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Breathing and Burning

This concept mainly deals with human respiration and the combustion process.

After reading the concept, students will be able to:

  • Identify the respiratory system organs.
  • Explain nostrils’ meaning and their functions.
  • Analyse the role of air sacs in lungs.
  • Draw a labelled lung structure diagram.
  • Understand the primary function of lungs in human body.
  • Know that during respiration exchange of gases take place in the lungs.


Each concept is explained to class 4 students using descriptions, illustrations, and concept maps. After you go through a concept, assess your learning by solving the two printable worksheets given at the end of the page.

Download the worksheets and check your answers with the worksheet solutions for the concept of Human Respiration and Combustion provided in PDF format.



Breathing is a physical process that involves taking in oxygen gas from the environment (i.e., inhalation) and giving out carbon dioxide gas in the environment (i.e., exhalation). Inside our body, there is the respiratory system that helps in breathing.

Respiratory System:

The respiratory system consists of a group of organs that help us breathe.


Parts of the Respiratory System:

1. Nostrils:

  • The holes of the nose are called nostrils.
  • We have two nostrils.
  • The nostrils are separated by a wall made of soft bone called the septum.

2. Nasal cavity:

  • The inner part of the nose is called the nasal cavity.
  • It is lined with hairy epithelial tissue.

3. Pharynx:

  • The nasal cavity opens into the pharynx, which is at the back of the mouth.
  • The pharynx is a common area for both the digestive and respiratory systems.
  • The pharynx opens into the larynx.

4. Larynx:

  • The larynx is also called the voice box, and it leads to the trachea.
  • A very thin and small flap-like structure called epiglottis guards the entrance of the trachea.
  • Epiglottis covers the trachea when we drink and eat.
  • The larynx moves up or down when we eat, allowing safe passage for air and food.

5. Trachea:

  • The trachea is also called the windpipe.
  • It continues from the larynx to the chest cavity.
  • It is situated in front of the esophagus.
  • The trachea is covered with cartilage from top to bottom, which prevents it from collapsing.
  • The cells present inside the trachea secrete mucous which prevents dust particles to enter the windpipe.

6. Bronchus:

  • The terminal part of the trachea divides into two tubes, and each is called a bronchus.
  • The right bronchus is divided into three bronchi, while the left is divided into two.

7. Bronchioles:

  • The bronchus divides into many narrow tubes called the bronchioles.
  • The narrowest and smallest tubes are called respiratory bronchioles and are about 0.5 millimetres in diameter.
  • The bronchioles are connected to the air sacs of our lungs.

8. Lungs:

  • The lungs are the primary breathing organs.
  • The gas exchange process takes place inside our lungs.
  • These are highly elastic and hollow bag-like structures covered with thin membranes and situated inside the chest cavity.
  • There are two lungs—the right one is larger than the left one.
  • The left lung is divided into two lobes, and the right one has three lobes.
  • Blood is supplied to the lungs by the heart.


Exchange of Gas Inside Lungs:

The exchange of gas takes place inside the air sacs of the lungs, which are supplied with numerous blood capillaries. The air sacs of the lungs are called alveolus in the singular and alveoli in the plural.The amount of oxygen gas is more in the air around us and lower inside our bodies. The amount of carbon dioxide gas is higher inside our bodies and lower in the air around us.
During the gas exchange, oxygen moves from the lungs to the blood. At the same time, carbon dioxide too passes from the blood to the lungs. This entire process occurs in the lungs between the alveoli and capillaries, which are found in the alveolar wall.As a result, oxygen and carbon dioxide can move through the respiratory system and the blood.Oxygen travels back to the heart through the blood and carbon dioxide in the alveoli is released out of the body by exhalation.


Respiratory System Diseases:

1. Asthma:

In this disease, the airways swell due to the presence of extra mucous. It makes breathing difficult and makes a whistling sound during inhalation.

2. Pneumonia:

In this case, there is inflammation of the air sacs of the lungs due to bacterial, viral and fungal infections. The air sacs get blocked by pus.

3. What Is Lung cancer?

It is a type of cancer that develops in the tissues of the lungs, mainly in the cells of the windpipe and the air sacs. Smoking is a significant cause of the occurrence of lung cancer.



Burning is a process where a substance is burned, and it reacts with oxygen to release energy. It is also known as combustion.Any chemical reaction that produces heat and light and involves oxygen is called combustion.Air is necessary for burning as it contains oxygen.

Example: When we burn wood, the carbon in the wood reacts with the oxygen present in the air.


Flammable Substances:

The substances that burn very easily are called flammable or combustible substances.


Dried leaves
Kerosene oil


Ignition Point:

  • Combustion of a substance begins when a substance reaches a particular temperature called its ignition point.
  • After reaching this temperature, the substance quickly reacts with oxygen to produce a flame.
  • Different substances have different ignition points.


New Words:

Blood capillaries: These are thin and small channels that connect arteries and veins.

Inflammation: Our body’s response when any germ enters the body. Here, the germ irritates the air sacs and makes them swollen.

Pus: It is an opaque, thick whitish or yellowish liquid that is mostly produced by the infected cells of the body.


Did You Know?

  • The larynx contains vocal cords, which vibrate and produce sound while we speak.
  • Patients with asthma use inhaler pumps. These pumps have medicines that expand the respiratory passage to help overcome shortness of breath and allow the person to breathe normally.
  • Children and women breathe faster than men.
  • The lungs are the only organs of the human body that can float on water.
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