During normal quite breathing, maximum work is done in which of the following?
Correct Answer: To overcome chest and lung elastic recoil
Description: Ans. a. To overcome chest and lung elastic recoil. (Ref. Guyton & Hall's Textbook of Medical Physiology 1 l,h/pg. 471; Fig. 37-1).During normal quiet breathing, all respiratory muscle contraction occurs during inspiration; expiration is almost entirely a passive process caused by elastic recoil of the lungs and chest cage. Thus, under resting conditions, the respiratory muscles normally perform "work" to cause inspiration but not to cause expiration.The work of inspiration can be divided into three fractions:(1) that required to expand the lungs against the lung and chest elastic forces, called compliance work or elastic work;(2) that required to overcome the viscosity of the lung and chest wall structures, called tissue resistance work; and(3) that required to overcome airway resistance to movement of air into the lungs, called airway resistance work.Mechanics of Pulmonary VentilationMuscles That Cause Lung Expansion and ContractionThe lungs can be expanded and contracted in two ways: (1) by downward and upward movement of the diaphragm to lengthen or shorten the chest cavity, and (2) by elevation and depression of the ribs to increase and decrease the anteroposterior diameter of the chest cavity.Normal quiet breathing is accomplished almost entirely by the first method, that is, by movement of the diaphragm. During inspiration, contraction of the diaphragm pulls the lower surfaces of the lungs downward. Then, during expiration, the diaphragm simply relaxes, and the elastic recoil of the lungs, chest wall, and abdominal structures compresses the lungs and expels the air.During heavy breathing, All the muscles that elevate the chest cage are classified as muscles of inspiration, and those muscles that depress the chest cage are classified as muscles of expiration. The most important muscles that raise the rib cage are the external intercostals, but others that help are the (1) sternocleidomastoid muscles, which lift upward on the sternum; (2) anterior serrati, which lift many of the ribs; and (3) scaleni, which lift the first two ribs. The muscles that pull the rib cage downward during expiration are mainly the (1) abdominal recti, which have the powerful effect of pulling downward on the lower ribs at the same time that they and other abdominal muscles also compress the abdominal contents upward against the diaphragm, and (2) internal intercostals.
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