Respiration
Respiration can be defined simply as breathing, or more specifically, as the physical and chemical processes by which oxygen is assimilated into a living organism and carbon dioxide and water are given off. The mechanics of breathing and its function have interested scientists since ancient times. The Greeks generally thought respiration was intended to cool the heart, although the followers of Hippocrates also thought that air provided some sort of nourishment to the body.
William Harvey, in his revolutionary 1628 description of the circulation of the blood, established that blood flowed through the lungs and then back into the heart for dispersion throughout the body. Harvey's discovery prompted new investigations into the nature of respiration. The British physiologist Richard Lower (1631-1691) showed clearly in 1669 that exposure to air in the lungs caused dark venous blood to take on a bright, scarlet color. From this, Lower concluded that the blood absorbed a substance vital to life from the air. This substance was then transported by the bloodstream and absorbed throughout the body. Lower's ideas, convincingly confirmed by experiment, gained rapid acceptance.
The British researchers Robert Boyle and John Mayow (1640-1679) investigated the properties of air and its vital role in sustaining life. Mayow, devising ingenious experiments, showed in 1674 that "nitro-aerial" particles are removed from the air in both combustion and respiration and are essential for life to continue. These vital particles, Mayow concluded, entered the bloodstream and were carried to the tissues; the particles gave arterial blood its bright redness as compared with venous blood. Two centuries later, Antoine-Laurent Lavoisier identified Mayow's nitro-aerial particles as oxygen. He also confirmed oxygen's role in respiration and showed that respiration, like combustion, liberates heat, carbon dioxide, and water.
Confirming a 1789 suggestion by Lazzaro Spallanzani, two French physicians, Julien Legallois in 1812 and Marie Flournes in 1837, located the center of neurological control of respiration in the medulla oblongata (brainstem). A German physician, Heinrich Magnus, reported the first measurement of blood gases in 1837. He found that arterial blood contains more oxygen and venous blood more carbon dioxide. Another German physician, Otto Funke, discovered hemoglobin in 1851. He and fellow German Ernst Hoppe-Seyler (1825-1895) then showed that hemoglobin is the blood's oxygen-carrying substance. Also in the 1800s, E. F. W. Pfluger demonstrated that the chemical changes that occur in respiration take place in the cells, not in the lungs. An understanding of respiration at high altitude was developed by Paul Bert (1833-1886), and the pH/oxygen status of blood was established early in the twentieth century by Lawrence Henderson (1878-1942) and Hasselbalch.
A form of periodic breathing called Cheyne-Stokes respiration was described by two Irish physicians, John Cheyne (1777-1836) in 1818 and William Stokes (1804-1878) in 1854. A patient with Cheyne-Stokes, a condition caused by central nervous system depression, stops breathing altogether for perhaps fifteen seconds, then begins again, very shallowly at first and then increasingly quickly and deeply, and then gradually ceases again, constantly repeating the cycle.
Contributions to respiratory knowledge were made by several outstanding investigators in the twentieth century. A Scottish physiologist practicing in England, John Scott Haldane, demonstrated in 1905 that breathing is controlled by the concentration of carbon dioxide in arterial blood. To assist his respiratory research, Haldane devised an apparatus for analyzing blood that became standard equipment in respiratory labs until its recent replacement by electronic devices. He also worked out a decompression technique for deep-sea divers to avoid the "bends," caused by gases in the blood.
The Danish physiologist Schack Krogh (1874-1949) and his wife Marie, a physician, demonstrated in 1910 that the absorption of oxygen by the blood was always accomplished by simple diffusion, never by secretion of oxygen by the alveoli. Schack Krogh also studied capillary physiology in depth, finding that muscle capillaries were open during work and partially closed at rest. After receiving the Nobel Prize in 1920 for his capillary work, Krogh went on to show that capillary opening and closing was controlled by both muscle action and hormones. He also devised a number of new methods and measuring devices in his investigations of respiratory gases and their movements, including a microtonometer to measure blood gas pressures, a rocker spirometer, and a bicycle ergometer.
A British biochemist, David Keilin, discovered the respiratory enzyme cytochrome and reported in 1925 that it was this enzyme that acted as a catalyst for oxygen-hydrogen combination in the cells. Otto Warburg, a German biochemist, identified iron within the cytochrome as the agent that activated the oxygen transfer.
Diseases of the repiratory system are a serious public health problem. According to the American Lung Association, close to 335,000 Americans die of lung disease each year. Lung disease is the number three killer in the United States, leading to one in seven death. As of 1998, more than 23 million Americans were living with chronic lung diseases such as asthma, emphysema, and chronic bronchitis.
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