Oxygen must be available to most of the cells of the body for them to produce energy and function normally. Carbon dioxide, a by-product of this aerobic metabolism, must be removed from these cells. Hydrogen ions are also produced by several metabolic pathways, and the respiratory system assists in their removal. The respiratory system therefore takes oxygen from the atmosphere, supplies it to the cells, and removes from the body most of the carbon dioxide and hydrogen ions produced by cellular metabolism. Other functions of the respiratory system include acid-base balance, phonation, pulmonary defense and metabolism, and the handling of bioactive materials.
Gas exchange occurs in the lungs, which are ventilated by the action of the respiratory muscles under the control of the respiratory center in the brain. At the same time, the right ventricle pumps venous blood through the lungs via the pulmonary circulation. In the pulmonary capillaries, carbon dioxide is exchanged for oxygen.
STRUCTURE OF THE RESPIRATORY SYSTEM
The respiratory system consists of the lungs, conducting airways, the parts of the central nervous system concerned with the control of the muscles of respiration, and the chest wall. The chest wall is formed of the rib cage and some of the muscles of respiration, including the diaphragm and intercostal muscles. The rib cage consists of the sternum, vertebrae, ribs, and associated cartilages. The diaphragm is the primary muscle of inspiration and is usually said to be responsible for about two-thirds of the air that enters the lungs during normal quiet breathing. It is innervated by the two phrenic nerves, which leave the spinal cord at the third through the fifth cervical segments. When the diaphragm contracts, its dome descends into the abdominal cavity, elongating the thorax. Because it is inserted into the lower rib margins, the lower ribs are also elevated during deep inspirations. The external intercostal muscles (and parasternal intercartilaginous muscles) raise and rotate the ribs, increasing the diameter of the chest and the transverse dimension of the lower portion of the chest, playing a major role during inspiration. These muscles are innervated by the intercostal nerves, leaving the spinal cord at the first through the eleventh thoracic segments. Accessory muscles of inspiration, sternocleidomastoid, scalenes, and pectoralis major contract during forced or dyspneic inspiration. These neck muscles are responsible for the retractions seen at the suprasternal notch during labored inspirations.
During normal quiet breathing, exhalation is passive and does not require muscle contraction. The muscles of active or forced expiration are the internal intercostal muscles and the muscles of the abdominal wall, including the rectus abdominus, external and internal oblique muscles, and transversus abdominus. The abdominal muscles are innervated by the lower six thoracic and first lumbar spinal nerves. Contraction of the internal intercostal muscles depresses the rib cage downward in a manner opposite to the actions of the external intercostals. Abdominal surgery impairs the ...