Inspired normoxic dry air has a pO2 of 21 kPa or 160 mmHg. Upon entering the lungs this is reduced, first by humidification, to 20 kPa (150 mmHg), and due to the constant removal of oxygen by the pulmonary capillaries, the pO2 in the alveolae is approximately 14 kPa (100 mmHg). In the blood, the oxygen tension is normally between 5 and 13 kPa (40-90 mmHg), venous to arterial side, respectively. Once the oxygen reaches the single cells in the tissue, the pO2 goes down to 0.5-2.5 kPa (4-20 mmHg), being the lowest in the mitochondria. It is therefore obvious that only small changes in oxygen pressure, uptake or delivery can greatly influence the amount of oxygen that can be utilized by the mitochondria for respiration, thereby possibly creating a supply-demand mismatch.
Additional keywords: air,pascal,kPa,tissue,lungs