From the lungs, the oxygen goes directly into the bloodstream, your body’s “assembly line.” Here’s how it works.
Your lungs contain millions of tiny air sacs, called alveoli, around which the blood flows. These sacs are like toy balloons filled with air and dangling in the liquid of your bloodstream. The air is forced into these sacs by atmospheric pressure. Then, following the Law of Gaseous Diffusion, the oxygen moves from the area of higher pressure in the alveoli to the red blood cells, where the pressure is lower (the red blood cells, remember, are now in effect “empty bottles,” having delivered their supply of oxygen and disposed of the returning wastes).
Remember, the limiting factors here include the number of red blood cells and the amount of hemoglobin they carry. Even if your lungs could process more oxygen, your body tissue still would not receive more unless there were more “bottles” to put it in for delivery.
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This is the role of the training effect. It produces more blood, specifically more hemoglobin, which carries the oxygen, more red blood cells that carry the hemoglobin, more blood plasma that carries the red blood cells, and consequently more total blood volume. Laboratory tests have repeatedly shown that men in good physical condition invariably have a larger blood supply than out-of-condition men of comparable size. An average-size man may increase his blood volume by nearly a quart in response to aerobic conditioning proportionately.
All of which means that there are now more “bottles” not only to deliver the oxygen, but more empties to carry away the wastes. The removal of carbon dioxide and other waste products is just as important in reducing fatigue and increasing endurance as the production of energy. It’s like your own home. You have to take out the trash and garbage regularly.
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When the “bottles” get to the tissue level, unload the oxygen and pick up the wastes from the tissue cells, the process by which they do it is described by that old word we all had so much fun with in Biology 1 in high school – osmosis. The oxygen and food particles, now in liquid form, pass through the cell membrane, and waste products exit the cell in the opposite direction. That is the whole life cycle right there: materials for nourishment and energy going in and wastes going out.
To complete the cycle, when the carbon dioxide and other wastes are carried away in the bloodstream via the veins and reach the lungs, the Law of Gaseous Diffusion now works in reverse. The pressure of the carbon dioxide in the veins is higher than it is in the alveoli, so it passes freely into the alveoli and is exhaled with the expired air.
The efficiency of the cycle, and its capacity, is a function of the training effect. Increase its workload and it increases its efficiency. Sit around and do nothing, and it deteriorates. It’s as simple as that.