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Symbol: O Atomic No: 8Oxygen
NitrogenFluorine Sulphur Data
Molecular weight 32.00

Density 1.335kg/m3 (at 15°C)

Boiling point -183.1°C (at 1 bar)

Importance in Man of Oxygen
Essential to aerobic metabolism & human life. Critical evolutionary interactions
Web Resources for Oxygen
Relevant Clinical Literature
UK Guidance

Element. Present in the atmosphere (21%) as the molecule O2 and higher up as O3 - Ozone. Odourless, colourless gas (at 15°C and 1 bar), pale blue as a liquid. Essential for survival as required for aerobic metabolism. The oxygenation of organic compounds is an essential feature of many biochemical processes.


Oxygen in Metabolism

  • Aerobic respiration, mitochondria
  • Basal oxygen consumption about 250ml/min for a body surface of 1.8m2.
    • Natural sleep reduces this by about 10%
    • Each 1°C fall in core body temperature will reduce by about 5%

Oxygen Delivery

Oxygen delivery = DO2 = CaO2 x CO

  • Where CaO2 is total oxygen content of arterial blood and CO is cardiac output.
CaO2 = (O2 carried by Hb) + (O2 in solution) = (1.34 x Hb x SpO2% x 0.01) + (0.023 x PaO2)
  • SO2 = percentage saturation of haemoglobin with oxygen
  • Hb = haemoglobin concentration in grams per 100 ml blood (g/dl)
  • PO2 = partial pressure of oxygen (0.0225 = ml of O2 dissolved per 100 ml plasma per kPa, or 0.003 ml per mmHg)

Although not tightly regulated it is actually one of the most important physiological variables in determining physiological organ viability. Oxygen content of the blood is determined predominantly by the concentration of haemoglobin and the oxygen saturation - and to a much lesser extent by the partial pressure. Accordingly as demonstrated in infective shock it is mixed venous oxygen saturation (SvO2) that correlates better with outcome than the more usually determined arterial oxygen saturation (SaO2).

See also:

LogoKeyPointsBox.pngVenous oxygen saturation is a better indicator of cardiac output than is arterial saturation. For a given oxygen uptake by the peripheral tissues the lower the cardiac output is, the less oxygen will remain in the venous blood returning from those tissues.

Peripheral oxygen saturation (SpO2) is even easier to determine with pulse oximetry but may not reflect oxygen delivery to more central organs. Supra-physiological oxygen delivery to the tissues does not show benefit in critical illness no doubt due to oxygen toxicity, with for example myocardial infarction where treatment with oxygen is associated with worse prognosis[1] fully fitting the known cardiovascular effects of hyperoxaemia. Patients with COPD have a higher mortality if given more oxygen than required[2] reinforcing recommendations to maintain pO2 at no more than 88-92%[3].

Normal adult male values at sea level (on Earth)

  • Oxygen content of arterial blood = CaO2 = 20.4 ml/100 ml
  • Oxygen content of mixed venous blood (for oxygen saturation (SvO2) = 75% and venous partial pressure of oxygen (PvO2) = 6 kPa ) = CvO2 = 15.2 ml/100 ml
  • The in vitro maximum oxygen carrying capacity is 1.39 ml O2/g Hb
  • The in vivo maximum oxygen carrying capacity is 1.34 ml O2/g Hb (Hüfner’s constant).

Neonatal oxygen delivery

The evidence base suggests no role for high inspired oxygen values in initial resuscitation of babies born at or after term as room air is associated with at least a 30% lower mortality than 100% oxygen[4]. Matters are more complex with pre-term children where a target range of oxygen saturation of (91 to 95%) is associated with lower mortality but more retinopathy of prematurity than a target of 85 to 89%[5]. The present international guidelines recommend initial resuscitation with blended oxygen and air to reach target oxygen saturations. However room air is an alternative, although it will risk hypoxaemia in some pre-term infants. 100% oxygen is not recommended for initial premature infant resusciation[4]. The variability of practice world wide reflect that oxygen therapy was established in neonates by 1780 and that it took some time after the 1998 WHO guidelines for evidence base reinforcement to change practice in cultures where oxygen supplementation availability was rarely an issue at birth. This started to happen in a major way in 2006.


Oxygen in Clinical Practice

"Pink" gas. Often useful for blue patients.

LogoWarningBox4.pngOxygen supports combustion but does not, of course, explode. Folk stories that smoking near oxygen equipment "may cause an explosion" abound, and are wrong. The one exception is if you have another inflammable vapour in the air so setting up a still next to your oxygen concentrator would be interesting. Smoking in an atmosphere even slightly enriched with oxygen may kill the smoker and has burnt their bed partner, due to secondary consequences of the enhanced burning of the cigarette causing a conflagration.

Any compressed gas cylinder can explode in a fire, but Oxygen is less frightening than Nitrous Oxide or Acetylene in this respect.

Oxygen Toxicity

Oxygen is potentially toxic, as a function of the

  • Protective mechanisms present (neonates are more sensitive than adults due to their differentially developed enzyme systems)
  • Partial pressure present
  • Time organism exposed to that partial pressure.

Free radical oxygen is actually used as a defence mechanism in the body, and can be generated by enzyme systems as well as being destroyed by scavenger enzyme systems. Such scavenger enzyme systems may be less effective in some tissues such as the retina. Ozone (O3) is toxic to most life forms at relatively low concentrations.

A popular, yet easy to oversimplify theory of aging and degenerative disease supposes the accumulation of oxidative damage over time.

Oxygen in evolution

Its key role is accepted, not only because its presence allows a much more complex and diverse biochemistry and fauna[6], but because it has driven organ structure[7] and has still active postulated selective roles[8].


  1. Wijesinghe M, Perrin K, Ranchord A, Simmonds M, Weatherall M, Beasley R. Routine use of oxygen in the treatment of myocardial infarction: systematic review. Heart (British Cardiac Society). 2009 Mar; 95(3):198-202.(Link to article – subscription may be required.)
  2. Effect of high flow oxygen on mortality in chronic obstructive pulmonary disease patients in prehospital setting: randomised controlled trial. l A Austin MA, Wills KE, Blizzard L, Walters EH, Wood-Baker R. BMJ 341:doi:10.1136/bmj.c5462
  3. O'Driscoll BR, Howard LS, Davison AG. BTS guideline for emergency oxygen use in adult patients. Thorax. 2008 Oct; 63 Suppl 6:vi1-68.(Link to article – subscription may be required.)
  4. a b Perlman JM, Wyllie J, Kattwinkel J, Atkins DL, Chameides L, Goldsmith JP, Guinsburg R, Hazinski MF, Morley C, Richmond S, Simon WM, Singhal N, Szyld E, Tamura M, Velaphi S. Part 11: neonatal resuscitation: 2010 International Consensus on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science With Treatment Recommendations. Circulation. 2010 Oct 19; 122(16 Suppl 2):S516-38.(Link to article – subscription may be required.)
  5. Carlo WA, Finer NN, Walsh MC, Rich W, Gantz MG, Laptook AR, Yoder BA, Faix RG, Das A, Poole WK, Schibler K, Newman NS, Ambalavanan N, Frantz ID, Piazza AJ, Sánchez PJ, Morris BH, Laroia N, Phelps DL, Poindexter BB, Cotten CM, Van Meurs KP, Duara S, Narendran V, Sood BG, O'Shea TM, Bell EF, Ehrenkranz RA, Watterberg KL, Higgins RD. Target ranges of oxygen saturation in extremely preterm infants. The New England journal of medicine. 2010 May 27; 362(21):1959-69.(Link to article – subscription may be required.)
  6. Jiang YY, Kong DX, Qin T, Zhang HY. How does oxygen rise drive evolution? Clues from oxygen-dependent biosynthesis of nuclear receptor ligands. Biochemical and biophysical research communications. 2010 Jan 8; 391(2):1158-60.(Link to article – subscription may be required.)
  7. Mess AM, Ferner KJ. Evolution and development of gas exchange structures in Mammalia: The placenta and the lung. Respiratory physiology & neurobiology. 2010 Jan 18.(Epub ahead of print) (Link to article – subscription may be required.)
  8. Bigham AW, Mao X, Mei R, Brutsaert T, Wilson MJ, Julian CG, Parra EJ, Akey JM, Moore LG, Shriver MD. Identifying positive selection candidate loci for high-altitude adaptation in Andean populations. Human genomics. 2009 Dec; 4(2):79-90.
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