Chapter Twenty-Two - The Respiratory System
I. Anatomy of the Respiratory System (p. 796; Fig. 22.1)
A. The principal organs of the respiratory system are the nose, pharynx, larynx, trachea, bronchi, and lungs. These organs serve to draw in air, exchange gases with blood, and expel the modified air.
1. The conducting division of the respiratory system consists of those that serve for airflow, mainly from the nostrils through the bronchioles.
2. The respiratory division consists of the alveoli and other distal gas-exchange regions.
3. The airway from the nose through the pharynx is often called the upper respiratory tract.
4. Regions from the larynx through the lungs compose the lower respiratory tract.
B. The Nose (p. 796; Figs. 22.2, 22.3; Transp. 401)
1. The nose has several functions. The nose warms, cleanses, and humidifies inhaled air, it detects odors, and it serves as a resonating chamber to modify the voice.
2. The external, protruding nose is supported by a framework of bone and cartilage.
a. The inferior half is supported by alar and lateral cartilages.
b. Dense fibroconnective tissue shapes the flared, lateral portion (ala nasi) of each nostril.
3. The nasal cavity extends from the anterior (external) nares to the posterior (internal) nares, or choanae. The dilated chamber inside the ala nasi is called the vestibule.
a. The vestibule is lined with stratified squamous epithelium and has stiff vibrissae (guard hairs) that block the inhalation of large particles.
4. The nasal septum divided the nasal cavity into right and left chambers called nasal fossae.
a. There is little open space in a nasal fossa because its lateral wall gives rise to the superior, middle, and inferior conchae that project toward the septum and occupy most of the fossa.
b. The conchae consist of mucous membranes supported by thin scroll-like turbinate bones.
c. Beneath each concha is a narrow air passage called a meatus. The narrowness of the passages and the turbulence caused by the conchae ensure that most air contacts the mucous membrane on its way through, enabling the nose to cleanse, warm, and humidify it.
d. Cilia of the respiratory epithelium continually beat toward the posterior nares and drive debris-laden mucus to the pharynx to be swallowed and digested.
C. The Pharynx (p. 799)
1. The pharynx is a muscular funnel extending 13 cm (5 in.) from the choanae to the larynx. It has three regions.
a. The nasopharynx lies posterior to the soft palate, receives the auditory tubes, and houses the pharyngeal tonsil.
b. The oropharynx lies between the soft palate and hyoid bone and contains the palatine and lingual tonsils.
c. At the level of the hyoid bone, the nasopharynx and oropharynx join to form the laryngopharynx, which extends to the larynx. It marks the end of the upper respiratory tract and is lined with pseudostratified epithelium.
D. The Larynx (p. 799; Figs. 22.4 - 22.6)
1. The larynx (voicebox) is a cartilaginous chamber. Its primary function is to keep food and drink out of the airway, but has evolved the additional role of producing sound.
2. The superior opening of the larynx, the glottis, is guarded by a flap of tissue called the epiglottis. During swallowing, the extrinsic muscles of the larynx pull it upward toward the glottis, and the epiglottis directs food and drink into the esophagus.
3. The vestibular folds play a greater role in keeping food and drink out of the airway.
4. The framework of the larynx consists of nine cartilages. These are the epiglottic, thyroid, and cricoid cartilages (all single), and the arytenoid, corniculate, and cuneiform cartilages (all paired).
5. The walls of the larynx are quite muscular.
a. Deep intrinsic muscles operate the vocal cords.
b. Superficial extrinsic muscles connect the larynx to the hyoid bone and elevate the larynx during swallowing.
6. The interior wall of the larynx has folds.
a. The superior pair are the vestibular folds, or false vocal cords.
b. The inferior pair are the true vocal cords. The intrinsic muscles control the vocal cords by pulling on the corniculate and arytenoid cartilages. As air passes through the cords, high-pitched sound occurs when the cords are pulled taut, and lower-pitched sounds occur when the cords are more relaxed. Loudness is determined by the force of air through the cords.
E. The Trachea (p. 801; Figs. 22.7, 22.8; Transp. 402)
1. The trachea is a rigid tube, 12 cm in length, with C-shaped cartilage rings to keep it from collapsing during inhalation.
2. The larynx and trachea are lined with pseudostratified epithelium, which provides a mucociliary escalator for removal of debris trapped in the mucus.
F. The Lungs, Bronchial Tree, and Alveoli (p. 802; Fig. 22.9)
1. Each lung is a conical organ, with its broad, concave base resting on the diaphragm. The lung receives the bronchus, blood and lymphatic vessels, and nerves through its hilum. The left lung is divided into two lobes; the right into three.
2. The Bronchial Tree (p. 803; Figs. 22.10, 22.11)
a. The bronchial tree is a system of highly branched air tubes, and begins with the primary bronchi that enter each lung from the trachea.
b. Each primary bronchus divides into secondary (lobar) bronchi that enter each lobe of the lung.
c. The next subdivision is that of tertiary bronchi that each supply a bronchopulmonary segment.
d. Tertiary bronchi give rise to bronchioles, the first air tubes to lack cartilage but with smooth muscle in their walls. The portion ventilated by each bronchiole is a primary lobule. Each bronchiole divides into 50-80 terminal bronchioles.
e. Each terminal bronchiole gives off smaller respiratory bronchioles that divide into alveolar ducts ending in alveolar sacs. Alveoli bud off respiratory bronchioles, and alveolar ducts and sacs.
3. Alveoli (p. 805; Fig. 22.12; Transp. 403)
a. To compensate for high metabolic rates and oxygen needs, each human lung has an immense surface area (70 square meters) for gas exchange. Animals with lower metabolic rates require less oxygen.
b. An alveolus consists mostly of squamous (type I) alveolar cells that are thin to allow for rapid gas diffusion through them. Around 5% are great (type II) alveolar cells that secrete pulmonary surfactant.
c. Also present within the lumens of the alveoli are alveolar macrophages (dust cells) that are the last line of defense against inhaled matter.
d. Each alveolus is surrounded with a basket of capillaries.
e. The respiratory membrane is made up of the wall of the alveolus, the endothelial wall of the capillary, and their fused basement membranes.
f. Pulmonary circulation has a very low blood pressure to prevent the alveoli from filling with fluid. The osmotic uptake of water overrides filtration and keeps alveoli dry.
G. The Pleurae (p. 806)
1. The surface of each lung is covered by visceral pleura. The chest cavity is lined with parietal pleura. Between the two is the potential space of the pleural cavity containing pleural fluid.
a. The pleura and pleural fluid serve to reduce friction during chest expansion, to create a pressure gradient, and for compartmentalization (to prevent infections).
II. Mechanics of Ventilation (p. 806)
A. Pressure and Flow (p. 806)
1. The process of breathing in is inhalation; breathing out is exhalation.
2. The pressure that drives respiration is atmospheric (barometric) pressure.
3. Air moves into the lungs because the volume of the lungs is increased, thus dropping intrapulmonary pressure.
4. During exhalation, the intrapulmonary pressure is above atmospheric pressure, so air leaves the lungs.
B. Inspiration (p. 807; Fig. 22.13; Transp. 404)
1. Changing the volume within the lungs is accomplished by stimulation of the diaphragm by the phrenic nerves, which causes a downward contraction, and stimulation of the external intercostal muscles to raise the ribs.
2. The chest cavity expands, and the parietal pleura clings to it. The visceral pleura attached to the lungs is carried along, due to intrapleura pressure. The lungs expand.
3. Another force that expands the lungs is warming of the inhaled air. As the inhaled air warms, it helps to expand the lungs.
C. Expiration (p. 808)
1. It takes muscular effort to inhale, burning calories and ATP. Exhalation, on the other hand, is a passive process.
2. Exhalation is due to relaxation of the diaphragm and external intercostal muscles that, because of elastic recoil, return to their original position. When this occurs, air pressure is greater inside the lungs (in a now smaller volume) than outside the body, and air leaves the lungs.
3. To force a deeper expiration, the internal intercostal muscles contract and depress the ribs.
D. Resistance to Airflow (p. 808)
1. One factor that affects resistance to airflow is pulmonary compliance - the ease with which the lungs expand.
2. The lungs normally expand easily, but compliance may be reduced by airway obstruction or degenerative lung diseases.
3. Airflow is also affected by the diameter of bronchioles. During bronchoconstriction, the diameter of bronchioles is decreased, reducing compliance. During bronchodilation, compliance is restored to normal.
E. Alveolar Surface Tension (p. 809)
1. Alveoli have a thin film over the surface of their epithelium which could potentially cause a collapse of the alveoli were it not for the presence of surfactant.
2. Surfactant reduces surface tension within the alveoli, also reducing the chance that the alveoli will collapse and "stick" to themselves during ventilation.
3. Premature infants often have a deficiency of pulmonary surfactant and experience great difficulty breathing.
F. Alveolar Ventilation (p. 809)
1. Not all the air that enters the lungs reaches the alveoli to be available for gas exchange. Dead air is air in the lungs that cannot exchange gases with blood, and in the conducting division is called anatomic dead space.
2. Physiologic (total) dead space is the sum of anatomic dead space and any pathological dead space that may exist. In healthy people, the anatomic and physiologic dead spaces are identical.
3. Alveolar ventilation rate gives the most directly relevant measure of the body's ability to get oxygen to the tissues.
G. Nonrespiratory Air Movements (p. 809)
1. Air movements including yawning, sneezing, coughing, laughing, crying, and others are considered to be nonrespiratory air movements.
2. Sneezing is triggered by irritants in the nasal cavity; coughing is triggered by irritants in the lower respiratory tract.
H. Measurements of Ventilation (p. 809; Figs. 22.14, 22.15; Transp. 405; Table 22.1)
1. A spirometer is used to measure expired breath.
2. Four spirometric measures are called respiratory volumes: tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. Others, called respiratory capacities, are calculated by adding two or more of the respiratory volumes. Fig. 22.15, p. 810 (Transp. 405) shows the relationships between these measures.
3. Restrictive disorders, such as pulmonary fibrosis, reduce compliance and vital capacity.
4. Obstructive disorders do not reduce respiratory volumes but reduce the speed of airflow.
I. Patterns of Breathing (p. 811; Table 22.2)
1. Various patterns of breathing are defined in Table 22.2, p. 811.
III. Neural Control of Ventilation (p. 812)
A. Control Centers in the Medulla Oblongata (p. 812; Fig. 22.16; Transp. 406)
1. Breathing relies on repetitive stimulation from the brain.
2. The medulla oblongata contains inspiratory (I) neurons, which fire during inhalation, and expiratory (E) neurons, which fire during forced expiration.
3. The medulla has two respiratory nuclei.
a. The inspiratory center (dorsal respiratory group, or DRG) is composed of I neurons that stimulate the muscles of inspiration.
b. The other medullary nucleus is called the expiratory center, or ventral respiratory group (VRG). Its I neurons inhibit the inspiratory center when deeper expiration is needed.
B. Control Centers in the Pons (p. 813)
1. The pons regulates ventilation by means of a pneumotaxic center in the upper pons and an apneustic center in the lower pons.
a. The pneumotaxic center sends a continual stream of impulses to the inspiratory center of the medulla. When impulse frequency increases, breathing becomes shallower and faster. Conversely, if frequency declines, breathing is slower and deeper.
b. The role of the apneustic center is still hypothetical.
C. Afferent Connections to the Brainstem Nuclei (p. 813)
1. The brainstem respiratory centers receive input from the limbic system, hypothalamus, chemoreceptors, and the lungs themselves.
2. Emotional state, especially anxiety, alter respiratory rate and depth.
3. Chemoreception of oxygen, carbon dioxide, and pH levels result in adjustments to pulmonary ventilation.
4. The vagus nerves transmit sensory input about irritants in the airways, and the medulla returns signals that initiate bronchoconstriction or coughing.
5. Excessive inflation triggers the inflation (Hering-Breuer) reflex that stops inspiration.
D. Voluntary Control (p. 813)
1. Voluntary control over pulmonary ventilation originates in the motor cortex of the frontal lobe of the cerebrum, which sends impulses down the corticospinal tracts to the respiratory neurons in the spinal cord, bypassing the brainstem respiratory centers.
2. There are limits to voluntary control. Even when a person has held their breath until they pass out, a breaking point is finally reached where autonomic controls override volition, forcing breathing to resume.
IV. Gas Exchange and Transport (p. 814)
A. Composition of Air (p. 814; Table 22.3)
1. Air is a mixture of gases, each of which contributes a share of the total atmospheric pressure called its partial pressure.
2. Partial pressures are important because they determine the rate of diffusion of a gas, and therefore strongly affect the rate of gas exchange between the blood and alveolar air.
3. The composition of gases in inspired and alveolar air is shown in Table 22.3, p. 814.
B. The Air-Water Interface (p. 814; Fig. 22.17; Transp. 407)
1. The greater the partial pressure of oxygen in the alveolar air, the more oxygen dissolves in the blood (a restatement of Henry's Law).
2. At the alveolus, the blood is said to unload carbon dioxide and load oxygen.
C. Alveolar Gas Exchange (p. 814; Figs. 22.18 - 22.21; Transp. 408-410)
1. Alveolar gas exchange is a process of oxygen loading and carbon dioxide unloading in the lungs.
a. The efficiency of both processes depends on the length of stay of an erythrocyte in an alveolar capillary, and how long it takes for oxygen and carbon dioxide to reach equilibrium in the capillary blood.
2. Factors that affect the efficiency of alveolar gas exchange include: concentration gradients of the gases, solubility of the gases, membrane thickness, membrane area, and ventilation-perfusion coupling (the ability to match, or adjust ventilation and perfusion to each other, dependent upon where lung damage is located).
D. Gas Transport (p. 817)
1. Oxygen (p. 817; Fig. 22.22; Transp. 411)
a. Hemoglobin consists of four protein (globin) chains, each with one heme group. Each heme can bind 1 O2 to the ferrous ion at its center: one hemoglobin molecule can carry up to 4 O2. If only one oxygen is carried on the hemoglobin, it is still referred to as oxyhemoglobin (HbO2).
b. The poisonous effect of carbon monoxide stems from its competition for the same binding site as oxygen.
c. The oxyhemoglobin dissociation curve is shown in Fig. 22.22, p. 818; Transp. 411. When the first heme group binds a molecule of oxygen, hemoglobin changes shape to facilitate uptake of a second molecule, which promotes the uptake of a third and then the fourth. This accounts for the rapidly rising midportion of the curve.
2. Carbon Dioxide (p. 818)
a. Carbon dioxide is transported in three forms: as carbonic acid in the plasma, as carbaminohemoglobin, and the remainder is dissolved in the blood as a gas.
E. Systemic Gas Exchange (p. 818; Fig. 22.23; Transp. 412)
1. Carbon Dioxide Loading (p. 818)
a. As a result of metabolic activity, the partial pressure of carbon dioxide is relatively high in the tissues. Thus, carbon dioxide diffuses into the blood and carried in the three forms noted.
b. In an exchange called the chloride shift, most of the bicarbonate diffuses out of the RBCs in exchange for chloride ions diffusing in. Most of the hydrogen ions bind to hemoglobin or oxyhemoglobin, which thus buffers the intracellular pH.
2. Oxygen Unloading (p. 818)
a. When oxyhemoglobin unloads its oxygen, it is called deoxyhemoglobin.
b. When oxyhemoglobin in the blood reaches an area in the tissues with a much lower partial pressure of oxygen (in metabolically active tissues), the oxyhemoglobin unloads its oxygen, which then diffuses into the tissues.
F. Alveolar Gas Exchange Revisited (p. 819; Fig. 22.24; Transp. 413)
1. The reactions that occur in the lungs are essentially the reverse of systemic gas exchange.
a. As hemoglobin loads oxygen, its affinity for hydrogen ions declines. Hydrogen ions dissociate from the hemoglobin and bind with bicarbonate ions diffusing from the plasma into RBCs. Chloride ions diffuse back out of the RBC (reverse chloride shift). The reaction of hydrogen ions and bicarbonate ions reverse the hydration reaction and generates free carbon dioxide, which diffuses into the alvelous and is exhaled.
G. Adjustment to the Metabolic Needs of Individual Tissues (p. 819; Fig. 22.25; Transp. 414)
1. Hemoglobin unloads more oxygen into the tissues that need it most. Four factors adjust the rate of oxygen unloading: ambient partial pressure of oxygen, temperature, the Bohr effect, and DPG.
a. The Bohr effect refers to increased HbO2 dissociation in response to low pH.
b. Since erythrocytes meet their energy needs solely by anaerobic metabolism, one of their metabolic intermediates is DPG (2,3-diphosphoglycerate), which binds to hemoglobin and promotes oxygen loading. An elevated body temperature and a number of hormones stimulate DPG synthesis, thus promoting oxygen unloading to the tissues.
2. The rate of carbon dioxide loading is also adjusted to varying needs of tissues. A low level of oxyhemoglobin (HbO2) enables the blood to transport more CO2, a phenomenon known as the Haldane effect. A high metabolic rate keeps oxyhemoglobin levels relatively low and thus allows more CO2 to be transported.
V. Blood Gases and the Respiratory Rhythm (p. 821; Fig. 22.26)
A. The purpose of respiration is to maintain pH, oxygen, and carbon dioxide levels in the blood within homeostatic limits. Thus, the brainstem respiratory centers monitor these conditions in the blood by various means.
1. Peripheral chemoreceptors are the aortic and carotid bodies located in the aortic arch and in the carotid arteries.
a. The aortic bodies send signals to the medulla by way of the vagus nerves.
b. The carotid bodies send signals to the medulla by way of the glossopharyngeal nerves.
2. The central chemoreceptors are close to the surface of the medulla oblongata, and primarily monitor the pH of the cerebrospinal fluid (CSF).
A. Hydrogen Ions (p. 821)
1. The pH of the CSF is the strongest stimulus to respiration. Once it is in the CSF, CO2 reacts with water and releases H+.
2. The pH of the blood is normally 7.40 + 0.05. If the blood pH falls below 7.35, a state of acidosis exists, which can trigger hyperventilation, or "blowing off" the excess CO2.
3. If the pH of the blood rises above 7.45, we have a state of alkalosis, which inhibits respiration to allow metabolic CO2 to accumulate.
B. Carbon Dioxide (p. 822)
1. The effect of arterial partial pressure of CO2 is mostly an indirect one, mediated by the effects of pH on CSF. Yet some evidence suggests that CO2 has some effect when pH remains stable.
a. At the beginning of exercise, the rising blood CO2 level may directly stimulate the peripheral chemoreceptors and trigger an increase in ventilation more quickly than the central chemoreceptors do.
C. Oxygen (p. 822)
1. Oxygen concentration usually has little effect on respiration.
a. A moderate drop in partial pressure of oxygen does stimulate the peripheral chemoreceptors, but this effect is overridden by the fact that hemoglobin binds more hydrogen ions under these conditions, raising blood pH. This indirectly inhibits respiration.
b. Only when the partial pressure of oxygen drops below 60 mmHg does the stimulatory effect of hypoxia override the inhibitory effect of pH increase.
2. The main chemical stimulus to pulmonary ventilation is the concentration of hydrogen ions in the CSF and tissue fluid of the brain.
VI. Respiratory Disorders (p. 823)
A. Oxygen Imbalances (p. 823)
1. Hypoxia is classified according to cause.
a. Hypoxemic hypoxia is caused by inadequate pulmonary gas exchange.
b. Ischemic hypoxia results from inadequate circulation, as in congestive heart failure.
c. Anemic hypoxia is due to anemia, and the blood cannot carry enough oxygen.
d. Histotoxic hypoxia occurs when tissues cannot use oxygen, due to poisoning.
2. Excessive oxygen is also dangerous. Oxygen toxicity develops rapidly after breathing pure oxygen for a few hours, which leads to nervous tissue damage and destruction of enzymes, and, ultimately, death.
B. Chronic Obstructive Pulmonary Diseases (p. 823)
1. Chronic obstructive pulmonary disease (COPD) refers to any disorder in which there is a long-term obstruction of airflow and thus reduced pulmonary ventilation.
a. Asthma occurs when allergens trigger the release of inflammatory chemicals that cause bronchoconstriction and sometimes suffocation.
b. Smoking can lead to chronic bronchitis, which involves inflammation of the bronchi and immobilization of the cilia. Smokers develop a chronic cough that is needed to bring up sputum.
c. In emphysema, alveolar walls break down and the lung exhibits larger but fewer alveoli. Thus there is much less respiratory membrane available for gas exchange. People with emphysema need three to four times the normal amount of energy just to breathe.
C. Smoking and Lung Cancer (p. 824; Fig. 22.27)
1. Lung cancer is the most common type of cancer and accounts for more deaths than any other form. The most important cause of lung cancer is smoking cigarettes.
2. Of the three forms of lung cancer, the most common is squamous-cell carcinoma. Adenocarcinoma, almost as common, originates in the mucous glands of the lamina propria. The least common, but most deadly form, is small-cell (oat-cell) carcinoma that begins in the primary bronchi.
3. Most smokers have a cough, although a cough may also indicate lung cancer. Coughing up blood is the sign most people take seriously. Lung cancer metastasizes very rapidly, and prognosis is generally poor.
D. Infectious Diseases (p. 825)
1. The common cold, or acute coryza, is caused by a number of viruses, spread by direct contact of fingers with the mucous membranes of the nose and eyes. The best prevention is frequent hand washing.
2. Pneumonia is a lower respiratory infection caused by bacteria, viruses, fungi, or protozoans, but most commonly by the bacterium Streptococcus pneumoniae. Alveoli become filled with fluid, interfering with gas exchange.
3. Tuberculosis is caused by the bacterium Mycobacterium tuberculosis. The lung tissue forms fibrous nodules, called tubercles, around the bacteria.
CHAPTER ESSAY: Diving Physiology and Decompression Sickness (p. 825)
i. People have been diving underwater for many hundreds of years, often carrying air in containers with them. Today's scuba divers used pressurized air tanks to ensure delivery of air to the diver.
ii. Breathing pressurized (hyperbaric) gas presents problems, since under higher pressure, nitrogen dissolves more readily in the blood. This can lead to nitrogen narcosis, or "rapture of the deep".
iii. Holding one's breath and rising rapidly to the surface can lead to pulmonary barotrauma, which can be fatal. However, taking a deep breath at the surface, then holding it for a dive, and turning to the surface poses no problem.
iv. Ascending too rapidly can leads to the "bends" or decompression sickness, also called caisson disease. This condition can be treated by placing the diver in a hyperbaric chamber that is then slowly decompressed.
VII. Connective Issues (p. 827)
A. Interactions between the respiratory system and other organ systems are listed on p. 827.
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