The primary function of the respiratory system is supplying oxygen to the blood and expelling waste gases, of which carbon dioxide is the main constituent, from the body. This is achieved through breathing: we inhale oxygen and exhale carbon dioxide. Respiration is achieved via inhalation through the mouth or nose as a result of the relaxation and contraction of the diaphragm. The air, in essence oxygen, then passes through the larynx and trachea to enter the chest cavity. The larynx, or voice box, is located at the head of the trachea, or windpipe. In the chest cavity, the trachea branches off into two smaller tubes called the bronchi, which enter the hilus of the left and right lungs. The bronchi are then further subdivided into bronchioles. These, in turn, branch off to the alveolar ducts, which lead to grape-like clusters called alveoli found in the alveolar sacs. The anatomy of the respiratory system is shown in Fig. 1.1. The walls of alveoli are extremely thin (less than 2 ¡m) but there are about 300 millions of alveoli (each with a diameter about 0.25 mm). If one flattens the alveoli (in an adult), the resulted surface can cover about 140 m2.
The lungs are the two sponge-like organs which expand with diaphragmatic contraction to admit air and house the alveoli where oxygen and carbon dioxide diffusion regenerates blood cells. The lungs are divided into right and left halves, which have three and two lobes, respectively. Each half is anchored by the mediastinum and rests on the diaphragm below. The medial surface of each half features an aperture, called a hilus, through which the bronchus, nerves, and blood vessels pass.
When inhaling, air enters through the nasal cavity to the pharynx and then through the larynx enters the trachea, and through trachea enters the bronchial tree and its branches to reach alveoli. It is in alveoli that the exchange between the oxygen in the air and blood takes place through the alveolar capillaries. Deoxygenated blood is pumped to the lungs from the heart through the pulmonary artery. This artery branches into both lungs, subdividing into arterioles and metarterioles deep within the lung tissue. These metarterioles lead to networks of smaller vessels, called capillaries, which pass through the alveolar surface. The blood diffuses
FIGURE 1.1: Anatomy of the respiratory system (top view); the zoomed in picture of a bronchiolus branch and alveolar ducts (bottom view)
waste carbon dioxide through the membranous walls of the alveoli and takes up oxygen from the air within. The reoxygenized blood is then sent through metavenules and venules, which are tributaries to the pulmonary vein. This vein takes the reoxygenized blood back to the heart to be pumped throughout the body for the nourishment of its cells.
Ventilation is an active process in the sense that it consumes energy because it requires contraction of muscles. The main muscles involved in respiration are the diaphragm and the external intercostal muscles. The diaphragm is a dome-shaped muscle with a convex upper surface. When it contracts it flattens and enlarges the thoracic cavity. During inspiration the external intercostal muscles elevate the ribs and sternum and hence increase the space of the thoracic cavity by expanding in the horizontal axis. Simultaneously, the diaphragm moves downward and expands the thoracic cavity space in the vertical axis. The increased space of the thoracic cavity lowers the pressure inside the lungs (and alveoli) with respect to atmospheric pressure. Therefore, the air moves into lungs. During expiration, the external intercostal muscles and diaphragm relax the thoracic cavity which is restored to its preinspiratory volume. Hence, the pressure in the lungs (and alveoli) is increased (becomes slightly positive with respect to atmospheric pressure) and the air is exhaled. At low flow rate respiration, i.e., 0.5 L s-1 when lying on ones back, almost all movement is diaphragmatic and the chest wall is still. At higher flow rates, the muscles of the chest wall are also involved and the ribs move too. Different people breathe differently in terms of using the diaphragm to expand the lungs or the chest wall muscles. For instance, breathing in children and pregnant women is largely diaphragmatic. Without going through the pulmonary physiology in detail, it is necessary to introduce a few pulmonary parameters that will be referred to when we discuss the lung sound analysis.
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