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Let's talk about the process that happens internally:
When you inhale, your diaphragm contracts and moves downward. The diaphragm is a dome-shaped muscle that separates your chest from the abdomen. When you the diaphragm creates a space in the chest cavity allowing your lungs to expand. The rib cage is also pulled upwards and outwards.
When this happens, your lungs are able to suck air through your nose. The air then travels through your trachea (windpipe) and into your lungs. Inside the lungs, the air goes through the bronchial tubes and then enters the alveoili (the grape-like air sacs).
The alveoili is surrounded by thin cappillaries that allows blood to collect oxygen from your lungs through a protein called hemoglobin. 
At the same time, blood deposits carbon dioxide in the air sac from the right side of the heart through the pulmonary artery.
Meanwhile, the blood that has been refilled with oxygen (called Oxygen-rich blood), is carried through the body i a network of cappillaries passing throught he pulmonary vein. The vein then distributes the blood through the left side of the heart whic then pumps in to the organs of the body.
On the other hand, when you exhale, the diaphragm relaxes, moves upward and fills the chest cavity. As the cavity ges smaller, carbon dioxide, which is still inside your lungs is forced out to the windpipe and then through your nose or mouth. 
To put it in simple terms, breathing is like a baloon. Your diaphragm expands to accomodate air, and if you press the baloon, the air is let out, contracting it.
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