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S E C T I O N 8.2 The Mammalian Respiratory System E X P E C TAT I O N S List the steps in the path taken by air as it moves from the outside of the animal to the internal gas exchange site in the lungs. Describe the role played by each part of the respiratory tract. Explain how the mammalian respiratory system is adapted to reduce water loss. Figure 8.11 All mammals, including marine mammals such as seals and whales, have lungs. What do these seals, which spend most of their lives in the ocean, have in common with a desert rat which might never see so much as a puddle in its life? Despite their very different habitats, the respiratory systems of all mammals share the same basic features. As you have seen, respiration refers to all parts of the process that supplies oxygen to body cells and rids the body of carbon dioxide. In mammals, respiration can be subdivided into the following: Breathing, which can be further divided into inspiration, the act of taking air into the lungs, and expiration, the act of breathing out; External respiration, the exchange of oxygen and carbon dioxide between air and blood; Internal respiration, the exchange of oxygen and carbon dioxide between blood and the cells of the surrounding tissue (discussed in Chapter 9); and Cellular respiration, the complex series of chemical reactions that take place mainly in the mitochondria of cells. The Respiratory Tract Lungs, with their many folds and fine membranes, are delicate, fragile structures. As a result, they must be shielded not only to prevent water loss, but also to guard against damage. The lungs of mammals are located deep within the body, where they are protected by the bone and muscular 256 MHR • Internal Systems and Regulation structure of the thoracic cavity. This adaptation means that a suitable passageway becomes necessary to allow air to move from the external environment to the respiratory surface deep inside the animal. The organs of the lung system therefore include a number of different structures, each with an important role to play. The following paragraphs trace the passage of air through these organs, using the human respiratory system as an example. The Upper Respiratory Tract The air first enters the nostrils (in humans and many other animals, it can also enter via the mouth). The nostrils conduct the air into the hollow nasal passages where several things occur. Thin bones, called turbinates, hang suspended from the nasal chambers. Their presence increases the surface area of these chambers. The turbinates are covered with a thin membrane that secretes mucus, which moistens the air. The epithelial linings of the nasal chambers and the turbinate bones are well supplied with capillaries, which serve both to warm the incoming air and also to increase its relative humidity. This warming and moistening helps to protect the delicate tissues of the lungs. The air then passes successively through the pharynx, the glottis, and the larynx (see Figure 8.12). The pharynx is the section of alimentary canal that connects the mouth and nasal cavity to the larynx and esophagus. The glottis is the opening of the trachea, the passageway that conducts air to the lungs. This opening is protected by the epiglottis, a flap-like structure that helps to prevent food from entering the trachea. The pharynx is the intersection between the trachea and the esophagus, the passageway for food. The larynx, or “voice box,” houses the vocal cords, which are held securely in place by the cartilaginous material present in the walls of the larynx (Figure 8.13). The larynx contains the two folded structures of the vocal cords. When you breathe normally, there is a large gap between the two cords. When you prepare to speak, muscles around the larynx contract, bringing the cords closer together. The passage of air through this narrower space causes the cords to vibrate, producing a sound. The pitch of the sound varies with the length of the cords: a long cord produces a low sound, while a shorter cord produces a higher sound. At puberty, the vocal cords of males grow quickly. This often produces a “breaking” quality in the voice until the vocal cords finish growing. PLAY To view the pathway of air through the respiratory tract, refer to your Electronic Learning Partner. sinus nasal cavity nostril sinus hard palate opening of auditory tube oral cavity nasopharynx tongue uvula tonsil epiglottis pharynx glottis vocal cords esophagus larynx trachea Figure 8.12 Together, the nasal passages, glottis, pharynx, larynx, and trachea are referred to as the upper respiratory tract. epiglottis base of tongue vocal cord larynx glottis vocal cords inner lining of trachea trachea Figure 8.13 A cross section of the larynx, showing the vocal cords. Air passing between the cords causes them to vibrate, producing sound. You can change the pitch of the sound you make by expanding or tightening the glottis; the glottis tighter the glottis, the higher the sound. Pitch is also determined by the length of the cords. The cords tend to be longer in men than in women, which is why men tend to have deeper voices. The Breath of Life • MHR 257 Figure 8.15 The alveolar tissue of the lung. The walls of the alveoli are only a single cell thick. Figure 8.14 The interior of the nasal passage After passing through the larynx, air goes down the flexible tube of the trachea. In mammals, the trachea is commonly called the “windpipe.” The trachea is supported in part by semicircular cartilage rings. These rings prevent the trachea from collapsing and are arranged so they do not interfere with the passage of food down the esophagus, which is adjacent to the trachea. The nasal and other passages of the upper respiratory tract are lined with ciliated cells that secrete mucus. The mucus traps foreign particles such as dust and bacteria, while the continual beating of the cilia helps to propel this material back into the nose and throat where it can be expelled by coughing or sneezing. When you catch a cold, more mucus is secreted, which is why you find yourself repeatedly blowing your nose. BIO FACT The average healthy human adult produces about 0.9 L of mucus every day. The Lower Respiratory Tract At about the level of your armpit the trachea branches into two smaller passageways called bronchi (singular bronchus). One bronchus enters each lung (see Figure 8.16). Here, each bronchus subdivides many times to produce a network of finer and finer tubes called bronchioles. Like the trachea and nasal passages, the bronchi and bronchioles are lined with a ciliated mucous membrane. 258 MHR • Internal Systems and Regulation Each bronchiole ends in a grape-like cluster of tiny sacs called alveoli (singular alveolus). It is in these sacs, which are always kept moist, that the actual exchange of gases takes place. If you could take all of the alveoli in the average human lung and spread them out on a smooth surface, they would cover approximately 70 to 90 m2 — an area about the size of a tennis court. The wall of each sac is one cell thick and is adjacent to a network of tiny capillaries (Figure 8.15). These capillaries are the site for the exchange of oxygen and carbon dioxide in the body. While most of the exchange of gases takes place through simple diffusion, a process of facilitated diffusion accounts for some (possibly as much as 30%) of the oxygen transfer. This allows the blood to take up oxygen more quickly than would otherwise be possible. The transport of oxygen across the alveolar membrane is facilitated by a particular protein-based molecule in the alveolar cell wall. The entire arrangement of bronchioles and alveoli is kept in a relatively permanent position by elastic connective tissue that fills the spaces between the individual structures. In addition, the alveoli are lined with a lipoprotein-based lubricating film that helps to keep them from collapsing. Wo rd LINK The word “lung” comes from the Old English word lungen, meaning “light in weight.” The lungs are the lightest organs of the body. Because of their many air sacs, they will float in water. The lungs of a livestock animal, particularly a sheep or cow, are sometimes called its “lights.” alveoli nasal cavity: filters, warms, and moistens incoming air pharynx: connects the nasal cavity and larynx epiglottis medulla oblongata larynx: contains the vocal cords esophagus trachea: carries air to the bronchi bronchus: carries air to the lung O2 -rich blood capillary network right lung bronchiole: carries air to alveoli left lung diaphragm alveolus: site of external respiration C O2 -rich blood Figure 8.16 The path of air through the human respiratory tract. The complex structure of the lungs serves to maintain a moist respiratory surface across which gases can be exchanged between the external environment and the body. Each lung is divided into lobes. The right lung has three lobes, while the left lung has only two (to accommodate the heart). A lobe is made up of a number of lobules, each with its own bronchiole. The lungs themselves are enveloped in layers of tissue called pleura (singular pleuron). This flexible membrane contains the lungs while still allowing them to expand and contract during SECTION inspiration and expiration. Each pleuron is made up of two layers separated by a thin film of lubricating fluid. The condition known as pleurisy occurs when the pleura become inflamed, typically as a secondary infection related to pneumonia or other thoracic diseases. Pleurisy can be extremely painful and requires prompt medical attention. REVIEW 1. K/U Explain how the two basic requirements for gas exchange are met by the structure of the mammalian lung. 4. MC Some cold-relief medications work by inhibiting the production of mucus. What side effects would you expect from these products? 2. K/U In what ways does the respiratory tract alter incoming air to prepare it for gas exchange in the lung? What organs of the respiratory tract are involved in this? 5. I Jarvic 7 is an artificial heart. Design a device that could act as an artificial lung. Provide detailed reasoning to support your design. 3. C “A lung is an inside-out gill.” Make one list of the ways in which this statement is true, and another list of the ways in which this statement is false. How can you summarize your findings in a single sentence? UNIT PROJECT PREP If you have decided to study a respiratory disorder in your Unit Project, make sure you can describe the location and function of the affected respiratory structures. The Breath of Life • MHR 259