The manifestations and causes of hypoxia

Signs and symptoms
Arterial oxygen tension can be measured by blood gas analysis of an arterial blood sample, and less reliably by pulse oximetry, which is not a complete measure of circulatory oxygen sufficiency. If there is insufficient blood flow or insufficient hemoglobin in the blood (anemia), tissues can be hypoxic even when there is high arterial oxygen saturation.

Cyanosis
Headache
Decreased reaction time, disorientation, and uncoordinated movement.
Impaired judgment, confusion, memory loss and cognitive problems.
Euphoria or dissociation
Visual impairment A moderate level of hypoxia can cause a generalized partial loss of color vision affecting both red-green and blue-yellow discrimination at an altitude of 12,000 feet (3,700 m).
Lightheaded or dizzy sensation, vertigo
Fatigue, Drowsiness or tiredness
Shortness of breath
Palpitations may occur in the initial phases. Later, the heart rate may reduce significantly degree. In severe cases, abnormal heart rhythms may develop.
Nausea and vomiting
Initially raised blood pressure followed by lowered blood pressure as the condition progresses.
Severe hypoxia can cause loss of consciousness, seizures or convulsions, coma and eventually death. Breathing rate may slow down and become shallow and the pupils may not respond to light.
Tingling in fingers and toes
Numbness

Causes
Oxygen passively diffuses in the lung alveoli according to a concentration gradient, also referred to as a partial pressure gradient. Inhaled air rapidly reaches saturation with water vapour, which slightly reduces the partial pressures of the other components. Oxygen diffuses from the inhaleded air, to arterial blood, where its partial pressure is around 100 mmHg (13.3 kPa).In the blood, oxygen is bound to hemoglobin, a protein in red blood cells. The binding capacity of hemoglobin is influenced by the partial pressure of oxygen in the environment, as described by the oxygen–hemoglobin dissociation curve. A smaller amount of oxygen is transported in solution in the blood.

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. Hypoxia can result from a failure at any stage in the delivery of oxygen to cells. This can include low partial pressures of oxygen in the breathing gas, problems with diffusion of oxygen in the lungs through the interface between air and blood, insufficient available hemoglobin, problems with blood flow to the end user tissue, problems with the breathing cycle regarding rate and volume, and physiological and mechanical dead space Experimentally, oxygen diffusion becomes rate limiting when arterial oxygen partial pressure falls to 60 mmHg (5.3 kPa) or below.

Almost all the oxygen in the blood is bound to hemoglobin, so interfering with this carrier molecule limits oxygen delivery to the perfused tissues. Hemoglobin increases the oxygen-carrying capacity of blood by about 40-fold, with the ability of hemoglobin to carry oxygen influenced by the partial pressure of oxygen in the local environment, a relationship described in the oxygen–hemoglobin dissociation curve. When the ability of hemoglobin to carry oxygen is degraded, a hypoxic state can result.