Oxygen partial pressure refers to the partial pressure of blood oxygen, which is the pressure generated by oxygen molecules physically dissolved in the blood. The arterial blood partial pressure of oxygen is largely dependent on the partial pressure of oxygen in the inhaled gas and the functional status of external respiration. The partial pressure of venous blood oxygen may reflect internal respiration. Hypoxemia may occur in respiratory and cardiac diseases, as well as in severe trauma, shock, and multiple organ insufficiency. For partial pressure of oxygen, if it is reduced, it may be seen in some diseases, such as hypopnea of the respiratory center, like encephalitis, pulmonary hemorrhage, pulmonary embolism. Carbon dioxide paralysis, poisoning by certain drugs, neuromuscular diseases like multiple sclerosis, myasthenia gravis, and problems with the thorax, plus acute pancreatitis, chronic bronchitis, emphysema, can lead to a decrease in oxygen partial pressure. Physiologic changes in oxygen partial pressure, which decline with age, with changes in body position, and with chronic smoking may also lead to a decrease in oxygen partial pressure.