Intercostal Muscles

(fig. 1-41). The lungs are cone-shaped organs that lie in the thoracic cavity. Each lung contains thousands of alveoli with their capillaries. The right lung is larger than the left lung and is divided into superior, middle, and inferior lobes. The left lung has two lobes, the superior and the inferior. Pleurae The pleurae are airtight membranes that cover the outer surface of the lungs and line the chest wall. They secrete a serous fluid that prevents friction during movements of respiration. Mediastinum The mediastinum is the tissue and organs of the thoracic cavity that form a septum between the lungs. It extends from the sternum to the thoracic vertebrae and from the fascia of the neck to the diaphragm. The mediastinum contains the heart, the great blood vessels, the esophagus, a portion of the trachea, and the primary bronchi. Diaphragm The diaphragm is the primary muscle of respiration. It is a dome-shaped muscle and separates the thoracic and abdominal cavities. Contraction of this muscle flattens the dome and expands the vertical diameter of the chest cavity. Intercostal Muscles The intercostal muscles are situated between the ribs. Their contraction pulls the ribs upward and outward, resulting in an increase in the transverse diameter of the chest (chest expansion). Inhalation is the direct result of the expansion caused by the action of the diaphragm and intercostal muscles. The increase in chest volume creates a negative (lower than atmospheric) pressure in the pleural cavity and lungs. Air rushes into the lungs through the mouth and nose to equalize the pressure. Exhalation results when the muscles of respiration relax. Pressure is exerted inwardly as muscles and 1-35 HM3F0140 ALVEOLI ALVEOLAR DUCT PULMONARY ARTERY BLOOD FLOW BRONCHIOLE PULMONARY VEIN PULMONARY ARTERIOLE CAPILLARY NETWORK ON SURFACE OF ALVEOLUS BLOOD FLOW BLOOD FLOW PULMONARY VENULE Figure 1-40.—Bronchiole and alveoli. HM3F0141 ALVEOLAR WALL ALVEOLUS PULMONARY CAPILLARY O2 CO2 Figure 1-41.—Pulmonary exchange at alveolus.