Answer:
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Cause it does not have to be followed by and believed by the theory itself.
The aortic valve prevents backward flow of blood into the left ventricle.
Oxygen and carbon dioxide is exchanged by the millions of alveoli in the lungs and the capillaries that envelop them. we inhale oxygen (which is then exchanged with carbon dioxide in the lungs) and then we exhale the carbon dioxide. although, the air that we breathe usually consists more nitrogen than oxygen.
