The Fuel of Life
Understanding Oxygen's Role in Exercise
Oxygen Saturation (O2) This is not intended to diagnose, but to inform. See if a physician if you are concerned about your respiratory health. It is a fact of life that our bodies survive by specific elements, specific to this post oxygen (O2). O2 from the environment is inhaled in exchange for an exhale of carbon dioxide (CO2). Through alveoli, small sacs, bodily gases are exchange through capillaries. Hemoglobin (Hb) is the primary transporter of O2 to tissues and has 2 states: oxygenated Hb & deoxygenated Hb. This allows us to measure a percentage of oxygen in the blood (PaO2, Pa meaning partial pressure). PaO2 outside of a lab setting is usually measured by a finger pulse oximeter. As a quick non-invasive measurement, a useful tool, but should be used with caution having many factors that can offset accuracy (1,2). As a guide we recommend their use. As a general medical guide 90% O2 saturation is the standard for concern, however because of bias 92% is a more conservative level that should be considered (2). Inadequate oxygen in the blood (hypoxemia) and in the tissue (hypoxia) can lead to serious consequences. In exercise O2 demands increase exponentially, in proportion to the metabolic stimulus placed on the body (3). For a brief period, the body goes into “oxygen debt” fallings short of the O2 demand. This is very quickly adapted to, by negative feedback. Decreases in blood pH trigger chemoreceptors signaling the medulla oblongata that the body is not in acute homeostasis (4). The response is an increase in cardiac output by increasing the HR. As the body approaches the balance needed for oxygen demand and the RR is decreased, this is called steady state (5). As a side note, the respiratory system is the most rapid physiological response to blood pH balance (6). The cycle of achieved steady state can accomplished at different levels with several underlying factors. At the termination of exercise O2 remains at a higher than resting state, decreasing linearly as homeostasis is achieved and recovery demands are met; this is referred to as “excess post-exercise oxygen consumption” (EPOC)(7). PaO2 doesn’t need to be unnecessarily measured.
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Sources:
1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5535407/ (pulse oxy bias)
2. https://www.thoracic.org.au/journal-publishing/command/download_file/id/34/filename/TSANZ-AcuteOxygen-Guidelines-2016-web.pdf
3. Quantitative Human Physiology: An Introduction. Joseph Feher – Elsevier Academic Press (2017), – Chap 6.4, pp. 593
4. Exercise Physiology: From Theory and Application to Fitness and Performance 9th ed – McGraw Hill (2015), - Chap 10, pp. 235
5. https://www.ncbi.nlm.nih.gov/pubmed/28818484
6. http://www.anaesthesia.med.usyd.edu.au/resources/lectures/acidbase_mjb/control.html
7. Exercise Physiology: From Theory and Application to Fitness and Performance 9th ed – McGraw Hill (2015), - glossary, pp. G-4