One day Boojho was eagerly waiting to meet his grandparents who were coming to the town after a year. He was in a real hurry as he wanted to receive them at the bus-stop. He ran fast and reached the bus-stop in a few minutes. He was breathing rapidly. His grandmother asked him why he was breathing so fast. Boojho told her that he came running all the way. But the question got stuck in his mind. He wondered why running makes a person breathe faster. The answer to Boojho’s question lies in understanding why we breathe. Breathing is a part of respiration. Let us learn about respiration.
WHY DO WE RESPIRE? In Chapter 2 you learnt that all organisms are made of small microscopic units called cells. A cell is the smallest structural and functional unit of an organism. Each cell of an organism performs certain functions such as nutrition, transport, excretion and reproduction. To perform these functions, the cell needs energy. Even when we are eating, sleeping or reading we require energy. But, where does this energy come from? Can you say why your parents insist that you should eat regularly? The food has stored energy, which is released during respiration.
Therefore, all living organisms respire to get energy from food. During breathing, we breathe in air. You know that air contains oxygen. We breathe out air which is rich in carbon dioxide. The air we breathe in is transported to all parts of the body and ultimately to each cell. In the cells, oxygen in the air helps in the breakdown of food. The process of breakdown of food in the cell with the release of energy is called cellular respiration. Cellular respiration takes place in the cells of all organisms. In the cell, the food (glucose) is broken down into carbon dioxide and water using oxygen. When breakdown of glucose occurs with the use of oxygen it is called aerobic respiration.
Food can also be broken down, without using oxygen. This is called anaerobic respiration. Breakdown of food releases energy. Human beings and many other animals undergo aerobic respiration. Carbon dioxide and water are also released in aerobic respiration.
You should know that there are some organisms such as yeast that can survive in the absence of air. They are called anaerobes. They get energy through anaerobic respiration. In the absence of oxygen, glucose breaks down into alcohol and carbon dioxide, as given below: in the absence of oxygen Glucose → alcohol + carbon dioxide + energy.
Our muscle cells can also respire anaerobically, but only for a short time, when there is a temporary deficiency of oxygen. During heavy exercise, fast running (Fig. 10.1), cycling, walking for many hours or heavy weight lifting, the demand for energy is high. But the supply of oxygen to produce the energy is limited. Then anaerobic respiration takes places in the muscle cells to fulfil the demand of energy:
Have you ever wondered why you get muscle cramps after heavy exercise? The cramps occur when muscle cells respire anaerobically. The partial breakdown of glucose produces lactic acid. The accumulation of lactic acid causes muscle cramps. We get relief from cramps after a hot water bath or a massage. Can you guess why it is so? Hot water bath or massage improves circulation of blood. As a result, the supply of oxygen to the muscle cells increases. The increase in the supply of oxygen results in the complete breakdown of lactic acid into carbon dioxide and water.
Breathing means taking in air rich in oxygen and giving out air rich in carbon dioxide with the help of respiratory organs. The taking in of air rich in oxygen into the body is called inhalation and giving out of air rich in carbon dioxide is known as exhalation. It is a continuous process which goes on all the time and throughout the life of an organism.
The number of times a person breathes in a minute is termed as the breathing rate. During breathing inhalation and exhalation take place alternately. A breath means one inhalation plus one exhalation. As a result more oxygen is supplied to our cells. It speeds up the breakdown of food and more energy is released.
A breath can be defined as an inhalation followed by an exhalation. Inhalation is the process of taking the air that contains oxygen inside the body. Exhalation is a process of releasing out air that contains carbon dioxide out of the body. Inhalation and exhalation take place alternatively in the breathing process.
HOW DO WE BREATHE?
Let us now learn about the mechanism of breathing. Normally we take in air through our nostrils. When we inhale air, it passes through our nostrils into the nasal cavity. From the nasal cavity, the air reaches our lungs through the windpipe. Lungs are present in the chest cavity. This cavity is surrounded by ribs on the sides. A large, muscular sheet called diaphragm forms the floor of the chest cavity. Breathing involves the movement of the diaphragm and the rib cage.
We take in the air present in the environment through our nostrils which travels through the nasal cavity. Then it moves through the windpipe and reaches the lungs. The lungs are located in the chest cavity which is surrounded by the ribs. On the floor of the chest cavity lays a muscle sheet called diaphragm. The lungs when releasing out air from the body which brings back the diaphragm and the ribs to their original positions.
During inhalation, ribs move up and outwards and diaphragm moves down. This movement increases space in our chest cavity and air rushes into the lungs. The lungs get filled with air. During exhalation, ribs move down and inwards, while diaphragm moves up to its former position. This reduces the size of the chest cavity and air is pushed out of the lungs. These movements in our body can be felt easily. Take a deep breath. Keep your palm on the abdomen, feel the movement of abdomen. What do you find?
To understand the expansion of the lungs, pull the rubbber sheet from the base downwards and watch the alloons. Next, push the rubber/plastic sheet up and observe the balloons. Did you see any changes in the balloons? What do the balloons in this model represent? What does the rubber sheet represent? Now, you should be able to explain the mechanism of breathing.
Breathing rate can be defined as the number of times a person breathes in a minute is called as breathing rate. An average adult human being breeds 15 to 18 times in a minute. While exercising, this rate can change up to 25 times a minute.
WHAT DO WE BREATHE OUT?
When you inhale (breathe in), air enters your lungs and oxygen from the air moves from your lungs to your blood. At the same time, carbon dioxide, a waste gas, moves from your blood to the lungs and is exhaled (breathe out). This process is called gas exchange and is essential to life.
As we inhale the air present in the surroundings sometimes various unwanted elements such as smoke and dust are also included in it. However, they get stuck in the hair in our nostrils but some of them can get through the nasal cavity.They thus cause irritation in the nasal cavity which makes us sneeze.
BREATHING IN OTHER ORGANISMS
Many animals have just cavities in their bodies just like human beings for example lions, elephants, goats, cows, snakes and birds.
Breathing in cockroach: A cockroach has small openings on the sides of its body. Other insects also have similar openings. These openings are called spiracles. Insects have a network of air tubes called tracheae for gas exchange. Oxygen rich air rushes through spiracles into the tracheal tubes, diffuses into the body tissue, and reaches every cell of the body. Similarly, carbon dioxide from the cells goes into the tracheal tubes and moves out through spiracles. These air tubes or tracheae are found only in insects and not in any other group of animals.
Earthworm: Recall from Chapter 9 of Class VI that earthworms breathe through their skins. The skin of an earthworm feels moist and slimy on touching. Gases can easily pass through them. Though frogs have a pair of lungs like human beings, they can also breathe through their skin, which is moist and slippery.
BREATHING UNDER WATER
Can we breathe and survive in water? There are many organisms which live in water. How do they breathe under water? You have studied in Class VI that gills in fish help them to use oxygen dissolved in water. Gills are projections of the skin. You may wonder how gills help in breathing. Gills are well supplied with blood vessels for exchange of gases.
DO PLANTS ALSO RESPIRE?
Like other living organisms, plants also respire for their survival as you have learnt in Class VI. They also take in oxygen from the air and give out carbon dioxide. In the cells oxygen is used to break down glucose into carbon dioxide and water as in other organisms.
In plants each part can independently take in oxygen from the air and give out carbon dioxide. You have already learnt in Chapter 1 that the leaves of the plants have tiny pores called stomata for exchange of oxygen and carbon dioxide.
Like all other living cells of the plants, the root cells also need oxygen to generate energy. Roots take up air from the air spaces present between the soil particles Can you guess what would happen if a potted plant is overwatered? In this chapter you learnt that respiration is a vital biological process. All living organisms need to respire to get the energy needed for their survival process. All living organisms need to respire to get the energy needed
for their survival.
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