Detailed Chapter Summary Chapter 23, The Respiratory System
23.1 Overview of the Respiratory System (p. 633) The respiratory system enables the blood to exchange gases with the air; serves for vocalization; provides a sense of smell; regulates blood pH; produces a hormone that regulates blood pressure; and creates pressure gradients that aid in the flow of lymph and blood and in expelling the contents of some abdominal organs.
The conducting division of the respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and most bronchioles; it serves only for airflow.
The respiratory division consists of respiratory bronchioles, alveoli, and other distal gas-exchange regions of the lungs.
The upper respiratory tract consists of the respiratory organs of the head and neck, extending from the nose through the larynx. The lower respiratory tract consists of the respiratory organs of the thorax, including the trachea, bronchi, and lungs.
23.2 The Upper Respiratory Tract (p. 634) The nose serves to warm and cleanse inhaled air, detect odors, and amplify the voice.
The nose extends from the nostrils to the posterior nasal apertures. The facial part of the nose is shaped by the maxillae, nasal bones, and the lateral and alar cartilages.
The nasal septum consists of the perpendicular plate of the ethmoid bone superiorly, the vomer inferiorly, and the septal nasal cartilage anteriorly. It divides the interior of the nose into right and left nasal fossae. The roof of the nasal cavity is formed by parts of the ethmoid and sphenoid bones, and the floor by the hard palate.
Each fossa has three scroll-like nasal conchae covered with a ciliated mucous membrane. Air flows through narrow spaces called the meatuses between the conchae. The conchae warm, humidify, and cleanse the air flowing over them.
The nasal cavity is lined with a sensory olfactory epithelium high in each fossa, and with a ciliated pseudostratified respiratory epithelium throughout the rest of the cavity. The respiratory epithelium traps airborne particles in its mucus and propels the mucus to the pharynx to be swallowed. The respiratory epithelium is well endowed with goblet cells, which produce mucus.
Erectile tissues of the inferior nasal concha swell and shrink, usually in one fossa at a time, to periodically shift airflow from one fossa to the other. This allows the less-ventilated fossa a chance to periodically recover from drying.
The pharynx is a muscular passage divided into nasopharynx, oropharynx, and laryngopharynx.
The larynx is a cartilaginous chamber beginning superiorly at the glottis and ending about 4 cm lower at the trachea. It is supported by nine cartilages bound to each other by intrinsic ligaments; two extrinsic ligaments attach the larynx to the hyoid bone above and the trachea below. The nine cartilages are solitary median epiglottic, thyroid, andcricoid cartilages, and paired arytenoid, corniculate, and cuneiform cartilages. The larynx has a pair of superior vestibular folds that exclude food and drink from the airway, and a pair of inferior vocal cords that function in speech. Extrinsic muscles of the larynx help to close it during swallowing, and its intrinsic muscles operate the vocal cords during speech.
23.3 The Lower Respiratory Tract (p. 638) The trachea is a 12-cm tube, supported by cartilaginous rings, that extends from the larynx above to the two main bronchi below. The ciliated mucosa of the trachea acts as a mucociliary escalator to remove inhaled debris, stuck in the tracheal mucus, from the respiratory tract. The C-shaped cartilage rings hold the trachea open during inspiration. The gap on the posterior side of the C is spanned by the trachealis muscle and allows for expansion of the esophagus during swallowing.
At its inferior end, the trachea branches into a right and left main bronchus, which convey air to the respective lungs.
Each lung is a conical organ extending from the superior apex to the inferior, broad base. Its extensive costal surface lies against the rib cage, and its indented mediastinal surface faces the heart. The mediastinal surface has a hilum through which it receives the main bronchi, pulmonary blood vessels, nerves, and lymphatics.
The right lung is shorter but broader than the left and is divided by two deep fissures into superior, middle, and inferior lobes. The left lung is taller but narrower and has only a superior and inferior lobe, separated by a single fissure. The left lung exhibits a cardiac impression where the heart presses against it.
Each lung has a bronchial tree—a branching system of air passages extending from the main bronchus to lobar bronchi (2 in the left lung and 3 in the right), segmental bronchi (8 in the left lung and 10 in the right), bronchioles, terminal bronchioles, and respiratory bronchioles. Each lobar bronchus supplies one lobe of the lung; each segmental bronchus supplies one bronchopulmonary segment. The terminal bronchioles are the end of the conducting division; all branches beyond this have alveoli and belong to the respiratory division.
The main bronchi are supported by C-shaped cartilages like the trachea. The lobar and segmental bronchi are supported by overlapping cartilage plates. When the air tubes reach about 1 mm or less in diameter and the cartilages cease to exist, they are considered bronchioles.
Respiratory bronchioles branch into 2 to 10 thin-walled alveolar ducts. Alveolar ducts end in grapelike clusters of alveolicalled alveolar sacs.
The epithelium of the airway is ciliated pseudostratified columnar from the trachea through the largest bronchioles; becomes ciliated simple columnar and then simple cuboidal in the bronchioles; and is stratified squamous from the alveolar ducts to the alveoli.
Smooth muscle in the wall of the airway allows for changes in airway diameter, especially in the bronchioles, since these are not impeded by reinforcing cartilage. Bronchoconstriction narrows the bronchioles and reduces airflow; bronchodilation widens them and increases airflow. Because of this capacity to constrict and dilate, and their large number, bronchioles are the most important point of control of respiratory airflow.
Each lung receives a pulmonary blood supply from the pulmonary artery and a systemic blood supply from the bronchial arteries. The pulmonary supply leads ultimately to capillaries that surround the alveoli and serve for gas exchange. The bronchial supply leads to capillaries that supply the metabolic needs of the larger lung tissues such as the bronchi and bronchioles.
An alveolus is a thin-walled sac surrounded by a basket of blood capillaries. It is composed of squamous and great alveolar cells and contains alveolar macrophages, the last line of defense against inhaled debris. The great alveolar cells secrete pulmonary surfactant, which prevents the alveoli from collapsing during expiration.
Gases are exchanged through the thin respiratory membrane composed of the capillary endothelial cells, the squamous alveolar cells, and their shared basement membrane.
The surface of each lung is a serous membrane called the visceral pleura. It continues as the parietal pleura, which lines the inside of the rib cage. The space between the pleurae is the pleural cavity, and is lubricated with pleural fluid. The pleurae reduce friction during breathing, contribute to the pressure gradients that move air into and out of the lungs, and help compartmentalize the thoracic cavity.
23.4 Neuromuscular Aspects of Respiration (p. 644) The diaphragm is the prime mover of pulmonary ventilation and accounts for about two-thirds of the airflow. The other one-third is due mainly to the external and internal intercostal muscles. Many other neck, thoracic, and abdominal muscles (accessory muscles of respiration) contribute to deep breathing.
Quiet inspiration is achieved mainly by contraction of the diaphragm and elevation of the ribs by the external intercostals. These actions enlarge the thoracic cavity in the vertical, transverse, and anteroposterior dimensions, causing an inflow of air.
Forced (deep) inspiration is aided by muscles that elevate the upper ribs (sternocleidomastoids, scalenes, pectoralis minor and major, serratus anterior, and part of the internal intercostals).
Quiet expiration is achieved when the inspiratory muscles relax and the thoracic structures recoil because of their own elasticity.
Forced expiration is aided by muscles that depress the ribs and sternum (internal intercostals, rectus abdominis) and compress the abdominal cavity (latissimus dorsi, transverse abdominal, oblique abdominals).
The respiratory rhythm is set by respiratory centers in the brainstem.
The medulla oblongata contains a nucleus called the ventral respiratory group (VRG) which serves as the primary pacemaker of the respiratory rhythm. It also has a dorsal respiratory group (DRG) that serves as an integrating center, receiving input from the pons, chemoreceptors in the medulla and arteries, and stretch and irritant receptors in the lungs. It acts on the VRG to modulate the breathing rhythm. produces inspiration and one called the ventral respiratory group with both inspiratory and expiratory roles in heavy breathing. The pons contains a pontine respiratory group that receives input from higher brain centers and issues output to the VRG and DRG to modify the breathing rhythm in relation to sleep, emotional excitement, speech, and other conditions.
The chemoreceptors that modify breathing are central chemoreceptors in the brainstem and peripheral chemoreceptors in the aorta and carotid arteries. They monitor the pH of the cerebrospinal fluid and the pH and the O2 and CO2 levels in the blood.
23.5 Developmental and Clinical Perspectives (p. 648) The respiratory system begins development as a pulmonary groove that grows from the floor of the pharynx around 3.5 weeks of gestation. This groove grows into a tube that forks into two lung buds, then branches extensively to form the bronchial trees. By weeks 8 to 9, the pericardium and diaphragm isolate the lungs and pleural cavities from the heart and abdominal cavity.
By 11 weeks of gestation, the fetus begins respiratory movements (fetal breathing) that moves amniotic fluid into and out of the respiratory tract. The neonate’s first breaths are very strenuous because of the need to inflate the alveoli with air. The pulmonary blood vessels also expand during these breaths, and the foramen ovale and ductus arteriosus gradually close to direct blood to the lungs.
With advancing age, the thoracic cage becomes less flexible and the depth and rate of breathing decline. Irritants and pathogens cannot be cleared from the lungs as easily, so elderly people become more susceptible to respiratory infections. Pneumonia is a common cause of death in old age.
Restrictive disorders of respiration stiffen the lungs and interfere with their inflation. Pulmonary fibrosis is an example. Obstructive disorders narrow the airway and interfere with the ease and speed of airflow. This can be caused by tumors, aneurysms, or bronchial congestion.
The chronic obstructive pulmonary diseases (COPDs) are chronic bronchitis and emphysema. These are usually caused by tobacco smoke, are among the leading causes of death in old age. Chronic bronchitis entails congestion of the airway with thick mucus and susceptibility to respiratory infection. Emphysema entails destruction of pulmonary alveoli. The COPDs also frequently lead to right-sided heart failure.
Asthma is the most common chronic respiratory disorder in childhood. It is usually an allergic reaction that causes bronchoconstriction, airway inflammation, and sometimes death by suffocation.
Lung cancer also is usually caused by tobacco smoke. Tumors replace functional respiratory tissue, cause bleeding lesions of the lung, and metastasize quickly to adjacent thoracic organs.