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Understanding the Concept of Aerobic Exercise Capacity

Your capacity for sustained physical activity depends on the amount of energy your body can produce in a process using oxygen called aerobic capacity. Your individual aerobic capacity depends on how well your cardiovascular system — your heart and blood vessels — works. It's the system that moves your blood through your lungs, where it's loaded with oxygen, and then distributes it to every part of your body, where the oxygen sustains life, nourishes your muscles and supports your ability to exercise.

The "motor" of the circulatory system is the heart. The heart is a pump made of live tissue: muscles, supportive tissue and a conduction system that produces the electrical signals that stimulate your heart's pumping action. An empty heart weighs an average of a little more than half a pound (250 to 300 grams) in females and between two-thirds and three-quarters of a pound (300 to 350 grams) in males. It has four chambers: the right atrium, right ventricle, left atrium and left ventricle.

The atria receive blood at low pressure. The right atrium receives venous blood returning to the heart from all over the body after it's been depleted of oxygen. The left atrium receives blood returning to the heart from the lungs after it's been enriched again with oxygen. The ventricles do most of the pumping. The right ventricle pumps blood to and through the lungs, while the left ventricle maintains the circulation of blood throughout the body, to all its organs and tissues. Blood flows through the heart in only one direction, thanks to a system of valves that open and close at just the right time. How hard your heart has to work varies depending on many factors, including your activity level.

On average, a human heart pumps about 2.4 ounces (70 milliliters) of blood per heartbeat — a measure that's known as "stroke volume."

The heart of an individual at rest beats, on average, 72 times per minute (this is your "heart rate"), which results in a cardiac output as follows:
  • 1.3 gallons (5 liters) of blood per minute
  • 1,900 gallons (7,200 liters) per day
  • 700,000 gallons (2,628,000 liters) per year
  • 48 million gallons (184 million liters) over an average life span of 70 years

And that output is just to meet the body's basic metabolic needs at rest: about 3.5 milliliters of oxygen per kilogram of body mass per minute. This resting metabolic rate is designated as one metabolic equivalent, which is expressed as "1 MET." When you exercise, your body's muscles require more oxygen, so your blood flow increases to meet that need; your heart rate may increase threefold, and your stroke volume may double. This increases the cardiac output of a person of average fitness from about 1.3 gallons (5 liters) per minute to between 4 and 5 gallons (15 and 20 liters) per minute and of a top athlete to as much as 10 gallons (40 liters) per minute. Not only does the blood flow increase, but more oxygen is extracted from each unit of blood. As a result of these changes, the metabolic level of a person of average fitness exercising at peak capacity increases to about 12 METs and of a top athlete running a 4:17 mile (or a 22.5-kilometers-per-hour pace) may increase to 23 METs.

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