No. 10 1. Trachea 2. Bronchi 3. Lungs 4. Pleura 5. mediastinum Section 4 The Trachea The trachea is a tube approximately 2.0 cm in diameter and 11 cm long. Location: It is located in the midline of neck and upper thorax, and in front of the esophagus. It extends from the larynx at the lower border of the cricoid cartilage at the level of sixth cervical vertebra to the level of the sternal angle (corresponding) to the lower border of the fourth thoracic vertebra, where it divides into the right and left principal bronchi. Composition: The wall of trachea is mainly composed of about 15～30 C-shaped rings of hyaline cartilage keep the lumen of the trachea open. But the posterior wall of the trachea is closed by the tracheal muscle and lies against the anterior surface of the esophagus permitting the esophagus to expand during swallowing. Carina of trachea: It is a sagittal semilunar ridge inside the bifurcation of trachea, which can be seen through bronchoscope and used as a guide to the bronchi. Section 5 The Bronchi The trachea terminates at the level of fourth thoracic vertebra and divides into the right and left principal bronchi. The right principal bronchus is about 2～3 cm long and is shorter, wider and more vertical in position than the left. The left principal bronchus is longer, about 4～5 cm long, finer and less vertical. So the foreign objects from the trachea usually pass to the right bronchus. Bronchial tree: As entering the lungs, the principal bronchus branches to form lobar bronchi which enter the lobes of lung. The lobar bronchi continue to branch, forming still smaller bronchi called the segmental bronchi, which are subdivided into the bronchioles. ramify into even smaller tubes called the terminal bronchioles. These branches resemble a tree and so are called the bronchial tree. Section 6 The Lungs The lungs are the essential organs of respiration. Healthy lungs always contain some air so they may float in water. In the newborn which has not breath, the lungs will not float in water. At birth the lungs are pinkish-white but they turn dark trey with age and become almost black due to carbonaceous deposits. Location: The lungs are situated one on each side within the thorax, and separated from each other by the heart and other contents of the mediastinum. Ⅰ. The External Features of Lungs Each lung is shaped somewhat like a cone, with an apex, a base, two surfaces and three borders. The right lung is shorter than the left one because the right dome of the diaphragm is higher, and it is wider because the heart and pericardium bulge more to the left. 1. One apex and one base 1) The apex of lung is rounded and extends to about 2～3 above the level of the medial one-third of the clavicle. 2) The base of lung is concave and related to the diaphragm which separates the right lung from the liver and the left lung from the stomach, spleen and liver, so the base of lung is also called the diaphragmatic surface. 2. Two surfaces The lung possesses a costal surface and a medial surface. 1) The costal surface is smooth, convex and related to the inner surface of the ribs, costal cartilages and intercostal spaces. 2) The medial surface is related to the mediastinum, so it is also called the mediastinal surface. The hilum of lung and root of lung: Near the center of the medial surface, there is a depression called the hilum of lung (hilus). The hilus is the region where the structures that form the root of the lung-that is, the bronchus, blood vessels, lymphatics and nerves-enter or leave the lung. The structures entering and emerging the hilum is called the root of lung, which is short broad pedicle and consists of the bronchi, pulmonary artery and veins, nerves, bronchial vessels, lymphatics and lymph nodes. 3. Three borders The borders of lung includes anterior, posterior and inferiors. The anterior border is thin and sharp, having a deep notch at the forth and fifth intercostals spaces in the left lung, called the cardiac notch of left lung, beneath which is the lingula of left lung. The posterior border is round. The inferior border is also sharp and separates the base of lung from the costal and medial surfaces. Ⅱ. The Lobes and Segments of Lungs Fissures and lobes: Each lung is divided into superior and inferior lobes by an oblique fissure. The right lung is further divided by a horizontal fissure, which bounds a middle lobe. The right lung therefore has three lobes, whereas the left has only two. Bronchopulmonary segments: Each lung is subdivided by connective tissue partitions into smaller units called bronchopulmonary segments. Each bronchopulmonary segment represents the portion of the lung that is supplied by a specific tertiary bronchus. Each lung has ten segments. The bronchopulmonary segments are important surgically because a diseased segment can be removed without having to remove an entire lobe or the entire lung. Also, disease does not spread so easily across the partitions that separate the segments, so pathology tends to be confined to one or several segments rather than spreading freely throughout the lungs. Section 7 The Pleura Each lung is enclosed in a double-walled the pleura. Both layers of the pleura are formed of serous membrane that lines the inner surface of the thorax and the surface of the thorax and the surfaces of lungs. The portion of the pleura that adheres firmly to the lungs is the visceral pleura. The portion that lines the walls of the thoracic cavity is the parietal pleura. Ⅰ. The Parietal Pleura The serous membrane lining the inner surface of chest wall is called the parietal pleura. According to the regions the parietal pleura is divided into four portions: ① the costal pleura, ② he diaphragmatic pleura, ③ the mediastinal pleura , ④ the cupola of pleura. Ⅱ. The Visceral Pleura The pleura is reflected from the mediastinum to the surface of lung, where it is called visceral pleura covering the lungs and extending into the fissures of lung. Below the root of lung the mediastinal pleura extends as a double layer to the mediastinal surface of lung. This double layer is called the pulmonary ligament. Ⅲ. The Pleural Cavity and Recesses Ⅰ) The Pleural Cavity The visceral and parietal layers are continuous at the hilus of the lung. between the two layers of the pleura is an extremely narrow pleural cavity, which is filled with pleural fluid. The pleural fluid is secreted by the pleura, and it acts as a lubricant to reduce the friction between the two layers during respiratory movements. The two pleural cavities are separated from each other by the mediastinum. Ⅱ) The Pleural Recesses Costodiaphragmatic recess: In quiet breathing the inferior border of the lung does not completely extend the inferior margin of the pleural reflexion, so that the costal and diaphragmatic pleurae are in contact with each other here, the intervening narrow slit termed the costodiaphragmatic recess. In quiet respiration the lower limit of lung is about 5 cm above the lower limit of the pleura. Costomediastinal recess: A similar condition present behind the sternum, between the costal and mediastinal pleurae it is termed the costomediastinal recess. Ⅳ. The Projection of the Inferior Margins of Lungs and Pleurae On the surface of the body, the projection of the inferior margins of the lungs and pleurae are shown in table 1: The inferior margins of pleurae is the costodiaphragmatic lines of reflexion of the pleurae Table 1 The inferior margins of lungs and pleurae Midclavicular Midaxillary Posterior median line line line Inferior 6th fib 8th rib On the level of T10 margin of spinal process lungs Inferior 8th rib 10th rib On the level of T12 margin of spinal process pleurae Section 8 The Mediastinum The mediastinum is generally defined as the interval between the right and left pleural sacs. Location: It extends from the sternum in front to vertebral column behind, and from the thoracic inlet above to the diaphragm below. Its lateral wall is the mediastinal pleura of both sides. Division: The mediastium is divided into superior and inferior mediastina by the line drawn horizontally from the sternal angle to the lower border of 4th thoracic vertebra. The inferior mediastinum is subdivided into an anterior mediastinum in front of pericardium, a middle mediastinum containing the pericardium with heart and great vessels, and the posterior mediastinum between pericardium and vertebral column. The posterior mediastinum extends to the lower border of the 8th thoracic vertebra, its main contents are the bronchi, esophagus, vagus and phrenic nerves, thoracic duct etc.
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