Without oxygen, energy cannot be released from food molecules to fuel cellular processes. Carbon dioxide is the product of cellular reactions, and is harmful in high concentrations. Respiration provides a way to take in oxygen and release carbon dioxide from the body. The respiratory system is divided into two parts that work together during respiration.
Upper Respiratory Tract
The nose is made up of bone, cartilage, and muscle. Air enters through the external nares, or nostrils, which are lined with hairs that remove larger debris from the inhaled air. The air flows into the nasal cavity, which is located behind the nose. It is comprised of nasal passages, called meatuses. Nasal conchae curl into the cavity to create the meatuses.
The nasal cavity is lined with a mucous membrane that contains hair-like structures called cilia. This membrane contains goblet cells that secrete mucus. Together, the mucus and cilia trap particles that enter the nasal cavity with the air. The cilia push the foreign particles towards the pharynx, where they are swallowed and broken down in the stomach.
The mucous membrane also contains a network of blood vessels. Heat from the blood vessels is transferred into the nasal cavity and warms the air. Water evaporates from the mucus and also contributes to warming the air.
Several sinus cavities are located around the nasal cavity. They are filled with air and are lined with the mucous membrane that continues from the nasal cavity. Mucus forms in the sinuses and drains into the nasal cavity.
The pharynx is located behind the tongue and serves as a passageway for air from the nasal cavity and food from the oral cavity. Food enters the esophagus during swallowing, which causes a cartilage flap, called the epiglottis, to close over the opening of the larynx to prevent food from entering the airway.
Lower Respiratory Tract
The epiglottis remains open when swallowing is not taking place and air enters into the larynx, which contains the vocal cords. The vocal cords vibrate when air is forced over them, and a voice is produced.
The trachea is a tube that is about 12 centimeters long. It is lined with a mucous membrane and cilia, just like the nasal cavity. The mucus further warms incoming air and the cilia move trapped particles towards the pharynx to be swallowed.
The trachea gives rise to the primary bronchi that supply the right and left lungs. The right bronchus divides into three lobar bronchi that supply different parts of the lung: superior, middle, and inferior. The left bronchus divides into the superior and inferior bronchi. These secondary bronchi further divide into tertiary, or segmental, bronchi, of which there are about eight to 10 in each lung.
The bronchi branch into the bronchioles, which further divide until they supply the alveolar ducts. The alveolar ducts terminate at the alveolar sacs, which are comprised of microscopic air sacs called alveoli.
The alveoli are surrounded by microscopic blood vessels that begin as arterioles that carry blood away from the heart and to the alveoli in the lungs. These arterioles become capillaries where gas exchange takes place between the air and the blood. This is called external respiration. The capillaries become venules that carry oxygenated blood back to the heart for circulation throughout the body.
The organs of the respiratory system work together to warm and clean the air that brings oxygen to the alveoli sac within the lungs. Carbon dioxide is removed from the blood and exits the body via the pathway by which the air originally entered the lungs. This process is called respiration.
Reference
Shier, D., Butler, J., Lewis, Ricki. Hole's Human Anatomy & Physiology. New York: McGraw-Hill, 2002.
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