Zoology Notes: 013 Chapter 9

Chapter 9. The Respiratory System The term respiration is defined as the union of oxygen with food in the cell, and the subsequent release of energy for work or for heat, with the release of carbon dioxide and water. General Functions The respiratory system functions to supply oxygen for the metabolic needs of the cells and to remove one of the waste materials of cellular metabolism, carbon dioxide. This involves the process of external respiration, absorption of O2, and removal of CO2 from the lungs, and the internal respiration, gaseous exchange between the cells of the body and their fluid medium. While the intake of oxygen and removal of carbon dioxide are the primary functions of the respiratory system, it plays other important roles in the body. The respiratory system helps regulate the balance of acid and base in tissues, a process crucial for the normal functioning of cells. It protects the body against disease-causing organisms and toxic substances inhaled with air. The respiratory system also houses the cells that detect smell, and assists in the production of sounds for speech. Anatomy of the Respiratory System The organs of the respiratory system extend from the nose to the lungs and are divided into the upper and lower respiratory tracts. The upper respiratory tract consists of the nose and the pharynx, or throat. The lower respiratory tract includes the larynx, or voice box; the trachea, or windpipe, which splits into two main branches called bronchi; tiny branches of the bronchi called bronchioles; and the lungs, a pair of saclike, spongy organs. The nose, pharynx, larynx, trachea, bronchi, and bronchioles conduct air to and from the lungs. They lead to the alveoli, the functional units of the lungs. Gaseous exchange between blood and air occurs only in the alveoli. Nose. The nose includes the external nose (external nares), the part of the upper respiratory tract that protrudes from the face, and nasal cavity: only a small part of the nasal cavity is in the external nose, most of it lying over the roof of the mouth. The functions of the nose are: 1. Serves as air passage 2. Warms the air 3. Filters the air 4. Serves as organ of smell 5. Aids in phonation Filtering prevents airborne bacteria, viruses, other potentially disease-causing substances from entering the lungs, where they may cause infection. Filtering also eliminates smog and dust particles, which may clog the narrow air passages in the smallest bronchioles. The nose filters Fig. 9.1. Illustration showing the different parts of the respiratory system. substances in two ways: 1. Vibrissae (the hairs that can be seen in the nose) filter out the coarsest bodies, such as insects. 2. Air currents passing over the moist mucosa in curved pathways deposit fine particles, such as dust, powder and smoke, against the wall. The cilia embedded in the mucous membrane wave 43

constantly, creating a current of mucus that propels the particles out of the nose or downward to the pharynx. These fine particles are subsequently conveyed to the pharynx and swallowed. If more particles are in the nasal passages than the cilia can handle, the particles build up on the mucus and irritate the membrane beneath it. This irritation triggers a reflex that produces a sneeze to get rid of the polluted air. The Pharynx. The pharynx is a musculo-membranous tube, about 5 inches in length, extending from the base of the skull to the esophagus. It is divided into three parts: 1. Nasopharynx – lies behind the nose with four openings – two from the auditory (Eustachian) tubes and two from the nose, the posterior nares. 2. Oropharynx – lies behind the mouth with a single opening, called the isthmus of the fauces, which communicates with the mouth. 3. Laryngopharynx – lies below the hyoid bone and behind the larynx which opens into the larynx and esophagus. The pharynx serves as a passageway for both the respiratory and digestive systems. It also assumes an important function in the formation of sound, particularly in the creation of vowel sounds. In addition to serving as an air passage, the pharynx houses the tonsils, lymphatic tissues that contain white blood cells. The white blood cells attack any disease-causing organisms that escape the hairs, cilia, and mucus of the nasal passages and pharynx. The tonsils are strategically located to prevent these organisms from moving further into the body. One tonsil, called the adenoids, is found high in the rear wall of the pharynx. A pair of tonsils, the palatine tonsils, is located at the back of the pharynx on either side of the tongue. Another pair, the lingual tonsils, is found deep in the pharynx at the base of the tongue. In their battles with disease-causing organisms, the tonsils sometimes become swollen with infection. When the adenoids are swollen, they block the flow of air from the nasal passages to the pharynx, and a person must breathe through the mouth. The Larynx. The larynx, or “voice box”, connects the pharynx with the trachea. Its opening is at the base of the tongue. It is broad superiorly and shaped like a triangular box. It joins with the trachea inferiorly, where it is narrower and round. It consists of nine cartilages united by extrinsic and intrinsic muscles as well as ligaments. While the primary role of the larynx is to transport air to the trachea, it also serves other functions. It plays a primary role in producing sound; it prevents food and fluid from entering the air passage that causes choking; and its mucous membranes and cilia-bearing cells help filter air. The cilia in the larynx waft airborne particles up toward the pharynx to be swallowed. The Trachea (Windpipe). The trachea is a membranous and cartilaginous tube. It lies in front of the esophagus and extends from the larynx on the level of the sixth cervical vertebra to the upper border of the fifth thoracic vertebra, where it divides into the two bronchi, one for each lung. The most distinctive feature of the trachea are the cartilages, which in most species are roughly Cor U-shaped pieces of hyaline cartilage that are incomplete dorsally. The number of cartilages in the wall of the trachea varies, not only among different species, but among different individuals of the same species. In some individuals, they are fused in places so that instead of being separate, distinct entities, they form a continuum. The “open” portion is toward the esophagus. The trachea functions as a simple passageway for air to reach the lungs. The Bronchi and Bronchioles. The two bronchi into which the trachea divides differ slightly, the right bronchus being shorter, wider and more vertical in direction than the left. They enter the right and left lung respectively, and then break up into a great number of smaller branches, which are called bronchial tubes and bronchioles. 44

Each bronchiole terminates in an alveolar duct which in turn terminates in one or more elongated saccules called alveolar sacs. The Lungs. The lungs (pulmones) are cone-shaped organs that fill the two lateral chambers of the thoracic cavity and are separated from each other by the heart and other contents of the mediastinum. The substance of the lungs is porous and spongy; owing to the presence of air, it crepitates when handled and floats in water. It consists of bronchioles and their terminal dilatations, numerous blood vessels, lymphatics, nerves and an abundance of elastic connective tissues. Each lobe of the lung is composed of many lobules, and into each lobule a terminal bronchiole enters then terminates in one or more alveolar sacs. Each sac presents on its surface numerous small pouches, or alveoli. The alveoli increase the surface area of the lungs for the exchange of gases without necessarily increasing the size of the lungs. Each lung is enclosed in a serous sac called pleura. The pleura is a thin, transparent membrane that forms the serous fluid. Mechanics of Breathing Quiet breathing is accomplished by the alternate contraction and Fig. 9.2. The alveoli. relaxation of the diaphragm and external intercostal muscles. Most of the air movement is accounted for by the action of the diaphragm.

Fig. 9.3 Demonstration of the mechanics of breathing (inhalation on the left and exhalation on the right.

Inspiration. When the diaphragm contracts, it descends and elongates the thoracic cavity. The contraction of the external intercostal muscles raises the ribs at the sternal end. This action forces the sternum outward, increasing the anterior–posterior diameter of the thorax. As the thorax enlarges, cohesion between the visceral and parietal pleurae causes both layers to expand, thereby enlarging the lungs. This reduces the pressure within the lungs (intrapulmonic pressure). The reduction in intrapulmonic pressure causes air to rush into the lungs. At the end of inspiration the pressure between the lungs and atmosphere is equalized. Expiration. Expiration during quiet breathing is a passive process, occurring as the diaphragm and external intercostal muscles relax. The thoracic cavity returns to its resting size and lungs recoil.

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Recoil contraction of the lungs increases intrapulmonic pressure, forcing the air out of the lungs at the end of the expiration the pressure between the lungs and atmosphere is equalized. Types of Breathing 1. Eupnea – normal, quiet breathing 2. Apnea – temporary cessation of breathing 3. Dyspnea – difficult breathing 4. Orthopnea – inability to breath easily in a horizontal position 5. Hyperpnea – an increased depth of breathing 6. Tachypnea – is the excessively rapid and shallow breathing Common Respiratory Disorders 1. Emphysema – a condition characterized by a breakdown of alveolar walls and loss of elasticity of the lungs, usually is a final stage of a lung disease which causes chronic bronchiolar obstruction. 2. Atelectasis – term used to describe any condition that causes collapse of alveoli in a localized region, an entire lung, or both lungs. 3. Asthma – an allergic reaction caused by substances in the air, such as pollen when inspired creating localized edema in the walls of the small bronchioles, secretion of thick mucus into their lumens; and spasm of their smooth muscular walls. 4. Pneumonia – an inflammation of the alveoli and supportive tissues, usually with an accumulation of fluid and blood cells in the alveoli. 5. Tuberculosis – caused by tubercle bacilli that invade the lungs, and initially there is acute inflammation in the area of the bacilli, with accumulation first of neutrophils and later macrophages. 6. Pulmonary edema – caused by an insufficiency of the left heart in pumping blood received from the lungs to the rest of the body, causing blood to back up into the pulmonary circulation. 7. Sinusitis – in acute stage in manifested by pain referred to the maxillary and frontal sinuses; the nose is plugged, and nasal and postnasal mucus discharged occur. 8. Epistaxis – (nose bleed) occasionally results in an animal because of the highly vascular nasal mucous membrane. Some race horses develop epistaxis during races, perhaps because of the high blood pressure involved. 9. Pharyngitis – (sore throat) is an inflammation of the pharynx and may involve the digestive system as well as the respiratory system and may result in tonsillitis.

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