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Structure and Function
Learning Tip Get permission to spend some time in the anatomy and physiology or biology laboratory at your school. Study the anatomical mod- els, in this case the models representing the respiratory system. Use all four senses by physically touching the various parts, naming them aloud as you do, while you also name their associated combining forms. Make a video of yourself while you do this activity so you can study the models again from home. Share your videos with your classmates! Flashpoint Aspiration is a term that is often used to describe food or fluids being sucked into the lungs. Learning Tip Take breaks from studying to get up and physically move. Physical activity energizes you, reduces your stress, and improves your ability to focus. In addition, research shows that allowing your brain to rest helps consolidate new information for improved recall later. Please note that using social media does not count as resting your brain!
While air moves through the upper airway, it is warmed, filtered, and humid- ified. Mucous membranes that line these structures contribute moisture to humidify the air. Cilia (tiny hairs) within the nasal cavity help filter the air by removing debris. The rich blood supply of all of these structures warms the air while it passes through. Sinus cavities serve to decrease the weight of the skull, provide resonance for the voice, and produce mucus, which helps eliminate microorganisms while it drains into the nasal cavities. Air moves toward the lower airway when it flows past the epiglottis and enters the trachea. The epiglottis acts as a doorway to the trachea and serves a vital protective function by opening to let in air and closing to keep out food and fluid. The trachea is approximately 5 inches long and gets its shape and strength from numerous rings of cartilage. It separates the upper and lower airways. When air flows through the tracheal entrance, it passes through the larynx . The air vibrates the vocal cords in the larynx to create sound when we talk. Air then flows down the trachea and into the lower airway , which consists of the bronchi and lungs. The bronchi split off into smaller bronchi and eventually into tiny bronchi- oles. The composition of the bronchi changes to less cartilage and more smooth muscle when they become smaller. The trachea and bronchi have ciliated mucous- membrane linings, which further moisten air and secrete mucus to trap debris that has been inhaled. Cilia move in a wavelike fashion to propel debris upward. The trachea and bronchi are extremely sensitive, and the presence of foreign parti- cles stimulates a powerful cough reflex that further helps to expel debris. The Lungs The lungs are divided into lobes ; the right lung has three lobes, and the left lung has two. The lungs are covered with two thin membranes known as the pleu- rae . The term intrapleural means within ( intra- ) the pleural space in general. The visceral pleura lies directly on the lungs, while the parietal pleura lines the inner wall of the thorax. The term interpleural refers to the specific area between ( inter- ) the visceral pleura and the parietal pleura. A small amount of pleural fluid lies within the space between the two membranes. This space is some- times referred to as a potential space because there is nothing there other than this tiny amount of fluid. While we breathe in and out, our lungs expand and contract. The elastic quality that allows the lungs to do this is sometimes called recoil . While we breathe, the pleural fluid between the visceral and parietal pleurae acts as a sort of lubricant, which helps to reduce friction as the lungs continually expand and contract. As air continues its journey into the lungs, it arrives at its final destination, the alveoli , which are microscopic-sized air sacs. We have approximately 300 million alveoli in each lung. They are covered with a delicate capillary bed (microscopic blood vessels) that provides a rich blood supply. The alveoli expand somewhat like tiny balloons during inspiration (also called inhalation) when air enters and fills them. They contract and partially deflate during expiration, when much of the air exits the lungs. Because the walls of the alveoli and the capillary beds are each just one cell thick, gases easily move back and forth across them. Excess carbon dioxide (CO 2 ) leaves the capillaries and moves into the air space within the alveoli and is then exhaled. Oxygen (O 2 ) moves from the air space in the alveoli into the capillary blood and is then distributed to various parts of the body via the circulatory system. We take oxygen into our lungs through the act of inhalation , or breathing in (also called inspiration), which is usually an unconscious act (Box 8-1). However, we may exert conscious control to take extra-large breaths or even hold our breath for a short time. At some point, however, we feel an overwhelming urge to breathe, which is triggered by a buildup of CO 2 . This buildup of CO 2 in the blood causes the blood to become more acidic. To be healthy, our blood must remain slightly alkaline—within the narrow range of 7.35 to 7.45 on the pH scale , which is a tool
Flashpoint Each lung contains approx- imately 300 million alveoli!
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