Subsequent measurements ( to ensure treatment is effective ), may be taken from a normal blood test taken from a vein, as there is little difference between the arterial and the venous pH . At very high levels, capillary blood ketone measurement becomes imprecise.
22.
Millikan's wartime work outlined three basic problems of oximetry : absence of suitable theory, inability to differentiate between blood and other tissues in the path of light, and inability to differentiate between arterial, venous and capillary blood, of which only arterial blood was relevant to oxygen measurement.
23.
Since the blood arriving in the pulmonary capillaries has a P _ _ 2 } of, on average, 45 mmHg ( 6 kPa ), while the pressure in the alveolar air is 105 mmHg, there will be a net diffusion of oxygen into the capillary blood, changing the composition of the 3 liters of alveolar air slightly.
