The Respiratory System

The term “respiration” includes the processes of oxygen uptake in the lungs and transfer to the bloodstream, oxygen uptake by cells all over the body, carbon dioxide removal from the cells and expiration from the lungs. The average adult breathes approximately 25,000 times per day.1

The upper respiratory tract’s main function is the movement of air between the outside and the lower respiratory tract. Additional functions include filtering, warming, humidifying air and protecting the airways from unintended aspiration. The components of the upper respiratory tract include the nasal cavity, mouth, pharynx, epiglottis and larynx. The pharynx or throat, which conducts air and swallowed food and liquids, connects the nasal cavity and mouth to the larynx. The epiglottis is a flap formed primarily of cartilage that closes during swallowing to protect the lower respiratory tract and to direct food to the digestive tract. The larynx is a small, triangular structure that leads into the lower respiratory system. It is muscular and contains the vocal chords as well.

An inverted tree is often used to represent the structures of the lower respiratory tract. The major components include the trachea and lungs. The right lung has three lobes and the left lung has two lobes. The lungs contain circulatory blood flow and airways for gas exchange. The branching airways within the lungs include the primary bronchi, secondary bronchi, and tertiary bronchi. Each succeeding branch is less rigid, narrower, shorter, more numerous and has greater surface area. The branching bronchi conduct air deep into the bronchioles of the lungs. The respiratory bronchioles continue to branch into terminal bronchioles, which lead into approximately 14 million alveolar ducts,2 lined with alveoli. There are more than 300 million alveoli,3 where oxygen and carbon dioxide exchange with the blood gases in a pulmonary capillary.