Hypoxia (medicine)
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 inhaled 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.
detectable_content:postscript