What is the major function of the respiratory system ?

The human respiratory system is a series of organs that are responsible for breathing (taking in oxygen and expelling carbon dioxide)

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when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.