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The upper respiratory tract consists of structures located outside the thoracic cavity; this includes the nose, nasopharynx, oropharynx, laryngopharynx, and larynx. These structures warm and humidify inspired air. They are also responsible for the senses of smell and taste, as well as chewing and swallowing. The lower respiratory tract consists of structures located inside the thoracic cavity; these include the trachea, bronchi, and lungs.
Air enters and leaves the respiratory system through the nose. Just inside the nostrils are small hairs called cilia that filter out dust and large foreign particles. The nasal cavity is separated from the mouth by a bony structure called the palate. A vertical plate of bone and cartilage (the septum) separates the nasal cavity into two halves. The cavity is lined with epithelium rich in goblet cells that produce mucus. Projecting from the lateral wall of each cavity are three bones called conchae. The conchae warm and moisten air as it flows past. At the same time, dust sticks to the mucus, which is then swallowed. Branches of the olfactory nerve (responsible for the sense of smell) penetrate the upper nasal cavity and lead to the brain. The sphenoid sinus (shown here) and the other paranasal sinuses (including the frontal, maxillary, and ethmoidal sinuses) drain mucus into the nasal cavity.
Just behind the nasal and oral cavities is a muscular tube called the pharynx. Commonly called the throat, the pharynx has three regions: The nasopharynx lies just behind the soft palate. It contains openings for the right and left auditory (eustachian) tubes. The oropharynx is a space between the soft palate and the base of the tongue. It contains the palatine tonsils (the ones most commonly removed by tonsillectomy) and the lingual tonsils, found at the base of the tongue. The laryngopharynx passes dorsal to the larynx and connects to the esophagus. Only air passes through the nasopharynx, whereas both food and air pass through the oropharynx and laryngopharynx.
The larynx prevents food and liquids from entering the trachea, acts as an air passageway between the pharynx and trachea, and produces sound. The larynx is formed by nine pieces of cartilage that keep it from collapsing; a group of ligaments bind the pieces of cartilage together and to adjacent structures. The epiglottis (which closes over the top of the larynx during swallowing to direct food and liquids into the esophagus) is the uppermost cartilage. The largest piece of cartilage is the thyroid cartilage, which is also known as the “Adam’s apple.” The mucous membrane lining the larynx forms two pairs of folds. The superior pair (called vestibular folds) close the glottis (the opening between the vocal cords) during swallowing to keep food and liquids out of the airway. They play no role in speech. The inferior pair, the vocal cords, produces sound when air passes over them. The opening between the cords is called the glottis.
The trachea lies in front of the esophagus; it is a rigid tube about 4.5 inches (12 cm) long and 1 inch (2.5 cm) wide. C-shaped rings of cartilage encircle the trachea to reinforce it and keep it from collapsing. The open part of the “C” faces posteriorly, giving the esophagus room to expand during swallowing. At the carina, the trachea branches into two primary bronchi, which are also supported by C-shaped rings of cartilage. The right bronchus is slightly wider and more vertical than the left, making this the most likely location for aspirated food particles and small objects to lodge. (Primary bronchi is shaded green in right lung.) Immediately after entering the lungs, the primary bronchi branch into secondary bronchi: one for each of the lung’s lobe (two on the left and three on the right). (These are shaded yellow in right lung.) Secondary bronchi branch into smaller tertiary bronchi (shaded in orange in right lung) The cartilaginous rings become irregular and disappear in the smaller bronchioles. Tertiary bronchi continue to branch, resulting in very small airways called bronchioles (shaded purple in right lung). Bronchioles divide further to form thin-walled passages called alveolar ducts. Alveolar ducts throughout the lungs terminate in clusters of alveoli called alveolar sacs, the primary structures for gas exchange.
The lung passages all exist to serve the alveoli, because it’s within the alveoli that gas exchange occurs. The alveoli are wrapped in a fine mesh of capillaries. The extremely thin walls of the alveoli, and the closeness of the capillaries, allow for efficient gas exchange. The exchange of air occurs through the respiratory membrane (which consists of the alveolar epithelium, the capillary endothelium, and their joined basement membranes). The inside of each alveolus is coated with a thin layer of fluid containing surfactant, a substance that helps reduce surface tension (the force of attraction between water molecules). This keeps the alveolus from collapsing as air moves in and out during respiration.
The lungs fill the pleural cavity. The primary bronchi and pulmonary blood vessels enter each lung through a slit on the lung’s medial surface, called the hilum. The top, or apex, of each lung extends about ½” (1.5 cm) above the first rib; the base of each lung rests on the diaphragm. The right lung is shorter, broader, and larger than the left. It has three lobes (superior, middle, and inferior) and handles 55% of the gas exchange. The right lung contains two fissures: the horizontal fissure and the oblique fissure. The left lung has only two lobes: the superior and inferior. It contains one fissure (the oblique fissure).
A serous membrane (the visceral pleura) covers the surface of the lungs, extending into the fissures. The parietal pleura lines the entire thoracic cavity. The space between the visceral and parietal pleurae is called the pleural cavity. The pleural cavity is a potential space; the two membranes are normally separated only by a film of slippery pleural fluid. The fluid in the pleural cavity lubricates the pleural surfaces, allowing the two surfaces to glide painlessly against each other as the lungs expand and contract. Also, because the pressure in the pleural cavity is lower than atmospheric pressure, it creates a pressure gradient that assists in lung inflation.
The main muscle responsible for pulmonary ventilation is the diaphragm. Inspiration: The external intercostal muscles pull the ribs upward and outward, widening the thoracic cavity; the internal intercostals help elevate the ribs; the diaphragm contracts, flattens, and drops, pressing the abdominal organs downward and enlarging the thoracic cavity. Air rushes in to equalize pressure. Expiration: The internal intercostal muscles relax; the diaphragm relaxes, bulging upward and pressing against the base of the lungs, reducing the size of the thoracic cavity; and air is pushed out of the lungs. During times of forced or labored breathing, accessory muscles of respiration assist with breathing. During deep inspiration, muscles of the neck (the sternocleidomastoids and scalenes) and the chest (the pectoralis minor) contract to help elevate the chest. During forced expiration, the rectus abdominis and external abdominal obliques contract to pull down the ribs and sternum, further reducing chest size and expelling air more rapidly.
The muscles used for breathing are skeletal muscles, which require nervous stimulation to contract. Unconscious breathing resides in the medulla and pons (parts of the brainstem). The medulla contains two interconnected centers that control breathing: the inspiratory center and the expiratory center. The inspiratory center (in medulla) is the primary respiratory center. It controls inspiration and, indirectly, expiration. The inspiratory center sends impulses to the intercostal muscles (via the intercostal nerves) and to the diaphragm (via the phrenic nerves). The inspiratory muscles contract, causing inhalation. Nerve output then ceases abruptly, causing the inspiratory muscles to relax. The elastic recoil of the thoracic cage produces exhalation. The apneustic and pneumotaxic centers are in the pons; these influence basic breathing rhythm. The expiratory center sends impulses to the abdominal and accessory muscles.
Sensory receptors throughout the body signal respiratory centers about needs. Peripheral chemoreceptors (in carotid and aortic bodies) detect low blood levels of oxygen and signal medulla to increase rate and depth of respirations to bring in more oxygen. Central chemoreceptors (in brainstem) detect increasing pH levels and signal the respiratory centers to increase rate and depth of breathing to blow off excess carbon dioxide and lower pH. Receptors in the lungs and chest wall detect stretching and signal respiratory centers to exhale. Hypothalamus and limbic systems alter breathing in response to emotions. Nerve cells in airways respond to irritants by signaling muscles to contract, causing cough or sneeze.
The intercostal muscles contract, pulling the ribs up and out; the diaphragm contracts and moves downward. This enlarges the chest cavity in all directions. The lungs expand along with the chest because of the two layers of the pleura. The parietal pleura is attached to the ribs; the visceral pleura covers the lungs. A thin film of fluid between the two pleura causes them to cling together like two pieces of wet paper. Also, the potential space between the two pleura maintains a pressure slightly less than atmospheric pressure (negative pressure). This is the intrapleural pressure. When the ribs expand and the parietal pleura pulls away, intrapleural pressure becomes even more negative. This has a suction-like effect, causing the visceral pleura to cling even tighter to the parietal pleura. The visceral pleura follows the parietal pleura, pulling the lung along with it. When the lungs expand, the volume of air in the lungs spreads throughout the enlarging space. This causes the pressure within the bronchi and alveoli (the intrapulmonic pressure) to drop. When the intrapulmonic pressure drops lower than the atmospheric pressure, air flows down the pressure gradient into the lungs.
The diaphragm and external intercostal muscles relax and the thoracic cage springs back to its original size. The lungs are compressed by the thoracic cage and intrapulmonary pressure rises. Air flows down the pressure gradient and out of the lungs.
Pulmonary compliance: This refers to the elasticity of lung tissue. Ventilation cannot occur unless the lungs and thorax can stretch and recoil. Diseases that cause scarring (such as tuberculosis or black lung disease) make the lungs stiff. The lungs have difficulty expanding and ventilation is impaired. Alveolar surface tension: The inner surface of each alveoli is covered with a thin film of water, which is necessary for gas exchange. However, water molecules are electrically attracted to each other; left alone, the water molecules will move toward each other and collapse the alveoli. (If the alveoli collapse, gas exchange cannot occur.) To avoid this problem, alveolar cells secrete surfactant, a lipoprotein that disrupts the electrical attraction between the water molecules. This lowers surface tension and prevents alveolar collapse.
Tidal volume: the amount of air inhaled and exhaled during quiet breathing Inspiratory reserve volume: the amount of air inhaled using maximum effort after a normal inspiration Expiratory reserve volume: the amount of air that can be exhaled after a normal expiration by using maximum effort Residual volume: the air (about 1300 mL) that remains in the lungs after a forced expiration; this air ensures that gas exchange continues even between breaths Vital capacity: the amount of air that can be inhaled and exhaled with the deepest possible breath (the tidal volume combined with the inspiratory and expiratory reserve volumes) Total lung capacity: the maximum amount of air that the lungs can contain (the vital capacity plus the residual volume)
The contribution of a single gas in a mixture of gases is called partial pressure. A gas’s partial pressure is symbolized by the letter “P” followed by the formula for the gas, such as Pco2. The partial pressures of oxygen and carbon dioxide vary between the air we breathe, the alveoli, arterial blood, and venous blood. These variations in pressure allow the body to absorb oxygen and expel carbon dioxide. The differences in partial pressures of O2 and CO2 on either side of the respiratory membrane cause O2 to move out of the alveoli and into the capillaries and CO2 to move out of the capillaries into the alveoli. The CO2 is later exhaled through the lungs. Optimum exchange depends on pressure gradient between oxygen in alveolar air and oxygen in incoming pulmonary blood, adequate alveolar surface area, and adequate respiratory rate.
Of the oxygen entering the body, only 1.5% is dissolved in blood plasma. The remaining 98.5% of oxygen travels to the lungs, where it forms a weak bond with the iron portion of hemoglobin to form oxyhemoglobin. Oxyhemoglobin travels to the cells, where the difference between venous and arterial pH breaks the bond, and oxygen is released to the tissues.
Carbon dioxide bound to hemoglobin forms carbaminohemoglobin. (Hemoglobin can transport both O2 and CO2 at the same time because they bind to different sites on the hemoglobin molecule.) Most carbon dioxide is carried in the form of bicarbonate ions (HCO3−). This occurs because, when CO2 dissolves in plasma, it reacts with the water in the plasma to form carbonic acid. Carbonic acid then dissociates into bicarbonate and hydrogen ions.