Class 7 Science Respiration in Plants and Animals Notes

The process of breathing is aided by the rhythmic expansion and contraction of the thoracic cavity. It is the intercostal muscles between the ribs and the diaphragm that make this possible. Breathing involves two steps:

1. Inspiration or Inhalation

2. Expiration or Exhalation

Inspiration: Breathing in or the entry of air from the atmosphere to the lungs is called inspiration. During inspiration, intercostal muscles contract and push the ribs u pwar ds a nd ou twar ds, making t he cone - s ha ped diaphragm flat. This causes an increase in the air pressure inside the alveoli. The atmospheric air, which is at higher pressure, rushes in through the respiratory tract to equalize the pressure. Finally, the exchange of air takes place in the alveoli surrounded by fine blood capillary. Oxygen is taken up by haemoglobin of RBCs of blood and carbon dioxide is given up by blood to the alveoli.

Expiration: The process of breathing out or expelling carbon dioxide and water from the lungs through the respiratory tract is called expiration. During expiration intercostal muscles relax, the ribs, move inward and the diaphragm becomes cone – shaped again. The volume of the chest cavity decreases and the air pressure inside the alveoli increases. Thus air is pushed out under increased pressure through the respiratory tract.

Cellular Respiration

In inhalation, there is more oxygen in the atmospheric air in the alveoli than in the blood capillaries. So the oxygen moves from the air to the blood. Here it combines with the haemoglobin (the red coloured pigment of Red blood cells) to form oxyhaemoglobin (Fig, 4).

The oxygenated blood goes from lungs to the blood vessels and returns to the heart. The heart pumps this blood throughout the body through the arteries. The arteries further subdivide many times to form capillaries which supply the cells of the body, the oxygen they need to breakdown food (glucose) and release energy. Carbondioxide and water are produced as waste products during cellular respiration, which need to be eliminated from the body. This carbondioxide is now absorbed by the blood in the blood capillaries which combine to form veins. This carbondioxide is exchanged with oxygen in the alveoli. The carbondioxide is then expelled from the body through exhalation or expiration.

Respiration in Other Animals

We have lungs for respiration. But all animals do not have lungs. Frogs, snakes, crocodiles, birds and mammals possess lungs. They have nostrils (not the nose) to inhale air. However, primitive animals like earthworm cockroach, mosquito, snail and fish are without lungs. Their respiratory organs are much different.

Respiration in Earthworm

Worms breathe through their skin, as they don’t have any lungs or nose. The mouth is used for eating organic and rotting material along with soil. Breathing through their skin allows them to stay underground for long periods. They also don’t have any eyes or ears but rather sense their way along with chemical and light sensitive cells.

The skin’s moisture plays a key role in how oxygen travels into the worm. The worm exposes itself to oxygen by either lying out in the open or burrowing into the soil. Oxygen meets the dampness of the skin and breaks down to be obsorbed through the small tiny blood vessels called capillaries just under the skins surface. The oxygen then travels through these blood streams and is pushed throughout the body with their five larger blood vessels that resemble hearts. Once through t he body, t he ca pilla ries pus h the wast e of carbondioxide back out the same capillaries through the skin and away from the worm.

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