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Effects of the Black Death
The Black Death, an epidemic of plague in Europe that lasted from 1347 to 1351, resulted in the deaths of almost one-quarter of Europe’s population. The Black Death was the first in a cycle of plagues in Europe that continued into the 18th century. Shown here, the French city of Marseille is devastated by a later outbreak of plague.
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In early medieval Europe, religious groups established hospitals and infirmaries in monasteries and later developed charitable institutions designed to care for the victims of vast epidemics of bubonic plague, leprosy, smallpox, and other diseases that swept Europe during the Middle Ages. The Benedictines were especially active in this work, collecting and studying ancient medical texts in their library at Monte Cassino near Salerno, Italy. St. Benedict of Nursia, the founder of the order, obligated its members to study the sciences, especially medicine. The abbot of Monte Cassino, Bertharius, was himself a famous physician.
During the 9th and 10th centuries Salerno became Europe’s center for medical care and education and was the site of the first Western school of medicine. By the 12th century other medical schools were established at the universities of Bologna and Padua in Italy, the University of Paris in France, and Oxford University in England.
In the 13th century, medical licensure by examination was endorsed and strict measures were instituted for the control of public hygiene. Representative scientists of this period include the German scholastic St. Albertus Magnus, who engaged in biological research, and the English philosopher Roger Bacon, who undertook research in optics and refraction and was the first scholar to suggest that medicine should rely on remedies provided by chemistry. Bacon, often regarded as an original thinker and pioneer in experimental science, was strongly influenced by the authority of Greek and Arabic medicine.
The period of the Renaissance, which began at the end of the 14th century and lasted for about 200 years, was one of the most revolutionary and stimulating in the history of mankind. Invention of printing and gunpowder, discovery of America, the new cosmology of Copernicus, the Reformation, the great voyages of discovery—all these new forces were working to free science and medicine from the shackles of medieval stagnation. The fall of Constantinople in 1453 scattered the Greek scholars, with their precious manuscripts, all over Europe.
The revival of learning in Western civilizations brought great advances in human anatomy. Some resulted from the work of artists, including Italian Leonardo da Vinci, who dissected human corpses to portray muscles and other structures more accurately. Andreas Vesalius, a Belgian anatomist, clearly demonstrated hundreds of anatomical errors introduced by Galen centuries earlier. Gabriel Falliopius discovered the uterine tubes named after him (see Fallopian Tube) and diagnosed ear diseases with an ear speculum. He described in detail the muscles of the eye, tear ducts, and fallopian tubes. Italian physician Girolamo Fracastoro recognized that infectious diseases are spread by invisible so-called seeds that can reproduce themselves. He founded modern epidemiology, the study of how diseases spread. The term syphilis, applied to the virulent disease then devastating Europe, was derived from his famous poem, “Syphilis sive Morbus Gallicus” (Syphilis or Disease of Gauls, 1530). Ambroise Paré introduced new surgical techniques and helped to found modern surgery.
The event that dominated 17th-century medicine and marked the beginning of a new epoch in medical science was the discovery of how the blood circulates in the body by the English physician and anatomist William Harvey. Harvey's “Essay on the Motion of the Heart and the Blood” (1628) established that the heart pumps the blood in continuous circulation. The Italian anatomist Marcello Malpighi advanced Harvey's work by his discovery of tiny blood vessels called capillaries, and the Italian anatomist Gasparo Aselli provided the first description of the lacteals, capillaries found in the lymphatic system. In England the physician Thomas Willis investigated the anatomy of the brain and the nervous system and was the first to describe diabetes mellitus. The English physician Francis Glisson advanced the knowledge of the anatomy of the liver, described the nutritional disorder rickets (sometimes called Glisson's disease), and was the first to prove that muscles contract when activity is performed. The English physician Richard Lower studied the anatomy of the heart, showed how blood interacts with air, and performed one of the first blood transfusions.
The French mathematician and philosopher René Descartes, who also made anatomical dissections and investigated the anatomy of the eye and the mechanism of vision, maintained that the body functioned as a machine. This view was adopted by the so-called iatrophysicists, such as Italian physician Sanctorius, who investigated metabolism, and the Italian mathematician and physicist Giovanni Alfonso Borelli, who worked in the area of physiology. Opponents of this view were the iatrochemists, who regarded life as a series of chemical processes, including Jan Baptista van Helmont, a Flemish physician and chemist, and Prussian anatomist Franciscus Sylvius, who studied the chemistry of digestion and emphasized the treatment of disease by drugs.
The English physician Thomas Sydenham, called the English Hippocrates, and later the Dutch physician Hermann Boerhaave, reestablished the significance of bedside instruction in their emphasis on the clinical approach to medicine. Sydenham carried out extensive studies on malaria and introduced the new treatment quinine, obtained from cinchona bark, into Europe in 1632. After the invention of the first compound microscope in 1590, Dutch scientist Antoni van Leeuwenhoek used this groundbreaking technology in 1676 to identify organisms later called bacteria. This was the first step toward recognition that microbes were the cause of infectious disease.
Vaccination with Cowpox
A cartoon satirizes British physician Edward Jenner, who is shown injecting patients with cowpox virus, resulting in their transformation into cows. Jenner lived from 1749 to 1823. His discovery that cowpox was an effective vaccine against smallpox was initially greeted with skepticism, but soon accepted. The procedure dramatically reduced the incidence of smallpox, once a major cause of death in the 18th century.
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The 18th century continued to be marked by unsupported theories. The German physician and chemist Georg Ernst Stahl believed that the soul is the vital principle and that it controls organic development; in contrast, the German physician Friedrich Hoffmann considered the body a machine and life a mechanical process. These opposing theories of the vitalists and the mechanists were influential in 18th-century medicine. The British physician William Cullen attributed disease to the excess or deficiency of nervous energy; and the physician John Brown of Edinburgh taught that disease was caused by weakness or inadequate stimulation of the organism. According to his theories, known as the Brunonian system, stimulation should be increased by treatment with irritants and large dosages of drugs. In opposition to this system, the German physician Samuel Hahnemann developed the system of homeopathy late in the 18th century, which emphasized small dosages of drugs to cure disease.
Other unusual medical practices developed toward the end of the 18th century include phrenology, a theory formulated by the German physician Franz Joseph Gall, who believed that examination of the skull of an individual would reveal information about mental functions. The theory of animal magnetism developed by the Austrian physician Franz Mesmer was based on the existence of a magnetic force having a powerful influence on the human body.
Despite these unorthodox medical practices, the end of the 18th century was marked by many true medical innovations. British physicians William Smellie and William Hunter made advances in obstetrics that established this field as a separate branch of medicine. The British social reformer John Howard furthered humane treatment for hospital patients and prison inmates throughout Europe. In 1796 British physician Edward Jenner introduced vaccination to prevent smallpox. His efforts both controlled this dreaded disease and also established the science of immunization.
Used to listen to sounds arising especially from the heart and lungs, a stethoscope has a two-part sound-detecting device at one end. The bell, bowl-shaped with a hole in the center, detects low-pitched sounds when the rim is pressed against the skin. The other side, called the diaphragm, has a thin, flat plastic cover. The diaphragm detects high-pitched sounds. A doctor hears these sounds through the earpieces of the stethoscope as they pass up the Y-shaped rubber tubing.
Herbert Wagner/Phototake NYC
Many discoveries made in the 19th century led to great advances in diagnosis and treatment of disease and in surgical methods. Medicine’s single most important diagnostic tool, the stethoscope, an instrument used to detect sounds in the body such as a heart beat, was invented in 1819 by French physician René-Théophile-Hyacinthe Laënnec. A number of brilliant British clinicians studied and described diseases that today bear their names. British physician Thomas Addison discovered the disorder of the adrenal glands now known as Addison's disease; Richard Bright diagnosed the kidney disorder, Bright's disease; British physician Thomas Hodgkin described a cancer of lymphatic tissue now known as Hodgkin's disease; British surgeon and paleontologist James Parkinson described the chronic nervous system disease called Parkinson disease; and the Irish physician Robert James Graves diagnosed the thyroid disorder exophthalmic goiter, sometimes called Graves' disease.
Medicine, like all other sciences, is subject to influences from other fields of study. This was particularly true during the 19th century, renowned for its great scientific innovations. For instance, the evolutionary theory proposed by Charles Darwin in On the Origin of Species by Means of Natural Selection (1859) revived interest in the science of comparative anatomy and physiology. And the plant-breeding experiments of the Austrian biologist Gregor Johann Mendel in 1866, although initially overlooked, eventually had a similar effect in stimulating studies in human genetics (see Heredity).
Since its accidental discovery in 1895, the X ray has been an important diagnostic and therapeutic tool. Produced by bombarding a target made of tungsten with high-speed electrons, X rays are absorbed by various tissues of the body in a predictable manner. They are absorbed by dense bone and pass through the soft tissue of internal organs. On a photographic plate, bone appears white and soft tissues appear gray. While diagnostic dental and medical X rays are low-intensity beams, high-intensity X rays, capable of destroying tissue, are used in the treatment of tumors. Rapidly dividing cancerous cells are especially vulnerable to X rays.
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German pathologist Rudolf Virchow pioneered development of pathology, the scientific study of disease. Virchow showed that all diseases result from disorders in cells, the basic units of body tissue. His doctrine that the cell is the seat of disease remains the cornerstone of modern medical science. In France, physiologist Claude Bernard performed important research on the pancreas, liver, and nervous system. His scientific studies, which emphasized that an experiment should be objective and prove or disprove a hypothesis, were the basis for the scientific method used today. Bernard's work on the interaction of the digestive system and the vasomotor system, which controls the size of blood vessels, was developed further by the Russian physiologist Ivan Petrovich Pavlov, who developed the theory of the conditioned reflex, the basis of human behaviorism.
A milestone in medical history occurred in the 1870s when French chemist Louis Pasteur and German physician Robert Koch separately established the germ theory of disease. Important in the development of this theory was the pioneering work of the American physician and author Oliver Wendell Holmes and of the Hungarian obstetrician Ignaz Philipp Semmelweis, who showed that the high rate of mortality in women after childbirth was attributable to infectious agents transmitted by unwashed hands (see Puerperal Fever).
Carbolic Steam Spray
The carbolic steam spray device invented by Joseph Lister in 1865 is credited with lowering the surgical mortality rate of the 1860s from 50 percent to approximately 12 percent. Drawing on the theories and discoveries of Louis Pasteur, Lister developed his own techniques of applying carbolic acid directly to wounds and surgical instruments, which led to the modern practice of antiseptic medicine.
Soon after the germ theory was recognized, the causes of such age-old scourges as anthrax, diphtheria, tuberculosis, leprosy, and plague were isolated. Pasteur developed a way to prevent rabies using a vaccine in 1885. In the last decade of the 19th century, German physician Emil von Behring and German bacteriologist Paul Ehrlich developed techniques for immunizing against diphtheria and tetanus.
New understanding of infectious diseases made surgery safer. Until the 1800s, surgeons operated in their street clothes, often without even washing their hands. Operating rooms, like other parts of hospitals, were filthy. About half of all surgery patients who survived the actual surgery typically died of infections that developed after the operation. The era of aseptic surgery, in which physicians used sterilized instruments and techniques to avoid infecting patients, was heralded by British surgeon and biologist Joseph Lister. With his introduction of an effective antiseptic, carbolic acid, Lister was able to successfully reduce mortality from wound infection (see Antiseptics). Rubber gloves were first worn during surgery in 1890, and gauze masks in 1896.
A color illustration depicts German scientist Robert Koch at work in his laboratory. Considered the founder of modern medical bacteriology, Koch isolated the bacillus that causes tuberculosis in 1882. He won the Nobel Prize for physiology or medicine in 1905.
THE BETTMANN ARCHIVE/Corbis
Another great advance in surgery came with the discovery of anesthesia. Until the 19th century, doctors used alcohol, opium, and other drugs to relieve pain during surgery. These medications could sometimes dull pain but could never completely mask it—patients often suffered from shock and died during surgery. In the United States, physician Crawford Long discovered the anesthetic effects of ether in 1842, and the dentist William Morton used ether in a tooth extraction in 1846. Ether and other anesthetics reduced surgical mortality and enabled surgeons to perform longer, more complicated operations.
A new tool for diagnosing internal diseases became available in 1895 when German scientist Wilhelm Roentgen discovered X rays. The Danish physician Niels Ryberg Finsen developed an ultraviolet-ray lamp, which led to an improved prognosis for some skin diseases (see Ultraviolet Radiation). In 1898 in France, Marie and Pierre Curie discovered radium, which was later used to treat cancer.
In 1898 British physician Ronald Ross proved the role of the mosquito as a carrier of the malarial parasite, a disease that has been widespread and sometimes fatal for most of human history. In 1900 United States Army physician Walter Reed and his colleagues, acting on a suggestion made by the Cuban biologist Carlos Juan Finlay, demonstrated that the mosquito is the carrier of yellow fever. This finding lead to better sanitation and mosquito control, resulting in the virtual elimination of this disease from Cuba and other areas.
Medicine's most revolutionary advances have occurred since 1900. By the end of the 20th century, medical advances helped to increase the average person's life expectancy by almost 30 years. As people lived longer, new medical challenges emerged. Heart disease, cancer, stroke, and other conditions often associated with aging replaced infectious diseases as the leading causes of death. Physicians began to devote greater attention to preventing disease and keeping patients healthy into advanced age. Biomedical research also shifted focus to the most basic causes of diseases, including defects in individual genes.
Discovery of Penicillin
British bacteriologist Alexander Fleming discovered penicillin in 1928. Penicillin, an important antibiotic derived from mold, is effective against a wide range of disease-causing bacteria. It acts by killing bacteria directly or inhibiting their growth.
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Infectious diseases that historically have killed millions of people each year were conquered early in the 20th century by improved sanitation, antibiotics, and vaccines.
German physician Paul Ehrlich showed around 1910 that a chemical compound, arsphenamine, could treat syphilis. He opened the era of chemotherapy, in which physicians use chemical compounds that act selectively to target specific diseases.
In the early 1930s, German and French scientists showed that sulfonamide was effective in treating streptococcal bacteria infections. This discovery led to the first family of so-called wonder drugs, the sulfonamide antibiotics. In 1938 British biochemists Howard Florey and Ernst Chain purified penicillin, the bacteria-destroying compound that Alexander Fleming observed in mold ten years earlier. Streptomycin, the first antibiotic for tuberculosis, was discovered in 1944 by American microbiologist Selman Waksman. Dozens of other antibiotics were subsequently discovered, each stronger and more effective against a broader range of bacteria.
Scientists learned more about how the body's immune system protects itself from infections, resulting in new tests for diagnosing infectious diseases and new vaccines to prevent them. The Wasserman blood test for syphilis was developed in 1906 and the tuberculin skin test for tuberculosis appeared in 1908. By the 1930s new techniques for growing viruses in the laboratory led to vaccines against viral diseases. These included a yellow fever vaccine in the late 1930s and the first effective influenza vaccine in the 1940s. The American physician Jonas E. Salk developed a polio vaccine in 1954. Later virologist Albert B. Sabin developed a safer oral polio vaccine, which was in wide use by the 1960s. Later came vaccines for other childhood diseases, including measles, German measles, mumps, and chicken pox.
Infectious diseases, once thought conquered by antibiotics, became a major concern again in the 1990s. New forms of tuberculosis and other diseases resistant to antibiotics spread. Concerns also arose over new or newly recognized microbes, such as human immunodeficiency virus (HIV), the cause of acquired immunodeficiency syndrome (AIDS), which became epidemic in 1981. As human populations grow and expand into wilderness areas, humans and animals come in closer contact. A number of diseases transmitted from animals have become problematic in recent years, including the hemorrhagic fevers caused by the Ebola and Marburg viruses, hantavirus pulmonary syndrome, and Lyme disease. In other areas, physicians recognized that an easily curable bacterial infection caused most peptic ulcers, a disease once blamed on stress and diet.
Polish-born American biochemist Casimir Funk introduced the term vitamine in 1912. Researchers later identified vitamins needed by the body to prevent deficiency diseases such as beriberi, rickets, scurvy, and pellagra. As better nutrition was developed and the quality of life improved, these diseases almost disappeared from industrialized countries (see Human Nutrition). But by the end of the 20th century, other nutritional disorders emerged. Studies conducted in the United States in the 1990s showed that more than 97 million Americans were overweight and risked health problems, such as heart disease and diabetes mellitus, commonly associated with obesity.
An endoscope provides a surgeon with an illuminated and magnified view of internal organs and body cavities without making sizable incisions. Endoscopes are easily maneuverable to reach inaccessible areas and they can be equipped with a variety of instruments, from knives to lasers.
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Modern Surgery Using an Operating Microscope
The use of operating microscopes for surgical procedures has greatly assisted surgeons with seemingly impossible types of surgery such as limb reattachment and eye and ear surgery. Operating microscopes are especially useful when individual nerve fibers and blood vessels must be realigned for attachment or repair.
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Operations that people once regarded as impossible became routine in the 20th century. Many of these surgical advances resulted from improved drugs or medical technology. Better drugs to prevent rejection of transplanted organs made transplantation of hearts, kidneys, livers, lungs, and other organs removed from donors possible. Patients were kept alive with artificial kidneys and temporary artificial hearts while awaiting a transplant (see Medical Transplantation). The heart-lung machine made it possible to stop and restart the heart during coronary bypass surgery. Small fiber-optic instruments called endoscopes led to the new field of minimally invasive surgery. These new tools made it possible to remove a diseased gallbladder or appendix, for example, through small slits rather than large incisions, greatly reducing the amount of anesthesia required during the surgery and lessening recovery time. Transfusions of blood, plasma, and other saline solutions, which went into use in the 1930s, helped prevent deaths from shock in surgery patients. In the 1990s, physicians even began performing surgery to repair defects in unborn infants.
Computerized Axial Tomography (CAT) Scan
A computerized axial tomography (CAT) scan produces a cross-sectional image of a part of the body by directing a narrow X-ray beam through the body. A CAT scan is a very accurate, painless, diagnostic tool allowing examination of the interior of the body without invasive procedures.
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New methods for viewing diseased structures inside the body improved diagnosis of disease beginning in the 1970s (see Radiology). A gamma camera detects radioactive medication that attaches to certain forms of cancer cells. Computed tomography (CT) scanners use X rays to produce lifelike three-dimensional images of body structures. Magnetic resonance imaging (MRI) scanners produce highly detailed images without X rays. Positron emission tomography (PET) detects very early warning signs of disease. Sonograms, or ultrasound, taken with high-frequency sound waves diagnose disease and monitor the progress of pregnancies. X rays and high-energy particles emitted by linear accelerators also are used to treat cancer. Lithotripsy uses high-frequency sound waves to destroy some kidney stones and gallstones, conditions that once required surgery.
Brain Activity in Bipolar Disorder
These positron emission tomography scans of the brain of a person with bipolar disorder show the individual shifting from depression, top row, to mania, middle row, and back to depression, bottom row, over the course of 10 days. Blue and green indicate low levels of brain activity, while red, orange, and yellow indicate high levels of brain activity.
Dr. Michael Phelps/Lewis Baxton/UCLA School of Medicine
Even in the early part of the 20th century, mental illness was almost a sentence of doom, and mentally ill persons were handled with cruel confinement and little medical aid. In the latter half of the century, successful therapy for some mental illnesses has greatly improved the prognosis for these diseases and has partly removed their stigma.
The theories advanced by Austrian physician Sigmund Freud were among the first attempts to understand malfunctioning of the mind, but the methods of psychoanalysis advocated by Freud and modified by his followers proved ineffective for treating certain serious mental illnesses. Two early attempts to treat psychotic illness were the destruction of parts of the brain in a procedure called lobotomy, introduced in 1935, and electroconvulsive therapy, devised in 1938. Lobotomy and less severe forms of psychosurgery are now used only rarely, and electroconvulsive therapy is primarily a treatment for depressive illness that has not responded to drug therapy.
A new era in treatment of schizophrenia, a severe form of mental illness, began in the early 1950s with the introduction of phenothiazine drugs. These drugs led to a new trend, deinstitutionalization, in which patients were released from mental hospitals and treated in the community. Valium (see Diazepam) and other benzodiazepine drugs went into wide use in the 1970s for treating anxiety and other emotional illness. Late in the century, there was growing awareness about the importance of diagnosing and treating clinical depression, a leading cause of suicide. Advanced imaging techniques that show the structural and functional differences in the brains of people with certain mental illnesses have opened the door for new treatment options.
Francis Crick and James Watson
The deoxyribonucleic acid (DNA) molecule is the genetic blueprint for each cell and ultimately the blueprint that determines every characteristic of a living organism. In 1953 American biochemist James Watson, left, and British biophysicist Francis Crick, right, described the structure of the DNA molecule as a double helix, somewhat like a spiral staircase with many individual steps. Their work was aided by X-ray diffraction pictures of the DNA molecule taken by British biophysicist Maurice Wilkins and British physical chemist Rosalind Franklin. In 1962 Crick, Watson, and Wilkins received the Nobel Prize for their pioneering work on the structure of the DNA molecule.
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The discovery of genes and their role in heredity and disease was one of the most important medical advances in history (see Genetics). In 1953 British biophysicist Francis Crick and American biochemist James Watson identified the double-helix structure of deoxyribonucleic acid (DNA). This discovery helped to explain how DNA carried genetic information. In the 1960s American biochemist Marshall Nirenberg added key details about how DNA determines the structure of proteins.
Indian-born American biochemist Har Gobind Khorana was the first to synthesize a gene in the laboratory in 1970, forging the way for scientists to develop ways to isolate, alter, and clone, or copy, genes. They applied these genetic engineering techniques to the diagnosis and treatment of diseases. Researchers identified genes associated with cancer, heart disease, mental illness, and obesity. With the genes identified, they worked on ways of modifying the genes to treat the disease. Gene therapy emerged as an experimental medical field that used genetically modified genes to treat diseases. In 2003 scientists completed the sequence of the human genome, in which they identified all the genes needed to make a human being (see Human Genome Project).
Correcting Genetic Diseases
Gene therapy may someday be able to cure hereditary diseases, such as hemophilia and cystic fibrosis, which are caused by missing or defective genes. In one type of gene therapy, genetically engineered viruses are used to insert new, functioning genes into the cells of people who are unable to produce certain hormones or proteins necessary for the body to function normally.
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Genetic engineering techniques enabled production of scarce human hormones and other materials for use as drugs. A new biotechnology industry started producing these materials for medical use. Scientists also began genetically modifying sheep and other animals to produce drugs in their milk.
In 1905, British scientist Ernest H. Starling introduced the word hormone to describe substances secreted by the endocrine glands that regulate body functions (see Endocrine System). The discovery of adrenaline, or epinephrine, in 1901 led to identification and isolation of other hormones. One of the most important advances was the discovery of insulin by Canadian scientists Frederick Banting and Charles H. Best and Scottish physiologist John J. Macleod in 1921. For years people with diabetes mellitus used insulin extracted from animal pancreases. In 1981, human insulin produced using biotechnology became available. American physicians made another major advance in endocrinology in 1949. They discovered that cortisone, an adrenal gland hormone, relieved inflammation. New discoveries about human sex hormones later led to the first birth control pills.
Ultrasound, or sound waves with frequencies above detection by the human ear, is commonly used in obstetrics to diagnose both the age and health of the developing fetus. An ultrasound-emitting device called a transducer is placed against the skin of the pregnant woman’s abdomen. The sound waves reflect in varying degrees when they contact tissues of different density and elasticity. The pattern of echoes is detected by the transducer and is converted into a moving image seen on a monitor. Ultrasound is also used in procedures involving the sampling of amniotic fluid or placental tissue. Outside of obstetrics, it is used to detect tumors, damage, or abnormalities in the liver, kidney, ovaries, eyes, and other organs. Because ultrasound waves pass readily through soft tissue but not through bone or gas, the technique cannot be used to scan parts of the body such as the brain, lungs, or intestines.
Alexander Tsiaras/Science Source/Photo Researchers, Inc.
Great advances were made in birth control with the improvement of intrauterine devices in the 1950s and the development of the birth control pill in 1960 by the American biologist Gregory Pincus. By the 1990s long-lasting hormonal implants and contraceptive injections such as Depo-Provera were developed. These options gave women more control in deciding whether to become pregnant. Voluntary sterilization, involving vasectomies in men and tubal sterilization in women, emerged as a popular way of permanent birth control. Unwanted pregnancies, however, remained a serious problem in the late 1990s. Researchers still sought more convenient and safer methods of birth control, including a male birth control pill.
By 1975 physicians were able to diagnose some congenital or inherited diseases before childbirth (see Birth Defects). Doctors take samples of placental cells (see Chorionic Villus Sampling) or of the amniotic fluid around the fetus (see Amniocentesis) to determine whether hereditary blood diseases, Down syndrome, defects of the spine, or other congenital diseases are present. Even the sex of a fetus may be known in advance.
In amniocentesis, a medical procedure generally performed during the fourth month of pregnancy, approximately one ounce of the amniotic fluid surrounding the fetus is drawn off for study. The examination of fetal cells contained in the sample can provide valuable information concerning developmental abnormalities of the fetus.
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In addition to advances in early diagnosis, progress occurred in identifying the causes of some birth defects. Excess alcohol consumption during pregnancy was linked to fetal alcohol syndrome, and inadequate intake of the vitamin folic acid was linked to spina bifida and other neural tube defects.
Advances in treating infertility, which prevents couples from having children, began with the world's first so-called test-tube baby born in the 1980s through in vitro fertilization. Other forms of assisted reproduction soon became available. Researchers in 1997 cloned a lamb from cells taken from an adult ewe. It led to speculation that human cloning could become another option in human reproduction.
False-Color Angiogram of a Healthy Human Heart
Coronary angiography, the X-ray examination of the heart after the injection of a radiopaque dye or contrast medium, is used to detect the presence and extent of coronary disease. The main coronary arteries appear as yellow ribbons across the heart.
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Heart disease emerged as one of the leading causes of death in Western countries by the end of the 20th century. Great advances occurred in diagnosis, treatment, and prevention of this widespread disease.
Diagnosis improved with the widespread use of cardiac catheterization in the 1950s. This procedure involves threading a slender tube into the heart to take measurements and identify blocked arteries. Less invasive diagnostic methods, such as thallium scans in which a special imaging camera detects the movement of thallium in heart muscle, provided additional diagnostic improvements.
These techniques led to a new era in surgical treatment of coronary heart disease, artery blockages that cause most heart attacks. Physicians began treating blocked coronary arteries with a variety of new techniques. The first bypass operation was performed in 1967 and involved the creation of a new route for blood supply to reach blood-starved heart muscles. In balloon angioplasty, developed in 1977, a deflated balloon is inserted into a narrowed artery. The balloon is then inflated at the site of the narrowing to widen it. Other surgical advances included replacement of diseased heart valves with artificial valves; implantation of pacemakers that maintain normal heart rhythm; use of temporary artificial hearts; and better methods for correcting hereditary defects in the heart.
New drugs were developed to treat angina pectoris, the chest pain of heart disease; high blood pressure; dangerous abnormalities in heart rhythm; and high blood cholesterol levels. Studies showed that drug treatment could reduce the risk of a heart attack or stroke. In the 1980s, aspirin went into wide use to prevent blood clots that cause many heart attacks. Emergency medical personnel also began using drugs that dissolve clots and stop a heart attack if given soon after symptoms develop.
Advances have been made in the prevention of heart disease. Studies have identified risk factors such as high blood pressure, high blood cholesterol, cigarette smoking, diabetes, obesity, and lack of exercise. Government health agencies and public health groups began public education programs to help people reduce heart disease risks. These preventive methods seem to be working—according to the American Heart Association, the death rate from coronary heart disease declined 26.3 percent between 1988 and 1998.
A patient undergoes radiation treatment for cancer of the spine. In this procedure the radioisotope cobalt 60 is used as the source of gamma radiation. A high dose of gamma radiation is guided by laser targeting to a localized area of treatment.
Martin Dohrn/Science Source/Photo Researchers, Inc.
Early detection and better treatment have resulted in major improvements in survival of patients with cancer. By 2000, 59 percent of people diagnosed with cancer were alive five years later, compared with only 25 percent in 1940. New drugs, surgical procedures, and ways of treating cancer with X rays and radioactive isotope radiation contributed to the improvement. In the 1990s, physicians used new knowledge about the human immune system to develop immunotherapy for some kinds of cancer, in which the immune system is stimulated to produce antibodies against specific invaders. Another form of immunotherapy is the use of monoclonal antibodies, genetically engineered antibodies that target specific cancer cells.
Screening tests for early detection of cancers of the cervix, prostate, breast, and colon and rectum (see Colorectal Cancer) became widely available. Researchers also made progress in identifying cancer genes that are associated with an increased risk of the disease and developed screening tests for some cancer genes. Advances in gene therapy also offered promise for new cancer treatments.
Health groups placed great emphasis in the second half of the century on cancer prevention through avoiding smoking and eating a diet rich in fresh fruits and vegetables. Despite these advances, the percentage of deaths from cancer increased from about 2 percent in 1900 to about 20 percent in 2000. Much of the rise, however, resulted from an increased proportion of older people, who are more vulnerable to cancer, and from cigarette smoking.
Advances in computer and Internet technologies created new possibilities for doctors and their patients in the early 1990s. Using computers to send live video, sound, and high-resolution images between two distant locations, doctors can easily examine patients in offices thousands of miles away. Rural patients no longer had to make long trips into urban centers to consult specialists.
In telemedicine, a computer fitted with special software and a video camera turns a live video image of a patient into a digital signal. This signal is transmitted over high-speed telephone lines to similar equipment at the doctor’s office, where it is converted back into a format that can be viewed live on a television screen. Telemedicine also includes machines specially designed to measure and record a patient’s vital signs at home, then transmit the information directly to a hospital nursing station. This electronic remote home care enables health care professionals to monitor a patient’s heart rate, temperature, blood pressure, pulse, blood-oxygen levels, and weight several times a day, without the patient ever having to leave home.
In addition to providing a vehicle for doctors and patients in remote locations to interact, telemedicine also enabled doctors in distant locations to share information. Patient charts, X rays, and other diagnostic materials can be transmitted between doctors’ offices. Moreover, doctors in rural areas of the world can observe state-of-the-art medical procedures that they would otherwise have had to travel thousands of miles to witness. Still in its infancy in the late 1990s, telemedicine may one day alleviate some of the regional inequalities inherent in modern medicine, not just between regions of North America, but also between developing countries and urban medical centers in the industrialized world.
Man on Life Support
Life support equipment keeps an elderly man alive at a hospital. Proponents of euthanasia believe that unnecessarily prolonging life in terminally ill patients causes suffering to the patients and their family members. Many societies now permit passive euthanasia, which allows physicians to withhold or withdraw life-sustaining treatment when directed to do so by the patient or an authorized representative.
Jan Halaska/Photo Researchers, Inc.
New medical, reproductive, and genetic technology in the second half of the 20th century led to increased concern about moral issues in medical treatment and research. By the 1990s, medical ethics, or bioethics, emerged as a recognized discipline that involved physicians, nurses, attorneys, theologians, philosophers, and sociologists.
Many bioethics issues involve the possible misuse of genetic engineering technology. The Human Genome Project led to identification of genes that raise an individual's risk of developing cancer, heart disease, mental illness, alcoholism, violent behavior, and other conditions. Tests to detect some of these disease-susceptibility genes became available in the 1990s.
These discoveries led to debate over whether genetic tests should be performed and how the results should be used. Should parents use such tests to screen their unborn infants? If a fetus tested positive, should it be aborted? If a woman tested positive for a breast cancer susceptibility gene, should the information be made available to insurance companies? Do insurers have a right to deny coverage to people with a genetic high risk for serious diseases? Do employers have a right to demand genetic screening tests before hiring people?
Genetic technology also offers the potential of eventually replacing defective genes with normal copies in human sperm and eggs. Some fear it will lead to mandatory eugenics programs, attempts to improve the hereditary traits of individuals or even entire races. Others argue that advances in genetic technology could eliminate defective genes and hereditary diseases from future generations.
An intense discussion about bioethics occurred in 1997 and 1998, after researchers in Scotland cloned the lamb, Dolly, from udder cells from an adult ewe. The experiment showed that it was possible to clone, or produce an exact genetic copy, of an adult mammal. Medical ethicists debate whether cloning of human beings should be permitted, as well as the potential effects on society.
Although abortion became legal in the United States in 1973, it still causes heated debate over the rights of the fetus and the pregnant woman, as well as the question of when a fetus becomes a human being. The availability of RU-486, also known as mifepristone, an inexpensive drug that induces abortion, led to concern that more people would use abortion for birth control. Ethical discussions centered on whether tissue from aborted fetuses should be used in medical research, treatment of disease, and organ transplants.
The right of terminally ill people to receive assistance in dying raised other ethical dilemmas. Physician-assisted suicide came to national attention largely through the efforts of Jack Kevorkian, a Michigan physician who helps people with terminal illnesses commit suicide. Opponents claim it is unethical for physicians to help patients commit suicide. Supporters counter that terminally ill patients have a right to determine the time and manner of their death. While the U.S. Supreme Court in 1997 ruled that states can ban physician-assisted suicide, that same year Oregon voters rejected an effort to repeal their law, the nation's first to legalize physician-assisted suicide.
Mammography is a special X-ray technique that is used to visualize soft tissues of the breast as a means for screening women for breast cancer. This mammogram shows calcification (dense white flecks) in a cancerous tumor. The nipple is to the left. The majority of breast cancers originate in the duct of the mammary, or milk-secreting, gland. The remainder arise in the glands themselves. Most tumors of either type show early evidence of invasive (malignant) behavior, but both may also exist in noninvasive forms.
Kings College Hospital/Science Source/Photo Researchers, Inc.
In the 1960s and 1970s, physicians and medical educators began to recognize a basic flaw in the health care system. Medicine traditionally was concerned with treating disease after symptoms appeared, resulting in treatment that was often very expensive. About 600,000 coronary bypass operations were performed annually in the United States in the 1990s, at a cost of $44,000 each. Medical officials recognized the advantage of preventing disease in the first place, rather than just treating it.
Medical schools began teaching students the importance of disease prevention. Some physicians specialized in a new field, preventive medicine, which emphasized keeping patients healthy. Practicing physicians spent more time counseling patients about smoking, excessive drinking, and other unhealthy practices. They did so by encouraging patients to avoid risk factors for disease; take periodic screening tests that detect disease early; and treat high blood pressure.
Yet by the late 1990s, many people still failed to use preventive services. Studies in 1997 estimated that 30,000 deaths per year could have been prevented if more people were immunized against influenza, pneumococcal pneumonia, and hepatitis B. Likewise, smoking, the leading preventable cause of death in the industrialized world, causes more than 4 million deaths worldwide each year.
Another dramatic change in medicine involved the idea that individuals have an important role in preventing diseases caused by an unhealthy lifestyle. Health care consumers grew more knowledgeable about medicine. Medical pages became a regular feature of major newspapers, news magazines, and television news programs. Some people subscribed to magazines and newsletters devoted entirely to health. Laypeople consulted books, such as the Physician's Desk Reference and The Merck Manual, once used only by professionals. They also tapped health information available on the Internet's World Wide Web (WWW). With this knowledge, consumers sought to become partners with their physicians in deciding the best ways of preventing, diagnosing, and treating disease.
Practitioners of chiropractic believe that many ailments and diseases can be healed by the manual adjustment of bone and tissue. Chiropractors offer non-surgical remedies for back and neck pain, headaches, and other conditions. Chiropractic is also considered a preventative health-care method.
Art Stein/Science Source/Photo Researchers, Inc.
A resurgence of interest developed in the 1990s in medical treatments not fully accepted by conventional medicine or biomedicine, which requires stringent scientific proof of safety and effectiveness before accepting a treatment. Such evidence is lacking for many approaches used in the medical systems and treatments known as alternative medicine in the United States. In Europe, these same approaches often are called complementary medicine. Growing public interest in nontraditional treatments led the NIH to open the National Center for Complementary and Alternative Medicine (formerly the Office of Alternative Medicine) in 1992, which encourages research on alternative medicine. The number of Americans using an alternative therapy rose from 33 percent in 1990 to more than 42 percent in 1997.
Alternative medicine emphasizes improving the quality of life for people with chronic illness; disease prevention; and treatments for conditions that conventional medicine cannot adequately control, such as arthritis, chronic pain, allergies, cancer, heart disease, and depression. A cornerstone of alternative medicine is the idea that the mind influences the health of the body.
Alternative medical systems include chiropractic, holistic medicine, and homeopathy. Chiropractors treat disease with spinal manipulation, massage, diet, and many other techniques. Holistic healers emphasize treatment of the whole person, including body, mind, emotions, spirit, and interactions with the family and environment. Homeopathic healers use substances that cause the very symptoms being treated. When treating a headache or nausea, for example, homeopathic healers administer herbs that in large doses cause headache or nausea. But they use very small doses that cause the patient no discomfort.
Specific alternative medical treatments include aromatherapy, inhaling oils from aromatic plants; massage techniques, including Rolfing and reflexology; biofeedback; iridology, in which the eye is used to diagnose certain diseases; and acupuncture. Some approaches, including chiropractic manipulation and acupuncture, have gained greater acceptance in conventional medicine. Some conventional biomedical studies have concluded that chiropractic manipulation is effective for low-back pain. A 1997 NIH report gave acupuncture limited endorsement for certain medical uses.
Organizations that educate the public about health fraud and quackery expressed concern about growing interest in some alternative medicine treatments. They emphasized the importance of receiving a conventional medical diagnosis, and exploring standard treatment options, before turning to alternative medicine.
The United States spends more on health care than any other country in the world. Spending in 1998 averaged $4,094 per person, compared to $2,689 in 1990, $1,052 in 1980, $341 in 1970, and $141 in 1960. The only countries that approached the United States in per capita spending were Switzerland ($2,412), Germany ($2,222), Luxembourg ($2,206), and Canada ($2,002). In the United States, spending on health care exceeded $1.1 trillion in 1998, up from $699.4 billion in 1990, $247.3 in 1980, $73.2 in 1970, and $26.9 billion in 1960.
Yet millions of Americans still do not have adequate access to health care because they lack insurance coverage. An estimated 44.2 million people had no health insurance in 1998. Access is a greater problem in the United States because most other industrialized countries have national health insurance systems that cover medical expenses. Since the 1960s, the United States Congress established and expanded programs to improve access to care. Medicare, the major program, covered about 38 million people over age 65 and people with disabilities in 1997. Another was Medicaid, a federal-state program that covers low-income people. During the 1990s, Congress considered and rejected proposals to establish a national health insurance system or extend government health care benefits to more people. The high costs of such a program were among the reasons for rejection.
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Diaphragm and Respiration
As the diaphragm contracts and moves downward, the pectoralis minor and intercostal muscles pull the rib cage outward. The chest cavity expands, and air rushes into the lungs through the trachea to fill the resulting vacuum. When the diaphragm relaxes to its normal, upwardly curving position, the lungs contract, and air is forced out.
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The flow of air in and out of the lungs is controlled by the nervous system, which ensures that humans breathe in a regular pattern and at a regular rate. Breathing is carried out day and night by an unconscious process. It begins with a cluster of nerve cells in the brain stem called the respiratory center. These cells send simultaneous signals to the diaphragm and rib muscles, the muscles involved in inhalation. The diaphragm is a large, dome-shaped muscle that lies just under the lungs. When the diaphragm is stimulated by a nervous impulse, it flattens. The downward movement of the diaphragm expands the volume of the cavity that contains the lungs, the thoracic cavity. When the rib muscles are stimulated, they also contract, pulling the rib cage up and out like the handle of a pail. This movement also expands the thoracic cavity. The increased volume of the thoracic cavity causes air to rush into the lungs. The nervous stimulation is brief, and when it ceases, the diaphragm and rib muscles relax and exhalation occurs. Under normal conditions, the respiratory center emits signals 12 to 20 times a minute, causing a person to take 12 to 20 breaths a minute. Newborns breathe at a faster rate, about 30 to 50 breaths a minute.
The rhythm set by the respiratory center can be altered by conscious control. The breathing pattern changes when a person sings or whistles, for example. A person also can alter the breathing pattern by holding the breath. The cerebral cortex, the part of the brain involved in thinking, can send signals to the diaphragm and rib muscles that temporarily override the signals from the respiratory center. The ability to hold one’s breath has survival value. If a person encounters noxious fumes, for example, it is possible to avoid inhaling the fumes.
A person cannot hold the breath indefinitely, however. If exhalation does not occur, carbon dioxide accumulates in the blood, which, in turn, causes the blood to become more acidic. Increased acidity interferes with the action of enzymes, the specialized proteins that participate in virtually all biochemical reaction in the body. To prevent the blood from becoming too acidic, the blood is monitored by special receptors called chemoreceptors, located in the brainstem and in the blood vessels of the neck. If acid builds up in the blood, the chemoreceptors send nervous signals to the respiratory center, which overrides the signals from the cerebral cortex and causes a person to exhale and then resume breathing. These exhalations expel the carbon dioxide and bring the blood acid level back to normal.
A person can exert some degree of control over the amount of air inhaled, with some limitations. To prevent the lungs from bursting from overinflation, specialized cells in the lungs called stretch receptors measure the volume of air in the lungs. When the volume reaches an unsafe threshold, the stretch receptors send signals to the respiratory center, which shuts down the muscles of inhalation and halts the intake of air.
Brown Smog Over Phoenix, Arizona
Smog is caused by industrial and automobile pollution. It is compounded by temperature inversions, which cause the air pollution to be kept in a particular area for extended periods. Continued exposure to smog can result in respiratory problems, eye irritation, and even death.
Eric Kamp/Phototake NYC
The respiratory system can be damaged by a variety of chemicals found in the environment, ranging from automobile fumes and industrial smoke to household cleaning products. Cigarette smoke, however, poses a particularly serious threat to the respiratory system because of the tar and other substances that enter the lungs. After a person smokes just one cigarette, for example, tar temporarily paralyzes the cilia of the upper and lower respiratory tracts. The tar also temporarily immobilizes the macrophages in the alveoli of the lungs. With the filtering and cleansing functions inactivated, the air passages and lungs are exposed not only to the irritating effects of tar but also to airborne bacteria, viruses, and other particles. These, along with the tar, settle in the mucous layers of the lungs. The paralyzed cilia recover after about one hour, but repeated smoking eventually kills them. Repeated smoking also causes mucus to build up in the lungs and block the smaller air passages. The blockage triggers a cough reflex—the familiar “smoker’s cough”—the lung’s effort to clear the airways. In addition, tobacco smoke contains over 40 chemicals known to cause cancer. Smoking is responsible for almost 90 percent of lung cancer cases among men, and more than 70 percent among women (see Smoking).
Cancerous Human Lung
This dissection of human lung tissue shows light-colored cancerous tissue in the center of the photograph. At bottom center lies the heart. While normal lung tissue is light pink in color, the tissue surrounding the cancer is black and airless, the result of a tarlike residue left by cigarette smoke. Most lung cancer begins in the cells lining the main air passages, or bronchi. In their cancerous state, these cells lack the cilia that normally catch and eliminate foreign particles inhaled into the lung. Mucous ordinarily cleared by bronchial cilia becomes trapped, blocking air passages. Lung cancer accounts for the largest percentage of cancer deaths in the United States, and cigarette smoking is directly responsible for the majority of these cases.
Martin Rotker/Phototake NYC
Workers in occupations that produce impurities released into the air are at high risk for respiratory illnesses. Sandblasters, stone cutters, quarry workers, miners, construction workers, people who install brake lining or insulation, people who work in shipyards or on farms, and people who pick cotton are among those at risk. In the United States, the Occupational Safety and Health Administration (OSHA) issues regulations that protect workers—requiring air masks with filters for certain jobs, for example. The Environmental Protection Agency (EPA) monitors and regulates the amount of pollutants released into the air. Despite these efforts, respiratory illnesses remain higher among workers who have significant exposure to air pollutants (see Environmental and Occupational Diseases).
Normal, everyday exposure to air pollution from cars and industrial emissions in the city also weakens the respiratory system of city-dwellers. Even if a person does not smoke, the city air gradually changes pink, healthy lung tissue to tissue darkened with particles of smog, dust, and other pollutants, making the lungs more vulnerable to infection. While outdoor pollutants pose threats to the respiratory system, a far greater threat is created by indoor air pollution. In homes and offices, a variety of substances, including cleaning compounds, air fresheners, synthetic carpets and furniture, and certain construction materials, can emit hazardous gases, which become highly concentrated in unventilated rooms. Individuals at greatest risk are those who spend most of their time indoors, children, the elderly, and people with a history of respiratory illnesses. Like outdoor air pollutants, indoor air pollutants weaken the lungs and invite infection. The long-term effects of air pollution are difficult to measure, but may include cancer and other serious diseases.
Respiratory Distress Syndrome
Acute respiratory distress syndrome is one of two human diseases caused by hantavirus. Dust containing virus-infected rodent feces becomes airborne and is inhaled. The virus embeds in the lungs where the infection begins. Flulike symptoms appear in about a week, followed by the collection of fluid and white blood cells in the lungs, causing respiratory failure, then death.
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The diseases and disorders of the respiratory system can affect any part of the respiratory tract and range from trivial to life-threatening. The nasal passages and pharynx, for example, are targets for the viruses that cause colds. These viruses infiltrate and destroy the cells of the nasal passage membranes. The immune system fights back by increasing blood flow to the area, bringing numerous virus-attacking white blood cells to the scene; this causes the membranes to swell, resulting in the stuffy nose associated with colds. Mucous secretions increase in response to the viral attack, creating the runny nose typical of colds. The infection can spread to the sinuses, the membrane-lined cavities in the head, as well as the lower respiratory tract and the middle ear.
The respiratory system is also subject to allergic reactions such as hay fever and asthma, brought about when the immune system is stimulated by pollen, dust, or other irritants. Hay fever is characterized by a runny nose, watery eyes, and sneezing. It usually occurs seasonally in response to abundant pollen in the air. In asthma, a person has difficulty breathing because the bronchi and bronchioles are temporarily constricted and inflamed. An asthma attack is typically mild, but can be severe enough to be life threatening.
Laryngitis, an inflammation of the larynx, is caused by a viral infection, irritants such as cigarette smoke, or by overuse of the voice. Laryngitis may cause hoarseness, or the voice may be reduced to a whisper until the inflammation subsides. Bronchitis is an inflammation of the membranes that line the bronchi or bronchioles. Bronchitis results from viral or bacterial infection or from irritating chemicals. Infections caused by bacteria or viruses can lead to pneumonia, a potentially serious condition of the lungs in which fluid and inflammation builds up in the alveoli, impeding the flow of oxygen and carbon dioxide between the capillaries and the alveoli.
Tuberculosis is caused by a bacterium that attacks the lungs and sometimes other body tissues as well. If infections in the lungs are left untreated, the disease destroys lung tissue. In the past, antibiotics have controlled tuberculosis, but recently, new antibiotic-resistant strains of the tuberculosis bacterium have evolved. These new strains now pose a significant public health problem.
In emphysema the alveolar tissue is partially destroyed and the remaining alveoli are weakened and enlarge. The bronchioles collapse on exhalation, trapping air in the alveoli. Over time this process impairs the ability to exchange oxygen and carbon dioxide with the circulatory system, leading to breathing difficulties. A noncontagious disease, emphysema results from multiple factors, including a genetic predisposition to the condition, smog, cigarette smoke, and infection.
Lung cancer develops in individuals with a genetic predisposition to the disease who are exposed to cancer-causing agents, such as tobacco smoke, asbestos, and uranium. Cancerous tumors may start in the bronchi, bronchioles, or in the alveolar lung tissue. If lung cancer is detected before the cancer has spread to other parts of the body, treatments are more effective, and the prognosis for full recovery is good. Unfortunately, 85 percent of lung cancer cases are diagnosed after the cancer has spread, and for these cases, the prognosis is very poor.
Respiratory Distress Syndrome (RDS) is the name for a cluster of symptoms that indicate severe malfunctioning of the lungs. In infants, RDS is termed Infant Respiratory Distress Syndrome (IRDS). Commonly found in premature infants, IRDS results when the alveoli fail to fully expand during inhalation. Expansion of the alveoli requires a chemical called surfactant, but in many premature infants, the alveoli are not developed enough to produce this vital substance. IRDS is treated by administering air and surfactant through a breathing tube until the alveoli begin producing surfactant on their own. Adult Respiratory Distress Syndrome (ARDS) results when lungs are severely injured, for example, in an automobile accident, by poisonous gases, or as a response to inflammation in the lungs. ARDS is a life-threatening condition with a survival rate of about 50 percent.
How Fish Breathe
A fish breathes by absorbing oxygen from the water it drinks. Water flows into the mouth, through the gills, and out of the body through gill slits. As water flows through the gills, the oxygen it contains passes into blood circulating through gill structures called filaments and lamellae. At the same time, carbon dioxide in the fish’s bloodstream passes into the water and is carried out of the body.
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The need to take in oxygen and expel carbon dioxide is almost universal among organisms. The movement of these gases between an organism and its environment, called gas exchange, is accomplished in a variety of ways by different organisms. In one-celled aquatic organisms, such as protozoans, and in seaweeds, sponges, jellyfish, and other aquatic organisms that are only a few cell layers thick, oxygen and carbon dioxide diffuse directly between the water and cells. Diffusion works for these simple organisms because all cells of the organism are within a few millimeters of an oxygen source.
Animals with many cell layers cannot rely on diffusion because cells several layers deep in the body would die before oxygen reached them. As a result, for gas exchange, more-complex animals require special respiratory organs, such as gills or lungs, in combination with circulatory structures, such as blood, blood vessels, and a heart. The earliest development of these gas exchange structures is seen in roundworms, microscopic invertebrates abundant in water and moist soil. In roundworms, oxygen diffuses through the skin into a fluid that fills an internal cavity. As the worm moves, the fluid sloshes around in the cavity, bringing oxygen into contact with the digestive system, reproductive organs, and other structures in the cavity. This primitive circulatory system is called an open circulatory system because the fluid is not contained within vessels. In clams an open circulatory system is combined with a heart that pumps fluid around the internal cavity. Clams also use gills, thin-walled filaments that are extensions of the body surface. Gills provide a more extensive surface area for gas exchange than the body surface alone, enabling clams and larger organisms to obtain the amount of oxygen they need. Fish have gills, a heart, and a closed circulatory system, one in which blood is transported in vessels by the pumping action of the heart.
Lungless salamanders, such as the one shown here, breathe through the mucous membrane in the mouth and throat and through the skin. Moisture is especially important to lungless salamders, because their skin must be wet in order to absorb oxygen. These animals shelter in damp places such as caves and the bottom of logs, only venturing out when the air is sufficiently humid.
Allen Blake Sheldon/Animals Animals
Relatively simple land-dwelling organisms, including some plants, fungi, and animals such as flatworms, accomplish gas exchange by diffusion. More-complex organisms, however, rely on specialized respiratory structures. Instead of gills, whose delicate filaments collapse unless supported by water, land animals use lungs. Located inside the body, lungs are formed by the infolding of membranes. The folds form a single balloon-like sac, as in amphibians; they may be arranged in stacks, as in the book lungs of spiders; or they are composed of millions of tiny air sacs, such as the lungs of most mammals. In virtually all vertebrates, a heart and a closed circulatory system work with the lungs to deliver oxygen and to remove carbon dioxide from cells.
Insects have a unique respiratory system made up of small tubes called tracheae. The tracheae connect all parts of the body to small openings on the surface of the insect. Oxygen and carbon dioxide are transported through the tracheae, and from the tracheae to the blood of the insect by diffusion. The blood of most insects is contained in an open circulatory system and is moved around the internal organs by a heart.
The respiratory system of birds, adapted for flight, is very different from that of land-bound animals. The lungs have two openings, one for taking in oxygen-filled air; the other for expelling carbon dioxide-laden air. Rather than ending up in alveoli, the air loops through the lungs so that the oxygen flow through the lungs is continuous. This design enables birds to obtain the amount of oxygen they need to power the extremely high energy demands of flight.
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