respiration

1. 1. The act or process of inhaling and exhaling; breathing. Also called ventilation.
   2. The act or process by which an organism without lungs, such as a fish or plant, exchanges gases with its environment.
2. 1. The oxidative process occurring within living cells by which the chemical energy of organic molecules is released in a series of metabolic steps involving the consumption of oxygen and the liberation of carbon dioxide and water.
   2. Any of various analogous metabolic processes by which certain organisms, such as fungi and anaerobic bacteria, obtain energy from organic molecules.

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Concept

Respiration is much more than just breathing; in fact, the term refers to two separate processes, only one of which is the intake and outflow of breath. At least cellular respiration, the process by which organisms convert food into chemical energy, requires oxygen; on the other hand, some forms of respiration are anaerobic, meaning that they require no oxygen. Such is the case, for instance, with some bacteria, such as those that convert ethyl alcohol to vinegar. Likewise, an anaerobic process can take place in human muscle tissue, producing lactic acid—something so painful that it feels as though vinegar itself were being poured on an open sore.

How It Works

Forms of Respiration

Respiration can be defined as the process by which an organism takes in oxygen and releases carbon dioxide, one in which the circulating medium of the organism (e.g., the blood) comes into contact with air or dissolved gases. Either way, this means more or less the same thing as breathing. In some cases, this meaning of the term is extended to the transfer of oxygen from the lungs to the bloodstream and, eventually, into cells or the release of carbon dioxide from cells into the bloodstream and thence to the lungs, from whence it is expelled to the environment. Sometimes a distinction is made between external respiration, or an exchange of gases with the external environment, and internal respiration, an exchange of gases between the body's cells and the blood, in which the blood itself "bathes" the cells with oxygen and receives carbon dioxide to transfer to the environment.

This is just one meaning—albeit a more familiar one—of the word respiration. Respiration also can mean cellular respiration, a series of chemical reactions within cells whereby food is "burned" in the presence of oxygen and converted into carbon dioxide and water. This type of respiration is the reverse of photosynthesis, the process by which plants convert dioxide and water, with the aid of solar energy, into complex organic compounds known as carbohydrates. (For more about carbohydrates and photosynthesis, see Carbohydrates.)

How Gases Move Through the Body

Later in this essay, we discuss some of the ways in which various life-forms breathe, but suffice it to say for the moment—hardly a surprising revelation!—that the human lungs and respiratory system are among the more complex mechanisms for breathing in the animal world. In humans and other animals with relatively complex breathing mechanisms (i.e., lungs or gills), oxygen passes through the breathing apparatus, is absorbed by the bloodstream, and then is converted into an unstable chemical compound (i.e., one that is broken down easily) and carried to cells. When the compound reaches a cell, it is broken down and releases its oxygen, which passes into the cell.

On the "return trip"—that is, the reverse process, which we experience as exhalation—cells release carbon dioxide into the bloodstream, where it is used to form another unstable chemical compound. This compound is carried by the bloodstream back to the gills or lungs, and, at the end of the journey, it breaks down and releases the carbon dioxide to the surrounding environment. Clearly, the one process is a mirror image of the other, with the principal difference being the fact that oxygen is the key chemical component in the intake process, while carbon dioxide plays the same role in the process of outflow.

Hemoglobin and Other Compounds

In humans the compound used to transport oxygen is known by the name hemoglobin. Hemoglobin is an iron-containing protein in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them. In the lungs, hemoglobin, known for its deep red color, reacts with oxygen to form oxyhemoglobin. Oxyhemoglobin travels through the bloodstream to cells, where it breaks down to form hemoglobin and oxygen, and the oxygen then passes into cells. On the return trip, hemoglobin combines with carbon dioxide to form carbaminohemoglobin, an unstable compound that, once again, breaks down—only this time it is carbon dioxide that it releases, in this case to the surrounding environment rather than to the cells.

In other species, compounds other than hemoglobin perform a similar function. For example, some types of annelids, or segmented worms, carry a green blood protein called chlorocruorin that functions in the same way as hemoglobin does in humans. And whereas hemoglobin is a molecule with an iron atom at the center, the blood of lobsters and other large crustaceans contains hemocyanin, in which copper occupies the central position. Whatever the substance, the compound it forms with oxygen and carbon dioxide must be unstable, so that it can break down easily to release oxygen to the cells or carbon dioxide to the environment.

Cellular Respiration

Both forms of respiration involve oxygen, but cellular respiration also involves a type of nutrient—materials that supply energy, or the materials for forming new tissue. Among the key nutrients are carbohydrates, naturally occurring compounds that consist of carbon, hydrogen, and oxygen. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances.

Glucose is a simple sugar produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and such complex sugars as sucrose (cane or beet sugar) and fructose (fruit sugar). In cellular respiration, an organism oxidizes glucose (i.e., combines it with oxygen) so as to form the energy-rich compound known as adenosine triphosphate (ATP). ATP, critical to metabolism (the breakdown of nutrients to provide energy or form new material), is the compound used by cells to carry out most of their ordinary functions. Among those functions are the production of new cell parts and chemicals, the movement of compounds through cells and the body as a whole, and growth.

In cellular respiration, six molecules of glucose (C₆H₁₂O₆) react with six molecules of oxygen (O₂) to form six molecules of carbon dioxide (CO₂), six molecules of water (H₂O), and 36 molecules of ATP. This can be represented by the following chemical equation: 6C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + 36 ATP

The process is much more complicated than this equation makes it appear: some two dozen separate chemical reactions are involved in the overall conversion of glucose to carbon dioxide, water, and ATP.

The Mechanics of Breathing

All animals have some mechanism for removing oxygen from the air and transmitting it into the bloodstream, and this same mechanism typically is used to expel carbon dioxide from the bloodstream into the surrounding environment. Types of animal respiration, in order of complexity, include direct diffusion, diffusion into blood, tracheal respiration, respiration with gills, and finally, respiration through lungs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration directly from the environment, meaning that there are no intermediate organs or bodily chemicals, such as lungs or blood. More complex organisms, such as sponges, jellyfish, and terrestrial (land) flatworms, all of which have blood, also breathe through direct diffusion. The latter term describes an exchange of oxygen and carbon dioxide directly between an organism, or its bloodstream, and the surrounding environment.

More complex is the method of diffusion into blood whereby oxygen passes through a moist layer of cells on the body surface and then through capillary walls (capillaries are small blood vessels that form a network throughout the body) and into the bloodstream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. Among the organisms that rely on diffusion into blood are annelids, a group that includes earthworms, various marine worms, and leeches.

In tracheal respiration air moves through openings in the body surface called spiracles. It then passes into special breathing tubes called tracheae that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air simply moves into the tracheae, while in large insects, body movements assist tracheal air movement. Insects and terrestrial arthropods (land-based organisms with external skeletons) use this method of respiration.

Much more complicated than tracheae, gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood. Fish and other aquatic animals use gills, as did the early ancestors of humans and other higher animals. A remnant of this chapter from humans' evolutionary history can be seen in the way that an embryo breathes in its mother's womb, not by drawing in oxygen through its lungs but through gill-like mechanisms that disappear as the embryo develops.

Lungs

Lungs are composed of many small chambers or air sacs surrounded by blood capillaries. Thus, they work with the circulatory system, which transports oxygen from inhaled air to all tissues of the body and also transports carbon dioxide from body cells to the lungs to be exhaled. After air enters the lungs, oxygen moves into the bloodstream through the walls of these capillaries. It then passes from the lung capillaries to the different muscles and organs of the body.

Although they are common to amphibians, reptiles, birds, and mammals, lungs differ enormously throughout the animal kingdom. Frogs, for instance, have balloon-like lungs that do not have a very large surface area. By contrast, if the entire surface of an adult male human's lungs were spread flat, it would cover about 750 sq. ft. (70 m²), approximately the size of a handball court. The reason is that humans have about 300 million gas-filled alveoli, tiny protrusions inside the lungs that greatly expand the surface area for gas exchange.

Birds have specialized lungs that use a mechanism called crosscurrent exchange, which allows air to flow in one direction only, making for more efficient oxygen exchange. They have some eight thin-walled air sacs attached to their lungs, and when they inhale, air passes through a tube called the bronchus and enters posterior air sacs—that is, sacs located toward the rear. At the same time, air in the lungs moves forward to anterior air sacs, or ones located near the bird's front. When the bird exhales, air from the rear air sacs moves to the outside environment, while air from the front moves into the lungs. This efficient system moves air forward through the lungs when the bird inhales and exhales and makes it possible for birds to fly at high altitudes, where the air has a low oxygen content.

Humans and other mammals have lungs in which air moves in and out through the same pathway. This is true even of dolphins and whales, though they differ from humans in that they do not take in nutrition through the same opening. In fact, terrestrial mammals, such as the human, horse, or dog, are some of the only creatures that possess two large respiratory openings: one purely for breathing and smelling and the other for the intake of nutrients as well as air (i.e., oxygen in and carbon dioxide out).

Real-Life Applications

Anaerobic Respiration

Activity that involves oxygen is called aerobic; hence the term aerobic exercise, which refers to running, calisthenics, biking, or any other form of activity that increases the heart rate and breathing. Activity that does not involve oxygen intake is called anaerobic. Weightlifting, for instance, will increase the heart rate and rate of breathing if it is done intensely, but that is not its purpose and it does not depend on the intake and outflow of breath. For that reason, it is called an anaerobic exercise—though, obviously, a person has to keep breathing while doing it.

In fact, a person cannot consciously stop breathing for a prolonged period, and for this reason, people cannot kill themselves simply by holding their breath. A buildup of carbon dioxide and hydrogen ions (electrically charged atoms) in the bloodstream stimulates the breathing centers to become active, no matter what we try to do. On the other hand, if a person were underwater, the lungs would draw in water instead of air, and though water contains air, the drowning person would suffocate.

Anaerobic Bacteria

Some creatures, however, do not need to breathe air but instead survive by anaerobic respiration. This is true primarily of some forms of bacteria, and indeed scientists believe that the first organisms to appear on Earth's surface were anaerobic. Those organisms arose when Earth's atmosphere contained very little oxygen, and as the composition of the atmosphere began to incorporate more oxygen over the course of many millions of years, new organisms evolved that were adapted to that condition.

The essay on paleontology discusses Earth's early history, including the existence of anaerobic life before the formation of oxygen in the atmosphere. The appearance of oxygen is a result of plant life, which produces it as a byproduct of the conversion of carbon dioxide that takes place in photosynthesis. Plants, therefore, are technically anaerobic life-forms, though that term usually refers to types of bacteria that neither inhale nor exhale oxygen. Anaerobic bacteria still exist on Earth and serve humans in many ways. Some play a part in the production of foods, as in the process of fermentation. Other anaerobic bacteria have a role in the treatment of sewage. Living in an environment that would kill most creatures—and not just because of the lack of oxygen—they consume waste materials, breaking them down chemically into simpler compounds.

Humans and Anaerobic Respiration

Even in creatures, such as humans, that depend on aerobic respiration, anaerobic respiration can take place. Most cells are able to switch from aerobic to anaerobic respiration when necessary, but they generally are not able to continue producing energy by this process for very long. For example, a person who exercises vigorously may be burning up glucose faster than oxygen is being pumped to the cells, meaning that cellular respiration cannot take place quickly enough to supply all the energy the body needs. In that case, cells switch over to anaerobic respiration, which results in the production of lactic acid, or C₃H₆O₃. One advantage of anaerobic respiration is that it can take place very quickly and in short bursts, as opposed to aerobic respiration, which is designed for slower and steadier use of muscles. The disadvantage is that anaerobic respiration produces lactic acid, which, when it builds up in muscles that are overworked, causes soreness and may even lead to cramps.

Lactic Acid in the Body

Eventually, the buildup of lactic acid is carried away in the bloodstream, and the lactic acid is converted to carbon dioxide and water vapor, both of which are exhaled. But if lactic acid levels in the bloodstream rise faster than the body can neutralize them, a state known as lactic acidosis may ensue. Lactic acidosis rarely happens in healthy people and, more often than not, is a result of the body's inability to obtain sufficient oxygen, as occurs in heart attacks or carbon monoxide or cyanide poisoning or in the context of diseases such as diabetes.

The ability of the body to metabolize lactic acid is diminished significantly by alcohol, which impairs the liver's ability to carry out normal metabolic reactions. For this reason, alcoholics often have sore muscles from lactic acid buildup, even though they may not exercise. Lactic acid also can lead to a buildup of uric acid crystals in the joints, in turn causing gout, a very painful disease.

Lactic Acid in Food and Industry

Lactic acid is certainly not without its uses, and it is found throughout nature. When lactose, or milk sugar, is fermented by the action of certain bacteria, it causes milk to sour. The same process is used in the manufacture of yogurt, but the reaction is controlled carefully to ensure the production of a consumable product. Lactic acid also is applied by the dairy industry in making cheese. Molasses contains lactic acid, a product of the digestion of sugars by various species of bacteria, and lactic acid also is used in making pickles and sauerkraut, foods for which a sour taste is desired.

A compound made from lactic acid is used as a food preservative, but the applications of lactic acid extend far beyond food production. Lactic acid is important as a starting material for making drugs in the pharmaceutical industry. Additionally, it is involved in the manufacturing of lacquers and inks; is used as a humectant, or moisturizer, in some cosmetics; is applied as a mordant, or a chemical that helps fabrics accept dyes, to textiles; and is employed in tanning leather.

Respiratory Disorders

In almost any bodily system, there are bound to be disorders, or at least the chance that disorders may occur. This is particularly the case with something as complex as the respiratory system, because the more complex the system, the more things that can go wrong. Among the respiratory disorders that affect humans is a whole range of ailments from the common cold to emphysema, and from the flu to cystic fibrosis.

The Common Cold

Colds are among the most common conditions that affect the respiratory system, though what we call the common cold is actually an invasion by one of some 200 different types of virus. Thus, it is really not one ailment but 200, though these are virtually identical, but the large number of viral causative agents has made curing the cold an insurmountable task.

When you get a cold, viruses establish themselves on the mucus membrane that coats the respiratory passages that bring air to your lungs. If your immune system is unsuccessful in warding off this viral infection, the nasal passages become inflamed, swollen, and congested, making it difficult to breathe.

Coughing is a reflex action whereby the body attempts to expel infected mucus or phlegm. It is essential to removing infected secretions from the body, but of course it plays no role in actually bringing a cold to an end. Nor do antibiotics, which are effective against bacteria but not viruses (see Infection). Only when the body builds up its own defense to the cold—assuming the sufferer has a normally functioning immune system—is the infection driven away.

Influenza and Allergies

Influenza, a group of viral infections that can include swine flu, Asian flu, Hong Kong flu, and Victoria flu, is often far more serious than the common cold. A disease of the lungs, it is highly contagious, and can bring about fever, chills, weakness, and aches. In addition, influenza can be fatal: a flu epidemic in the aftermath of World War I, spread to far corners of the globe by returning soldiers, killed an estimated 20 million people.

Respiratory ailments often take the form of allergies such as hay fever, symptoms of which include sneezing, runny nose, swollen nasal tissue, headaches, blocked sinuses, fever, and watery, irritated eyes. Hay fever is usually aggravated by the presence of pollen or ragweed in the air, as is common in the springtime. Other allergy-related respiratory conditions may be aggravated by dust in the air, and particularly by the feces of dust mites that live on dust particles.

Bronchial Ailments

Allergic reactions can be treated by antihistamines (see The Immune System for more about allergies), but simple treatments are not available for such complex respiratory disorders as asthma, chronic bronchitis, and emphysema. All three are characterized by an involuntary constriction in the walls of the bronchial tubes (the two divisions of the trachea or windpipe that lead to the right and left lungs), which causes the tubes to close in such a way that it becomes difficult to breathe.

Emphysema can be brought on by cigarette smoking, and indeed some heavy smokers die from that ailment rather than from lung cancer. On the other hand, a person can contract a bronchial illness without engaging in smoking or any other activity for which the sufferer could ultimately be blamed. Indeed, small children may have asthma. One treatment for such disorders is the use of a bronchodilator, a medicine used to relax the muscles of the bronchial tubes. This may be administered as a mist through an inhaler, or given orally like other medicine.

Tuberculosis and Pneumonia

More severe is tuberculosis, an infectious disease of the lungs caused by bacteria. Tuberculosis attacks the lungs, leading to a chronic infection with such symptoms as fatigue, loss of weight, night fevers and chills, and persistent coughing that brings up blood. Without treatment, it is likely to be fatal. Indeed, it was a significant cause of death until the introduction of antibiotics in the 1940s, and it has remained a problem in underdeveloped nations. Additionally, thanks to mutation in the bacteria themselves, strains of the disease are emerging that are highly resistant to antibiotics.

Another life-threatening respiratory disease is pneumonia, an infection or inflammation of the lungs caused by bacteria, viruses, mycoplasma (microorganisms that show similarities to both viruses and bacteria), and fungi, as well as such inorganic agents as inhaled dust or gases. Symptoms include pleurisy (chest pain), high fever, chills, severe coughing that brings up small amounts of mucus, sweating, blood in the sputum (saliva and mucus expelled from the lungs), and labored breathing.

In 1936, pneumonia was the principal cause of death in the United States. Since then, it has been controlled by antibiotics, but as with tuberculosis, resistant strains of bacteria have developed, and therefore the number of cases has increased. Today, pneumonia and influenza combined are among the most significant causes of death in the United States (see Diseases).

Lung Cancer and Cystic Fibrosis

Respiratory ailments may also take the form of lung cancer, which may or may not be a result of smoking. Cigarette smoking and air pollution are considered to among the most significant causes of lung cancer, yet people have been known to die of the disease without being smokers or having been exposed to significant pollution.

One particularly serious variety of respiratory illness is cystic fibrosis, a genetic disorder that causes a thick mucus to build up in the respiratory system and in the pancreas, a digestive organ. (For more about genetic disorders, see Heredity; for more on role of the pancreas, see Digestion.) In the United States, the disease affects about one in every 3,900 babies born annually. No cure for cystic fibrosis exists, and the disease is invariably fatal, with only about 50% of sufferers surviving into their thirties.

Lung complications are the leading cause of death from cystic fibrosis, and most symptoms of the disease are related to the sticky mucus that clogs the lungs and pancreas. People with cystic fibrosis have trouble breathing, and are highly susceptible to bacterial infections of the lungs. Coughing, while it may be irritating and painful if you have a cold, is necessary for the expulsion of infected mucus, but mucus in the lungs of a cystic fibrosis is too thick to be moved. This makes it easy for bacteria to inhabit the lungs and cause infection.

Where to Learn More

Bryan, Jenny. Breathing: The Respiratory System. New York: Dillon Press, 1993.

Cellular Metabolism and Fermentation. Estrella Mountain Community College (Web site). <http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookGlyc.html>.

Kimball, Jim. "The Human Respiratory System." Kim ball's Biology Pages (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/P/Pulmonary.html>.

Levesque, Mireille, Letitia Fralick, and Joni McDowell. "Respiration in Water: An Overview of Gills." University of New Brunswick (Web site). <http://www.unb.ca/courses/biol4775/SPAGES/SPAGE13.HTM>.

Llamas, Andreu. Respiration and Circulation. Milwaukee: Gareth Stevens, 1998.

Paustian, Timothy. Anaerobic Respiration. Department of Bacteriology, University of Wisconsin-Madison (Web site). <http://www.bact.wisc.edu/microtextbook/Metabolism/RespAnaer.html>.

Roca, Núria, and Marta Serrano. The Respiratory System, the Breath of Life. Illus. Antonio Tenllado. New York: Chelsea House Publishers, 1995.

Silverstein, Alvin, and Virginia B. Silverstein. The Respiratory System. New York: Twenty-First Century Books, 1994.

The various processes associated with the biochemical transformation of the energy available in the organic substrates derived from foodstuffs, to energy usable for synthetic and transport processes, external work, and, eventually, heat. This transformation, generally identified as metabolism, most commonly requires the presence of oxygen and involves the complete oxidation of organic substrates to carbon dioxide and water (aerobic respiration). If the oxidation is incomplete, resulting in organic compounds as end products, oxygen is typically not involved, and the process is then identified as anaerobic respiration. See also Metabolism.

The term “external respiration” is more appropriate for describing the exchange of O₂ and CO₂ between the organism and its environment. In most multicellular organisms, and nearly all vertebrates (with the exception of a few salamanders lacking both lungs and gills), external respiration takes place in specialized structures termed respiratory organs, such as gills and lungs. See also Lung; Respiratory system.

The ultimate physical process causing movement of gases across living tissues is simple passive diffusion. Respiratory gas exchange also depends on two convective fluid movements. The first is the bulk transport of the external medium, air or water, to and across the external respiratory exchange surfaces. The second is the transport of coelomic fluid or blood across the internal surfaces of the respiratory organ. These two convective transports are referred to as ventilation and circulation (or perfusion). They are active processes, powered by ciliary or muscular pumps.

In all vertebrates and many invertebrates, the circulating internal medium (coelomic fluid, hemolymph, or blood) contains a respiratory pigment, for example, hemocyanin or hemoglobin, which binds reversibly with O₂, CO₂, and protons. Respiratory pigments augment respiratory gas exchange, both by increasing the capacity for bulk transport of the gases, and by influencing gas partial pressure (concentration) gradients across tissue exchange surfaces. See also Blood; Hemoglobin; Respiratory pigments (invertebrate).

The physiological adjustment of organisms to variations in their need for aerobic energy production involves regulated changes in the exchange and transport of respiratory gases. The adjustments are effected by rapid alterations in the ventilatory and circulatory pumps and by longer-term modifications in the respiratory properties of blood.

Respiration is the absorption of oxygen and the output of carbon dioxide that take part in the metabolic processes in the body; it includes the burning of foodstuffs in the tissues, the transport of the gases in the blood and their exchange in the lungs. All but the smallest animals, for example single cells like amoeba, require specialized transport and exchange organs to provide sufficient supply and removal of the large quantities of gases involved. A vigorously exercising athletic human might take up as much as six litres of oxygen per minute, and excrete a similar amount or more of carbon dioxide.

It is usual to divide respiration into external and internal components, linked by the bloodstream that transports the gases but has little metabolism of its own. In mammals, external respiration — the ventilation of the lungs — is achieved by breathing, the mechanical basis of respiration: the terms are sometimes used synonymously. In fishes there is equivalent ‘ventilation’ of the gills with water. At rest an adult human inhales about 6-8 litres of air per minute, of which about two thirds gets to the alveoli and takes part in gas exchange, the rest remaining in the air passages. Fresh air contains 21% oxygen and almost no carbon dioxide. In the lungs, the entry of oxygen into the blood and the release of carbon dioxide results in exhaled gas having about 5% less oxygen and 5% more carbon dioxide. The gases pass in and out of the blood by passive diffusion, due to their relative pressures in the blood and in the alveolar gas, just as a fizzy drink will lose its gas and go flat when exposed to air. At rest an average-sized adult will take up about 250 ml of oxygen each minute, and exhale about 200 ml of carbon dioxide. In vigorous exercise these values can increase over 20-fold in a trained athlete as a result of the increased metabolism in the muscles. The blood in the capillaries leaving the lungs, and therefore in the arteries carrying it round the body, have the same pressures of oxygen and carbon dioxide as those in the alveoli, at least in health. In some lung and heart disease, involving a problem of gas transfer, the arterial blood may have a lower oxygen and a higher carbon dioxide pressure than that in the alveoli, causing hypoxia and hypercapnia. In other types of lung disease these abnormalities in the blood result from inadequate breathing — failure to ventilate the alveoli with sufficient fresh air.

In the blood almost all the oxygen is carried combined with haemoglobin in the red cells, while carbon dioxide is taken up in both plasma and red cells. Once the blood has reached the tissues, internal respiration takes over. Oxygen will diffuse from the capillaries into metabolizing cells, again passively following its pressure gradient. Vigorously contracting muscle cells use up almost all the available oxygen inside them, and strongly pull a fresh supply from the blood by diffusion. The oxygen combines with food materials, mainly sugars and fats, to release heat and energy for contraction of muscle, with water and carbon dioxide as the main waste products. The water enters the general water pool of the body, while the carbon dioxide enters the blood and is carried to the lungs mainly as bicarbonate but also in combination with hemoglobin and plasma proteins. An appreciable amount of carbon dioxide, unlike oxygen, is also free in solution in the plasma. Such aerobic metabolism occurs similarly in the great majority of body cells.

This understanding of the chemical basis of respiration was only developed in the eighteenth century, with the chemical identification of oxygen and carbon dioxide, mainly by Joseph Priestley (1733-1804) and Antoine-Laurent Lavoisier (1743-94). For two thousand years before then there were many speculations about the meaning of breathing, the main idea being that its function was to cool the blood by physically mixing air with it in the arteries and veins. The cooling function seemed proved by the fact that exhaled gas was usually warmer than inhaled air, and the mixing with blood was deduced from the fact that when arteries and veins were cut the blood flowing out was often frothy. (We now know that if large veins are cut they will suck air into the circulation, and it will pass through the heart and lungs and make the haemorrhaging blood bubbly.) In the second century ad Galen wrote ‘respiration is useful to animals for the sake of the heart, which to some extent requires the substance of the air and besides needs very greatly to be cooled because of its burning heat.’ Even primitive men must have known that breathing was necessary for life; according to the Judaeo-Christian tradition, breath created it. ‘And the Lord God formed man of the dust of the ground and breathed into his nostrils the breath of life; and man became a living soul’ (Genesis 2: 7). In the seventeenth century much experimental work established that one component of air was essential for life, and that another different fraction was exhaled. These were identified as oxygen and carbon dioxide a century later. There was much speculation about the ‘respiration’ of fetuses. Clearly they could not breathe air, since their lungs were full of liquid. Since the role of the blood circulation in gas exchange was not understood, the respiratory function of the placenta, the ‘external’ respiratory organ of the fetus, was not apparent.

Sophisticated and accurate methods of analysing respiratory gases were developed in the twentieth century, and the mechanisms of external respiration are now well defined. At about the same time development of biochemistry and, later, of molecular biology, led to an understanding of internal respiration. When oxygen combines with carbohydrates every molecule of oxygen creates one molecule of water and one in carbon dioxide. Thus the ratio of the exchange volumes of the two gases, the respiratory quotient, is 1; in practice this would only occur in someone living on a diet of, and metabolizing only carbohydrate — polished rice, for example. Fat and protein contain less oxygen than does carbohydrate, so more oxygen is needed for their consumption than the carbon dioxide that is produced. On an average diet in a developed country the ratio of carbon dioxide excreted to oxygen absorbed is about 0.8. However these values assume that there is enough oxygen for the metabolic needs of all the body — that we are at rest or in a state of entirely aerobic metabolism. ‘Aerobic exercise’ is exercise within this limit. If exercise is severe, not enough oxygen is available for the muscles, which pass an ‘anaerobic threshold’, and energy is provided additionally by the breakdown of carbohydrates without using oxygen and with the formation of lactic acid. This diffuses into the blood, causing a mild acidosis and acting on the bicarbonate to release carbon dioxide which is excreted in the lungs. (Lemon juice, put on self-raising flour, or bicarbonate of soda, will make it fizz as it releases carbon dioxide.) Thus in severe exercise we exhale more carbon dioxide than the oxygen we absorb. After the exercise we retain carbon dioxide in the body and most of the lactic acid is converted back into carbohydrate in the liver. Extra oxygen is needed for this, and the process is referred to as ‘repaying the oxygen debt’.

— John Widdicombe

See also breathing; breathing in exercise; carbon dioxide; haemoglobin; lungs; metabolism; oxygen.

The chemical process by which food is converted into a form of energy that can be used by the body. During this internal or cellular respiration, carbohydrates and fats (and proteins in extreme circumstances) are broken down either without oxygen (anaerobic respiration) or with oxygen (aerobic respiration) to form adenosine triphosphate (ATP). ATP is the only chemical which can be used directly as a source of energy by the body.

noun

The act or process of breathing: breath. See breath/breathlessness.

The gaseous exchange between cells of the body and the environment. Four stages exist: pulmonary ventilation, diffusion of gases in the alveoli, transport of gases in the blood to and from cells, and regulation of the process.

Any or all of the processes used to generate metabolic energy, mainly in the form of adenosine triphosphate, from the breakdown of food. See also cellular respiration.

Respiration is the exchange of gases (oxygen and carbon dioxide) between an animal and its environment. There are three phases to the process of respiration (gas exchange): 1) breathing, when an animal inhales oxygen and exhales carbon dioxide; 2) transport of gases via the blood (circulatory system) to the body's tissues; and 3) at the cellular level, when the cells take in oxygen from the blood and in return add carbon dioxide to the blood.

The conversion of oxygen by living things into the energy by which they continue life. Respiration is part of metabolism.

1. the exchange of oxygen and carbon dioxide between the atmosphere and the body cells, including inspiration and expiration, diffusion of oxygen from the pulmonary alveoli to the blood and of carbon dioxide from the blood to the alveoli, and the transport of oxygen to and carbon dioxide from the body cells.
2. cellular respiration, the metabolic processes by which living cells break down carbohydrates, amino acids and fats to produce energy in the form of ATP (adenosine triphosphate).

• abdominal r. — inspiration and expiration accomplished mainly by the abdominal muscles and diaphragm. Occurs in acute pleurisy because of pain in the chest and fixation of the thorax, and tick paralysis due to paralysis of the intercostal muscles.
• aerobic r. — oxidative transformation of certain substrates into secretory products, the released energy being used in the process of assimilation.
• anaerobic r. — respiration in which energy is released by chemical reactions in which free oxygen takes no part.
• artificial r. — that maintained by force applied to the body. Called also artificially assisted respiration.
• artificially assisted r. — see artificial respiration (above).
• Biot's r's — rapid, deep respirations with abrupt pauses in breathing. See also biot's respirations.
• cell r. — the processes in the living cell by which organic substances are oxidized and chemical energy is released.
• Cheyne–Stokes r. — breathing characterized by rhythmic waxing and waning of respiration depth, with regularly recurring apneic periods. See also cheyne–stokes respiration.
• cogwheel r. — breathing with jerky inspiration.
• controlled r. — during general anesthesia using an endotracheal tube with an inflated cuff, the animal's respiration can be controlled completely by compression alternating with relaxation on the rebreathing bag of the breathing circuit. See also intermittent positive-pressure ventilation.
• costal r. — the respiratory movements are mostly carried out by the chest wall.
• diaphragmatic r. — that performed mainly by the diaphragm.
• electrophrenic r. — induction of respiration by electric stimulation of the phrenic nerve.
• external r. — the exchange of gases between the lungs and the blood.
• internal r. — the exchange of gases between the body cells and the blood.
• Kussmaul's r. — see kussmaul's respiration.
• labored r. — see dyspnea.
• r. monitors — machines that monitor respiratory movement and efficiency are most desirable during anesthesia. They include rate monitors, apnea alarms, tidal and minute volume monitoring respirometers, infrared gas analyzers to measure carbon dioxide content of end-tidal air,
• paradoxical r. — that in which a lung, or a portion of a lung, is deflated during inspiration and inflated during expiration. See also paradoxical respiration.
• tissue r. — internal respiration.

The chemical process by which animals and plants release the energy that is stored in carbohydrates and other foodstuffs. In respiration, oxygen is taken up and carbon dioxide is given off.

Look up respiration in Wiktionary, the free dictionary.

Respiration may refer to:

• Respiration (physiology) and Breathing, the physiological process that enables animals to exchange carbon dioxide, the primary product of cellular respiration, for fresh air.
• Cellular respiration, the process in which nutrients are converted into useful energy in a cell
• Anaerobic respiration, cellular respiration without oxygen
• Aquatic respiration, the process of animals extracting oxygen from water
• Carbon respiration, a concept used in calculating carbon (as CO₂) flux occurring in the atmosphere
• Ecosystem respiration, measure of gross carbon dioxide production by all organisms in an ecosystem
• Root respiration, exchange of gases between plant roots and the atmosphere
• "Respiration" (song), a 1999 hip hop group single by Black Star

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