The human respiratory system consists of the nasal cavity, pharynx, trachea, various divisions of the bronchi and two lungs. Air flows through the nose or the mouth into the pharynx, through the trachea, down the right and left bronchi and then through the smaller bronchioles. When it reaches the end of the smallest bronchioles, the air enters tiny air-filled sacs in the lungs called alveoli.
The lungs are the primary functional organ in the respiratory system. The pharynx, trachea and bronchi simply serve to carry oxygenated air from the outside environment into the lungs and to return oxygen-poor air from the lungs to the outside environment. The main functional unit of the lungs is the alveolus. There are thousands of tiny alveoli in each lung. A capillary is wrapped around each alveoli, and this is the point at which oxygen diffuses into the blood and carbon dioxide diffuses out of the blood.
There are also several accessory organs and tissues in the respiratory system. The diaphragm is a muscle that allows for breathing by contracting to pull air into the lungs and then relaxing to push air out of the lungs. Inside the nasal passages are small hairs known as cilia. These tiny organs help capture dust and dirt so that it is not inhaled into the lungs and other airways.