Just like a candle under an inverted water glass, your fire is quickly extinguished without oxygen. Of the many tasks that your body performs the most critical one is how you extract oxygen from the air you breathe and convey it to the many trillions of cells where the oxygen is used to help generate energy.
You can live a life without thought (some people do), you can live a life without sex (if you had to), you can live for weeks without food, you can live days without water, but you can only live four minutes without oxygen. Oxygen is the breath of life.
Exercise physiologists know how to measure how effective you are in using oxygen. First they put a clip on your nose and a mouthpiece with tubing in your mouth, connect your expired air in a bag, and then start you on a treadmill or exercise bike for a few minutes. When you are at max effort, really puffing and puffing, the amount of oxygen that you use is determined by an oxygen meter. The amount is measured by volume per minute. This value goes by the term VO2 Max.
Like other body functions VO2 Max goes down with age, but not surprisingly the rate of decline depends strictly on your fitness level. My friend Steve Blair, now at the CDC, wrote the most critical paper on this topic in the JAMA "Physical Fitness and all Cause Mortality" (1). In it he showed that in fit people VO2 max declines at a graceful rate of a half a percent per year. But if you are the typical American couch potato your VO2 max declines at the rate of 2 percent per year, four times as fast, but still pretty slow over a short period of time. But multiply the 2 percent times 30 years and you see the dire prediction that results. Steve showed that fitness's high VO2 max level confers a huge survival advantage, so huge that it is calculated to represent a 30 year survival benefit. The fit person of 80 has the same risk as an unfit person of 50.
Three years ago I had my own personal VO2 Max measured in a lab at the University of Sydney by Maria Fiatarone, superb physiologist. On a treadmill I huffed and puffed and breathed and breathed. At the conclusion Maria said, "Congratulations, Walter, you're 50 years old." The calendar says 80, but the treadmill says 50, solely because I'm fit.
How old am I, really?
1) Blair SN Physical Fitness and All Cause Mortality JAMA 1989; 262:2395-2401.