Anatomy & Physiology

Anatomy & Physiology Study Guide • The width and the depth of the thoracic cavity are reduced and the ribs depressed because of the internal intercostal and transversus thoracic muscles. • The abdominal muscles, including the external and internal oblique, transversus abdominis, and rectus abdominis muscles, can assist the internal intercostal muscles in exhalation by compressing the abdomen and forcing the diaphragm upward. Modes of Breathing The respiratory muscles are used in various combinations, depending on the volume of air that must be moved into or out of the system. Respiratory movements are usually classified as quiet breathing or forced breathing according to the pattern of muscle activity during a single respiratory cycle. In quiet breathing , or eupnea , inhalation involves muscular contractions, but exhalation is a passive process. Inhalation typically requires the contraction of both the diaphragm and the external intercostal muscles. The relative contributions of these muscles can vary: • During diaphragmatic breathing or deep breathing, contraction of the diaphragm provides the necessary change in thoracic volume. The diaphragm contracts, air is drawn into the lungs and then passively exhaled as the diaphragm relaxes. • In costal breathing or shallow breathing, the thoracic volume changes because the rib cage alters its shape. Inhalation occurs as the external intercostal muscles contract; the ribs elevate and enlarge the thoracic cavity. Exhalation occurs when these muscles relax. Diaphragmatic breathing typically occurs at minimal levels of activity. As increased volumes of air are required, inspiratory movements become more substantial, and the contribution of rib movement rises. Even at rest, costal breathing can predominate when abdominal pressures, fluids, or masses restrict diaphragmatic movements. For example, pregnant women increasingly rely on costal breathing as the enlarging uterus pushes the abdominal viscera against the diaphragm. Forced breathing , or hyperpnea , involves active inspiratory and expiratory movements. Forced breathing calls on the accessorymuscles to assist with inhalation, and exhalation involves contraction of the internal intercostal muscles. Maximum levels of forced breathing involve the abdominal muscles in exhalation. Their contraction compresses the abdominal contents, pushing them up against the diaphragm and further reducing the volume of the thoracic cavity. Respiratory Rates and Volumes By modifying the number of breath per minute and the amount of air moved her breath the respiratory system can adapt itself to meet the demands of the body. When exercising at peak levels, the amount of air circulating in and out of the respiratory tract can be 50 times the amount moved at rest. Respiratory rate is defined as the number of breaths taken each minute. The average respiratory rate of a resting adult ranges from 12 to 18 breaths each minute, roughly one for every four heartbeats. Children breathe more rapidly, approximately 18–20 breaths per minute. To calculate the amount of air moved each minute, multiply the respiratory rate by the tidal volume VT. This value is called the Achieve Page 304 of 368 ©2018