The pathway by which atmospheric oxygen, which constitutes 21 % of the air we breathe at sea level, is transported from the air to the mitochondria in the muscle cells is shown diagrammatically in Exercises 2.1.
When we inhale, oxygenated air is transported deep into the lungs into the terminal air passages, which end in small sacklike projections called alveoli. Each alveolus is really only a very thin semispherical membrane, on the surface.
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The pathways by which oxygen is transported from atmospheric air to the active muscles of which run many small, equally thin-walled blood vessels, the capillaries of the pulmonary (lung) circulation. The alveolus is the meeting place of the oxygen in the air and the red blood cells contained in the blood. Red blood cells approaching the lung are oxygen depleted and carbon dioxide loaded (carbon dioxide is the major waste product of mitochondrial energy production).
As blood passes through the lungs, the carbon dioxide is released and a small fraction of the oxygen dissolves into the blood across the alveolar membrane, while a much larger fraction is taken up by the protein hemoglobin, which exists in the red blood cells and has a high attraction, or affinity, for oxygen. So combined, the oxygen is transported in the bloodstream to all the cells of the body, and the carbon dioxide is exhaled.
During exercise, the oxygen requirements of the active muscles can increase virtually instantaneously by as much as 20-fold, whereas the requirements of the inactive muscles remain unchanged. Clearly, some mechanism must exist to ensure that the increased blood flow during exercise goes to the right muscles. In brief, the increased activity of the skeletal muscles increases their demands for ATP to fuel the actin-myosin interactions. The metabolic breakdown products of ATP then act as a complex signal that causes both an increase in the rate at which fuels enter the mitochondria and an almost instantaneous dilation of the blood vessels supplying the active muscles. Simultaneously, these changes selectively increase both the energy and oxygen supplies to the active muscles.
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