A blood analysis apparatus is provided for use in a system in which blood from a patient is provided to an oxygenator 40 and in which blood from the oxygenator is provided back to the patient. The apparatus comprises a first group of sensors 6, 7 for directly measuring a corresponding first group of properties of the blood provided to or from the oxygenator； and a second group of sensors 1, 2, 3, 4 for directly measuring a corresponding second group of properties of a gas provided to or from the oxygenator. The apparatus also comprises a processor 60 for determining a value for the partial pressure of oxygen in the blood and/or a value for the partial pressure of carbon dioxide in the blood in dependence upon the first group of directly measured properties and the second group of directly measured properties. Preferably, the first group of sensors measures its corresponding property without coming into contact with the blood.
