Breathing, Gaseous exchange And Gaseous exchange in other organisms


THE various processes carried out by the body, e.g. movement, growth and reproduction, require the expenditure of energy. In animals this energy can be obtained only from the food they eat. Before the energy can be used by the cells of the body it must be set free from the chemicals of the food. This process of liberating energy is called respiration and involves the use of oxygen and the production of carbon dioxide.

Oxygen enters the animal's body from the air or water surrounding it. In the less complex animals the oxygen is absorbed by the entire exposed surface of the body, but in the higher animals there are special respiratory areas such as lungs is or gills. Excess carbon dioxide is usually eliminated from the same area. In the respiratory organ oxygen combines with the haemoglobin in the blood and is so carried to all living parts of the body where it is used in tissue respiration.

  An efficient respiratory organ has a large surface area, a dense capillary network or similar blood supply, a very thin epithelium separating the air or water from the blood vessels and, in land-dwelling animals, a layer of moisture over the absorbing surface. In many animals there is also a mechanism which renews the air or water in contact with or near the respiratory surface, a process called ventilation. In mammals, the respiratory organs are lungs.


The lungs are two thin-walled, elastic sacs lying in the thorax. They can be expanded or compressed by movements of the thorax in such a way that air is repeatedly taken in and expelled. They communicate with the atmosphere through the wind pipe or trachea, which opens into the pharynx.

In the lungs, a gaseous exchange takes place; some of the atmospheric oxygen is absorbed and carbon dioxide from thee blood is released into the lung cavities.

Lung structure. The trachea divides into two bronchi which enter the lungs and divide into smaller branches called bronchioles. These divide further and terminate in a mass of little thin-walled, pouch like air sacs or alveoli.

(a) AIR PASSAGES. Rings of cartilage keep the trachea and bronchi open and prevent their closing up when the pressure inside them falls during inspiration. The lining of the air passages is covered with numerous cilia. These are minute, cytoplasmic hairs which constantly flick to and fro. Mucus is secreted by glandular cells, also in the lining. Dust particles, bacteria, etc, which are carried in with the air during inspiration become trapped in the mucus film and, by the movements of the cilia, are swept away in it up to the larynx and into the pharynx where they are swallowed.

The epiglottis and other structures at the top of the trachea prevent large particles from entering the air passages, particularly during swallowing, Choking and coughing are reflex actions which tend to remove any foreign particles which accidentally enter the trachea or bronchi 

(b) ALVIBOLI. The alveoli have thin, classic walls consisting internally of a single cell layer, or epithelium, and beneath this, a dense network of capillaries supplied with de- oxygenated blood pumped from the right ventricle through the pulmonary artery, In one human lung there are about 350 million alveoli with a total absorbing surface of about 90 m2.

Gaseous exchange

The lining of the alveoli is covered with a film of moisture. The oxygen concentration in the blood is lower than in the alveolus, hence oxygen in the air space dissolves in the film of moisture and diffuses through the epithelium, the capillary wall, the plasma and into a red cell, where it combines with the haemoglobin. The capillaries reunite and eventually from the pulmonary veins which return the oxygenated blood to the left atrium of the heart, The low concentration of carbon dioxide in the alveoli stimulates the enzyme, carbonic anhydrase, in the blood to break down the hydrogen carbonate salts and liberate carbon dioxide. This gas diffuses into the alveoli and is eventually expelled Although nitrogen does dissolve in blood plasma, it plays no part in the chemical reactions of the body so the rates of diffusion into and out of the blood are the same.

Diffusion gradient. A steep diffusion gradient of oxygen is maintained by (i) replenishment of air in the air passages by ventilation, (ii) the very short distance between the alveolar lining and the blood, (iii) the combination of oxygen with haemoglobin, so removing oxygen from solution, (iv) the blood flow which constantly replenishes Oxygenated blood with deoxygenated blood. Similar factors work in the reverse direction for the diffusion of carbon dioxide. The conversion of hydrogen carbonate to carbon dioxide by carbonic anhydrase raises the concentration of carbon dioxide in the blood above that in the alveoli.

Rate of breathing

The rhythmical breathing movements are usually carried out quite unconsciously about 16 times a minute. They are controlled by a region of the brain which is very sensitive to the carbon dioxide concentration in the blood. If there is a rise in the carbon dioxide concentration of the blood reaching this region of the brain, nerve impulses are automatically sent to the diaphragm and rib muscles which increase the rate and depth of breathing. The concentration of carbon dioxide in the blood is most likely to rise during vigorous activity, and the accelerated rate of breathing helps to expel the rapidly accumulating carbon dioxide and to increase the amount of oxygen in the blood, so meeting the demands of increased tissue respiration.

By regulating the oxygen and carbon dioxide levels in the blood, the lungs are fulfilling a homeostatic function. At most times the rate of breathing can be controlled voluntarily, as in singing or when playing a wind instrument.

Lung capacity

The total capacity of the lungs, when fully inflated in an adult man, is about 54 litres, but during quiet breathing only about 500 cm3 of air is exchanged. This is called tidal air. During activity the thoracic movements are more extensive, and deep inspiration can take in another 2 litres while vigorous expiration can expel an additional 1 litres. The thorax cannot collapse completely, so that 1 litres of air can never be expelled. This residual air, which remains stationary in the alveoli, exchanges carbon dioxide and oxygen by diffusion with the tidal air that sweeps into the bronchi and air passages.

The nose

The ciliated epithelium and film of mucus which line the nasal passages help to trap dust and bacteria. The air is also warmed slightly before it enters the lungs. In addition, in the linings of the nasal cavity there are sensory organs which respond to chemicals in the air and confer a sense of smell.


The vocal cords are two folds protruding from the lining of the larynx. They contain ligaments which are controlled by muscles. When air is passed over them in a certain way they vibrate and produce sounds. The controlling muscles can alter the tension in the cords and the distance between them. In this way they vary the pitch and quality of the sounds produced.

Ventilation of the lungs

The exchange of air in the lungs is brought about by muscular movements of the thorax which alter its volume. The thorax is an airtight cavity enclosed by the ribs at the sides and the diaphragm below. The diaphragm is a muscular sheet of tissue extending across the body cavity between the thorax and abdomen. At rest, it is domeshaped, extending upwards into the thoracic cavity, with the liver and stomach immediately below it. Any change in the volume of the thorax is followed by the lungs, which are too thin to oppose the movements.

INSPIRATION. Luring inspiration the volume of the thorax is increased by two movements.

(a) The muscles of the diaphragm contract and cause it to flatten from its domed position.

(b) The lower ribs are raised upwards and outwards by contraction of the intercostal muscles which run obliquely from one rib to the next.

Both these movements increase the volume of the thorax and, consequently, the volume of the lungs which follow the movements. The increase in volume raises the capacity of the lungs so that atmospheric pressure forces air into them through the nose and trachea.

EXPIRATION. Expiration, or breathing out, results mainly from a relaxation of the muscles of the ribs and diaphragm.

The ribs move down under their own weight, and the organs below the diaphragm, under pressure from the muscular walls of the abdomen, push the relaxed diaphragm back into its domed position. The lungs, as a result of these movements and by virtue of their elasticity, return to their original volume.

In this way air containing less oxygen and more carbon dioxide and water vapour than when it entered the lungs is expelled from them. Usually, in quiet breathing the movements of the diaphragm alone are responsible for the ventilation of the lungs.

Pleural membranes. The pleural membrane is the lining which covers the outside of the lungs and the inside of the thorax. It produces pleural fluid which lubricates the surfaces in the regions of contact between the lungs and thorax. As a result, they can slide freely over one another with very little friction during the breathing movements.

Gaseous exchange in other organisms

(a) Green plants. The leaves and stem of a plant exchange Oxygen and carbon dioxide with the atmosphere by diffusion. Roots obtain their oxygen from the air dissolved in soil water or in air spaces.

(b) Micro-organisms. The surface area of microscopic plants and animals is large in comparison with their volume and the distance from the cell surface to the centre of the protoplasm is very small. Consequently, simple diffusion of gases is rapid enough to meet the respiratory needs of the organism and the diffusion gradients are maintained by the consumption of Oxygen and production of carbon dioxide in the protoplasm.

(c) Insects use their tracheal system for gaseous exchange

(d) Fish. The respiratory surface of a fish is provided by the gills, and ventilation is achieved by passing a current of water over them.

(e) Frog and tadpole. Gaseous exchange takes place through the skin, gills and lungs at various stages of the life cycle and in different situations.


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