Structure and Function of the Respiratory Tract in Dogs
Dr. Bari Spielman
The most forward portion of the respiratory tract is the nose. The external, visible portion of the nose consists of a fixed bony case and a moveable cartilage framework. The front portion of the nose is flattened and devoid of hair and is called the planum nasale, which includes the nares or nostrils. The nostrils are the entrance openings of the nasal cavity and are supported by cartilage.
What Is the General Structure of the Respiratory Tract?
The respiratory tract is a very structurally diverse system.
The nasal cavity is the air passageway within the facial area of the skull. It consists of right and left halves that are divided by the nasal septum, a thin-walled structure, which is part cartilage and part bone. Deep within the nose are numerous fine, paper-like bony plates that are lined with a mucous membrane called the turbinates. The blood supply to the turbinates is very extensive.
The olfactory region (area responsible for smell) is located in the back of the nasal cavity. The mucous membrane of this region contains special nerves designed for smell.
The pharynx consists of two parts, including the nasopharynx (associated with the respiratory tract), and oropharynx (associated with the digestive tract). It is a fairly large round cavity that is lined by soft membranes of the back of the throat.
The larynx is a round structure that is composed of muscles, several cartilages, and soft tissues. The cartilages at the beginning of the larynx are designed to open and close during breathing or swallowing. When the larynx is open, air passes from the nose to the trachea. During swallowing, the laryngeal opening is closed so that food does not fall into the trachea.
The trachea is a semi-rigid, flexible tube that connects the larynx to the bronchi of the lungs. It is made up of many C-shaped cartilages that are strung together, each alternating with an elastic ligament and muscle. The C-cartilages lie with the open area at the top. A soft membrane covers this open area in the cartilages.
The bronchial tree within the lungs begins at the bifurcation of the trachea with the formation of a right and a left mainstem bronchus. Each mainstem bronchus divides into lobar bronchi that supply the various lobes of the lung. Within the lobe of each lung, the lobar bronchi divide into smaller segmental bronchi. This process of branching continues until the respiratory bronchioles are formed. The bronchi are cylindrical tubes that are kept from being flattened by overlapping, curved cartilages. The bronchioles eventually give rise to alveoli, which are tiny saclike structures with very thin membranes that allow gases to pass to and from the lungs into the airways.
What Are the Functions of the Respiratory Tract?
The nose (along with the mouth) is responsible for taking air into the body. Both the fine hairs (cilia) that line the nasal cavity and the mucus that is produced by the cells of the nasal cavity work to filter debris and foreign material from the air before it enters the body. The nasal cavity also warms and moistens the air before it enters the trachea. The blood supply to this area is extensive and contributes to warming the inspired air. Moisture is added to the air by evaporation of mucosal secretions. As air passes over the back portion of the nose, the sense of smell is activated.
The nasopharynx functions as the passageway between the nasal cavity and the larynx. Air transported through this area passes very near the tonsils. The tonsils are a part of the immunologic system, and are capable of activating certain defense mechanisms of the body when foreign material and infectious agents are detected.
The larynx guards the entrance to the trachea and regulates both the inspiration and expiration of air. The valvular function of the larynx, which is created by the epiglottis and arytenoid cartilages, is vital to protecting the airway and to preventing the aspiration of food. The larynx also contains the vocal folds, which are necessary for vocalization, such as barking, whining and growling.
The trachea or windpipe serves to conduct air downward into the lungs. It is also lined by tiny hairs called cilia and mucus producing cells that trap debris and foreign substances. The trachea returns those substances to the mouth through the act of coughing.
The bronchi bring air from the trachea into the lungs. Like the trachea they are also lined with cilia and mucus producing cells.
The main function of the lungs is provide a huge surface area over which gases are exchanged between the body's circulation and the outside air. Oxygen is taken in from the atmosphere and carbon dioxide is exhaled from the blood. The physical act of breathing involves well-coordinated interactions between the lungs, the central nervous system, the diaphragm and the circulatory system.