Structure and Function of the Respiratory Tract in Cats

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What Is the Respiratory System?

The respiratory system is a series of tracts and organs responsible for respiration, without which life would not be possible. Respiration is the term used to describe breathing. It involves the inhalation of air and the intake of oxygen, as well as the exhalation of waste gases such as carbon dioxide from the lungs.

Besides breathing, the respiratory tract serves other important roles, such as the humidification and warming of air before it enters the body, the trapping and expelling of foreign substances, facilitation of the sense of smell, and the production of vocal sounds (e.g. meowing, purring). The respiratory system consists of the nasal passages, the back of the mouth (nasopharynx), the voice box (larynx), the windpipe (trachea), the lower airway passages, and the lungs.

Where Is the Respiratory Tract Located?

The respiratory tract is a large, contiguous system comprised of several structures. The respiratory system begins at the nostrils, involves several structures of the head, continues down the neck and ends at the lungs that lie in the chest cavity.

  • The nose is positioned in the center of the face.

  • The nasal passage is located between the nostrils and the back of throat. There are two passages, one on each side of the nose. They are separated by a bony plate or septum until they end at the nasopharynx.

  • The nasal cavity is surrounded by sinuses. The sinuses are air filled spaces within the bones of the skull. The major sinuses lie just below and above both eyes.

  • The pharynx is the structure that lies at the back of the mouth and throat. It is the cavity behind the tongue and nasal passage through which both food and air are transported to deeper structures. The portion of the pharynx that is part of the respiratory tract is referred to as the nasopharynx, and it connects the back of the nasal cavity to the larynx (voice box).

  • The larynx is located directly behind the base of the tongue and soft palate, and lies between the pharynx and the trachea (windpipe). The larynx covers the trachea during swallowing so that food does not enter into the windpipe.

  • The trachea is a cylindrical tube that runs from the base of the larynx to the beginning of the airways in the lungs. At its termination in the chest it splits into two branches, with one branch for each set of lungs (right and left). Within the chest it lies just above the base of the heart, and just next to and below the esophagus.

  • Once the trachea splits into two branches, the airway passages are called bronchi. The bronchi spread out into lung tissue and continue to divide into smaller and smaller hollow channels as they go further into the lungs. The airways eventually terminate in tiny air pockets within the lungs called alveoli.

  • There are two sets of lungs on either side of the chest cavity. They surround the heart and fill most of the chest between the base of the neck and the diaphragm. The diaphragm is the muscle that separates the chest cavity from the abdomen.

    What Is the General Structure of the Respiratory Tract?

    The respiratory tract is a very structurally diverse system.

  • 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.

    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 meowing.

  • 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 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.

  • 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.

  • What Are Some Common Diseases of the Respiratory Tract?

    There are many primary disorders that affect the respiratory tract. Generally speaking, coughing, sneezing, and/or difficult breathing are the most common signs seen with respiratory disease.

  • Diseases of the nasal cavity often cause nasal discharge, sneezing, and/or sterterous breathing (snoring or snorting sounds). Common diseases of the nasal cavity include:

    Rhinitis is an inflammation of the mucosa (lining) of the nasal cavity. It can be caused by infectious agents (bacteria, virus, fungal agents, parasites) or noninfectious disorders, including foreign bodies, allergies, trauma, dental disease, and environmental irritants. Sneezing and nasal discharge are commonly seen. Rhinitis may extend into the adjacent sinuses of the face, resulting in sinusitis.

    Neoplasia (tumors) may develop within the nose of dogs. Most nasal tumors are malignant cancers, such as adenocarcinomas, osteosarcoma, lymphosarcoma, and squamous cell carcinoma. Tumors may develop initially on one side of the nose, but with time may affect both nasal passages. Besides sneezing and nasal discharge, swelling of the face or around the eye may also be noted.

  • Pharyngeal disorders often present with stertor, gagging, or retching. Rarely, dogs may develop inflammatory polyps in this area. Polyps are usually benign soft tissue growths that grow from the lining of the back of the nasal passage, the pharynx, or the auditory (Eustachian) tube. They may obstruct the flow of air through the nasopharynx and cause problems within the middle ear.

  • Diseases of the larynx often cause respiratory distress and/or stridor (a high pitched audible wheezing sound). They may involve only the larynx or other parts of the upper airway as well.

    Laryngitis is inflammation of the larynx. It most commonly occurs with tracheitis and upper respiratory infections. Clinical signs may include coughing, nasal discharge, difficulty or noisy breathing and a change in voice.

    Brachycephalic syndrome is a condition of brachycephalic dogs in which several upper airway abnormalities occur together and seriously decrease the passage of air. Dogs with this syndrome may have congenitally small nostrils (stenotic nares), overly long soft palates that hang down into the nasopharynx, underdeveloped tracheas (tracheal hypoplasia), herniation of the vocal folds into the larynx (everted laryngeal saccules), and laryngeal paralysis. All of these conditions predispose the dog to respiratory difficulties, particularly if they become excited or overactive, are confined to small areas, are obese, or are exposed to heat and humidity. Clinical signs may be mild (noisy breathing, snoring, gagging or retching phlegm, exercise intolerance) to severe (respiratory distress, cyanosis, overheating, collapse, shock) with this condition.

    Laryngeal paralysis is an acquired disease of some older, large breed dogs. The cartilages that normally control the opening and closing of the larynx become paralyzed and the larynx does not open well. These dogs exhibit changes in their voice (hoarse, raspy bark), noisy breathing, and exercise intolerance. They may also overheat and collapse in respiratory distress.

    Polyps and tumors may also develop within the larynx.

  • Diseases of the trachea are most often associated with cough, respiratory distress, exercise intolerance, weakness, cyanosis (blue color to the normally pink tongue and gums) or fainting (syncope). Common disorders of the trachea include the following:

    Infectious tracheobronchitis (kennel cough) is a contagious upper respiratory disorder, and is common in dogs that have been exposed to other infected dogs, such as at a boarding facility, dog show, or playgroup. The disease is usually caused by a mixture of viruses (e.g. parainfluenza, adenovirus) and bacteria (especially Bordetella). The cough associated with kennel cough is often harsh, occurs in spasms, and may sound as though the dog is choking. At the end of the cough the dog make gag or retch. The trachea is often very sensitive, and any light manipulation of the neck results in spasms of coughing.

    Tracheitis or inflammation of the trachea may also occur with exposure to irritants such as smoke, chemicals, dust, or foreign bodies. Certain parasites may also migrate to the trachea, causing inflammation within the airway. Tracheitis occasionally develops after the use of endotracheal tubes during general anesthesia.

    Obstruction of the trachea may develop from the inhalation of foreign material, from the growth of tumors of the trachea, or from tumors or masses impinging on the trachea from surrounding tissues.

    Tracheal collapse is a disease seen most often in middle-aged to older toy breed dogs (e.g. miniature poodle, Yorkshire terrier, Pomeranian). In these dogs the soft membrane across the top of the C-cartilage of the trachea becomes weak and stretches out. This allows the C-cartilages to collapse onto themselves, making it difficult for air to pass into the lungs. The cough associated with tracheal collapse has been described as a "goose honk." It may occur with excitement, exercise, pressure on the neck from collars and leashes, and during eating or drinking. Spasms of coughing, respiratory distress, weakness and fainting are sometimes also seen with this condition.

    The most common disorder of the bronchial tree of dogs is bronchitis. Bronchitis is inflammation of the lower airways that may arise with infections (e.g. bacterial, viral, mycoplasmal, parasitic), irritants (e.g. smoke, dust, foreign material), allergies, diseases of the lungs, etc. The most common presenting sign is usually coughing. Bronchitis may either be acute or chronic in nature.

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