Answer:
Gas exchange is the delivery of oxygen from the lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs. It occurs in the lungs between the alveoli and a network of tiny blood vessels called capillaries, which are located in the walls of the alveoli.
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Explanation:
A covalent bond is formed when atons share electrons in their shell
This link should help you understand http://www.universetoday.com/20289/phases-of-the-moon/#
The heart, blood and blood vessels work together to service the cells of the body. Using the network of arteries, veins and capillaries, blood carries carbon dioxide to the lungs and picks up oxygen. From the small intestine, the blood gathers food nutrients and delivers them to every cell.
Cells, tissues, organs, organ systems, organisms