Respiration is a physical process by which living organisms take in oxygen from the surrounding medium and emit carbon dioxide. The term respiration is also used to refer to the liberation of energy, within the cell, from fuel molecules such as carbohydrates and fats. Carbon dioxide and water are the products of this process, which is sometimes called cellular respiration to distinguish it from the physical process of breathing. Cellular respiration is similar in most organisms, from such protists as the amoeba and paramecium to higher animals . Process of Respiration. Small organisms of the kingdoms Protista and Monera have no specialised respiratory mechanisms; rather, they rely on the diffusion of oxygen and carbon dioxide across the cell membrane. The concentration of oxygen in the organism is less than that of the surrounding air or water, and the concentration of carbon dioxide is greater.

As a result, oxygen diffuses into the organism, and carbon dioxide diffuses out. Respiration in plants and sponges is based on the same process.In aquatic lower animals that are more complex than sponges, a circulating medium, similar in composition to seawater, carries the respiratory gases from the outer tissues to cells that are distant from the site of gas exchange. In higher animals, specialised organs increase the area of exposure of the circulating fluid to the external medium, and circulatory systems carry the fluid to every part of the body. In addition, the circulation fluid, or blood , contains respiratory pigments complex organic molecules in which a ring structure containing a metal such as iron is combined with a protein.
The most common respiratory pigment is haemoglobin , which is present in the blood of most mammals and consists of an iron compound of hematin combined with a globulin. In certain insects the blood pigment is hemocyanin, a compound similar to haemoglobin, in which the iron is replaced by copper. The most important property of the respiratory pigments is an affinity for oxygen. The pigment forms a loose chemical combination with oxygen when exposed to an atmosphere rich in that element, as in the capillaries of respiratory organs such as gills or lungs. The oxygen compound is more acidic than the pigment and consequently absorbs sodium ions from the sodium carbonate-bicarbonate solution in the blood plasma, forcing the latter to release carbon dioxide. When the blood reaches the tissues, the oxygen balance is reversed; the blood pigment releases oxygen and, becoming more basic, also releases sodium ions, which combine with the carbon dioxide from the tissues to form sodium bicarbonate. The interchange of gases in the blood is called external respiration; that taking place in the body, between the blood and tissues, is called internal respiration. Respiration in Lower Animal Life. Aquatic animals carry on external respiration by means of gills , over which auxiliary respiratory mechanisms keep a constant current of fresh water flowing. The gills are branched to such an extent as to resemble feathers or plumes: In each branch fine blood vessels ramify so that the blood is separated from the water medium by two layers of cells, one being the wall of the fine blood vessel, or capillary, and the other being the epithelium of the gill. The gases are diffused readily through the epithelium, and the extended surface produced by the branching enables large quantities of blood to be oxygenated in a short time. In such air-breathing forms as the earthworm, respiration takes place through the capillaries in the skin; amphibious forms, such as the frog, respire through the skin and also by means of lungs . Insects breathe by means of air tubes, or tracheae, which open on the outside of the body and ramify through the tissues, carrying air to internal organs and structures. Reptiles and mammals respire solely by means of lungs; birds, however, have auxiliary air sacs in the body cavity and air spaces within certain bones, all of which connect with the lungs and act as aids to pulmonary respiration.
The respiratory and circulatory systems of air-breathing animals become adapted and modified for life in oxygen-deficient environments. For example, persons living in the Andes at altitudes of 3000 m (10,000 ft) or more have larger lungs, more highly branched capillary systems, and a faster heartbeat than people living at lower altitudes. Moreover, the blood of high-altitude dwellers contains 30 percent more red cells than the blood of persons Living at sea level; they are therefore able to make efficient use of the one-third less oxygen that is available.
Aquatic mammals generally have large, complex systems of veins for the storage of blood. The blood volume of whales and seals is up to 50 percent greater per kilogram of body weight than the blood volume of humans, which enables them to supply their tissues with oxygenated blood for a long period without breathing. Whales may remain submerged from 15 min to more than 1 hr, depending on the species. The elephant seal may stay underwater for 30 min. When a seal begins an underwater dive, its heartbeat slows from 150 beats per min to 10, and the oxygen content of the arterial blood is 20 percent. When the oxygen level drops to nearly 2 percent, the seal must surface.Human Respiration. In humans, as in the other vertebrates, the lungs are enclosed in the thorax. The ribs support the body wall of the thorax, which has a domed base formed by the diaphragm. The ribs slant downward and forward; when they are raised by the action of the intercostal muscles, the volume of the thorax is increased. The volume of the thorax is also increased by contraction of the muscles of the diaphragm. Within the thorax, the lungs are held close to the body wall by atmospheric pressure, and, when the thorax expands, the lungs also expand and become filled with air drawn through the upper respiratory passages. Relaxation of the muscles expanding the thorax allows the opposing set of muscles to return the chest to its naturally contracted position, forcing the air from the lungs. From 200 to 500 cu cm (12 to 30 cu in) of air are usually inhaled and exhaled at each inspiration; this volume is called tidal air. About 1500 cu cm (about 90 cu in) of additional air, called complemental air, can be inhaled on a forced inspiration and then exhaled; still another 1500 cu cm, called supplemental air, can be exhaled on a forced expiration. The sum of these three quantities is called the vital capacity. About 1500 cu cm of air always remains in the lungs and cannot be exhaled; this volume is called the residual, or alveolar, air.
The human lungs are roughly pyramidal in shape, conforming to the shape of the thorax. They are not strictly symmetrical: The right lung consists of three lobes; the left consists of two lobes and has, near the medial edge of the base, a cardiac notch into which the heart extends. On the medial side of each lung is the root, by which the lung is attached to the mediastinum, or central partition of the chest. The root consists of pleural folds, bronchi, and pulmonary arteries and veins. As the bronchus penetrates the substance of the lung, it divides and subdivides repeatedly until it ends in the lobule, the structural and functional unit of the lung. Accompanying the bronchus, the pulmonary arteries and veins divide at the same points, the arterioles and venules of the lobules being connected by a dense network of capillaries that lie in the walls of the air cells. Nerves from the pulmonary plexus and lymphatic vessels are also distributed in the same manner. Within the lobule, the bronchiole divides into terminal bronchi, each of which opens into a group of atria, or air spaces. Each of the atria opens in turn into a number of alveolar saccules, the walls of which are pouched out to form the numerous alveoli, or air cells, of the lobule.
The principal nervous centre for controlling the rate and depth of respiration is in the respiratory section of the vagus nucleus in the medulla oblongata . The cells of this nucleus are sensitive to the acidity of the blood, which reflects higher and lower concentrations of carbon dioxide in the blood plasma. When the acidity of the blood is high, as is usually caused by an excess of carbon dioxide, the respiratory centre stimulates the respiratory muscles to greater activity. When the carbon dioxide concentration is low, breathing is depressed.