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Gross Anatomy and the Lower Respiratory Tract II Lower Respiratory Tract Trachea The flow of air continues from the larynx to the trachea or windpipe. The trachea is about 10 cm (4 in.) long and 2 cm (3/4 in.) wide and extends from the larynx to the level of the fifth thoracic vertebrae. The presence of 16 to 20 C-shaped pieces of hyaline cartilage located along the trachea prevents this airway from closing. The last cartilage in the trachea exhibits a projection from the anterior surface that extends into the lumen. This projection, the carina, is very sensitive to particulate material and causes coughing when stimulated. The openings of the Cshaped cartilages face the posterior of the trachea and contain the trachealis smooth muscle. This muscle constricts when someone coughs, increasing the force of the cough. The trachealis muscle also constricts during an asthmatic reaction, shrinking the airway and making it harder to breathe. Structurally the trachea consists of three concentric tissue layers: the mucosa, submucosa, and adventitia. This arrangement is continuous throughout the remainder of the upper respiratory system. The layer facing the lumen, the mucosa, is an epithelial layer. The cell types found in the trachea are pseudostratified, ciliated, columnar epithelia. In addition prominent elastic fibers can be found in the lamina propria. The submucosa contains blood vessels, nerves, assorted connective tissues, and mucus glands. The outer adventitia is mostly areolar connective tissue that is continuous with the hyaline cartilage and connects the trachea to the surrounding structures and tissues. The Bronchi and Bronchioles The bronchial tree is so named because it resembles a tree that has been turned upside-down. The trachea would be the trunk, the progressively smaller bronchi and bronchioles are the branches, and the alveoli are the leaves. Branching of the trachea into the right and left primary bronchi occurs after the last cartilage in the trachea at the level of the seventh thoracic vertebrae. The right primary bronchus is wider, shorter, and more vertical than the left primary bronchus because the left primary bronchus and lung must accommodate the heart. The primary bronchi branch to form the secondary or lobar bronchi. There are three secondary bronchi on the right and two on the left. The secondary bronchi continue to branch into the tertiary or segmented bronchi, which also continue the branching pattern. Approximately 23 successive branches lead to the bronchioles. A bronchiole is a tube with a diameter of less than 1 millimeter (mm). When the measurement reaches less than 0.5 mm, a bronchiole is termed a terminal bronchiole. The bronchial tree is lined by pseudostratified, ciliated, columnar epithelia with goblet cells dispersed among the columnar cells. Plates of cartilage are found in the walls of the bronchial tree, with the amount of cartilage and the number of plates decreasing as the bronchi and bronchioles become smaller. The terminal bronchioles do not contain cartilage plates; these tubes are small enough to stay open without cartilage. Smooth muscle is found throughout the system, even into the respiratory zone. Lungs, detail aveoli. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United Detail aveoli. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/). The Lungs The lungs are two of the largest organs of the body, but they are among the lightest. Along with the heart, the lungs take up nearly all of the space in the thorax, superior to the diaphragm. As an organ, the lung is made up of airway tubes and alveoli, giving it little weight. Elastic connective tissues in the stroma of the lungs allow them to expand with incoming air and recoil when expelling air. The lungs contain a large amount of surface area in order to efficiently support the exchange of oxygen and carbon dioxide. The hilus (meaning depression or pit) of the lungs is an indentation on the medial side of the lungs and the point of entry of blood vessels, primary bronchi, nerves, and lymphatics. This collection of vessels and nerves makes up the root of the lung. The tip or apex of the lungs is a blunted point found just above the clavicles. The posterior, lateral, and anterior sides of the lungs are surrounded by the ribs. These areas are called the costal surfaces of the lungs referring to the costal cartilage surrounding them. The flat, inferior surface of the lungs is found superior to the diaphragm and referred to as the base of the lung. Since, the liver is found on the right side of the body and inferior to the diaphragm, the insertion of the diaphragm is slightly raised on the right. Consequently the right lung is usually slightly shorter than the left. The lungs extend from the first costal cartilage to the tenth thoracic vertebrae. The lungs consist of a right lung and a left lung. Even though the right lung is slightly shorter than the left, the left lung has about 10 percent less mass than the right due to the cardiac notch on the medial side of the left lung. The heart is tucked into this notch. The heart, the right lung, and the left lung, are each located in their own anatomical compartment in the upper thorax. The right lung is divided into three lobes. A horizontal fissure separates the superior and middle lobes, and an oblique fissure separates the middle and inferior lobes. The smaller left lung contains only two lobes. An oblique fissure separates the superior and inferior lobes on this side. Each lobe is divided into bronchopulmonary segments separated by connective tissue septa. There are a total of 10 of these segments and each contains a tertiary bronchiole, a pulmonary and bronchial artery, and a lymphatic branch. The presence of these segments aids in further isolating parts of the lungs to prevent the spread of infection or disease. Connective tissue further divides the segments into lung lobules, the smallest anatomical unit in the lungs. A lobule is hexagonal in shape and less than a centimeter in diameter. Each lobule contains a terminal bronchiole and its associated alveoli. The connective tissue associated with lobules may be blackened by tobacco smoke or pollution from the environment. Image below compares the lungs from a smoker with those from a nonsmoker. Lungs. Usually, lungs are a pink with a relatively smooth surface. The lungs on the right are blackened and irregularly shaped from the accumulation of material inhaled with tobacco smoke. This material can be cleared from the lungs, but only over a long period of time and only if the person stops smoking. Cigarette smoking causes the deaths of over 440,000 people in the United States each year, including people exposed to second-hand smoke. The financial losses associated with these deaths are in excess of 200 billion dollars a year. There are over 4,000 chemical compounds that are created when the more than 600 chemical ingredients in cigarettes are burned and inhaled. More than 50 of the chemicals produced by the burning of a cigarette are classified as carcinogens. A partial list of these chemicals includes: acetone, acetic acid, ammonia, arsenic, benzene, butane, cadmium, carbon monoxide, formaldehyde, lead, naphthalene, methanol, nicotine, tar, and toluene. The direct effect of many of the chemicals in the smoke is to paralyze and destroy cilia. When the cilia cannot function, mucus accumulates in the lung tubules. Opportunistic bacterial and other microorganisms utilize the mucus to grow and colonize the lungs. Many of these organisms create pathologic conditions inside the lung. Smoker’s cough is a condition that exists because the mucus and other debris accumulate in the lung tissue. These accumulations stimulate the cough reflex as means to clear the lungs. The inhaled chemicals can also contribute to the breakdown of connective tissue fibers, especially elastin. The degeneration of the connective tissue can lead to emphysema, a form of chronic obstructive pulmonary disease (COPD). Blood and nerve supply The lungs have a dual blood supply. The pulmonary artery brings oxygen-poor blood from the right ventricle of the heart. This blood passes through the pulmonary capillaries, where some carbon dioxide will leave the blood and a large amount of oxygen will be acquired. The newly oxygenated blood enters the pulmonary veins and returns back to the left side of heart. The pulmonary circulation holds about 500 milliliters (ml) of blood, or about 10 percent of the body’s supply. About 75 ml of blood is in the pulmonary capillaries for gas exchange at any one time. The blood supply that nourishes the tissues of the lungs arrives through the bronchial artery, which branches off of the aorta and carries oxygen-rich blood to support the lung tissues. The bronchial supply anastomoses with the pulmonary vessels, and a mixture of blood leaves through the bronchial and pulmonary veins. Blood passes through the lungs at a rate equal to cardiac output, or about five liters per minute. Blood pressure measured in the pulmonary circulation is less than it is in corresponding vessels of the systemic circulation. To some extent this decrease results from the decreased resistance found in this shorter pulmonary pathway. The mean pulmonary arterial pressure of about 15 mmHg is adequate to push blood through the pulmonary capillary network and into the left side of the heart. The low hydrostatic pulmonary capillary pressure of 7-9 mmHg only produces a small amount of fluid filtration across the capillary wall. Under normal conditions, the lymphatic vessels readily remove this filtrate. Under conditions where left atrial pressure rises dramatically, such as in mitral valve stenosis or congestive heart failure, pulmonary capillary pressure will also rise, increasing the rate of capillary filtration. If the lymphatic system cannot keep up with the higher filtration rate, fluid will accumulate in the alveoli as pulmonary edema. This can interfere with the exchange of oxygen, leading to cyanosis, decreased activity tolerance, etc. The pulmonary circulation responds to hypoxia differently than the systemic circulation. The systemic circulation dilates under hypoxic conditions causing an increased blood flow through the tissues. The arterioles of pulmonary circulation constrict selectively in cases of alveolar hypoxia. This constriction diverts blood to areas in the lungs that are better ventilated. This helps ensure adequate gas exchange. Nerves from the pulmonary plexus enter the lungs at the hilus. These nerves contain a mixture of visceral sensory and autonomic nerve fibers that follow the bronchial tree and blood vessels. Parasympathetic nerve stimulation results in bronchoconstriction, constriction of the bronchioles, while sympathetic nerve stimulation results in bronchodilation, dilation of the bronchioles. Pleura Each lung is found in a pleural cavity bounded by the pleural membrane, a double sided membrane that contains a thin layer of pleural fluid. The visceral pleural is a mucus membrane that covers the lungs and folds over at the hilus. The folded membrane continues and becomes the parietal pleura, which lines the inner wall of the thoracic cavity. The space between the two membranes is called the pleural cavity or space. From 1 to 15 ml of pleural fluid is found on the facing surfaces of the pleural membranes. This fluid helps to lubricate the membrane surfaces so that the movement of the lungs during inhaling and exhaling does not cause frictional damage to the tissues. The fluid also lightly holds the two membranes together so that they move together as the chest wall expands and contracts. Since pleural fluid is mainly a filtrate of plasma, the factors that affect the amount of pleural fluid production and removal are the same factors that govern interstitial fluid volumes in most regions of the body. The main factors include capillary hydrostatic pressure, capillary colloid osmotic pressure, the permeability (“leakiness”) of the capillaries, and the rate of fluid removal by the lymphatics. Under normal conditions, there is a slow but steady turnover of pleural fluid, but under certain conditions excess fluid can build up, and impede lung expansion. This condition is known as a pleural effusion and may be secondary to blockade of venous drainage by tumors, increased capillary permeability because of infections, etc. The pleural sac extends below the lungs, to the level of the twelfth thoracic vertebrae. Samples of the pleural fluid can be safely taken from this area. Normal pleural fluid is clear and pale yellow in color. It has very few cells free in the fluid. The majority (75 percent) of these cells are macrophages. About 23 percent of the cells are lymphocytes, with an assortment of cells making up the remaining 2 percent. Pleural Transudates vs. Exudates Since a pleural effusion may be caused by a number of different conditions, inspection of the pleural fluid is often be used to help determine the cause of the effusion. The fluid can either be classified as a transudate or an exudate, depending on if the protein content is significantly increased (exudates) or not (transudate). A transudate forms when the forces contributing to capillary exchange are altered, such as with venous blockade that increases hydrostatic pressure, or when lymphatic drainage is impeded. An exudate occurs when the permeability of the capillaries increases, such as with infections or localized cancers. Gross examination of pleural fluids can help to distinguish between transudates and exudates. Both conditions produce an increase in fluid volume. Transudates will appear similar to normal pleural fluid. It will appear clear and slightly yellow in color. Exudates will be cloudy and gray in appearance. Transudates will also have a normal cell count and distribution. Exudates will have an increased total cell count. For example, an increase in lymphocytes occurs in tuberculosis, and an increase in neutrophils occurs in bacterial infections.