The lung is the organ for gas exchange; it transfers oxygen from the air into the blood, and carbon dioxide (a waste product of the body) from the blood into the air. To accomplish gas exchange, the lung has two components: airways and alveoli. The airways are branching, tubular passages that allow air to move in and out of the lungs.
The wider segments of the airways are the trachea and the two bronchi (going to either the right or left lung). The smaller segments are called bronchioles. At the ends of the bronchioles are the alveoli, thin-walled sacs. (The airways and alveoli can be conceptualized as bunches of grapes with the airways analogous to the stems and the alveoli analogous to the grapes.) Small blood vessels (capillaries) run in the walls of the alveoli, and it is across the thin walls of the alveoli where gas exchange between air and blood takes place.
Breathing involves inspiration followed by exhalation. During inspiration, muscles of the diaphragm and the rib cage contract, expanding the size of the chest (as well as the airways and alveoli) causing negative pressure within the airways and alveoli. As a result, air is sucked through the airways and into the alveoli. During exhalation, the same muscles relax to their resting positions, shrinking the chest and creating positive pressure within the airways and alveoli. As a result, air is expelled from the lungs.
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