Science Study Notes on Human Respiratory System

By Komal|Updated : June 18th, 2020

In this article, we are discussing the topic of the human respiratory system. Questions on this topic are generally asked in the REETUPTETCTET and other teaching examinations. If you go through the previous year's question papers, you will find that every year a good number of questions are asked on this topic.

Human Respiratory System

  • The respiratory system is an oxidation process involving the burning up of food substances such as carbohydrates, fats and proteins with the tissue to form carbon dioxide, water and the release of energy.
  • The primary organs of the respiratory system are the lungs, which carry out this exchange of gases as we breathe.

The respiratory organs in Human are as follow-

  1. Nosal Cavity
  2. Pharynx
  3. Larynx
  4. Trachea
  5. Lung

Nosal cavity

  • A pair of holes are present above the mouth and in human beings, it is called a nostril.
  • On the wall of the Nosal cavity and Nosal holes, mucous glands are found through which mucus is secreted.
  • Dust particles and microbes are trapped by Nosal hairs and semi-liquid material present in the nose.


  • It is a Muscular membranous channel connecting the nasal cavity to the larynx and the oral cavity to the oesophagus.
  •  It helps in breathing, ingestion of food and speech.


  • The part of the respiratory system which connects the pharynx with the trachea is called Larynx
  • its main function is to produce sound hence also known as the voice box. It also helps in coughing, swallowing and the safety of respiratory tracks.
  • At the entrance point of the larynx, a thin blade-like structure exists which is called Epiglottis, which ensures that the larynx closes during the food intake so that food cannot enter the respiratory system and this process occurs normally and automatically.


  • The larynx is connected to the trachea which is 12 cm long having a tube-like structure.
  • The trachea is hung like an inverted tree in the chest cavity of the human body. Here, both main branches of the trachea are called primary bronchiole.
  • The rightmost bronchiole is divided into three branches and the leftmost bronchiole is divided into two branches.


  • Lungs are sponge-like cylindrical in structure and their colour is red.
  •  There are two lungs in the thoracic cavity i.e. right lung and left lung. The right lung lingers while the left lung is made from two lobs.
  •  Each lung is surrounded by a membrane which is called a pleural membrane, which is filled with some fluid.
  • The right lung is slightly greater than the left lung.

The process of respiration can be divided into four parts:

  1. External respiration
  2. Internal respiration
  3. Transportation of gases
  4. Cellular respiration

1. External respiration: it is a process through which an organism exchanges respiratory gasses from the surrounding. It can be divided into two parts

(a) Breathing

(b) Exchange of gases

(a) Breathing:

In all mammals, a pair of spongy lungs are present and air inhales and exhales at a definite rate which is called breathing.


In this phase, the atmospheric air enters the lungs and consequently, the lungs are expanded because of the increase in the volume of the chest cavity. Thus in this process low pressure is developed inside the lungs and that’s why atmospheric air enters.

Expiration: in this phase, the absorbed atmospheric air of the lungs is pumped out and due to the contraction of the internal intercostal muscles, ribs again come back to their original position and the absorbed air of the lungs is pumped out. This pumping of the air from the lungs is called expiration.

 (b) Exchange of gases:

  • The exchange of gases takes place inside the lungs due to the pressure difference in the process of diffusion.
  • The direction of the diffusion of oxygen and carbon dioxide are opposite to each other. 

2. Internal respiration:

  • Inside the body, gaseous exchange takes place between blood and tissue fluid which is called internal respiration.

3. Transportation of gases:

  • The process in which the oxygen from the lungs goes to the cells and the carbon dioxide goes from cells to the lungs is called the transportation of gases.
  • Transportation of gases is completed with the help of the blood circulatory system.
  • Transportation of oxygen: oxygen is transported by a red pigment substance which is called haemoglobin. It directly reacts with oxygen and forms an unstable compound. And now this compound reaches every cell of the body and decomposes to supply oxygen.
  • Transportation of carbon dioxide – transportation of co2  by the haemoglobin takes place in which co2 is reached from the cells of the body to the lungs. 

4. Cellular respiration: The process of oxidization of glucose is called cellular respiration.

Types of Respiration: Respiration is of two types i.e. anaerobic respiration and aerobic respiration.

a.Anaerobic respiration:

  • When the process of respiration takes place in the absence of oxygen then it is called Anaerobic respiration.
  • The last product of anaerobic respiration is pyruvic acid.
  • During the process energy of ATP of 4 molecules is generated in which the energy of ATP of two molecules is utilised in completing the process and the rest of the energy of 2 molecules is utilised.
  •  The final product of anaerobic respiration in animal tissue like skeletal muscle cells is lactic acid.
  • Lactic acid causes pain in muscles if we do excess exercise.




 b.Aerobic respiration:
When oxidation of food takes place in the presence of oxygen, it is called aerobic respiration. At the end of aerobic respiration, 36 energy of ATP is released and 55 to 60% of energy is utilised and the rest of the energy is dissipated as heat.




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