RESPIRATION

the process by which animals, including human beings, take in the oxygen required for survival and release carbon dioxide that accumulates in their bodies as a result of the expenditure of energy. Single-celled organisms (e.g., amoebas) have no organs for respiration; they simply exchange carbon dioxide and oxygen across their cell membranes. Higher animals, however, have special respiratory mechanisms. Insects have a network of tubes (tracheae) that open to the outside and bring oxygen into the body. Fish and several aquatic invertebrates have gills, which permit the exchange of gases with the surrounding water; a pumping mechanism then carries the oxygenated blood through the animal's body. Some fish and amphibians use their thin, moist, vascular skin for respiratory exchange. Land-dwelling vertebrates have lungs that vary in complexity. The lungs are inflated by various types of pumping mechanisms, which cause oxygen to enter in response to a change in air pressure. Oxygen is transported throughout the body by bonding with hemoglobin pigments in red blood cells. Hemoglobin also acts to carry carbon dioxide from body tissues back to the lungs where it is exhaled. The human respiratory system consists of the nasal cavity, throat (pharynx), voice box (larynx), windpipe (trachea), bronchi, and lungs. The ribs and muscles of the chest wall, as well as the diaphragm, help in the expansion and contraction of the lungs, although they are not considered part of the respiratory system. The pharynx and larynx are sections of the airway that leads to the lungs. The larynx is joined to the trachea, which in turn branches into the left and the right main bronchi. The bronchi continue to subdivide, finally terminating in the tiny alveolar ducts, which are filled with capillaries, or small blood vessels. Gases from the air in the alveoli are exchanged with gases contained in the pulmonary capillary blood by means of diffusion. Hemoglobin transports oxygen to the rest of the body and returns carbon dioxide to the lungs to be expired. Freshly oxygenated blood leaves the lungs through the pulmonary veins and is carried to the left atrium of the heart, where it is then pumped throughout the body. The movements of breathing are regulated by several nerve centres in the brain. Cells in the brainstem are believed to control the rhythm and depth of breathing, while the vagus, or 10th cranial, nerve controls the blood vessels and bronchi of the lungs. Because the body stores practically no oxygen, any interference with ventilation that lasts more than a few minutes can cause death. Asphyxiation, drowning, and loss of chest muscle power brought on by drugs or disease can all cause acute respiratory failure and death. Gas transport in the lungs depends on the relationship of the pressure of oxygen within the alveoli and the atmospheric pressure. At high altitudes, the atmospheric pressure is greatly reduced, and the delicate balance between oxygen and carbon dioxide in the body is often upset. The body gradually adapts to these changes, but the unacclimatized person who does physical work at a high altitude will experience dizziness, nausea and vomiting, and headaches. The lungs are susceptible to a wide variety of diseases because they are constantly in contact with the external environment and the impurities it contains. General symptoms of lung diseases include coughing; production of sputum, which may be tinged with blood; chest pain; and shortness of breath. Colds and influenza are caused by viral infections. Bacterial diseases that attack the lungs include bacterial pneumonia and pulmonary tuberculosis. Both diseases were very serious until the discovery of antibiotic therapy brought them under control. The lungs are particularly vulnerable to allergic diseases. When the smooth muscle of the bronchial tree comes into contact with foreign particles (e.g., pollen), it releases histamine, which stimulates the muscle to contract. Also, any general allergic reaction causing inflammation of blood vessels is likely to be felt most severely in the highly vascular lungs. Asthma is an allergic disease characterized by contraction of the smooth muscle of the airway and an inability to clear mucus, resulting in difficult breathing. Acute bronchitis (congestion of the bronchi) can be brought on by a virus or exposure to harmful gases. In its chronic form it results in obliteration and obstruction of the bronchi. Emphysema also causes destruction of alveoli. Both diseases are associated with cigarette smoking and air pollution. Lung cancer is also linked to cigarette smoking. the process by which animal organisms take up oxygen and discharge carbon dioxide in order to satisfy their energy requirements. In the living organism, energy is liberated, along with carbon dioxide, through the oxidation of molecules containing carbon. The term respiration also denotes the exchange of the respiratory gases (oxygen and carbon dioxide) between the organism and the medium in which it lives and between the cells of the body and the tissue fluid that bathes them. With the exception of energy used by animal life in the deep ocean, all energy used by animals is ultimately derived from the energy of sunlight. The carbon dioxide in the atmosphere in conjunction with the energy of sunlight is used by plants to synthesize sugars and other components. Animals consume plants or other organic material to obtain chemical compounds, which are then oxidized to sustain vital processes. This article considers the gaseous components of air and water, the natural respiratory habitats of animals, and the basic types of respiratory structures that facilitate gas exchange in these environments. Although the acquisition of oxygen and the elimination of carbon dioxide are essential requirements for all animals, the rate and amount of gaseous exchange vary according to the kind of animal and its state of activity. In the Table the oxygen consumption of various animals is expressed in terms of millilitres of oxygen per kilogram of body weight per hour, reflecting the gas demands of different species at rest and in motion. A change in the chemical composition of the body fluids elicits a response from the central nervous system, which then excites or depresses the machinery of external respiration. Additional reading General features of the respiratory process August Krogh, The Comparative Physiology of Respiratory Mechanisms (1941, reissued 1968), is classic in its field. Julius H. Comroe, Jr., Physiology of Respiration: An Introductory Text, 2nd ed. (1974) covers the basic aspects of respiration in mammals. More recent texts include John Widdicombe and Andrew Davies, Respiratory Physiology (1983), a good introduction; Peter Sebel et al., Respiration: The Breath of Life (1985), an overview of respiration, the respiratory system, and its diseases; N. Balfour Slonim and Lyle H. Hamilton, Respiratory Physiology, 5th ed. (1987); and Allan H. Mines, Respiratory Physiology, 2nd ed. (1986). F. Harold McCutcheon, Organ Systems in Adaptation: The Respiratory System, in D.B. Dill (ed.), Handbook of Physiology, sect. 4, Adaptation to the Environment (1964), pp. 167191, discusses respiration in relation to the environment, including chemical regulation, gas transport, and evolutionary patterns. Stephen C. Wood (ed.), Evolution of Respiratory Processes: A Comparative Approach (1979), compares respiratory processes in modern animals to gain insights into evolutionary changes. David J. Randall et al., The Evolution of Air Breathing in Vertebrates (1981), begins with the aquatic ancestral form. Respiration in animals Introductions to the field are provided by G.M. Hughes, Comparative Physiology of Vertebrate Respiration, 2nd ed. (1974); Rufus M.G. Wells, Invertebrate Respiration (1980), a short but useful study; F. Reed Hainsworth, Animal Physiology: Adaptations in Function (1981), which includes chapters on respiration, circulation, temperature, and energetics and their interplay; William S. Hoar, General and Comparative Physiology, 3rd ed. (1983), in which phylogeny in animal functions is used as a framework for depicting animal physiology; Martin E. Feder and Warren W. Burggren, Skin Breathing in Vertebrates, Scientific American, 253(5):126142 (Nov. 1985); Knut Schmidt-Nielsen, Animal Physiology: Adaptation and Environment, 3rd ed. (1983), which explains systematically how animals cope with their environments; and a supplement to it, C. Richard Taylor, Kjell Johansen, and Liana Bolis (eds.), A Companion to Animal Physiology (1982), which probes certain topics, including respiratory physiology. See also V.B. Wigglesworth, The Principles of Insect Physiology, 7th ed. (1972, reprinted 1982), an excellent introduction to the form and function of insect respiration. C. Ladd Prosser, Oxygen: Respiration and Metabolism, ch. 5 in C. Ladd Prosser (ed.), Comparative Animal Physiology, 3rd ed. (1973), pp. 165211, is a comprehensive chapter on oxygen and its role. Charlotte P. Mangum, Oxygen Transport in Invertebrates, The American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 248(5):R505R514 (May 1985), provides a succinct overview of oxygen-carrying proteins.The principles of gas exchange in animals and humans are discussed in Malcolm S. Gordon, Animal Physiology: Principles and Adaptation, 4th ed. (1982), a consideration of the mechanisms of gas exchange among animals; Gas Exchange and Circulation, in R. McNeill Alexander (ed.), The Encyclopedia of Animal Biology (1987), pp. 5065; and Handbook of Physiology, sect. 3, The Respiratory System, vol. 4, Gas Exchange, ed. by Leon E. Farhi and S. Marsh Tenney (1987), a critical, comprehensive presentation of physiological knowledge and concepts.The interplay between respiration, circulation, and metabolism is outlined by Ewald R. Weibel, The Pathway for Oxygen: Structure and Function in the Mammalian Respiratory System (1984); R. Gilles (ed.), Circulation, Respiration, and Metabolism: Current Comparative Approaches (1985), essays on oxygen transport and utilization in animals; and C.R. Taylor et al., Adaptive Variation in the Mammalian Respiratory System in Relation to Energetic Demand, Respiration Physiology, 69(1):1127 (July 1987), an entire issue devoted to the subject. Alfred P. Fishman Fred N. White Alfred P. Fishman Robert A. Klocke Ewald R. Weibel