O2 Basics by Dr AA Khan #1

Back To Basics – O2 Physiology


The composition of dry air is 20.98% O2, 0.04% CO2, 78.06% N2, and 0.92% other inert constituents such as argon and helium. The movement of this oxygen from the outside environment to the cells within the tissues, and the transport of carbon dioxide in the opposite direction is defined as physiological respiration. Biologically, respiration can also be defined as a process of producing cellular energy. Where oxygen is present in this metabolic process aerobic respiration take place i.e. Glucose + Oxygen = Carbon Dioxide + Water + Energy.


Physiological respiration includes two processes:

  1. External respiration: the absorption of O2 and removal of CO2 from the body as a whole
  2. Internal respiration: the utilization of O2 & production of CO2 by cells and the gaseous exchanges between the cells and their fluid medium.


The O2 delivery system in the body consists of the lungs and the cardiovascular system. O2 delivery to a particular tissue depends on the amount of O2 entering the lungs, the adequacy of pulmonary gas exchange, the blood flow to the tissue, and the capacity of the blood to carry O2. The amount of O2 in the blood is determined by the amount of dissolved O2, the amount of haemoglobin in the blood, and the affinity of the haemoglobin for O2.


Clinical Pearls:

  • BREATHING is physiological respiration i.e. inhaling oxygen from the air and exhaling carbon dioxide
  • Aerobic respiration = Long term energy using glucose + oxygen.
  • Anaerobic respiration = Short term energy using only glucose, resulting in lactic acid production.
  • Tissue O2 levels depend on : the amount of O2 inhaled, pulmonary gas exchange efficiency, blood flow and the blood capacity to hold O2
  • Blood O2 capacity depends on: amount of dissolved O2, amount of haemoglobin present, and the ability of the haemoglobin to transport O2.



Dr. Aadil Ahmed Khan

MBChB, BMedSc, HMedSc (Physiology)

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