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Respiratory  system : 

Respiration : 

   This broad process can be divided into  2  processes: 

  1. External respiration :  It is the process of absorption of oxygen and removal of carbon dioxide. Here , no chemical changes occur.

  2.  Internal  respiration :  It is the process where oxygen is used to oxidise food to produce energy in the form of ATP and carbon dioxide. Here , chemical changes occur.

 They  moreover, can be  direct ;  no involvement of special organs just cells involved  or indirect ; involvement of special organs like : skin, buccal cavity and lungs .

  According to need of oxygen  there are 2 types of respiration : 

  1. Aerobic respiration : oxygen is must.

  2. Anaerobic respiration : oxygen is absent or very limited.

 Differences : 

 Anatomy : 

  Respiratory  tract  : 

Sequence : 

Nostril →nasal chamber → internal nares → pharynx →larynx→ trachea → bronchi 

  1. Nasal cavity  :  

 Parts : 

  •  Vestibule:  opening of two nostrils which possess or are lined with  skin from  which vibrissae grow .

 This protects entry  of large dust particles, so they act as a filtering chamber .

  •    Respiratory  part  : 

  Here , there are 3 conchas richly supplied by blood vessels. They keep the air entering the chamber in equilibrium with the body's temperature. This is lined by ciliated columnar epithelium 

  • Olfactory part : 

   This possesses olfactory epithelium  for sense of smell.

  1. Pharynx 

  3 sections : 

  1. Nasopharynx :  lined by  pseudo stratified epithelium    which possess pharyngeal tonsils.

  2. Oropharynx : back portion of the  mouth that contains palatine tonsils for immune defense.

  3.  Laryngopharynx : bottom section of pharynx where the respiratory tract divides into the esophagus and larynx .

  Both oro and laryngo pharynx are lined by stratified squamous epithelium.

  1. Larynx 

 Commonly called voice box .  The voice box in birds lie below the larynx called syrinx.

  Cartilage that makes  up larynx :

  1. Arytenoid : hyaline  as well as elastic; a pair 

  2. Cricoid : hyaline; unpaired 

  3. Thyroid : hyaline  ; unpaired ; largest

  4. Corniculate :  elastic ; paired 

  5. Cuneiform : elastic ; paired

  6. Epiglottis : elastic ; unpaired

  A pair of  Vocal cords here, are made up of  yellow dense connective tissue and in between these two  there is a gap called rima -glottis.

  1. Trachea 

This  lies in the media sterna  , inferior to the larynx  in front of the oesophagus.

It is supported by twenty C shaped  hyaline cartilage  which prevents the collapse of the lumen.

It is internally lined  by ciliated  pseudo stratified epithelium.

 Trachea bifurcates  at the level of T4 cartilage .

  1.  Bronchial intercom

 After bifurcation this intercom   begins .  It starts with primary bronchus  then divides to form secondary bronchus  which  again divides to form tertiary bronchioles. This further divides to form bronchioles, then  to terminal bronchioles   . Later it divides to respiratory bronchioles and lastly to alveolar duct and ends to pouch like alveoli.

 Cells of alveoli : 

  There are 3 type of cells  called pneumocyte here :

  1. Type 1 pneumocyte : more in  number , this helps in gaseous epithelium  and are of simple squamous epithelial cells.

  2. Type 2 pneumocyte :   less in number , helps in secreting respiratory surfactant  which is chemically phospholipid  to prevent collapse of alveoli sac. COVID 19 virus  affects this cell to cause asphyxia (oxygen  starvation) as surfactants are deficient causing collapse of alveoli and filling with mucous . They can supplement type 1 cells on it’s deficiency. 

  3. Dust cells :  they are phagocytic cells in lungs .

 Respiratory  organ : 

The main respiratory organ in  humans is LUNG. 

 This is triangular, pink colored inside a pleural cavity .

  It is divided into right and left portions .

They can be studied under following difference table :


Right lung Left lung 
1. It is 3 lobed  with 2 fissures ; oblique and horizontal1. It is 2 lobed and 1 fissure.
2. Wider and shorter .2. Narrower and longer 
3. Three secondary bronchi 3. Two secondary bronchi
4. More vertically placed bronchi so, more chance to infection4.  Less vertically placed bronchi so, less  chance of protozoan infection.
5. No cardiac notch 5. Cardiac notch is  present.


 Pleural cavity  possess pleural fluid which is for ;

  1. Reduce friction while breathing.

  2.  Acts as shock absorbers.

Functions of lungs : 

  1.  It throws out harmful carbon dioxide out of the body.

  2. It also supplies oxygen to the body .

  3.  This also maintains homeostasis  by releasing water vapours and cools the body.