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Class 7 Science: Respiration in Organisms (Exam Notes)

Chapter 10: Respiration in Organisms

The cells of living organisms require a constant supply of oxygen to release energy. The energy released is used to carry out cellular processes.

Human respiratory system

Breathing

Breathing is the process by which an organism takes in oxygen continuously and releases carbon dioxide into the environment.

1. Breathing is a part of respiration and it is also referred to as external respiration.
2. Breathing merely involves exchange of gases between the organism and the environment.
3. Breathing is purely a physical process.
4. Breathing is brought about by coordination between lungs, ribs and the diaphragm. The diaphragm is a thin sheet of skeletal muscle that separates the thoracic cavity from the abdominal cavity.
5. It is a mechanical process that involves two steps: inhalation and exhalation.

(a) Inhalation is the action of taking in air rich in oxygen. During inhalation, the rib cage moves outwards and the diaphragm contracts to move downwards. As the rib cage expands, the space in the chest cavity increases, allowing air rich in oxygen to enter the lungs.(b) Exhalation is the action of giving out air rich in carbon dioxide to the environment. During exhalation, the ribs move inwards and the diaphragm relaxes to return to its normal position. The contraction of the rib cage reduces the size of the chest cavity. Now air rich in carbon dioxide is driven out of the lungs.

Respiration	Breathing
Respiration involves both physical and chemical processes.	Breathing is a purely physical process.
Respiration involves oxidation of food to release energy.	Breathing is the action of taking in oxygen and giving out carbon dioxide.
Respiration is the sum of external and internal respiration.	Breathing is the sum of inhalation and exhalation.
Respiration releases energy.	Breathing does not release any energy.
Respiration takes place in all the cells of the body.	Breathing takes place only in the organs involved in the process.
Respiration involves breathing and oxidation of food inside the cell.	Breathing is a part of respiration.

Breath and breathing rate

1. One complete breath accounts for the sum of one inhalation and one exhalation.
2. Breathing rate is the number of times a person breathes in one minute. The normal breathing rate in an individual during rest is 15–18 breaths per minute. The breathing rate increases drastically during exercise and running.

Respiratory system in human beings

Different organs of the respiratory system include the nostrils, larynx, trachea, bronchi, a pair of lungs, bronchioles and alveoli.

Mechanism of breathing

(a) During inhalation, the nostrils take in the air, which is moistened by the mucus secreted by the inner lining of the nose.

1. The mucous lining present in the respiratory tract keeps the passage moist from the nasal cavity to the lungs.
2. Mucus and the hair in the nose entangle the dirt, pollen and other dust particles and prevent them from entering our nose.

(b) Air passes through the pharynx and enters the trachea, which is made up of rings of cartilage. The trachea is also called the wind pipe.(c) The trachea bifurcates into two stems called bronchi (singular: bronchus).(d) Lungs are situated inside the chest cavity and they rest on a large muscular sheet called the diaphragm.

1. The diaphragm forms the floor of the chest cavity. When you breathe in, your diaphragm and rib cage get into action.
2. The diaphragm is protected by the rib cage.
3. The diaphragm plays an important role in inhalation and exhalation. The movement of the diaphragm is brought about by special musculature.

(e) Each bronchus enters the lung on either side and gives out repeated branches named bronchioles inside the lung.(f) Bronchioles supply individual cells of the lung named alveoli (singular: alveolus). Each lung is made up of 300 million alveoli.

1. A group of alveolar cells is surrounded by blood vessels.
2. Oxygen that enters the lungs is exchanged with carbon dioxide from the tissues at this alveolar region.
3. Carbon dioxide is taken out in the same path through which oxygen entered.

(g) Carbon dioxide is released through the nostrils by the process of exhalation.(h) During exercise, the breathing rate increases so as to obtain more oxygen, which in turn oxidizes more food to release more of the energy required.

Respiration in earthworms

Earthworms exhibit cutaneous respiration, i.e. respiration by skin.

1. Earthworms do not possess special organs for respiration.
2. They always secrete a slimy secretion onto the skin, which keeps it moist.
3. The skin in these animals is moist and slimy, which makes it permeable for oxygen to diffuse in and carbon dioxide to diffuse out of the body.
4. As earthworms cannot breathe in waterlogged conditions, they come out onto the soil during the rainy season to obtain the oxygen required for respiration.

Respiration in insects

An insect body is covered with air holes called spiracles.

1. Air enters through these air holes on the body and finally reaches the trachea.
2. The trachea is a network of fine air tubes extending throughout the body.
3. The trachea helps in circulating oxygen throughout the body.
4. The oxygen in the air diffuses into the tissues and is ultimately absorbed by the cells.
5. The carbon dioxide released by the cells is carried by the trachea and given out through the spiracles.

Respiration in fishes

Fish are aquatic vertebrates which respire through structures called gills. This is called branchial respiration.

1. Gills are present on either side of the head and are supplied by rich blood vessels.
2. Fish obtain oxygen dissolved in water.
3. During respiration, water enters the body through the mouth, passes through the gills and comes out of the operculum.
4. Exchange of gases takes place in the gills of fish, supplied by numerous blood vessels.
5. They accept oxygen into the body and expel carbon dioxide.

Respiration in frogs

A frog is an amphibian having two lives, one in water and one on land.

1. Frogs are the only organisms which exhibit cutaneous, branchial and pulmonary respiration during different stages of their life cycle.
2. Adult frogs can breathe through their skin. The skin is moist and slimy, helping the animal to respire through the skin under water as well as on land. This is termed cutaneous respiration.
3. Adult frogs on land respire through lungs. Lungs are the respiratory organs which help in the exchange of gases. This is termed pulmonary respiration.
4. Tadpoles, the larval stages of the frog, live in water. They respire through their gills as they do not possess well developed lungs. This is termed branchial respiration.