen in the blood, although the two conditions often occur simultaneously, since a decrease in blood oxygen typically corresponds to a decrease in oxygen in the surrounding tissue. However, hypoxia may be present without hypoxemia, and vice versa, as in the case of infarction. Several other classes of medical hypoxia exist. Causes Hypoxia can result from various causes which can be categorised as: anemic hypoxia, cellular hypoxia, generalised, or hypoxic hypoxia, pulmonary hypoxia, stagnant hypoxia, increased oxygen consumption due to a hypermetabolic state, or any combination of these. The three fundamental causes of hypoxia at the tissue level are low oxygen content in the blood (hypoxemia), low perfusion of the tissue, and inability of the tissue to extraxt and use the oxygen in the blood. Generalised, or hypoxic hypoxia may be caused by: Hypoventilation – failure of the respiratory pump due to any cause (fatigue, barbiturate poisoning, pneumothorax, etc.) Low-inspired oxygen partial pressure, which may be caused by breathing air at low ambient pressures due to altitude, by breathing hypoxic breathing gas at an unsuitable depth, by breathing inadequately re-oxygenated recycled breathing gas from a rebreather, life support system, or anesthetic machine, or hypoxia of ascent in freediving. Airway obstruction, choking, drowning. Abnormal pulmonary function Chronic obstructive pulmonary disease (COPD) Neuromuscular diseases or interstitial lung disease Malformed vascular system such as an anomalous coronary artery Altitude effects When breathing the ambient air at high altitudes (above 3048 metres/10,000 feet), the human body experiences altitude sickness and hypoxemia due to a low partial pressure of oxygen, decreasing the carriage of oxygen by hemoglobin.