At the end of this post, students should be able to:
1. Describe the different types of respiratory systems.
2. List the characteristics of a respiratory surface.
3. Draw and label some of the respiratory organs of some animals.
4. Explain the various mechanisms of respiration in some animals, e.g. mammals.
5. Describe the mechanism of exchange of gases through the stomata of plants.


Respiration is defined as a process in living organisms ‘ cells involving enzyme catalysed chemical reactions in which food substances or glucose are broken down within cells, to release energy, in the presence or absence of oxygen. In other words, respiration is a biochemical activity of the cell in which glucose is broken down

by a series of reactions controlled by enzymes to release energy. In mammals, oxygen is breathed in and it breaks down the food substances (glucose) to release energy, carbon dioxide and water. Carbon dioxide and water are regarded as waste products of respiration. The energy so released is used by living organisms for various life processes.

Importance of energy to living organisms

1. Energy is required for transportation of materials within the body.
2. It is needed by animals for movement.
3. It is required for the maintenance of body temperature with heat energy.
4. It is required for the synthesis of substance such as protoplasm and other body materials needed for growth and repair of the body.
5. Energy is required for reproduction.
6. It is important in excretion in living organisms.
7. Energy is required in the transmission of nerve impulses and co-ordination.
8. It helps to produce sound e.g. speaking in human.
9. It helps to produce electrical shocks for offence or defence as in catfish.
10. Energy is required by all organisms during reproduction.

Types of respiration

There are two major types of respiration.

These are:
i. Aerobic respiration
ii. Anaerobic respiration

i. Aerobic respiration: aerobic respiration is the type of respiration which requires oxygen to break down glucose (substrates) into water, carbon dioxide and energy (ATP).
ii. Anaerobic respiration: anaerobic respiration is the type of respiration which does not require the presence of oxygen to provide energy. During anaerobic respiration, glucose is broken down to yield carbon dioxide, alcohol (ethanol) and energy.

phases or stages of respiration

Respiration takes place in two phases or stages. These are external and internal (tissue) respiration.
1. External respiration (breathing): external respiration or breathing is defined as the exchange of gases between the environment and the respiratory organs of living organisms. In other words, external respiration simply involves the breathing in of air or oxygen into the respiratory organs such as lungs or gills otherwise called inspiration or inhalation and the breathing out of carbon dioxide and water vapour into the atmosphere otherwise called expiration or exhalation.
2. Internal (tissue) respiration: internal or tissue respiration is defined as the oxidation of organic food substances within the cells leading to the release of energy, carbon dioxide and water. In other words, tissue respiration takes place when the oxygen taken in is used up by the individual cells in the body for the oxidation of food substances. Carbon dioxide, water and energy are given out by these cells in return. Internal, tissue or cellular respiration can be represented by the equation:
C6H12O6 + 6O2(R) 6H2O + Energy (Glucose) (Oxygen) (Water) (Carbon dioxide) ATP
In summary, the main purpose of respiration is to generate energy required by the body for various life processes.


For efficient respiration (gaseous exchange) to take place, the following condition must prevail or be available:
1. Respiratory medium: respiratory medium refers to the surroundings of the organism from which it obtains oxygen. Examples of respiratory media are air and water.
2. Respiratory organ: this is the organ such as lungs in mammals and gills in fish through which oxygen is taken in to the body and carbon dioxide and water vapour are given out. The taking in of one gas, e.g. oxygen and the given out of another, e.g. carbon dioxide and water vapour is called gaseous exchange.
3. Transport medium: this is a system which moves the dissolved gases, e.g. carbon dioxide which needs to be eliminated from the body. Example of transport medium in mammal is the blood.
4. Ventilation: the movement of water or air in one direction over the respiratory surface is called ventilation. This helps to renew the air or water in contact with or near a respiratory surface. Ventilation mechanism as in human breathing, increases the rate of gaseous exchange.
5. Respiratory surface: the respiratory surface refers to the definite surface of the body such as the surface of the lungs or a gill where gaseous exchange takes place.

characteristics of respiratory surfaces

Respiratory surfaces, be it in plant or animal must have the following characteristics:
i. Respiratory surfaces must be moist because gases diffuse in solution through them.
ii. It must be permeable to allow gases to pass in and out of them.
iii. It must be thin walled to shorten diffusion distance and make diffusion easier and faster.
iv. It must have adequate supply of transport medium, e.g. blood.
v. It must have a large surface area to aid easy diffusion of gases.
vi. Respiratory areas must be highly vascularised, i.e. equipped with capillaries or similar network to bring in or take away diffused gases.


Respiratory system includes all the structures and organs of living organisms associated with the process of taking oxygen into the body and giving out carbon dioxide and water vapour into the environment.
Organisms adopt different types of respiratory systems depending on their types, complexity, size and the habitat in which they are found.

Different Organisms and their Respiratory Systems

Organisms Respiratory system/organ
1. Unicellular organisms, Body surface
e.g. Amoeba and paramecium
2. hydra and tapeworm cell membrane
3.earthworm wet skin or body surface
4. fishes e.g. Tilapia gills
5. Arthropod, e.g. insects tracheal system
6. Arachnids, e.g. spiders lung books
7. Tadpoles gills
8. Reptiles, e.g. lizard lungs
9. Amphibians, e.g. mouth, skin and lungs
Toads and frogs
10 Aves, e.g. birds lungs
11 Mammals lungs
12 Flowering plants stomata and lenticels



There are no special respiratory organs in unicellular and aquatic organisms such as Amoeba and Paramecium. Oxygen that dissolves in water diffuses into the body while carbon dioxide goes out of the body through the entire body surface. The cell membrane acts as the respiratory surface. The concentration of oxygen in water is higher than that inside the body, hence it will diffuse into all parts of the body. On the other hand, carbon dioxide inside the body is higher than that of the water, hence it will diffuse out of the body. The process of gaseous exchange in unicellular organisms is made possible by simple diffusion. These organisms have large surface area to volume ratio hence diffusion through the body surface alone is enough to satisfy its gaseous exchange needs.
Insects generally use the tracheal system for gaseous exchange. The insects have tiny openings in their abdomen called spiracles through which oxygen diffuses into the body and carbon dioxide diffuses out. The spiracles lead into certain tubes called tracheae (singular trachea). These tracheae further divide into smaller tubes called tracheoles and these tiny tubes get into the tissues and cells through the body fluid.
Insects such as cockroach, grasshopper etc. Use the tracheal system.
The insects perform breathing movements by compressing its body dorso ventrally (i.e. inwards from both sides along its entire body) and then relaxes it. When it compresses its body, it becomes flattened and air is expelled from the tracheae through the spiracles. When it relaxes its body air enters into the tracheae.
During the process of compression and relaxation of the body, oxygen in the air taken in dissolves in the body fluid at the end of the tracheoles from where it enters the various cells. Carbon dioxide from the cells diffuses into the body fluid then into the tracheae and finally comes out of the insect body through the spiracles.

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