No matter what you're doing---watching TV or running a marathon, chores around the house or hitting the weights at the gym---one of your body's most important tasks is to supply oxygen to the cells. Two major physiological cycles are involved in accomplishing this goal: breathing brings oxygen into the lungs and circulation distributes it throughout the body.
Inhalation
The first step in supplying cells with oxygen is to get the oxygen from the outside air into the lungs, which are specialized oxygen-transfer organs. In fact, inhaling air isn't actually accomplished through any activity on the part of the lungs themselves, says Linda Costanzo, Ph.D., in her textbook "Physiology." Rather, she states, the lungs are passive receptacles that simply fill with air as the muscles of inhalation (including the intercostal muscles of the rib cage) create a vacuum in the chest.
Blood Oxygenation
The lungs are full of capillaries, or tiny blood vessels, that are routed very close to the tissue-thin interior surface of the lungs. Oxygen-deficient blood is pushed through these vessels by the heart, which works like a mechanical pump. Oxygen in the lungs from inhaled air then passes directly through the tissue and into the blood flowing through capillaries, according to Dr. Costanzo. The reason for this is the same as the reason that ink spilled into one end of a swimming pool will eventually be distributed evenly throughout the pool---substances passively move from areas of higher concentration to areas of lower concentration.
Circulation
Oxygen-rich blood leaves the small vessels of the lungs and makes its way toward the heart, pulled by the pumping mechanism of the heartbeat. From here, the heart pumps the blood out to all the body cells, which (because they're constantly working hard and consuming oxygen) are oxygen deficient compared to the blood. For the same reason that oxygen passively diffuses from the oxygen-rich lungs to the oxygen-poor blood, oxygen now passes out of the capillaries and into the cells. According to the American Heart Association, it takes approximately 60 to 80 heartbeats per minute to provide enough oxygen to body cells under resting (non-exercise) conditions.
Cellular Metabolism
Cells use the oxygen to burn carbohydrate and other fuels. This reaction is a part of the chemical process called metabolism, which provides the cells with energy. Biochemists Mary Campbell, Ph.D. and Shawn Farrell, Ph.D. explain that chemical "burning" of nutrients is very similar to physical burning of, for instance, wood. In both cases the reaction requires oxygen, releases energy and produces carbon dioxide. While chemical "burning" doesn't involve an actual flame, it nevertheless produces vast amounts of energy that can either be stored or utilized immediately for muscle movement and other cellular activities.
Carbon Dioxide Removal
The carbon dioxide produced by the cells is a metabolic waste product, and must be removed from the body. Because working cells contain high concentrations of carbon dioxide and the blood contains little carbon dioxide, the chemical passes out of the cells and into the blood stream (just like oxygen, but in the opposite direction). The pumping action of the heart returns blood---now depleted of oxygen but rich in carbon dioxide---from the body cells and circulates it back through the heart to the lungs. Here, carbon dioxide passes out of the blood and into the lungs, while oxygen passes from the lungs into the blood, completing the cycle. According to the University of Virginia, an average person breathes oxygen in and carbon dioxide out about 15 to 20 times each minute while at rest. Increased activity increases the cells' need for oxygen (and also increases their production of carbon dioxide), which necessitates more rapid breathing.
References
- "Physiology"; Linda S. Costanzo, Ph.D.; 1998
- "Biochemistry"; Mary K. Campbell, Ph.D., Shawn O. Farrell, Ph.D.; 2009
- American Heart Association: Heart Rate
- University of Virginia: Respiratory Rate
