[2] Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise.

Mild, non-damaging intermittent hypoxia is used intentionally during altitude training to develop an athletic performance adaptation at both the systemic and cellular level.

[19] In severe hypoxia, or hypoxia of very rapid onset, ataxia, confusion, disorientation, hallucinations, behavioral change, severe headaches, reduced level of consciousness, papilloedema, breathlessness,[17] pallor,[20] tachycardia, and pulmonary hypertension eventually leading to the late signs cyanosis, slow heart rate, cor pulmonale, and low blood pressure followed by heart failure eventually leading to shock and death.

[33] If tissue is not being perfused properly, it may feel cold and appear pale; if severe, hypoxia can result in cyanosis, a blue discoloration of the skin.

While HIE is associated in most cases with oxygen deprivation in the neonate due to birth asphyxia, it can occur in all age groups, and is often a complication of cardiac arrest.

[47] In order to support continuous growth and proliferation in challenging hypoxic environments, cancer cells are found to alter their metabolism.

[59] In systemic tissues, oxygen again diffuses down a concentration gradient into cells and their mitochondria, where it is used to produce energy in conjunction with the breakdown of glucose, fats, and some amino acids.

[65] G-LOC, or g-force induced loss of consciousness, is a special case of ischemic hypoxia which occurs when the body is subjected to high enough acceleration sustained for long enough to lower cerebral blood pressure and circulation to the point where loss of consciousness occurs due to cerebral hypoxia.

[67] This can be caused by alterations in respiratory drive, such as in respiratory alkalosis, physiological or pathological shunting of blood, diseases interfering in lung function resulting in a ventilation-perfusion mismatch, such as a pulmonary embolus, or alterations in the partial pressure of oxygen in the environment or lung alveoli, such as may occur at altitude or when diving.

Some chronic neuromuscular disorders such as motor neuron disease and muscular dystrophy may require ventilatory support in advanced stages.

Such situations may lead to unconsciousness without symptoms since carbon dioxide levels remain normal and the human body senses pure hypoxia poorly.

[63]: 997–99 Anemia is typically a chronic process that is compensated over time by increased levels of red blood cells via upregulated erythropoetin.

In air breathing vertebrates this is based on lungs to acquire the oxygen, hemoglobin in red corpuscles to transport it, a vasculature to distribute, and a heart to deliver.

[89] Hypoxic ventilatory response (HVR) is the increase in ventilation induced by hypoxia that allows the body to take in and transport lower concentrations of oxygen at higher rates.

[91] There are several potential physiologic mechanisms for hypoxemia, but in patients with chronic obstructive pulmonary disease (COPD), ventilation/perfusion (V/Q) mismatching is most common, with or without alveolar hypoventilation, as indicated by arterial carbon dioxide concentration.

Hypoxemia normally stimulates ventilation and produces dyspnea, but these and the other signs and symptoms of hypoxia are sufficiently variable in COPD to limit their value in patient assessment.

[6] The brain has relatively high energy requirements, using about 20% of the oxygen under resting conditions, but low reserves, which make it specially vulnerable to hypoxia.

but under conditions when there is insufficient oxygen available, increased blood flow may not be sufficient to compensate, and hypoxia can result in brain injury.

In addition, presynaptic neurons release large amounts of glutamate which further increases Ca2+ influx and causes catastrophic collapse in postsynaptic cells.

If the hypoxia is not too severe, cells can suppress some of their functions, such as protein synthesis and spontaneous electrical activity, in a process called penumbra, which is reversible if the oxygen supply is resumed soon enough.

[81] Energy metabolism in the affected area shifts from mitochondrial respiration to anaerobic glycolysis almost immediately, with concurrent reduction of effectiveness of contractions, which soon cease.

Arterial flow must be restored to return to aerobic metabolism and prevent necrosis of the affected muscle cells, but this also causes further damage by reperfusion injury.

The formation of new blood vessels is necessary for continued tumor growth, and is also an important factor in metastasis, as the route by which cancerous cells are transported to other sites.

[8] Both acute and chronic hypoxia and hypercapnia caused by respiratory dysfunction can produce neurological symptoms such as encephalopathy, seizures, headache, papilledema, and asterixis.

[8] Anemic Hypoxia is caused by a deficit in oxygen-carrying capacity, usually due to low hemoglobin levels, leading to generalised inadequate oxygen delivery.

[54] Prevention can be as simple as risk management of occupational exposure to hypoxic environments, and commonly involves the use of environmental monitoring and personal protective equipment.

Studies of miners and astronomers working at 3000 meters and above show improved alveolar PO2 with full acclimatization, yet the PO2 level remains equal to or even below the threshold for continuous oxygen therapy for patients with chronic obstructive pulmonary disease (COPD).

[8] Decreasing metabolic rate by reducing body temperature lowers oxygen demand and consumption, and can minimise the effects of tissue hypoxia, especially in the brain, and therapeutic hypothermia based on this principle may be useful.

[104][105] It is the definitive treatment for severe decompression sickness, which is largely a condition involving localized hypoxia initially caused by inert gas embolism and inflammatory reactions to extravascular bubble growth.

This is recorded in underwater diving incidents, where drowning has often been given as cause of death, high altitude mountaineering, where exposure, hypothermia and falls have been consequences, flying in unpressurized aircraft, and aerobatic maneuvers, where loss of control leading to a crash is possible.