|MadSci Network: General Biology|
G'day Troy As a person breathes, the body consumes oxygen and makes carbon dioxide and water vapour. A person breathing normal air (20% oxygen) typically only uses about a quarter of the oxygen in the air that is breathed in. The rest is breathed out along with nitrogen and carbon dioxide and water vapour. So the answer to your question depends upon the volume of (originally fresh) air that is available to re-breathe. Let's assume this is ten times the volume breathed in at each breath (you can easily see how to work out for other multiples than ten). Then the air breathed out will be richer in carbon dioxide and water vapor like this: First breath 15% oxygen 5% carbon dioxide , diluted 10 to 1 by the unused-yet air. So (with complete mixing) the reservoir of formerly fresh air is now: 19.5% oxygen 0.5% carbon dioxide. As you can see the carbon dioxide level builds up and each breath gets continually evermore contaminated. After ten breaths the air is 15% oxygen and 4% carbon dioxide. (If you find computing fun, write a little program to simulate this) After 20 breaths 11% oxygen and 8% carbon dioxide. Clearly there is a limit to how long this can continue. The result will be carbon dioxide poisoning, or as some people like to say hypercapnia. Here are some results I found for you on the web. See if you can find some better ones by using these search phrases in Google: "elevated CO2" effects "elevated CO2" effects human beings “hypercapnia” human health 1) Hypercapnia lowers the shivering threshold and increases core cooling rate in humans. It has intense effects on human temperature regulation. Research work, both current and past have been done on this subject. Johnston CE, DA Elias, AE Ready and GG Giesbrecht. "Hypercapnia lowers the shivering threshold and increases core cooling rate in humans". Aviat Space Environ Med. 67: 438-444, 1996 V Lun, JCL Sun, GG Giesbrecht and IB Mekjavic. Shivering thermogenesis during acute hypercapnia. Can J Physiol Pharmacol. 72: 238-242, 1994. 2) Effects of Hypoxia and Hypercapnia on Human Red Blood Cell ATP Release. I found a mention of this as part of a project details of which are given below: Upstate Medical University Department of Medicine Student Research Fellowship - Summer 1999 Mentors: Dr. Tawfic Hakim, Associate Professor, Department of Medicine Dr. Jeffrey Freedman, Associate Professor, Department of Physiology Project. 3) Effect of carbon dioxide on diaphragmatic function in human beings. "...Our results show that contractility was reduced with hypercapnia (when end-tidal carbon dioxide was 7.5 per cent or higher), although hypocapnia (end-tidal carbon dioxide, 3 per cent) had no effect on diaphragmatic contractility. We also studied the development of diaphragmatic fatigue before and during carbon dioxide breathing...Electromyographic signs of fatigue appeared at a lower tension-time index during hypercapnia than during normocapnia, indicating that endurance is diminished during hypercapnia." The New England Journal of Medicine.Volume 310:874-879 April 5, 1984 Number 14 4) Physical effects on human beings are described in the extract given below: "With respect to the direct health effects of CO2-enriched air, it is known that very high concentrations of atmospheric CO2 can produce a state of hypercapnia (Nahas et al., 1968; Brackett et al., 1969; van Ypersele de Strihou, 1974) or an excessive amount of CO2 in the blood that typically results in acidosis (Poyart and Nahas, 1968; Turino et al., 1974), which is a serious and sometimes fatal condition characterized in humans by headache, nausea and visual disturbances. However, several studies have indicated that these phenomena have little to no negative impact on human health until the CO2 concentration of the air reaches approximately 15,000 ppm (Luft et al., 1974; Schaefer, 1982 5) Van Ypersele de Strihou, C. 1974. Acid-base equilibrium in chronic hypercapnia. In: Carbon Dioxide and Metabolic Regulations. G. Nahas and K.E. Schaefer (Eds.). Springer-Verlag, New York, NY, pp. 266. What this all adds up to, Troy, is you are ok until you total breaths equal the volume of the originally fresh air you have available to breathe. Beyond that take heed of any nausea, headaches or visual disturbances. So take care and have fun with your diving! John
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