Pathophysiology
After mixing with water vapor and expired CO2 in the lungs, oxygen diffuses down a pressure gradient to enter arterial blood where its partial pressure is around 100 mmHg (13.3 kPa). Arterial blood flow delivers oxygen to the peripheral tissues, where it again diffuses down a pressure gradient into the cells and into their mitochondria. These bacteria-like cytoplasmic structures strip hydrogen from fuels (glucose, fats and some amino acids) to burn with oxygen to form water. The fuel's carbon is oxidized to CO2, which diffuses down its partial pressure gradient out of the cells into venous blood to be exhaled finally by the lungs. Experimentally, oxygen diffusion becomes rate limiting (and lethal) when arterial oxygen partial pressure falls to 40 mmHg (5.3 kPa) or below.
If oxygen delivery to cells is insufficient for the demand (hypoxia), hydrogen will be shifted to pyruvic acid converting it to lactic acid. This temporary measure (anaerobic metabolism) allows small amounts of energy to be released. Lactic acid build up (in tissues and blood) is a sign of inadequate mitochondrial oxygenation, which may be due to hypoxemia, poor blood flow (e.g., shock) or a combination of both. If severe or prolonged it could lead to cell death.
Read more about this topic: Hypoxia (medical)