Basic Anatomy
There are two lungs, and each surrounds the heart in the chest cavity. The right lung is made of three lobes: the upper, middle and lower sections. The left lung is slightly smaller than the right lung because it is embedded in the chest cavity with the heart. The left lung only has two lobes, upper and lower.
Inhaling
When a person inhales, the chest expands and the diaphragm pushes against the lungs. This causes the lungs to expand and air enters into the cavity. Air enters the lungs through the trachea, which is connected to the mouth. Air travels through the trachea into alveoli, which are balloon-like structures responsible for gas exchange. The alveoli are surrounded by blood vessels that deliver blood for the exchange of oxygen.