The respiratory system is made up of the nose, nasal cavity, pharynx, larynx, trachea, bronchi, lungs, alveoli, and the diaphragm. The nose aids with air filtration, and the nasal cavity warms and humidifies the air before it begins heading down into the trachea to eventually reach the millions of alveoli that are contained within each lung. The alveoli are wrapped in a capillary bed, and the process of diffusion allows carbon dioxide to move from the blood stream into the alveoli and oxygen to move from the alveoli into the blood. This diffusion of gasses happens every time a person inhales and exhales, and that process is called breathing, or ventilation.
Ventilation occurs because of the movement of the diaphragm. The diaphragm is a large muscle located at the bottom of the rib cage, and it separates your abdomen from your chest cavity. When the diaphragm contracts, the muscle moves down. This enlarges the overall size of the chest cavity. When the chest size increases, a pressure decrease inside the chest occurs. This essentially creates a partial vacuum inside the chest, so atmospheric air rushes in to equalize the pressure. When the diaphragm relaxes, it moves up. Consequently, the size of the chest cavity decreases, pressure increases, and air is exhaled. The inverse relationship between the pressure and the volume of a gas is Boyle's Law and is one of the basic gas laws.