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| External Respiration |
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| Cellular respiration is common to all organisms - plants or animals. However, the external respiration, that is, the mechanism of gaseous exchange is different for different organisms. |
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| In plants, it is through pores over the body surface. |
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| In animals, the various ways of gaseous exchange are also called the breathing mechanisms. There are various structural modifications for gaseous exchange according to the environment in which they live. The area where the gaseous exchange takes place is called the respiratory surface. The exchange takes place by diffusion and thus, the respiratory surface should be permeable |
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| a) to allow the gases to pass through a thin membrane (1mm or less) b) to allow effective diffusion richly supplied with blood vessels or bodily fluids |
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| c) to allow maximum uptake of oxygen having a large surface area |
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| d) to allow maximum uptake of oxygen in minimum time. |
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| Earthworm has a segmented cylindrical body covered by a thin cuticle below which is epidermis. This skin is always kept very moist by the secretion of mucous from the epidermis and the body fluids from the excretory pores. It always lives in moist soil especially during the day. This prevents their skin from drying or desiccation. |
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| Gaseous Exchange in Earthworm |
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| The epidermal layer has blood capillaries which have looped out from the vascular system circulating the blood. These blood capillaries are so close to the skin that the gases can diffuse from the surroundings into and out of the blood through the skin and the capillary walls. The blood contains haemoglobin in solution which circulates the gases round the body. |
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| Fish has specialised structures called the gills to carry out exchange of gases with water, the medium in which they live. |
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| Gills for Aquatic Male Respiration |
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| The region between the buccal cavity (mouth) and the oesophagus is called the pharynx. In the pharyngeal region, the wall on either side shows slits which open to the exterior. These slits are called the gill slits. The gill slits are separated by a tissue called the gill arch or the branchial arch. There are four pairs of gill arches separating five pairs of gill slits. |
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| Respiratory Structure in Fish - The Gills |
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| From each gill arch arise two rows of filaments which are arranged in a V-shaped manner. The gill arch along with the filaments is called a gill. |
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| Each filament is made up of a plate-like structures called lamellae which have a rich supply of blood capillaries. Thus the barrier between the blood capillaries and the water is only a few cells thick. The lamellae also serve to increase the surface area greatly. |
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| Along the gill arch run the blood vessels which give off branches into the filaments and the lamellae. The whole arrangement on either side is covered over by a movable cover called the operculum. It consists of muscles and thin layers of bone. |
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| Mechanism |
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| Water moves through the gills by the coordinated action of mouth and the operculum. |
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| Taking in water and thereby oxygen is called inspiration and exit of water is called expiration. |
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| Mechanism of Respiration in Fish |
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| Inspiration |
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| When the mouth is open, the space inside buccal cavity increases which reduces the pressure inside. This makes the water enter the buccal cavity. At the same time, the water pressure outside presses the posterior end of the operculum against the body preventing the entry of water into the gills from the other end. The opercular cavity is enlarged by the contraction of the muscles in the operculum. This decreases the pressure in the opercular cavity and draws water from the buccal cavity into the opercular cavity. This water flows over the gill filaments during which the exchange of gases between the capillaries and the water take place. |
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| Counter Current System |
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| The blood in the capillaries flow in the opposite direction to the flow of water. This is called countercurrent system. This ensures maximum intake of oxygen (80%) by the blood. This is because the blood in the capillaries always encounter water that has greater concentration of oxygen. This creates a concentration gradient for a longer time so that oxygen can diffuse easily into the blood vessel from water. |
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| The presence of lamellae also greatly increases the surface area of absorption. The gill filaments at the end overlap with each other. This further slows down the flow of water out of the gills giving more time for the exchange of gases. |
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| Expiration |
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| The mouth and the entrance to the oesophagus close and the floor of the buccal cavity rises. This pushes the water out into the opercular cavity. This water under high pressure force opens the operculum at the posterior end and moves out into the surrounding. |
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Respiratory Structures in Insects |
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| The respiratory system in insects is called the tracheal system. It involves the diffusion of oxygen directly from the atmosphere into the air-filled tubes. Thus, the diffusion is through air and hence, is more efficient than the diffusion through water (300,000 times more) or tissues (1,000,000 times more). |
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| In cockroach, the tracheal system consists of 10 pairs of spiracles, located laterally on the body surface. Of these, 2 pairs are thoracic and 8 pairs are abdominal. The spiracles are guarded by fine hairs to keep the foreign particles out and by valves that function to open or close the spiracles as required. The spiracles open into small spaces called the atria that continue as air tubes called the tracheae. The tracheae are fine tubes that have a wall of single layered epithelial cells. The cells secrete spiral cuticular thickenings around the tube that gives support to the tubes. |
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| The tracheal tubes branch further into finer tracheoles that enter all the tissues and sometimes, even the cells of the insect. The ends of the tracheoles that are in the tissue are filled with fluid and lack the cuticular thickenings. |
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| The main tracheal tubes join together to form three main tracheal trunks- dorsal, ventral and lateral. At some places, the trachea enlarge to form air sacs which are devoid of cuticle and serve to store air. |
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| Mechanism |
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| The first four pairs of spiracles are involved in inspiration or drawing in of air (that is oxygen-rich). This air passes through the trachea and the air sacs to reach the tracheoles. |
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| Mechanism of Respiration in Insects |
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| The ends of the tracheoles are filled with fluid. This end enters into the tissue. The ends of the tracheoles are also devoid of cuticle and therefore the respiratory surface is very thin making the diffusion of oxygen into the cells easy. As respiration occurs in the cell, the products of respiration accumulate in the cell and force the fluid in the tracheoles to enter the tissue. The exit of fluid creates low pressure in the tubes and draws in more oxygen to the tissues where it is needed. |
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| The carbon dioxide produced is detected by the chemoreceptors which make the muscles near the spiracles contract. This pushes the air out. The last six pairs of spiracles are involved in expiration of air. |
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| Thus, in cockroach there is ventilation or circulation of air as the oxygen-rich air is inhaled through the first four spiracles and the carbon dioxide-rich air is exhaled through the remaining six pairs of spiracles. |
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| In insects, therefore, the respiratory system is independent of the circulatory system. |
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