Class 12 Respiratory System notes NEET AIIMS

Respiratory System

Respiration is a Physico-chemical process involving the exchange of gases, oxidation of food, and liberation of energy.

Purpose of respiration: Energy production.

Purpose of respiration: Energy production.

Types of Respiration

Gases may be exchanged between the body and environment directly or there may be specialised respiratory organs/organ system for respiration.

On this basis the respiration may be:

1.   Direct Respiration: This occurs in unicellular worms, roundworms etc). It is the direct exchange of gases between body cells and water.

2.   Indirect Respiration: No direct exchange of gases between body cells and the surrounding medium. Organisms during indirect respiration have specialized respiratory organs for respiration.       

eg.    ¨ Annelids & amphibians– Skin.   

      ¨ Crustaceans, molluscans, fishes & some amphibians– Gills.        

      ¨ Snail, some amphibians, all reptiles, birds and mammals– lungs.

On the basis of use of O2 respiration may be:

(1)       Anaerobic Respiration

ü    Oxygen is not required. A process occurs in the cytoplasm.

ü    Due to incomplete oxidation less energy is released.

ü    End products are lactic acid and alcohol.

ü    It occurs in RBCs, white muscle fibres, and internal parasites (but parasites in the blood perform aerobic respiration due to dissolved O2 in the blood).

(2) Aerobic Respiration

ü    Oxygen is necessary and the process occurs in mitochondria.

ü    Due to complete oxidation more energy is released.

ü    End products are CO2 and H2O.

ü    It occurs in higher plants and higher animals.

       There are 3 stages of aerobic respiration

1.   External Respiration

2.   Internal Respiration

3.   Cellular Respiration

1.   External Respiration: It is a gaseous exchange between the environment and the lungs.

2.   Internal Respiration: It is the gaseous exchange between the lungs and blood and also between blood and tissue cells.

3.   Cellular Respiration: It is the oxidation of organic compounds in cells and energy is released.

Respiration in animals:


1.   Amphibians during hibernation respire only through skin.

2.   The earthworm has dissolved haemoglobin ( Erythrocruorin to transport gases).

 Respiratory System– It is divided into 2 parts:

(1) Respiratory tract   and (2) Lungs

(1) Respiratory Tract: Passage of air from external nostrils to the lungs. 

1.   External nares: 2 openings in the lower part of the nose (called dirhynous condition). They open in the vestibule.

2.   Vestibule: Has hairs, sweat & oil glands. 2 vestibules are separated by the nasal septum.

Note: Nasal septum in the vestibule is of Hyaline cartilage while posteriorly it is of mesethmoid bone.

3.   Nasal chambers: Composed of bones which have bony-projections called turbinals/turbinates. Due to these turbinates, the nasal passage is long and zig-zag, so it helps in air-conditioning of inspired air. The turbinals are Nasoturbinals (on the nasal bone),  Maxilloturbinals (on maxilla) and Ethmoturbinals (on the ethmoid bone).

       Schniderian membrane— Epithelial lining of nasoturbinals (also called the olfactory epithelium). It is composed of neurosensory epithelium. Bowman’s glands secrete mucus to keep it moist. It receives smell sensations.

4. Nasopharynx: Nasal chambers open by internal nares in the nasopharynx. It is a part of the pharynx. Soft palate separates the nasopharynx from the oropharynx.

Note: Pharynx is the common chamber for both food and air. So, we can breathe with the mouth also.    


5.   Larynx: A hollow box-like structure on the trachea. It has 2 functions— to close the lower respiratory tract and sound production. It is composed of:

(i)   3 large unpaired cartilages – Thyroid, Cricoid & Epiglottis

(ii)   3 smaller paired cartilages –Arytenoid, Corniculate & Cuneiform

a. Thyroid:

ü    Largest cartilage of the larynx. It is composed of Hyaline cartilage.

ü    It is C-shaped (incomplete dorsally).

ü    It protrudes ventrally to form a prominence in males called ‘Adam’s apple’. It is a secondary sexual character.

b. Cricoid cartilage:

ü    It is signet-ring shaped i.e., complete cartilage of hyaline. It is broad dorsally and narrows ventrally.

c. Arytenoid cartilage:

ü    It is also composed of hyaline. It is pyramid-shaped and the base of arytenoid is broad and the apical part is pointed.

ü    These are present just above cricoid cartilage on the dorsal surface of larynx.

d.  Corniculate cartilages:

ü    2 small conical cartilages whose bases articulate with the apices of arytenoid cartilages.

e. Cuneiform cartilages:

ü    2 small club-shaped cartilages lie anterior to the corniculate cartilages.

f.  Epiglottis:

üA leaf-shaped elastic cartilage. ü    It is attached by its stem to the posterior aspect of the thyroid cartilage at the angle.