Epithelial tissues cover most organs and cavities within the body, forming a barrier to keep different body systems separate and regulate the exchange of materials.
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The oxygen we inhale is absorbed in the alveoli, tiny air sacs in the lungs, where it diffuses into the bloodstream to be transported to the rest of the body.
Oxygen is transported to body cells by binding to hemoglobin in red blood cells, which are carried through the bloodstream to tissues where oxygen is released for cellular respiration.
Cells need oxygen for the functions of mitochondria, which are involved in energy production.
Blood collects waste from all parts of the body and carries it to the liver and kidneys for disposal.