The Respiratory System

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The air inside the lungs is never replaced completely. It is always a mixture of the air left inside and the air inspired. In other words, the air in the lungs is oNy becoming better and worse with each inspiration and expiration.

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Qualitywise, the expired air differs from inspired air in the following respects :

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The air inside the lungs is never replaced completely. It is always a mixture of the air left inside and the air inspired. In other words, the air in the lungs is oNy becoming better and worse with each inspiration and expiration.

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Qualitywise, the expired air differs from inspired air in the following respects :

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2.

It contains more carbon dioxide.

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3.

It contains more water vapour.

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4.

It is warmer (or at the same temperature as

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that of the body).

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It may contain some bacteria.

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What is Respiration?

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Respiration is a vital process in all living organisms. It goes on non-stop throughout life.

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This chapter explains the various aspects related to respiration — the raw material used, the end products formed and the amount of energy liberated, etc.

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Some experiments to demonstrate the mechanism of breathing are very interesting.

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Need for Respiration:

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There are five important points to remember about this chemical reaction in respiration.

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This part of respiration, yielding energy, occurs inside the living cells and hence, it is better known as cellular or tissue respiration.

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The breakdown of glucose to carbon dioxide and water does not occur in a single step but in a series of chemical steps.

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Some of these steps occur in the cytoplasm of the cell and some inside the mitochondria.

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Each breakdown step is due to a particular enzyme.

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The energy liberated in the breakdown of the glucose molecule is not all in the form of heat, but a large part of it is converted into chemical energy in the form of ATP - a chemical substance called adenosine triphosphate.

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The essential steps of cellular respiration are the same in plants and animals.

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Animals need more Energy:

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The need for the production of energy is greater in animals than in plants.

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This is because animals consume more energy in doing physical work.

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They have to move about for obtaining food or run away to escape enemies.

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They have to chew their food and have to take after their eggs or young ones, and so on.

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Birds and mammals including ourselves have also to produce a lot of heat for keeping their body warm. This heat comes through respiration in the cells.

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The amount of heat to keep the body warm is quite large.

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Think about the cold winter days when the outside temperature is far below our body temperature.

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We are constantly losing heat to the outside air, and more of it has to be continuously produced to make up for the loss.

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Liver cells in particular produce much heat, and the muscle cells also contribute to it.

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The energy used in all cellular activities is obtained from the oxidation of glucose (C6H 20 6), a carbohydrate.

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Glucose:

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If the simple carbohydrate (glucose) is not available directly, the cells may break down the proteins or fats to produce glucose for respiratory needs.

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Think for a while about the wild animals which are totally flesh-eaters.

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The main constituent of their diet is a protein with very few carbohydrates.

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The excess amino acids absorbed through protein- digestion are broken down in the liver to produce sugar (glucose) and the nitrogenous part is converted into urea which gets excreted out.

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The glucose thus produced may be used immediately or may get stored in the liver cells as glycogen for future needs.

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A similar process takes place in humans if they take excessively protein-rich food.

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Aerobic and Anaerobic Respiration:

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In animals, there is normally aerobic respiration using oxygen.

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Anaerobic respiration (in the absence of oxygen) is only exceptional in some cases as in the tapeworms living inside the human intestines.

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Anaerobic respiration may occur even in our own body in the fast-working skeletal muscles temporarily.

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During continuous physical exercise as fast running, walking over long distances, swimming, wrestling, weight-lifting, etc., our muscles work too fast but not getting enough oxygen.

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In this situation, the muscles are working in the absence of oxygen (anaerobic respiration) to provide extra energy.

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The product of anaerobic respiration in such muscles is lactic acid.

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Accumulation of lactic acid gives the feeling of fatigue.

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This is a condition that may be called oxygen debt.

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When you rest after such exercise, the lactic acid gets slowly oxidized by the oxygen later available and then the “debt is cleared” producing carbon dioxide in the process.

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Aerobic respiration in animals:

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The chemical changes taking place in aerobic respiration in animals are the same as in aerobic respiration in plants.

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Thus by taking 180 g of glucose the energy released is 686 kilocalories, or if expressed in kJ (kilojoules) the energy released is about 2890 (686 x 4 2) kJ.

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Anaerobic respiration in animals:

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In animal cells, particularly in the skeletal muscle cells, anaerobic respiration may occur when they have to work very fast with insufficient oxygen.

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It is a slow process.