oxygen

Oxygen is transported across the alveolar membrane and enters plasma. Some is then taken up by hemoglobin and some remains in plasma. Under normal conditions - that is at a standard atmosphere, which is defined as 1013 hPa or 760 mm Hg - 100 ml of arterial blood carries about 19 ml of oxygen as oxyhaemoglobin and only 0.3 ml in solution. Hence, the latter is often ignored. However only the oxygen in the plasma is available for transport through the capillary wall into the tissues and the concentration or tension determines the rate.

Oxygen has to dissociate from hemoglobin to be available. The plasma oxygen tension breathing air with oxygen at a partial pressure (Dalton's Law) of 2 tenths of an atmosphere (21% of 1 atm abs) is about 95 mm Hg. Increasing the oxygen inspired to 100% multiplies the amount in solution by a factor of 5 - hence (Henrys Law) the amount Carried in solution is multiplied by five to 1.5 ml at 3 atm abs it is 4.5 ml per 100 ml blood, which is the normal arterial - venous difference at rest. Hence, all the requirements of the body can be met by the oxygen in the plasma. However, the gradient is what is so important in therapy - over 2000 mm Hg can be achieved - a more than twenty fold increase. Consequently, life can be supported with Blood for a short time and the paper was published in 1959. Used Properly oxygen is the most powerful therapeutic tool in medicine. We Need to ensue our medical students are taught properly but after 25 Years in this school, we have only just established oxygen therapy in the curriculum.

Philip James M.D.
Wolfson Hyperbaric Medicine Unit
University of Dundee
Reprinted with Permission