Blood gas tension Blood gas tension Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension; the most common gas tensions measured are oxygen tension (PO), the carbon dioxide tension (PCO) and carbon monoxide tension (PCO). The subscript "x" in each symbol represents the source of the gas being measured; ""a"" meaning arterial, ""A"" being alveolar, ""v"" being venous, ""c"" being capillary. Blood gas tests (such as arterial blood gas tests) measure these partial pressures. PO – Partial pressure of oxygen at sea level (765 mmHg) in arterial blood is between 75
Fetal hemoglobin of the chorionic plate, where it bathes the chorionic villi that carry umbilical capillary beds, thereby allowing gas exchange to occur between the maternal and fetal circulation. Deoxygenated maternal blood drains into open ended intervillous venules to return to maternal circulation. Due to the admixture of oxygenated and deoxygenated blood, maternal blood in the intervillous space is lower in oxygen than arterial blood. As such, fetal hemoglobin must be able to bind oxygen with greater affinity than adult hemoglobin in order to compensate for the relatively lower oxygen tension of the maternal blood supplying the chorion. Fetal hemoglobin's affinity for