Editing Generalized hypoxia (section)

== 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.<ref name="Manninen and Unger 2016" /> 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 extract and use the oxygen in the blood.<ref name="Bhutta et al 2022" /> Generalised, or hypoxic hypoxia may be caused by:
*[[Hypoventilation]]<ref name="Manninen and Unger 2016" />  – failure of the respiratory pump due to any cause (fatigue, [[Barbiturate overdose|barbiturate poisoning]], [[pneumothorax]], etc.)
*Low-inspired oxygen partial pressure, which may be caused by breathing air at low ambient pressures due to altitude,<ref name="Manninen and Unger 2016" /><ref name="Bhutta et al 2022" /> by breathing hypoxic [[Breathing gas#Classification by oxygen fraction|breathing gas]] at an unsuitable depth, by breathing inadequately re-oxygenated recycled breathing gas from a [[rebreather]],<ref name="Elliott 1997" /> [[life support system]], or [[anesthetic machine]], or [[hypoxia of ascent]] in [[freediving]].<ref name="Lindholm 2006" />
*[[Airway obstruction]], [[choking]],<ref name="Manninen and Unger 2016" /> [[drowning]].
*Abnormal [[pulmonary]] function{{cn|date=November 2022}}<!-- plausible but no ref -->
**[[Chronic obstructive pulmonary disease]] (COPD)<ref name="Cleveland" />
**Neuromuscular diseases or [[interstitial lung disease]]<!-- plausible but no ref -->
*Malformed vascular system such as an [[Anomalous left coronary artery from the pulmonary artery|anomalous coronary artery]]{{cn|date=November 2022}}

===Altitude effects===
{{see also|Altitude sickness}}
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.
* Above 3000 metres (10,000 feet) - ambient pressure 69.7kPa, about 14.6kPa partial pressure of oxygen – enough hypoxic stimulation to cause increased ventilation
* Above 3700 metres (12,000 feet) - 64.4kPa, about 13.52kPa P<sub>O<sub>2</sub></sub> – first irritability appears
* Above 5500 metres (18,000 feet) - 50.6kPa, about 10.6kPa P<sub>O<sub>2</sub></sub> – severe symptoms
* Above 7600 metres (25,000 feet) - ambient pressure 37.6kPa absolute, 7.9kPa partial pressure of oxygen – consciousness lost{{cn|date=November 2022}}

While breathing pure oxygen at ambient pressure, from an oxygen cylinder or other source, the maximum altitude a human can tolerate{{clarify|Is this stable for long term exposure, or only tolerable for limited periods? At what level of exertion?|date=November 2022}} while their body is at atmospheric pressure is 13,700 metres (45,000 feet),{{cn|date=October 2022}} , where atmospheric pressure is about 14.7kPa.<!-- plausible PPO2 to remain conscious for long periods--> This is a function of the partial pressure of oxygen in the breathing gas, and is also dependent on level of exertion which affects the oxygen requirements of metabolism, cardiovascular fitness, and acclimatization to altitude which affects the available hemoglobin and can vary significantly between individuals. {{clarify|This altitude is possibly better expressed as a value for ambient pressure|date=November 2022}}