Chapter 10: THE RESPIRATORY SYSTEM

10 the respiratory system Chapter Outline the respiratory system: Built for gas exchange Airways are pathways for moving air Gases are exchanged in the lungs RESPIRATION = GAS EXCHANGE In gas exchange, oxygen and carbon dioxide diffuse down a pressure gradient Gases are exchanged across a thin, moist respiratory surface When hemoglobin binds oxygen, it helps maintain the steep pressure gradient Breathing at altitude and underwater breathing: air in, air out When you breathe, air pressure gradients reverse in a cycle How much air is in a “breath”? how GASes are EXCHANGED AND TRANSPORTed Alveoli are built for gas exchange Hemoglobin is the oxygen carrier Hemoglobin and blood plasma both carry carbon dioxide Controls Over BREATHING A respiratory pacemaker in the brain sets the basic rhythm of breathing Carbon dioxide is the main trigger for controls over the rate and depth of breathing Other controls help match air flow to blood flow Only minor aspects of breathing are under conscious control RESPIRATORY SYSTEM disorders: tobacco, irritants, and apnea Tobacco is an avoidable threat Irritants cause other disorders Apnea is a condition in which breathing controls malfunction Pathogens and Cancer in the Respiratory System Connections: The respiratory system in homeostasis The respiratory system provides oxygen and removes waste carbon dioxide from all cells in the body SUMMARY Review questions self-quiz critical thinking explore on your own your future Objectives Understand how body processes generate a need to acquire oxygen and dispose of carbon dioxide. Describe the gradients that the respiratory gases follow in their routes into and out of the body. Understand how the human respiratory system functions and how it works in coordination with other systems of the body. Explain the controls over the breathing processes. List some of the things that can go wrong with the respiratory system and explain the mechanisms through which the breakdown in the system occurs. Key Terms respiratory system pharynx larynx epiglottis trachea bronchus, -chi vocal cords lungs diaphragm pleurae bronchioles alveolus, -oli respiration respiratory surface respiratory cycle inspiration expiration pneumothorax tidal volume vital capacity respiratory membrane infant respiratory distress syndrome oxyhemoglobin carbaminohemoglobin carotid bodies aortic bodies bronchitis emphysema asthma pneumonia influenza tuberculosis lung cancer squamous cell carcinomas adenocarcinomas large-cell carcinomas small-cell carcinomas Lecture Outline Smoking poses a threat to human health and survival. Smoking puts the body at increased risk for cancer, high blood pressure, and elevated levels of “bad” cholesterol. The respiratory system functions to bring oxygen into, and carbon dioxide out of, the body. The Respiratory System: Built for Gas Exchange Airways are pathways for moving air. The respiratory system brings in oxygen that each body cell requires and takes away carbon dioxide that every cell generates. Through the nasal cavities of the nose, air enters and leaves the respiratory system; the nasal cavities are separated by a septum of cartilage and bone. Hair and ciliated epithelium filter dust and particles from the air. Blood vessels warm the air and mucus moistens it. The paranasal sinuses lie just above the cavities and are linked to them by channels. Air moves via this route: nasal cavities >>> pharynx >>> larynx >>> vocal cords (the gap between the cords is the glottis) >>> trachea >>> bronchi (one bronchus goes to each lung). The trachea leads from the larynx downward to branch into two bronchi, which are lined with cilia and mucus to trap bacteria and particles. The vocal cords at the entrance of the larynx vibrate when air passes through the glottis, allowing us to make sounds; during swallowing, the glottis is closed to prevent choking. Gases are exchanged in the lungs. Lungs are a pair of organs housed in the rib cage above the diaphragm; the two lungs are separated by the heart. Each lung is enclosed by a pair of thin membranes called pleurae (singular: pleura); the pleural membrane is folded in a manner that forms a pleural sac leaving an intrapleural space filled with a lubricating intrapleural fluid. Inside the lungs, bronchi narrow to form bronchioles ending in respiratory bronchioles. Tiny clustered sacs called alveoli (singular: alveolus) bulge out from the walls of the respiratory bronchioles. Together the alveoli provide a tremendous surface area for gaseous exchange, with the blood located in the dense capillary network surrounding each alveolar sac. Respiration = Gas Exchange Respiration is the overall exchange of inhaled oxygen from the outside air for exhaled carbon dioxide waste. In gas exchange, oxygen and carbon dioxide diffuse down a pressure gradient. Air is 78% nitrogen, 21% oxygen, 0.04% carbon dioxide, and 0.96% other gases. Partial pressures for each gas in the atmosphere can be calculated; for example, oxygen’s is 160 mm Hg. Oxygen and carbon dioxide diffuse down pressure gradients from areas of high partial pressure to areas of low partial pressure. Gases are exchanged across a thin, moist respiratory surface. Gases enter and leave the body by diffusing across thin, moist respiratory surfaces of epithelium; the speed and extent of diffusion depends on the surface area present and on the partial pressure gradient. When hemoglobin binds oxygen, it helps maintain the steep pressure gradient. Hemoglobin is the main transport protein. Each protein binds four molecules of oxygen in the lungs (high oxygen concentration) and releases them in the tissues where oxygen is low; by carrying oxygen away from the lungs, the gradient is maintained. Breathing at Altitude and Underwater Hypoxia occurs when tissues do not receive enough oxygen; at high altitudes the partial pressure of oxygen is lower than at sea level, so that hyperventilation may occur. Underwater, divers must breathe pressurized air from tanks and avoid nitrogen narcosis, where nitrogen dissolves into the body, including the brain; divers must also ascend to the surface slowly to prevent nitrogen bubbles in the blood—the “bends” or decompression sickness. Breathing: Air In, Air Out When you breathe, air pressure gradients reverse in a cycle. The respiratory cycle is the continuous in/out ventilation of the lungs and has two phases: Inspiration (inhalation) draws breath into the airways. Expiration (exhalation) moves a breath out of the airways. During the cycle, the volume of the chest cavity increases then decreases, and the pressure gradients between the lungs and outside air reverse. This works because the air in the airways is the same pressure as the outside atmosphere. Pressure in the alveoli (intrapulmonary pressure) is also the same as the outside air. The basic respiratory cycle. To inhale, the diaphragm contracts and flattens, muscles lift the rib cage upward and outward, the chest cavity volume increases, internal pressure decreases, air rushes in. To exhale, the actions listed above are reversed; the elastic lung tissue recoils passively and air flows out of the lungs. Active exhalation involves contraction of the abdominal muscles to push the diaphragm upward, forcing more air out. Another pressure gradient aids the process. The lungs are stretched to fill the thoracic cavity by a slight difference between the intrapulmonary pressure (higher) and the intrapleural pressure (lower). In a collapsed lung (pneumothorax), air enters the pleural cavity, disrupting the normal expansion and contraction of the lungs. How much air is in a “breath”? About 500 ml of air (tidal volume) enters and leaves the lungs with each breath. A human can forcibly inhale 3,100 ml of air (inspiratory reserve volume) and forcibly exhale 1,200 ml (expiratory reserve volume). The maximum volume that can be moved in and out is called the vital capacity (4,800 ml for males, 3,800 ml for females). A residual volume of about 1,200 ml remains in the lungs and cannot be forced out. Sometimes food enters the trachea rather than the esophagus; it can be forced out by the Heimlich maneuver, which forces the diaphragm to elevate, pushing air into the trachea to dislodge the obstruction. How Gases Are Exchanged and Transported Ventilation moves gases into and out of the lungs; it is different from respiration, which is the actual exchange of gases between the blood and cells. In external respiration, oxygen moves from the alveoli to the blood; carbon dioxide moves in the opposite direction. In internal respiration, oxygen moves from the blood into tissues and vice versa for carbon dioxide. Alveoli are built for gas exchange. Each alveolus is only a single layer of epithelial cells surrounded by a thin basement membrane and a net of lung capillaries, also with thin basement membranes. Between the two basement membranes is a film of fluid. Together the system forms the respiratory membrane. The partial pressure gradients are sufficient to move oxygen in and carbon dioxide out of the blood, passively. Pulmonary surfactant is a secretion produced by the alveoli that reduces the surface tension of the film to prevent collapse of the alveoli; infant respiratory distress syndrome occurs in premature babies who lack the ability to make the surfactant. Hemoglobin is the oxygen carrier. Blood cannot carry sufficient oxygen and carbon dioxide in dissolved form as the body requires; hemoglobin helps enhance its capacity to carry gases by transporting oxygen. Oxygen diffuses down a pressure gradient into the blood plasma >>> red blood cells >>> hemoglobin, where it binds at a ratio of four oxygens to one hemoglobin to form oxyhemoglobin. Hemoglobin gives up its oxygen in tissues where partial pressure of oxygen is low, blood is warmer, and pH is lower; all three conditions occur in tissues with high metabolism. When tissues are chronically low in oxygen, red blood cells produce DPG (2, 3-diphosphoglycerate), which decreases the affinity of hemoglobin for oxygen, allowing more oxygen to be released to the tissues. Hemoglobin and blood plasma both carry carbon dioxide. Because carbon dioxide concentration is higher in the body tissues than in blood, it diffuses into the blood capillaries. Seven percent remains dissolved in plasma, 23% binds with hemoglobin (forming carbaminohemoglobin), and 70% is in bicarbonate form. Bicarbonate and carbonic acid formation is enhanced by carbonic anhydrase, an enzyme located in the red blood cells. Reactions that make bicarbonate are reversed in the alveoli where the partial pressure of carbon dioxide is low. Homeostasis Depends on Controls over Breathing A respiratory pacemaker in the brain sets the basic rhythm of breathing. Automatic mechanisms ensure a regular cycle of ventilation. Clustered nerve cells in the medulla coordinate the signals for the timing of exhalation and inhalation; the pons fine tunes the rhythmic contractions. The nerve cells are linked to the diaphragm muscles and the muscles that move the rib cage; during normal inhalation, nerve signals travel from the brain to the muscles causing them to contract and allowing the lungs to expand. Normal exhalation follows relaxation of muscles and elastic recoil of the lungs. Carbon dioxide is the main trigger for controls over the rate and depth of breathing. The nervous system is more sensitive to levels of carbon dioxide and uses this gas to regulate the rate and depth of breathing. Sensory receptors in the medulla detect hydrogen ions produced when dissolved carbon dioxide leaves the blood and enters the cerebrospinal fluid bathing the medulla. The drop in pH in the cerebrospinal fluid triggers more rapid and deeper breathing to reduce the levels of carbon dioxide in the blood. Changes in the levels of carbon dioxide, oxygen, and blood pH are also detected by carotid bodies, located near the carotid arteries, and aortic bodies, located near the aorta; both receptors signal increases in ventilation rate to deliver more oxygen to tissues. Other controls help match air flow to blood flow. When the rate of blood flow in the lungs is faster than the air flow, the bronchioles dilate to enhance the air flow and thus the rate of diffusion of the gases. When the air flow is too great relative to the blood flow, oxygen levels rise in the lungs and cause the blood vessels to dilate, increasing blood flow. Only minor aspects of breathing are under conscious control. Reflexes can stop breathing during swallowing or coughing. Conscious cessation of breathing is only possible for a few minutes. Respiratory System Disorders: Tobacco, Irritants, and Apnea Tobacco is an avoidable threat. Smoking has both immediate effects like loss of cilia function and long-term effects like lung cancer. Even one cigarette can cause damage as well as hurt those around you through secondhand smoke. Irritants cause other disorders. Bronchitis, caused by air pollution, cigarette smoke, or infection, leads to increased mucus secretions, interference with ciliary action, and eventual inflammation and possible scarring of the bronchial walls. If bronchitis progresses so that more of the bronchi become scarred and blocked with mucus, emphysema may result; here alveoli also begin to break down, further eroding the ability to breathe. Asthma occurs in response to various allergens; smooth muscles in the bronchiole walls contract in spasms, mucus rushes in, and breathing becomes difficult. Steroid inhalers may be needed to relieve symptoms. Apnea is a condition in which breathing controls malfunction. Apnea is a brief interruption in the respiratory cycle; breathing stops and then resumes spontaneously. Aging and obesity increase the risk of sleep apnea, a potentially damaging disorder. Pathogens and Cancer in the Respiratory System Airborne pathogens have easy access to the airways and lungs. Pneumonia occurs when inflammation in lung tissue and the buildup of fluids makes breathing difficult; pneumonia can sometimes occur when infections that start in the nose and throat, such as from influenza, spread. Tuberculosis arises from infection by the bacterium Mycobacterium tuberculosis; the disease destroys patches of lung tissue and can cause death if untreated. Lung cancer is a major killer. While there are many risk factors, exposure to tobacco smoke is the most significant. Rates are rising in women to parallel increased rates of women smoking. Squamous cell carcinomas, adenocarcinomas, large-cell carcinomas and small-cell carcinomas account for 90% of lung cancer cases. Connections: The Respiratory System in Homeostasis The respiratory system provides oxygen and removes waste carbon dioxide from all cells in the body. Suggestions for Presenting the Material The presentation of respiration can be effectively accomplished by following the sequence of the text. The outline begins with human respiratory organs and their functions, then focuses on mechanisms of gas exchange, and concludes with mention of respiratory problems. The most convenient way to present the human respiratory system is to display Figure 10.1 and follow the pathway of an inhaled breath. The names of most of the structures in Figure 10.1 are familiar to students except the most critical structures—the alveoli. These deserve special attention. When describing various respiratory surfaces, emphasize the one common feature they all share—moisture—and why moisture is important to gas exchange. While describing the glottis and epiglottis, be sure to include a note on the Heimlich maneuver. Be sure to stress the passivity of the lungs (compare them to a balloon) during ventilation, and emphasize the role of the diaphragm and rib muscles. Highlight the differences between the ways oxygen and carbon dioxide are transported in the blood. Respiratory control is a fitting capstone to previous discussions. Emphasize that the respiratory organs have some nonrespiratory functions such as coughing, sneezing, speech, yawning, regulation of pH, and sense of smell. Indicate how these functions play pivotal roles in homeostasis. Classroom and Laboratory Enrichment Construct or purchase a working model of the lungs, chest cavity, and diaphragm. Use a chart or dissectible mannequin to locate major organs of the human respiratory system. Exhibit a model of a human larynx and trachea. Display a freeze-dried or preserved preparation of a sheep’s lungs. Show how the measurement of oxygen consumption can be used to determine rate of metabolism by using a simple respirometer (see a biological supply catalog), or use a computer simulation of the experiment. Have the students use a spirometer to measure respiratory air volumes. Compare the results of smokers and nonsmokers. Ask for volunteers to debate “nonsmokers’ versus smokers’” rights. Ask them to provide data, rather than emotional attacks. Have a person who has visited a high altitude location report on the breathing discomforts she/he experienced. Invite a scuba diver to discuss the special gas composition and gas exchange problems associated with deep dives. Classroom Discussion Ideas Using cigarette ads gathered from magazines published during the past 40 years, show the change in public attitude toward smoking. How does the experimental “smokeless” cigarette work? Why is the American Medical Association so adamantly opposed to its approval by the Food and Drug Administration? Why has the message concerning the dangers of smoking not had its intended impact on teenagers and altered their taking up of the habit? Should smokers be subject to higher insurance rates (medical and automobile) due to the higher costs they incur, forcing the insurance companies to pass these costs on to everyone? Should smoking be banned from public places to “protect” nonsmokers from secondhand smoke? What is happening in the condition called “hiccups”? What causes it? What are the best short-term remedies? On what physiological principles (if any) are they based? Why is the best body position for public speaking and singing: a standing or “sitting tall” position? The air at high altitudes is sometimes described in everyday language as “thin.” How does this translate in technical terms? Can you die from holding your breath? Explain the neural mechanisms that are operating here. What are the functions of the sinuses? “Respiratory distress syndrome” (hyaline membrane disease) is the primary cause of respiratory difficulty in immature newborns. What are its symptoms and cause? How is it treated? Term Paper Topics, Library Activities, and Special Projects Vertebrate muscular activity is dependent on an oxygen supply carried by the blood (specifically red cells). When demand exceeds supply, an oxygen debt is incurred; activity slows or stops. Such is not the case with insects with their indefatigable flight muscles. Investigate why this difference exists. Certain woodwind musicians have perfected a breathing technique that allows the production of continuous sound from the instrument. Investigate reports of this technique, and interview a practitioner. Arrange a demonstration, if possible. Check a book of world records for the longest continuous bout of hiccups. What is the “ultimate” cure for a persistent case? How many people in the United States die of simple choking each year? What age is most affected? What is the most commonly lodged object? How might the anatomy and physiology of persons who were born and raised at very high altitudes be different from those who were born and raised in the lowlands and have only recently become acclimated to high elevations? Investigate the effects of air pollution on respiratory functions. Document the various legal attempts to ban smoking in public places. How do these laws differ from state to state? Videos, Animations, and Websites VIDEOS Films for the Humanities and Sciences The Respiratory System http://ffh.films.com/id/16272/The_Respiratory_System.htm Respiratory System Animated videos of the human respiratory system. http://health.howstuffworks.com/human-body/systems/respiratory/adam-200022.htm Argos Medical – The Respiratory System A brief 3D animation of the respiratory system http://www.youtube.com/watch?v=HiT621PrrO0 WISC – Online: The Respiratory System Animated view of respiration http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP15104 ANIMATIONS University of Texas San Antonio Anatomy of a breath http://teachhealthk-12.uthscsa.edu/studentresources/AnatomyofBreathing3.swf WEBSITES The National Heart, Lung, and Blood Institute Provides research, training, and education program to promote the prevention and treatment of heart, lung, and blood diseases http://www.nhlbi.nih.gov/index.htm Possible Responses to Review Questions The diagram on page 195 of the text corresponds to Figure 10.1. Answers are as follows. Left side, top to bottom: oral cavity, epiglottis, pleural membrane, lung, pleural membrane, diaphragm. Right side, top to bottom: nasal cavity, pharynx, larynx, trachea, bronchial tree, lung, division of trachea into bronchioles. Respiration is the movement of air in and out of the lungs; aerobic cellular respiration is the metabolism of glucose in cells using oxygen as a terminal electron acceptor in the mitochondrial electron transport chain. The partial pressure gradient refers to the relative concentration of gases in air as compared between the concentration of each gas in the air in the lung and the concentration of each gas in the tissues of the lung. A high gradient is needed to “push” oxygen, for example, across the respiratory surface into the alveoli. If the gradient is low, diffusion is less efficient. Oxyhemoglobin is hemoglobin that has bound four oxygen molecules (one per heme). Oxyhemoglobin forms in the red blood cells in the capillaries servicing the alveolar sacs. Moisture, temperature, pH, and the pressure gradients in the lungs and the intrapleural space work together to drive oxygen into the blood. Hemoglobin binds oxygen in the lungs where partial pressure of oxygen is high, blood is cooler, and pH is higher; under the opposite conditions (in the tissues) it gives up oxygen. Carbon dioxide, on the other hand, is released from hemoglobin under the influence of conditions of the lungs. Essentially, when comparing gas exchange in the lungs with gas exchange in the tissues, oxygen and carbon dioxide move onto and off of hemoglobin under contrasting conditions that essentially balance the flow of these two gases in the body. When hemoglobin binds oxygen, it removes it from blood plasma, effectively keeping the concentration of oxygen in the blood “low.” This ensures that the partial pressure of oxygen in the lungs will be higher and so will continue to drive the passage of oxygen from the lungs into the blood. Carbon dioxide, produced by metabolically active tissues, will in part dissolve in plasma, bind to hemoglobin (carbaminohemoglobin), or be converted into bicarbonate. The blood will then carry these forms to the lungs for exhalation (carbon dioxide will be released from hemoglobin and reconverted from bicarbonate). Nervous control over the rate and depth of breathing is used to help clear the blood of carbon dioxide. Clusters of nerves in the medulla are linked to muscles of the diaphragm and ribs and serve as a pacemaker for ventilation. One cluster of nerves tells you to breath, the other controls expiration. Rapid or deep breathing stimulates stretch receptors in airways that signal for inhibition of contraction and subsequent exhalation. Levels of oxygen and carbon dioxide will also be used to regulate the rate and depth of breathing. Breathing rate increases when you exercise because the rate of metabolism has increased in your tissues. Oxygen is being depleted and carbon dioxide is being released at high rates. The release of carbon dioxide in particular is rapidly sensed by the nervous system, sending signals to the diaphragm and ribs resulting in deeper and faster breathing. In essence, ventilation increases to try to prevent oxygen debt. Your heart rate increases in order to keep pace with the need to regulate blood flow through the lungs, where gas exchange needs to take place. Possible Responses to Critical Thinking Questions The more effective treatment for a person suffering from carbon monoxide poisoning would be the use of pure oxygen rather than fresh air. The reason for this is because the “fresh” air would contain only about 20% oxygen compared to the pure (100%) oxygen from a tank. This helps displace the carbon monoxide from the hemoglobin. Hyperventilation increases the blood pH and decreases the level of carbon dioxide in the blood. These two events would affect the neural respiratory center by depressing the rate of breathing. Obviously, this would be of an advantage to divers and swimmers who must hold their breath for extended periods of time. When the diaphragm moves upward, the volume of the chest cavity decreases, putting pressure on the sac-like lungs to deflate and expel air. This outward rush of air can be easily detected during a violent sneeze and cough. Incidentally, this is also the mechanism operating in the Heimlich maneuver that is very effective in removing objects from the airways during choking. There is no way that a human could reconfigure his/her respiratory system to remain underwater without breathing for an entire hour. The carbon dioxide levels would build up too fast and oxygen cannot be reserved anywhere to supply the needs of cells for that period of time. The sooner an individual begins to smoke, the longer that smoke has the opportunity to damage the lungs and the rest of the body. The child can expect the following: a shortened life; frequent lung infections due to physical damage to the tissues from smoke; decreased breathing capacity as the lung tissue is damaged, becomes scarred, and ceases to be as elastic as it should be; heart disease; and possibly cancer. Should she continue to smoke when she has her own children, not only do they risk suffering ill effects during gestation, but they are likely to become smokers themselves. Possible Responses to Explore on Your Own Questions Most states, at a minimum, should have a Department of Environmental Quality, or something similar. Large cities or regions may have their own separate websites with local data, but this data should also be transmitted to the state according to EPA rules. Most states, if not individual communities, will monitor gaseous emissions, particulates, and toxins in the air. Almost all of these pollutants come from industrial output or cars, and each state may have different levels that are considered “acceptable” based on local industry and population. Environmental quality websites should contain downloadable reports on yearly pollutant data that students can read for free. 134 Chapter Ten The Respiratory System 1243 134 Chapter Ten The Respiratory System 1243 134 Chapter Ten The Respiratory System 135 134 Chapter Ten The Respiratory System 135 134 Chapter Ten The Respiratory System 135