Figure 13-1 Bony chest skeleton.
The lungs continuously provide oxygen to and remove carbon dioxide from the circulatory system. The power required for breathing comes from the intercostal muscles and the diaphragm. These muscles act as a bellows to suck air into the lungs. Expiration is passive. The control of breathing is complex and is controlled by the breathing center in the medulla of the brain.
Inspired air is warmed, filtered, and humidified by the upper respiratory passages. After passing through the cricoid cartilage of the larynx, air travels through a system of flexible tubes, the trachea. At the level of the fourth or fifth thoracic vertebra, the trachea bifurcates into the left and right bronchi. The right bronchus is shorter, wider, and straighter than the left bronchus. The bronchi continue to subdivide into smaller bronchi and then into bronchioles within the lungs. Each respiratory bronchiole terminates in an alveolar duct, from which many alveolar sacs branch off. It is estimated that there are more than 500 million alveoli in the lungs. Each alveolar wall contains elastin fibers that allow the sac to expand with inspiration and to contract with expiration by elastic recoil. This system of air-conducting passages is illustrated in Figure 13-2.
The lungs are subdivided into lobes: the upper, middle, and lower on the right, and the upper and lower on the left. The lungs are enveloped in a thin sac, the pleura. The visceral pleura overlies the lung parenchyma, whereas the parietal pleura lines the chest wall. The two pleural surfaces glide over each other during inspiration and expiration. The space between the pleura is the pleural cavity.
To describe physical signs in the chest accurately, the examiner must understand the topographic landmarks of the chest wall. The landmarks of clinical importance are as follows:
Suprasternal notch Sternomanubrial angle Midsternal line Midclavicular lines Anterior axillary lines Midaxillary lines
Figure 13-2 System of air-conducting passages.
Posterior axillary lines Scapular lines • Midspinal line
Figure 13-3 illustrates the anterior and lateral views of the thorax, and Figure 13-4 illustrates the posterior thorax.
The suprasternal notch is located at the top of the sternum and can be felt as a depression at the base of the neck. The sternomanubrial angle is often referred to as the angle of Louis. This bony ridge lies approximately 5 cm below the suprasternal notch. When you move your fingers off the ridge laterally, the adjacent rib that you feel is the second rib. The interspace below the second rib is the second intercostal space. Using this as a reference point, you should be able to identify the ribs and interspaces anteriorly. Try it on yourself.
To identify areas, several imaginary lines can be visualized on the anterior and posterior chest in Figures 13-3 and 13-4. The midsternal line is drawn through the middle of the sternum. The midclavicular lines are drawn through the middle points of the clavicles and parallel to the midsternal line. The anterior axillary lines are vertical lines drawn along the anterior axillary folds parallel to the midsternal line. The midaxillary lines are drawn from each vertex of the axilla parallel to the midsternal line. The posterior axillary lines are parallel to the midsternal line and extend vertically along the posterior axillary folds. The scapular lines are parallel to the midspinal line and pass through the inferior angles of the scapulae. The midspinal line is a vertical line that passes through the posterior spinous processes of the vertebrae.
Rib counting from the posterior chest is slightly more complicated. The inferior wing of the scapula lies at the level of the seventh rib or interspace. Another useful landmark can be found by having the patient flex the neck; the most prominent cervical spinous process, the vertebra prominens, protrudes from the seventh cervical vertebra.
Only the first 7 ribs articulate with the sternum. The 8th, 9th, and 10th ribs articulate with the cartilage above. The 11th and 12th ribs are floating ribs and have a free anterior portion.
Anterior axillary line
Posterior axillary line
Anterior axillary line
Posterior axillary line
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