The lungs are the respiratory organs of the body. The respiratory tract starts from the nasal cavity and enters the lungs as bronchi via the trachea. The bronchi then branch into numerous bronchioles and the bronchioles branch and end as air-filled sac known as alveoli.

Air enters the body through the nasal cavity. From the nasal cavity, air passes through the pharynx and the larynx to the trachea. The trachea is lined with mucus-producing goblet cells and ciliated epithelia that propel foreign particles trapped in the mucus toward the pharynx. The end of the trachea splits into the right and left lungs, which are not identical. The larger right lung has three lobes, while the smaller left lung has two lobes. As air passes down the trachea to the lungs, it is diverted through bronchi beginning with the two primary bronchi. Each bronchus divides into secondary, then into tertiary bronchi, which further divide to create smaller diameter bronchioles that split and spread through the lung.

The terminal bronchioles then subdivide into respiratory bronchioles which subdivide into alveolar ducts. Numerous alveoli (sing. alveolus) and alveolar sacs surround the alveolar ducts. The alveolar ducts are attached to the end of each bronchiole; each duct ends in approximately 100 alveolar sacs. Each sac contains 20-30 alveoli that are 200-300 microns in diameter. Alveoli are made of thin-walled, parenchymal cells that are in direct contact with capillaries of the circulatory system. This ensures that oxygen will diffuse from alveoli into the blood and that carbon dioxide produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled.

Below the lungs is the diaphragm, which contracts and relaxes to facilitate inhaling and exhaling.

• The air that moves from the external environment into the body pass through the nasal cavity where it is warmed, humidified, and surveyed for particulates.

• As air moves out of the nasal cavity, it moves into the pharynx, larynx, trachea, the primary bronchi (right and left lung), secondary and tertiary bronchi, bronchioles, then alveolar sacs where gas exchange occurs with the capillaries.

• Components in the respiratory system allow for protection from foreign material; these include mucus production in the lungs and cilia in the bronchi and bronchioles to move matter out of the system.

• The exchange of gases takes place between the parenchymal cells of the alveolar sacs and the surrounding blood capillaries.

Key Terms

alveolus: a small air sac in the lungs, where oxygen and carbon dioxide are exchanged with the blood

mucus: a sticky or slimy material that is present on the inner lining of the respiratory tract

cilia: small hair-like protrusions that catch dirt and bacteria in the air

nasal cavity: a large, air-filled space above and behind the nose in the middle of the face

pharynx: a tube of skeletal muscle lined with respiratory epithelium; located between the nasal cavity and the trachea

larynx: a cartilaginous structure that produces the voice, prevents food and beverages from entering the trachea, and regulates the volume of air that enters and leaves the lungs

trachea: tube composed of cartilaginous rings and supporting tissue that connects the lung bronchi and the larynx; provides a route for air to enter and exit the lung

goblet cells: mucus-secreting cells

ciliated epithelial: cells in the lining of the trachea with small hair-like protrusions

bronchiole: branch of bronchi that are 1 mm or less in diameter and terminate at alveolar sacs

capillaries: one cell thick small blood vessels that join arteries to veins

diaphragm: a muscular sheet at the bottom of the thorax that contracts and relaxes to support inhaling and exhaling