Nutrition Science and Everyday Application - beta v 0.1

552 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
Figure 10.1. Anaerobic vs aerobic metabolism. Note that carbohydrate is the only fuel utilized
in anaerobic metabolism, but all three macronutrients can be used for fuel during aerobic
metabolism.
PHYSICAL ACTIVITY DURATION AND FUEL USE
The respiratory system plays a vital role in the uptake and delivery of oxygen to muscle cells
throughout the body. Oxygen is inhaled by the lungs and transferred from the lungs to the
blood, where the cardiovascular system circulates the oxygen-rich blood to the muscles. The
oxygen is then taken up by the muscles and can be used to generate ATP. When the body is
at rest, the heart and lungs are able to supply the muscles with adequate amounts of oxygen
to meet the energy needs for aerobic metabolism. However, during physical activity, your
muscles need more energy and oxygen. In order to provide more oxygen to the muscle cells,
your heart rate and breathing rate will increase. The amount of oxygen that is delivered to
the tissues via the cardiovascular and respiratory systems during exercise depend on the
duration, intensity and physical conditioning of the individual.
• During the first few steps of exercise, your muscles are the first to respond to
the change in activity level. Your lungs and heart do not react as quickly, and
during those beginning steps, they can’t yet increase the delivery of oxygen. In
order for our bodies to get the energy that is needed in these beginning steps, the
muscles rely on a small amount of ATP that is stored in resting muscles. The stored
ATP is able to provide energy for only a few seconds before it is depleted. Once the