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respiratory system
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Respiratory system
The system of organs involved in the acquisition of oxygen and the elimination of carbon dioxide by an organism. The lungs and gills are the two most important structures of vertebrates involved in the phase known as external respiration, or gaseous exchanges, between the blood and environment. Internal respiration refers to the gaseous exchanges which occur between the blood and cells. Certain other structures in some species of vertebrates serve as respiratory organs; among these are the integument or skin of fishes and amphibians. The moist, highly vascular skin of anuran amphibians is important in respiration. Certain species of fish have a vascular rectum which is utilized as a respiratory structure, water being taken in and ejected regularly by the animal. Saclike cloacal structures occur in some aquatic species of turtles. These are vascular and are intermittently filled with, and emptied of, water. It is thought that they may function in respiration. During embryonic life the yolk sac and allantois are important respiratory organs in certain vertebrates. See Yolk sac
Structurally, respiratory organs usually present a vascular surface that is sufficiently extensive to provide an adequate area of absorption for gaseous exchange. This surface is moist and thin enough to allow for the passage of gases.
The shape and volume of the lung, because of its pliability, conforms almost completely to that of its cavity. The lungs are conical; each has an apex and a base, two surfaces, two borders, and a hilum. The apex extends into the superior limit of the thoracic cavity. The base is the diaphragmatic surface. The costal surface may show bulgings into the intercostal spaces. The medial surface has a part lying in the space beside the vertebral column and a part imprinted by the form of structures bulging outward beneath the mediastinal pleura. The cardiac impression is deeper on the left lung because of the position of the heart.
For convenience the lung may be divided into anatomical areas. The bronchial tree branches mainly by dichotomy. The ultimate generations, that is, the respiratory bronchioles, alveolar ducts, and alveoli constitute all of the respiratory portion of the lung. The trachea and extrapulmonary bronchi are kept open by C-shaped bars of hyaline cartilage. When in their branching the bronchi and bronchioles are reduced to a diameter of 1 mm or less, they are then free of cartilage and are called terminal bronchioles. One of the terminal bronchioles enters the apex of a secondary lobule of the lung. These secondary lobules are anatomic units of the lung, whose hexagonal bases rest on the pleura or next to a bronchiole or blood vessel. Finer lines divide the bases of the secondary lobules into smaller areas. These are the bases of primary lobules, each served by a respiratory bronchiole. See Lung, Respiration
The blood supply to the lung is provided by the pulmonary and the bronchial arteries. The nerves which supply the lung are branches of the vagus and of the thoracic sympathetic ganglia 2, 3, and 4. Efferent vagal fibers are bronchoconstrictor and secretory, whereas the afferents are part of the arc for the breathing reflex. Efferent sympathetic fibers are bronchodilators; hence, the use of adrenalin for relief of bronchial spasm resulting from asthma. See Nervous system (vertebrate)
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