Thursday, 2 February 2017

Human Respiratory System notes for competitive exams | Biology notes

Human Respiratory System

Respiratory System

  • The respiratory system is involved in the intake and exchange of oxygen and carbon dioxide between an organism and the environment.
  • The primary organs of the respiratory system are lungs, which carry out this exchange of gases as we breathe.
  • All those organs comes under respiratory system which help in exchange of  gases are – Nasal passage, Pharynx, Larynx or Voice box, Trachea, Bronchi, Bronchioles, Lungs etc.

Nasal passage :

  • Nasal passage is a channel for air flow through the nose and its inner wall is lined with mucous membrane.
  • Mucous contains innumerable tiny hair like cell that  prevents the particles of sand, bacteria or other small organisms from entering into the body.
  • Mucous makes the air wet entering into the body and equalises it with the temperature of the body.


  • Muscular membranous channel connecting the nasal cavity to the larynx and the oral cavity to the esophagus; it enables breathing, ingestion of food and speech.

Larynx or Voice box :

  • The part of the respiratory system which connects the pharynx with trachea is called Larynx or voice box and it main function is to produce sound.
  • At the larynx entrance gate there is a thin blade-like door, which is called epiglottis which ensure that the larynx closes during the food intake so that food cannot enter the respiratory system .


  • It enters into the thoracic cavity and divided into two bronchi i.e. right and left.
  • Right bronchi enter into the right lungs after being divided into three branches.
  •  Left bronchi enter into the left lungs after being divided into only two branches.


  • The structure of lung is like sponge and its colour is  red.
  •  There are two lungs in the thoracic cavity i.e. right lung and left lung.
  •  Each lung is surrounded by a membrane which is called pleural membrane.
  • Size of right lung is greater in comparison to left lung.
The process of respiration can be divided into four parts :
  1. External respiration
  2. Transportation of gases
  3. Internal respiration
  4. Cellular respiration

1. External respiration: 

 It can be divided into two parts
(a) Breathing
(b) Exchange of gases

Mechanism of Breathing :

The process of taking air into and expelling it from the lungs is called breathing.
Inspiration :
  • At this stage, air from the environment enters into the lungs through the nasal passage, due to increases in the dimension of thoracic cavity a low pressure is formed in the lungs and air enters into the lungs from environment. This air continues to enter until the pressure of air inside and outside the body became equal.
Expiration : In this process air comes out of the lungs.

 Exchange of gases :

  • The exchange of gases takes place inside the lungs. This gaseous exchange takes place on the basis of concentration gradient through normal diffusion.
  • The exchange of oxygen and carbon dioxide gases takes place due to their difference in partial pressures..

2.Transportation of gases :

  • The process of transportation of oxygen from lungs to the cells and transportation of carbon dioxide from cells to lungs is called transportation of gases.
  • Transportation of gases takes place through blood.
(i) By mixing with plasma : Carbon dioxide forms carbonic acid after mixing in plasma. Transportation of 7% carbon dioxide takes place in this form.
(ii) In the form of bicarbonates : 70% part of carbon dioxide in the form of bicarbonates is transported. It mixes with potassium and sodium of blood and forms potassium bicarbonate and sodium bicarbonate.

3.Internal respiration :

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

4.Cellular respiration

The process of oxidization of glucose is called cellular respiration.
Types of Respiration: Respiration is o two types i.e. anaerobic respiration and aerobic respiration.

1.Anaerobic respiration:

  • When oxidation of food occur in the absence of oxygen ,it is called anaerobic respiration.
  • During this only 2 ATP molecules are produced from one molecule of glucose.
  •  Final product of anaerobic respiration in animal tissue like skeletal muscle cell is lactic acid.
  • Lactic acid causes the pain in muscles  if we do excess exercise


  • 2.Aerobic respiration:

    When oxidation of food takes place in the presence of oxygen , it is called aerobic respiration.