Anatomy & Physiology

Anatomy & Physiology Study Guide pulmonary ventilation. Alveolar ventilation acts as a preventative measure to keep carbon dioxide from building up in the alveoli and ensures a continuous supply of oxygen that keeps pace with absorption into the bloodstream. The Respiratory Cycle A respiratory cycle is a single cycle of inhalation (inspiration) and exhalation (expiration). At the start of the respiratory cycle, there is no movement of air, and the intrapulmonary and atmospheric pressures are equal. Inhalation begins with the fall of intrapleural pressure that accompanies the expansion of the thoracic cavity. Over the period, intrapulmonary pressure drops and then begins to rise as air flows into the lungs. At the start of exhalation, intrapulmonary and intrapleural pressures rise rapidly. Air is forced out of the lungs. At the end of expiration, air movement again ceases when the difference between intrapulmonary pressure and atmospheric pressures has been eliminated. The amount of air moved into the lungs during inhalation is equal to the amount transferred out of the lungs during respiration. That amount is the tidal volume. The Mechanics of Breathing The contraction of skeletal muscles - specifically, those that insert on the rib cage—and the diaphragm, which separates the thoracic and abdominopelvic cavities, changes the volume in the lungs. Because of the nature of their articulations with the vertebrae, when elevated, the ribs swing outward and increase the depth of the thoracic cavity. The Respiratory Muscles The most important muscles used in breathing are the diaphragm and the external intercostal muscles. These muscles are active during normal breathing at rest. When the depth and frequency of respiration must be increased markedly, the accessory respiratory muscles become active. These muscles include the internal intercostal, sternocleidomastoid, serratus anterior, pectoralis minor, scalene, transversus thoracis, transversus abdominis, external and internal oblique, and rectus abdominis muscles. Inhalation is an active process involving one or more of the following muscles: • Contraction of the diaphragm flattens the floor of the thoracic cavity. Air is drawn inward by increasing the thoracic cavity volume. Diaphragmatic contraction is responsible for roughly 75 percent of the air movement in normal breathing at rest. • Contraction of the external intercostal muscles assists in inhalation by elevating the ribs. This action contributes roughly 25 percent to the volume of air in the lungs at rest. • Contraction of accessory muscles, including the sternocleidomastoid, serratus anterior, pectoralis minor, and scalene muscles, can assist the external intercostal muscles in elevating the ribs. These muscles increase the amount and speed of rib movement. Depending on the level of respiratory activity, exhalation is either passive (involves relaxation of muscles) or active (using muscles to force air out). Active respiratory activity may require one or more of the following muscles:

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