Animals need oxygen to break down absorbed food substances in order to release energy the body uses to carry out its activities. One of the products that results from the digestion of starch is glucose, a compound that reacts with oxygen in the cells to release energy, water and carbon dioxide. Water is used by the cells but carbon dioxide is a waste product that must removed from the body.
Small animals
Gas exchange in small organisms such as Amoeba takes place across the entire body surface by diffusion. With their large surface area, relative to the volume, single celled animals can successfully get oxygen and get rid of carbon dioxide by diffusion across the surface. Some small multicellular organisms use the same method because their energy needs are small and all the cells are near the surface. To achieve the latter, the cnidarians ( eg Hydra) are hollow, while the platyhelmiths ( eg tapeworms) are have a flattened body. For the exchange of gases to proceed, the body surface must be most. Therefore these organisms are restricted to aquatic environments.
Large animals
Large organisms cannot exchange gases through the surface alone because with a small surface area and a large volume it would take too long for gases to transverse the long distance from the surface to the inner cells. Therefore, larger organisms are provided with specialised internal structures that increase the surface area for gas exchange. These include tracheae, gills and lungs. An effective gas exchange organ should have the following attributes
(1) Be moist
(ii) Have thin walls that allows easy diffusion of gases
(iii) Have large surface area/volume ratio
(iv) Be protected from mechanical damage eg gills protected by the operculum, lungs by the ribs.
1. Earthworms
To avoid dehydration the earthworm keeps itself under the soil where it is cool and moist. It also covers itself with mucus, which helps it to remain cool. Gas exchange takes place across the entire surface of the body. The earthworm’s body surface is suited for this because it is both thin and richly supplied with capillaries. The earthworms whole body surface is actually a respiratory organ.
2. Insects
Insects have a tracheal system consisting of a network of tracheae that open to the outside through a spiracle. A valve controls the opening of the spiracle. The tracheae are passages that branch into smaller tubes, the tracheoles. The tracheoles penetrate the insect’s body until they are in contact with individual body cells where the exchange of gases occurs.
The movement of oxygen into the trachea is made possible by the body movements of the insect. As the muscles contract, carbon dioxide is forced out of the body and when the muscles relax oxygen enters the body through the spiracles.
3. Fish
Fish has four pairs of gill arches which support gill lamellae. The lamellae form a double row arranged in a V - shape
Gas exchange in fish takes place in the gills. The gills are composed of filaments that are folded into plate-like lamellae and protected against injury by a flap of hard tissue called the operculum. The direction of the flow of the water over the lamellae is opposite that of the blood. This kind of arrangement is called countercurrent flow and its major advantage is that it increases the amount of oxygen that can be taken up by the fish.
Countercurrent Flow
Two factors need to be born in mind under the countercurrent mechanism. One is that the water medium has a higher concentration of oxygen than the blood flowing in the lamella capillaries. The second factor is that the direction of the blood flow in the lamellae is opposite that of the water.
Therefore, since the water has a higher content of oxygen it readily unloads its oxygen to the blood. As it does so, its oxygen content becomes less. However, as the water flows over the gills it continually meets blood that has even lower oxygen than it does. This results in water that always has a higher content of oxygen than the blood it meets.
Countercurrent flow enables the fish blood to continue receiving O2 from the water, which contains a higher concentration of oxygen than the blood along the length of the capillary. In this way, the fish will extract more O2 from the water than would have been the case if the flow of the blood and water were in the same direction.
4. Gas Exchange in Land Vertebrates
Land-dwelling animals cannot use gills because it would be impossible to protect them from desiccation. Therefore, land vertebrates have lungs. Lungs are adapted for gas exchange by having a sponge-like texture and air spaces that enormously increases their surface area. In addition, they are thin walled and provided with a rich capillary network.
Negative Pressure Breathing and Positive Pressure Breathing
Ventilation (aeration) of the lungs is either by negative-pressure breathing or by positive-pressure breathing. Negative-pressure breathing is characteristic of mammals and birds. Amphibians use both types of ventilation. The meanings of negative and positive pressure breathing will become clearer as we go along.
Gas Exchange in Amphibians
Most of the gas exchange in amphibians is through the skin and not through the lungs, as one would imagine. The skin is well adapted for gas exchange. A frog’s skin for example is kept moist by a mucous substance produced by numerous glands present in the skin. In addition, the skin is quite thin, loose and well supplied with blood. Amphibians cannot move far into the dry land because they need water for reproduction and secondly because they must avoid desiccation.
The frog, a positive pressure breather, fills its lungs by forcing air into them; A, the floor of the mouth is lowered, drawing air in through the nostrils. B. with the nostrils closed and the glottis open, the frog forces air into its lungs by raising the floor of the mouth. C, the mouth cavity is ventilated rhythmically for a period. D, the lungs are emptied by contraction of the body wall musculature and by elastic recoil of the lungs.
During breathing an amphibian, e.g., a frog closes its mouth and lowers the floor of the buccal cavity to create a large volume. This lowers air pressure in the buccal cavity and more air is forced into the mouth. This is an example of negative-pressure breathing. Next, the frog closes its nostrils and raises the floor of the buccal cavity forcing the air into the lungs. This is an example of positive pressure breathing.
Gas Exchange in Reptiles
The reptilian skin is composed of scales that are waterproof and airtight. The lungs are therefore the only organs that are concerned with gas exchange.
Gas Exchange in Birds
Avian or bird respiratory system is composed of a lung and eight to nine air sacs that extend into the wings, abdomen and the neck. The windpipe branches into a bronchus which gives rise to smaller parabronchi. Gas exchange takes place across the surface of the parabronchi.
When the bird inhales, air goes to the posterior air sacs. The air sacs store large quantities of air. During exhalation (breathing out) the air moves into the lungs. A second inhalation will force the air out of the lungs and into the anterior air sacs. A second exhalation will make the air in the anterior sacs to escape out of the lungs.
Therefore, a complete circuit of air through the avian lung requires two inhalations and two exhalations.
This unidirectional (one way) movement of air within the avian respiratory system allows the blood capillaries serving the lungs to extract as much O2 as possible from the air. The presence of the air sacs makes the avian lung is so efficient that birds can remain active even at very high altitudes where mammals of the same size are barely able to move
Gas Exchange in Mammals
The mammalian respiratory system consists of the nasal cavity, pharynx, glottis, larynx, trachea, bronchi, bronchioles and air sacs (alveoli).
The respiratory passages are lined by the presence of cilia and mucus-secreting cells, which keep them moist and trap dust, dirt and bacteria. The lungs are encased a rib cage. The rib cage is composed of the breastbone or sternum and ribs. At the bottom of the rib cage is a muscular diaphragm.
Breathing in mammals. During inhalation, the rib cage is pulled outwards and upwards by the contraction of the intercostal muscles, while the diaphragm is lowered or flattened. The volume of the thoracic cavity is increased, the air enters the lungs. During exhalation, the intercostal relax, the rib cage is lowered and pulled inwards while the diaphragm contracts and is raised. This reduces the volume of the thoracic cavity, and the air is forced out of the lungs.
During inhalation, the intercostal muscles of the ribs contract, pulling the rib cage upwards and outwards. This stretches the diaphragm outwards, creating a large empty space in the thoracic cavity. The air is then drawn into the lungs. During inhalation the intercostal muscles relax, the rib cage falls back to its original position and the diaphragm arches upwards. The result is that the volume of the thoracic cavity is reduced and the air is expelled from the lungs.
The amount of air inhaled at each breath is called the tidal volume and is about 500 ml for an adult human being. Only about 350 ml of the tidal volume reaches the lung. The remaining 150-ml of air, known as the dead space volume remains within the respiratory passages.
Under a strong physical exercise, the lungs can hold up to between 4500 ml and 5000 ml of air. This is called the vital capacity of the lungs. It is impossible to empty the lungs completely. Some air always remains in the lungs, which is called the residual volume.
Control of Breathing
Breathing is under the control of a centre in the medulla oblongata. The breathing centre is very sensitive to changes in the concentration of CO2 and the pH of the blood. An increase in CO2 concentrations increases the rate and the depth of breathing. Sometimes excessive vigorous exercise can reduce the level of CO2 concentration in the body so much so that the breathing centre stops sending signals to the lungs. When this happens, breathing stops, causing loss of consciousness or fainting.
Effects of Smoking on Health
Smoking is of great risk to health. Smoking reduces a persons live expectancy. It has been estimated that a twenty – five year old who smokes two packs of cigarettes a day has his life expectancy shortened by 8.3 years. The greater the number of cigarettes one smokes daily the shorter his life expectancy.
Cigarette smoking is the major cause of cancer in both men and women. Nicotine can be absorbed into the blood stream to exert its additive effects. Lung cancer is the most widely known and most harmful effect of smoking. Tobacco smoking is also associated with chronic bronchitis, emphysema, coronary artery disease, peripheral vascular disease and stroke. The harmful components of tobacco include tar, carbon monoxide, nitrogen dioxide and nitric oxide.
Cancer of the lungs, due to smoking, starts first by thickening the mucus producing cells that line the bronchi. This is followed by a loss of cilia so that it is impossible to prevent dust and dirt from settling in the lungs. Later cancer cells appear in the thickened lining. Some cancer cells break loose and penetrate the other lung tissues spreading the cancer. The incidence of cancer of the pharynx, mouth, oesophagus, bladder and pancreas appears to be higher in smokers than in non-smokers.
Cigarette smokers are 4 to 25 times likely to suffer from emphysema than non-smokers. Smoking thickens the lining of the bronchioles obstructing the free movement of air so that breathing becomes difficult and the air is trapped within the alveoli. The trapped air may cause the walls of the alveoli to break and the thickening of the surrounding capillary vessels, thereby reducing the amount of oxygen reaching the brain. This causes the heart to beat faster in order to get enough oxygen to the brain. In the end, this damages the heart.
Smoking is responsible for deaths due to coronary heart disease.
Pregnant mothers who smoke run the risk of having stillborn babies and babies who are underweight. Such children are usually underdeveloped and are socially unadjusted.
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