Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
Imagine that you're exercising. You're working up a sweat, you're breathing
hard, your heart is thumping, blood is coursing through your vessels to deliver
oxygen to the muscles to keep you moving, and you sustain the activity for more
than just a few minutes. That's aerobic exercise (also known as "cardio" in gym lingo); any activity that you can
sustain for more than just a few minutes while your heart, lungs, and muscles
work overtime. In this article, I'll discuss the mechanisms of aerobic exercise:
oxygen transport and consumption, the role of the heart and the muscles, the
proven benefits of aerobic exercise, how much you need to do to reap the
benefits, and more.
It all starts with breathing. The average healthy adult inhales
and exhales about 7 to 8 liters of air per minute. Once you fill your lungs, the
oxygen in the air (air contains approximately 20% oxygen) is filtered through
small branches of tubes (called bronchioles) until it reaches the alveoli. The alveoli are microscopic sacs where oxygen diffuses (enters) into the blood. From there, it's a beeline direct to the heart.
Getting to the heart of it
The heart has four chambers that fill with blood
and pump blood (two atria and two ventricles) and some very active
coronary arteries. Because of all this action, the heart needs a fresh supply of
oxygen, and as you just learned, the lungs provide it. Once the heart uses what
it needs, it pumps the blood, the oxygen, and other nutrients out through the
large left ventricle and through the circulatory system to all the organs,
muscles, and tissues that need it.
A whole lot of pumping going on
Your heart beats approximately 60-80 times
per minute at rest, 100,000 times a day, more than 30 million times per year,
and about 2.5 billion times in a 70-year lifetime! Every beat of your heart
sends a volume of blood (called stroke volume -- more about that later), along
with oxygen and many other life-sustaining nutrients, circulating through your
body. The average healthy adult heart pumps about 5 liters of blood per minute.
Oxygen consumption and muscles
All that oxygen being pumped by the blood is
important. You may be familiar with the term "oxygen consumption." In science,
it's labeled VO2, or volume of oxygen consumed. It's the amount of oxygen the
muscles extract, or consume from the blood, and it's expressed as ml/kg/minute
(milliliters per kilogram of body weight). Muscles are like engines that run on
fuel (just like an automobile that runs on fuel); only our muscles use fat and
carbohydrates instead of gasoline. Oxygen is a key player because, once inside the
muscle, it's used to burn fat and carbohydrate for fuel to keep our engines
running. The more efficient our muscles are at consuming oxygen, the more fuel
we can burn, the more fit we are, and the longer we can exercise.
I've heard that exercising at a lower heart rate burns more fat but fewer calories than exercising at a higher heart rate. Which form of exercise (slow or fast) will help me lose weight the fastest?
Author: Richard Weil, M.Ed., CDE
Medical Editor: Melissa Conrad Stöppler, MD
At lower intensities of exercise, muscles burn a higher percentage of fat than carbohydrate, but not necessarily more total fat, or more total calories, than at higher intensities. This is a subtle distinction, but it's an important one. Here's some background to help understand why.
You get more bang for the buck when muscles burn fat compared with carbohydrate, because fat has more than twice the number of calories (nine calories vs. four calories per gram). Fat is the high-test fuel.