Breathing. It's so easy you can do it in your sleep. Although respiration is a function we can do without conscious effort, it involves a lot of parts. One of the most important parts of the respiratory system is the lungs, of which you have two. They take up a lot of space in your chest. The ribcage protects the lungs. The muscle underneath them is the diaphragm. The diaphragm allows you to inhale and exhale air.
You inhale air through your mouth and nose. The air then travels down your aptly named windpipe. The air gets filtered through tiny hairs before it fills your lungs. Red blood cells collect oxygen from the lungs and carry it to all the parts of the body. These same cells collect carbon dioxide, a waste gas, and transports it back to the lungs for exhalation. When your body is active, your muscles need more oxygen. This is why when you run you breath harder. Your body is regulating how much oxygen your muscles need or anticipate needing.