Exercise 7 Respiratory System Mechanics

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Respiratory System Mechanics

O B J E C T I V E S 1. To explain how the respiratory and circulatory systems work together to enable gas exchange among the lungs, blood, and body tissues 2. To define respiration, ventilation, alveoli, diaphragm, inspiration, expiration, and partial pressure 3. To explain the differences between tidal volume, inspiratory reserve volume, expiratory reserve volume, vital capacity, residual volume, total lung capacity, forced vital capacity, forced expiratory volume, and minute respiratory volume 4. To list various factors that affect respiration 5. To explain how surfactant works in the lungs to promote respiration 6. To explain what happens in pneumothorax 7. To explain how hyperventilation, rebreathing, and breathholding affect respiratory volumes


he physiological functions of respiration and circulation are essential to life. If problems develop in other physiological systems, we can still survive for some time without addressing them. But if a persistent problem develops within the respiratory or circulatory systems, death can ensue within minutes. The primary role of the respiratory system is to distribute oxygen to, and remove carbon dioxide from, the cells of the body. The respiratory system works hand in hand with the circulatory system to achieve this. The term respiration includes breathing—the movement of air in and out of the lungs, also known as ventilation—as well as the transport (via blood) of oxygen and carbon dioxide between the lungs and body tissues. The heart pumps deoxygenated blood to pulmonary capillaries, where gas exchange occurs between blood and alveoli (air sacs in the lungs), oxygenating the blood. The heart then pumps the oxygenated blood to body tissues, where oxygen is used for cell metabolism. At the same time, carbon dioxide (a waste product of metabolism) from body tissues diffuses into the blood. The deoxygenated blood then returns to the heart, completing the circuit. Ventilation is the result of muscle contraction. The diaphragm—a domeshaped muscle that divides the thoracic and abdominal cavities—contracts, making the thoracic cavity larger. This reduces the pressure within the thoracic cavity, allowing atmospheric gas to enter the lungs (a process called inspiration). When the diaphragm relaxes, the pressure within the thoracic cavity increases, forcing air out of the lungs (a process called expiration). Inspiration is considered an “active” process because muscle contraction requires the use of ATP, whereas expiration is usually considered a “passive” process. When a person is running, however, the external intercostal muscles contract and make the thoracic cavity even larger than with diaphragm contraction alone, and expiration is the result of the internal intercostal muscles contracting. In this case, both inspiration and expiration are considered “active” processes, since muscle contraction is needed for both. Intercostal muscle contraction works in conjunction with diaphragm muscle contraction. 87


Exercise 7


Atmospheric pressure Parietal pleura Thoracic wall Visceral pleura Pleural cavity Transpulmonary pressure 760 mm Hg 756 mm Hg 4 mm Hg



Intrapleural pressure 756 mm Hg ( 4 mm Hg)

Lung Diaphragm Intrapulmonary pressure 760 mm Hg (0 mm Hg) (b)

F I G U R E 7 . 1 Respiratory volumes. (a) Opening screen of the Respiratory Volumes experiment. (b) Intrapulmonary and intrapleural relationships

Respiratory System Mechanics


Respiratory Volumes
Ventilation is measured as the frequency of breathing multiplied by the volume of each breath, called the tidal volume. Ventilation is needed to maintain oxygen in arterial blood and carbon dioxide in venous blood at their normal levels—that is, at their normal partial pressures. [The term partial pressure refers to the proportion of pressure that a single gas exerts within a mixture. For example, in the atmosphere at sea level, the...
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