The job of your respiratory system is very simple: To bring oxygen into your body, and remove the carbon dioxide from your body. Your body needs oxygen to survive.
Oxygen is used by your cells as it performs the functions of life. As your body uses oxygen, your cells produce another gas known as carbon dioxide. Too much carbon dioxide can be toxic, even deadly. For this reason, it is important that your body have a way to get rid of it.
The main organs in your respiratory system are your lungs. Your lungs are, in their simplest form, nothing more than sacs. As you breathe in, you fill these sacs with fresh oxygen-rich air. Your heart pumps blood into the walls of your lungs where it absorbs oxygen and releases carbon dioxide. As you exhale, or breathe out, you release the carbon dioxide-rich air into the space around you. With each breath you take, you are taking oxygen in, and putting carbon dioxide out.
From your lungs, blood returns back into your heart where it is pumped out to the rest of your body, carrying oxygen along with it.