Key terms

Respiratory systemThe body system responsible for gas exchange between the body and the external environment
Pharynx (throat)Tube connected the nose/mouth to the esophagus
Larynx (voice box)Tube forming a passage between the pharynx and trachea
TracheaTube connecting the larynx to the bronchi of the lungs
BronchiBranches of tissue stemming from the trachea
BronchioleAirway that extends from the bronchus
AlveoliStructures of the lung where gas exchange occurs
DiaphragmThoracic muscle that lays beneath the lungs and aids in inhalation/exhalation

The respiratory system

The process of physiological respiration includes two major parts: external respiration and internal respiration. External respiration, also known as breathing, involves both bringing air into the lungs (inhalation) and releasing air to the atmosphere (exhalation). During internal respiration, oxygen and carbon dioxide are exchanged between the cells and blood vessels.
Respiration begins at the nose or mouth, where oxygenated air is brought in before moving down the pharynx, larynx, and the trachea. The trachea branches into two bronchi, each leading into a lung. Each bronchus divides into smaller bronchi, and again into even smaller tubes called bronchioles. At the end of the bronchioles are air sacs called alveoli, and this is where gas exchange occurs.
An important structure of respiration is the diaphragm. When the diaphragm contracts, it flattens and the lungs expand, drawing air into the lungs. When it relaxes, air flows out, allowing the lungs to deflate.

Common mistakes and misconceptions

  • Physiological respiration and cellular respiration are not the same. People sometimes use the word "respiration" to refer to the process of cellular respiration, which is a cellular process in which carbohydrates are converted into energy. The two are related processes, but they are not the same.
  • We do not breathe in only oxygen or breathe out only carbon dioxide. Often the terms "oxygen" and "air" are used interchangeably. It is true that the air we breathe in has more oxygen than the air we breathe out, and the air we breathe out has more carbon dioxide than the air that we breathe in. However, oxygen is just one of the gases found in the air we breathe. (In fact, the air has more nitrogen than oxygen!)
  • The respiratory system does not work alone in transporting oxygen through the body. The respiratory system works directly with the circulatory system to provide oxygen to the body. Oxygen taken in from the respiratory system moves into blood vessels that then circulate oxygen-rich blood to tissues and cells.