The respiratory system is the body system responsible for breathing. It includes the lungs and a series of tubes and passageways ­that allow air into and out of the body. The respiratory system helps sustain life by bringing oxygen, essential for life, to the body’s cells, while at the same time getting rid of carbon dioxide, a waste product.

The respiratory system is divided into two parts for descriptive purposes, though it is important to note that there is no physical division between the two:

• Upper respiratory tract. This includes the nose, nasal cavities, sinuses, throat (pharynx), voice box (larynx) and the windpipe (trachea).
  
  
• Lower respiratory tract. Consists of the lungs, including the bronchi, bronchioles and alveoli.

During respiration, three gases are exchanged between the atmosphere and the body: oxygen, carbon dioxide and nitrogen. The respiratory system combines with the circulatory system (the heart and blood vessels) to help deliver life-giving oxygen to the cells of the body. The respiratory system has three primary functions:

• To bring oxygen into the body when a person inhales
  
  
• To eliminate carbon dioxide from the body when a person exhales
  
  
• To help maintain body fluids at a stable acid–base balance