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Oxygen enters the animal’s body from the air or water surrounding it as oxygen is quite soluble in water. In less complex singled celled organisms, the entire exposed surface of the body absorbs oxygen but is higher organisms there are special respiratory organs such as the lungs or grills where oxygen is taken up and carbon dioxide is excreted. In the respiratory organ, blood receives oxygen, and then transports it to all the living parts of the body where it is used in tissue respiration.

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To function efficiently, a respiratory organ has a large surface area for rapid oxygen uptake and a very good supply of blood in a dense network of blood capillaries, which are separated from the surrounding air or water by an epithelium made up of a very thin layer of cells.

In land-dwelling animals the whole absorbing surface of the organ is covered by a layer of moisture. In many animals there is also a mechanism which constantly changes and renews the air or water in contact with, or near, the respiratory surface. This is called ventilation.

Respiratory Structure and Breathing Mechanisms in Animals

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Protozoa

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Protozoa; e.g. amoeba, paramecium have no special respiratory organs and exchange of gasses takes place at the surface membrane of the organism through simple diffusion. Oxygen diffuses from the surrounding water into the organism as its concentration on the outside is higher. On passing through the membrane it diffuses to all parts of the protoplasm. Carbon dioxide passes to the outside in a similar manner.

Insects

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In insects, oxygen reaches the tissues directly through a system of breathing tubes called the trachea, and the carbon dioxide produced in respiration is excreted to the exterior mainly by the same path. The trachea branch repeatedly until they terminate in very fine tubes or tracheoles which penetrate the tissues of the body. They are lined with a thin layer of cuticle, thickened in spiral bands to keep them open against the internal pressure of the body fluids.

The walls of the trachea and tracheoles are permeable to gasses; oxygen can diffuse through them to reach the cells, and carbon dioxide can diffuse in the reverse direction into the tubes. The network of tracheoles is most dense in the region of very active muscles, e.g. in the flight muscles in the thorax. The tracheas are open to the atmosphere by pores in the cuticle called spiracles. The entrance to the spiracles is usually supplied with muscles which enable it to open and close. Many insects’ posses two pairs of spiracles in the thorax and eight pairs in the first eight abdominal segments.

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The spiracles close when the insect is not active and therefore needs little oxygen, and this closure helps to reduce the loss of water by evaporation from the internal tissues. In active adults’ insects, the exchange of air in the tracheal system is added by a ventilation process. Ventilation is brought about rhythmic concentration and relaxation of the abdomen. The direction of the compression can be either vertical or longitudinal (telescopic). This compression of the abdomen increases the pressure in the blood, which squeezes the tracheae along their length. This causes the expulsion of air from the trachea through the spiracles.

When the muscles relax, the abdomen springs back into shape, and the tracheae expand and draw in air. Accumulation of lactic acid in the muscles and tissues occurs during vigorous activities. This causes an increase in the osmotic concentration in the tissue fluids, causing fluids to drain out of the tracheoles, and thus enabling faster flow of oxygen to the tissues.

Fish

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Dissolved oxygen in water is absorbed by fish through special organs called gills. In most fish there are four gills lying below the operculum on each side. Each gill consists of a curved bony gill bar bearing many long filaments; the filaments bear smaller filaments, and these in turn divide into still smaller branches. Blood vessels run through the gill bar, and branches run down from them onto the filaments. The great number of filaments provides a very large surface for diffusion of oxygen from the surrounding water through their thin walls.

Oxygen combined with hemoglobin in the red blood cells and is carried in the blood capillaries from the gills to the rest of the body. Water bearing dissolved oxygen enters the mouth and passes over the gills and out of opercula a) Lowering the floor of the mouth reduces the pressure in the mouth cavity. Water then enters the mouth, while the free edges of the opercula are kept firmly close against the body wall by the higher pressure of the water outside them.

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b) When the floor of the mouth is raised; the volume of the mouth cavity is decreased. The increased water pressure in the mouth closes the two in-turned folds of skin along the upper and lower jaws and forces water across the gill filaments. The movement of water is helped by a simultaneous outward movement of the opercula, causing water to flow from the front to the back of the mouth cavity and over the gills c) The mouth and the opercula then close. A fold of skin along the free edge of each operculum is forced outwards by the pressure of the water inside it, and water escapes between the operculum and the body wall.

Frog

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In frogs, the skin, the lining in the mouth, and the lungs are involved in gaseous exchange. Oxygen diffuses into the blood in the capillary network through the capillary walls, in either the skin or the lings. It then combines with hemoglobin and is then transported by the circulatory system around the body. The skin is smooth and moist, thin-walled capillaries. Oxygen enters the blood by first dissolving in the film of moisture covering the body and then diffusing through the skin, through the walls of the capillaries and into the blood.

Carbon dioxide produced in respiration is eliminated from the body from the blood in the reverse direction, out of the blood vessels, through the skin and into the atmosphere. When active, the frog is able to take in additional supply of oxygen through its lung in the body cavity. The lungs can be expanded may times their relaxed size. There are no regular, rhythmic breathing movements. Air is forced into the lungs spasmodically as the need arises, by gulping movements of the mouth. The moist skin lining the frog’s mouth is also a respiratory surface.

Human

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The lungs are the organs in human where gaseous exchange take place. Oxygen from the atmospheric air is absorbed by blood and carbon dioxide carried by the blood is released into the atmosphere. Breathing (inspiration and expiration) is the process by which air is brought into contact with the alveoli of the lungs for gaseous exchange. The lungs are two thin-walled elastic sacs lying inside the rib-cage in the thorax are both covered with smooth elastic membrane, the pleural membrane. The gas exchange system includes the nasal passage, pharynx, larynx, trachea, bronchi, lungs and the muscles involved in inspiration and expiration.

 

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The conducting part of the respiratory system begins with the nasal cavity and continues across the pharynx to a branching system of tubes inside the lungs. The largest of these tubes, the trachea, or windpipe, is typically about 2 cm in diameter and 10 cm long. The trachea is supported at intervals by C-shaped cartilages embedded in its wall. Each cartilage extends around the trachea, its ends being separated by a short gap located at the back. At the top of the trachea, where it joins the pharynx, is the larynx. It acts as a valve which is closed during swallowing, sealing the trachea against the entry of food. In humans, the larynx can be used in the production of speech.

 

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The epiglottis, a flap of tissue supported by cartilage, projects forwards from the larynx and assists in directing food towards the esophagus. At its base, the trachea divided into a left and right bronchus, each of which is further subdivided into smaller tubes called bronchi and supported by cartilage in a similar way to the trachea. The branching system divides each lung into a number of distinct lobes. Within each lobe, the tubes of the bronchial tree continue to subdivide and eventually give rise to narrow unsupported tubes less than 1 mm in diameter. These are the bronchioles which subdivide again and finally end in minute air-filled sacs called alveoli. The alveoli are well supplied with a network of blood capillaries on the surface, so that gas exchange can take place.

Breathing Mechanism

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Ventilation of the lungs (expansion and contraction) is brought about by the movements of the thorax. The thorax is an air-tight cavity, enclosed at the sides by the ribs and the tissues covering them, and by the diaphragm, a sheet of muscle that separates the thorax and the abdomen. When the diaphragm is relaxed, it is dome-shaped, extending upwards into the thoracic cavity, with the liver and stomach lying immediately below it.

Inspiration (Breathing in)

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The volume of the thorax is increased by two movements: i) the diaphragm contracts and flattens out; ii) the outer inter-costal muscles which run obliquely from one rib to the next, contract and pull the lower ribs upwards and outwards. As the thorax expands the capacity of the lungs increases, and there is a consequent fall in the air pressure, or a partial vacuum, within them. As a result the pressure of the atmosphere, which is higher, forces air into the lungs through the trachea via the nose and mouth.

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Expiration (Breathing out)

During expiration, the outer inter-costal muscles, and the diaphragm relax. The ribs move down under their own weight, and the organs lying just below the diaphragm, under pressure from the muscular abdominal wall, push the diaphragm back into its dome shape again. As a result the lungs return back to their original volume, increasing the air pressure in them and part of the air they contain is expelled.

Info Science

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The exchange of gases means the operation that supplies oxygen from inhaled air to blood and removes carbon dioxide from blood. These operations are carried out through a thin membrane that is about 0.001 millimeters thick. This membrane makes up the walls of the alveoli and the capillary vessels. Oxygen and carbon diffuse rapidly from higher pressure area to lower one through that membrane. This is how the exchange of gases takes place.