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Unformatted text preview: Chapter 36 EXTERNAL RESPIRATI ON This chapter examines the reasons for and advantages of breathing, introduces some aspects of the mechanics of external respiration and gas exchange between the atmosphere and blood, and considers the key regulators of respiratory activity. The reader should attempt to integrate this material on respiratory activity into the functioning of the other vital life support processes. A. Why breathe? Large multicellular animals cannot depend on simple diffusion mechanisms to exchange gases between tissues en masse and the external environment; nor does their internal environment tolerate being subjected passively to variations in what prevails outside. The mutual participation of the circulatory and respiratory systems circumvents this potential hazard. Simultaneously, respiratory mechanisms provide for a number of more specialized functions: • Respiration provides a continuous supply of oxygen for general metabolic needs, as in oxidative phosphorylation. • In the processes of energy production, respiration recycles lactate as part of the Cori cycle. • Respiration removes excess carbon dioxide produced by metabolism and aids in the maintenance of acid-base balance. In ruminants, other gases produced within the rumen may be absorbed into the blood and eliminated by respiratory activity. • Respiration contributes to thermoregulation (see Chapter 61) by providing a means of dissipating body heat through two mechanisms: conductive, or direct heat flow, to warm incoming atmospheric air to body temperature; and evaporative, by vaporizing moisture in the airways to saturate the incoming air. • Respiration provides a mechanism for vocalization, and provides an air stream for minimizing thermal trauma of the buccal cavity from ingestion of very hot food and drink. • Through the generation of positive or negative pressure within the thorax, respiration aids in venous return to the heart (Chapter 34), in the function of the digestive tract (Chapter 43 and Box 46-2), and in the process of parturition at the end of pregnancy (Chapter 70). 2007 version – page 304 Figure 36-1. The lung and pulmonary alveoli. The upper diagram shows the right lung of a sheep viewed mediastinally, or from the left. The bronchial tree is shown in dotted outline as it is not visible in the intact lung. The lobes are apical (a); cardiac (c); diaphragmatic (d); and mediastinal (m). The fine terminal structures of the airways are shown in the exploded views. Note that along with the pneumocytes, or alveolar epithelial cells, capillaries are integral components of the alveolar wall. For the present, this introduction is mainly concerned with the passage of gases from inspired air across the moist alveolar wall of the lung and into the blood, and vice versa. Diffusion mechanisms, introduced in Chapter 12, Section A, are applicable, with concentration differences for each gas providing the driving force. CO 2 is much more diffusible than is O 2 . ....
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- Fall '08
- phrenic nerve