Human Respiratory System Notes | Study Biology Class 11 - NEET

NEET: Human Respiratory System Notes | Study Biology Class 11 - NEET

The document Human Respiratory System Notes | Study Biology Class 11 - NEET is a part of the NEET Course Biology Class 11.
All you need of NEET at this link: NEET

Respiratory System

Respiration: A process in which oxidation of organic compounds occurs in cell and energy is released is called as respiration.The Respiratory SystemThe Respiratory System

Stages of Respiration

According to scientist G.S. Carter there are three stages in respiration:

(i) External Respiration (Ventilation): Gaseous exchange between environment and lung.
(ii) Internal Respiration:

Gaseous exchange between lungs and blood.
Gaseous exchange between blood and tissue fluid.

(iii) Cellular RespirationOxidation of organic compounds in cell in which energy is released.

Structure of Respiratory System

  • Respiratory Tract (Conducting zone) - A passage from external nostrils to lungs or – A passage by which air enters into lungs
  • Lungs (Exchange zone) - Anatomical organ which is the actual site for exchange of gases. 

Nose & Nasal Passage

  • The respiratory path begins from the Nose and the Nasal passage. In rabbit, at the apex of nose, a pair of external-nares are present. This is termed as Dirhynous condition. Each external nare opens into a nasal passage or Nasal Chamber. Both the Nasal Chamber are separated from each other by hyaline cartilage.

Human Respiratory System Notes | Study Biology Class 11 - NEET

  • The anterior small part of each Nasal Chamber is called Nasal vestibule. It is formed by the invagination of the skin in embryonic stages ; so this part contains hair and sebaceous glands.
  • In this manner dust particles can not enter in. The remaining part of the nasal-passage is surrounded by 3 types of bones, namely the nasal, maxilla & ethamoid.
  • At several places in the nasal-passage these bones form out growths. These outgrowths are called Turbinal bones or Turbinates or chonchae. Due to these outgrowths the nasal-passage is long and spiral. 
  • This helps in making the temperature of air equal to the body temperature before it reaches the lungs and does not allow unwanted materials like dust to reach the lungs. A coat of epithelium is present on the turbinal bones of maxilla and ethmoid. This is pseudostratified columnar ciliated glandular epithelium (PSCCGE).
  • PSCCGE is a simple epithelium i.e., made up of only 1 layer of cells; but it appears to be stratified because it has cells of varying length. This epithelium has Goblet Cells, which secrete mucous. This mucous binds the dust particles and bacteria. The cilia present on the epithelium continuously propel the mucous into the pharyngeal cavity. On the turbinal bone of nasal-bone a hair less cillialess membraneous covering is present, known as Schneiderian membrane. This membrane is Olfactory. Both the nasal-passages, through a pair of internal-nares or choanae open into the naso pharyngeal cavity.
    Functional division of Nasal Passage
    (i) Vestibular Region:- skin, hairs, sebaceous glands.
    (ii) Respiratory Region:- PSCCGE , Goblet cells.
    (iii) Olfactory Region:- Schneiderian membrane or neuro sensory epithelium

Bucco-Pharyngeal Cavity

It is divided into two parts. Anterior part is called the Buccal cavity and the posterior part is called Pharyngeal cavity

  • Between the buccal and the Nasal cavity hard palate followed by soft palate is present. 
  • The terminal part of soft plate is called Uvula. Uvula is bent towards the pharyngeal cavity. This divides the pharyngeal cavity incompletely into two chambers. 
  • Upper chamber is the Nasopharyngeal chamber and the Lower chamber is Oropharyngeal chamber.

At the time of swallowing of food, the Uvula lifts up and covers the internal-nares and so prevents the food from entering the nasal-passage. In the pharyngeal chamber, 2 slit like apertures are present. Dorsal-pore is called the gullet which opens into the oesophagus, so this is the path of food. Ventral-pore is called the glottis; and it opens into the Larynx; and so this is the respiratory-passage.

Near the glottis a flat cartilage called the Epiglottis is present. At the time of swallowing food this cartilage covers the glottis, So the breathing stops. Pharynx is the only part where food and air passage mix together.

Larynx (Voice Producing Organ)

It is present in anterior part of trachea so it is considered as modification of trachea. It is a box like structure composed of two cartilage (on the basis of structure) four cartilage (in shape) & six cartilage (in number). These cartilage of larynx are held together by ligament & membranes. 

1. Thyroid Cartilage

  • Largest cartilage of larynx.
  • Composed of hyaline cartilage.
  • It is 'C' shaped cartilage so, incomplete cartilage.
  • Ventrally it is broad & laterally it is narrow but absent on dorsal surface. So, this cartilage makes complete ventral surface & few part of each lateral surface of larynx.

Thyroid CartilageThyroid Cartilage

  • On anterior part of thyroid epiglottis is attached which is composed of elastic cartilage.
  • In human males, ventral surface of thyroid makes a process called as ADAM'S APPLE which is a secondary sexual character of male.

2. Cricoid Cartilage

  • It is composed of hyaline cartilage.
  • It is signet ring shaped cartilage.

Human Respiratory System Notes | Study Biology Class 11 - NEET

  • Present just below thyroid cartilage.
  • Dorsal surface of this cartilage is broad and ventral surface is narrow.

3. Arytenoid Cartilage

  • Two in number.
  • Paired cartilage of larynx.
  • Composed of hyaline cartilage.

Human Respiratory System Notes | Study Biology Class 11 - NEET

  • Pyramid shape cartilage which is broad in base & pointed on ends.
  • They are present just upon cricoid cartilage in the dorsal surface of larynx.
  • One end of vocal cords is attached with arytenoid & other end is attached with thyroid cartilage.

4. Cartilage of Santorini

  • It is composed of elastic cartilage.
  • Two in number.
  • Present in the form of node like structure at the end of arytenoid so they are called as bands of arytenoid.
  • When epiglottis closes the glottis, it falls on cartilage of santorini so, this protects epiglottis from falling inside the glottis cavity.

In larynx two pair of vocal cords are present:
(i) Anterior pair is called as false vocal cord these are composed of membranes. They are pink in colours & they don't help in phonation. They provide moisture to true vocal cords
(ii) Posterior pair: It is true vocal cords. They are composed of sheet of yellow fibrous connective tissue, yellow in colour. Usually they are in relax position so, when air passes through these vocal cords no sound is produced but by the contraction in laryngeal muscles these vocal cord are stretched so distance between vocal cords decreases & when air passes through these vocal cords due to vibration sound is produced in the form of laryngeal voice which is converted into true speech by the help of lips and tongue due to presence of well developed speech centre in brain.


  • It is a 10-12 cm long tube like structure present  in complete length of neck upto anterior part of thoracic cavity In complete length of trachea 16-20 'C' shaped rings are present which prevent  trachea from collapsing. 
  • These are composed of hyaline cartilage. These rings are incomplete on dorsal surface of trachea. On dorsal surface, trachealis muscles are present which are involuntary in nature & help in the dilation of trachea during forceful breathing.

In the histology of wall of trachea, 4 layers are present (Inside to outwards)
1. Mucosa: 3 sub layer

Epithelium: PSCCGE
Lamina propria: Reticular fibrous connective tissue
Muscularis mucosa: Longitudinal & Circular muscle fibres.

2. Submucosa: Areolar connective tissue
3. Cartilaginous Layer: 'C' shape rings of hyaline cartilage.
4. Tunica Adventitia: White fibrous connective tissue.

Bronchial tree (B. T.) & Respiratory Tree (R. T.) 

  • When trachea enter into thoracic cavity, it divides in two branches called as primary bronchus. Branches of primary bronchus upto terminal bronchioles  makes bronchial tree.
  • Terminal bronchioles divide to form respiratory bronchioles & branches of respiratory bronchioles makes respiratory tree.

Bronchial  TreeBronchial  Tree

  • In bronchiole tree, cartilage rings are present, while these are absent in respiratory tree.

  • Gaseous exchange occurs in respiratory tree while it is absent in bronchial tree.

  • Volume of air which is filled in B.T. is a part of dead space volume. which doesn't take a part in gaseous exchange. (150 ml)


  • A pair of lungs are present in the thoracic-cavity. Lungs are covered by a double-membrane which are called the Pleural-membranes
  • Outer membrane is the Parietal Pleura and inner-membrane is the Visceral pleura. Both these membranes are derived from the mesoderm. 

Human Respiratory System Notes | Study Biology Class 11 - NEET

  • In between both the membranes a very narrow cavity called Pleural-cavity is present. In this cavity, a very thin layer of pleural fluid is present (about 150 ml). 
  • Sometimes due to bacterial infection the amount of this fluid increases. So, the organism feels a difficulty in breathing (dyspnoea). This is termed as pleurisy or pleural effusion disease. 
  • In human being right lung made up of 3 lobes & left lung made up of 2 lobes.The right lung of Rabbit is made up of 4 lobes and the left lung is made up of 2 lobes.

For Rabbit:

Right lung - 4 lobes: Anterior azygos, Right anterior, Right posterior, Posterior azygos

Left lung - 2 lobes: Left anterior, Left posterior

For Human:

Right lung (625 gm) - 3 lobes: Anterior lobe, Middle lobe, Posterior lobe

Left lung (575 gm) - 2 lobes: Left anterior, Left posterior    


  • Each lobe is further divided into several lobules by septa of connective tissue. 
  • Each lobule is further divided into several air-sacs; and in the end, each air-sac is lastly divided into 3 or 4 alveoli; which are also termed as the units of lungs.
  • Structural & functional unit of lungs is called alveoli Approximately 300 million alveoli are present in both lung. Inner (alveolar) surface area of both lungs is approximately 100 m2.
  • Wall of alveoli consist of two layers, outer layer is composed of yellow fibrous 
  • C.T., inner layer is composed of simple squamous epithelium
    (i) Squamous cells are called as pneumocytes.
    (ii) Most of the pneumocytes (Pneumocyte-I) help in gaseous exchange while few pneumocytes (Pneumocyte-II) which are larger in size secrete LECITHIN (Phospholipid) this acts as surfactant which prevents alveoli from remaning collapse by reducing its surface tension.

Alveoli of lungsAlveoli of lungs

  • Alveoli internal surface of is termed as the Respiratory surface. It is derived from the endoderm of the embryo.
  • Rest whole lung is mesodermal. The middle part of alveoli wall is made up of connective tissue. It is richly supplied with blood capillaries, a dense network of blood capillaries is found in alveoli.
  • These blood capillaries come from pulmonary artery. Pulmonary artery divides into blood capillaries after reaching in lungs. These capillaries from a dense network in the walls of alveoli. All these capillaries combine to form pulmonary vein at the another end. These veins carry pure blood to the left auricle of the heart. There are small pores present in the walls of alveoli. These pores make diffusion of gases easy. These pores are called pores of Kohn. It is the characteristic feature of mammalian lungs, that there is no central cavity, Mammalian lungs are solid and spongy.
  • Muscles are absent in the lungs of mammals. So the power of self-contraction and self-expension is absent in these lungs. (Sucken lungs)

Thoracic cage: Coverings of thoracic cavity makes thoracic cage.

Anterior surface: Clavicle bones , Neck
Posterior surface: Diaphragm.
Dorsal surface: Vertebral column & Ribs
Ventral surface: Sternum & ribs.
Lateral surface: Ribs


A muscular septum which is found only in mammals (and crocodile). Normal shape of it is dome like which divides body cavity in two parts upper thoracic cavity & lower abdominal cavity.

DiaphragmDiaphragmIn central region of diaphragm, central tendon is present  it is pierced by 3 structures:
(i) Oesophagus
(ii) Aorta
(iii) Posterior vena cava Radial muscles are present in diaphragm

They originate from periphery & inserted in central region of diaphragm. By the contraction in these muscles, diaphragm become flatten in shape, so, volume of thoracic cavity increases Therefore, diaphragm helps in inspiration.

  • Intercostal muscles (ICM): Space between two ribs is called inter costal space in which 2 types of muscles are present.
  • External ICM (EICM): They originate from dorsal part of upper rib & insert on ventral part of lower rib. By the contraction in this muscles, rib & sternum shifts upward & outward. So they helps in inspiration.
  • IICM: They originate from dorsal part of lower rib & insert in ventral part of upper rib. By the contraction in these muscles, ribs & sternum shifts downward & inward respectively. so  it helps in forceful expiration which is a voluntary activity, So contraction of IICM is under the control of cerebrum.
The document Human Respiratory System Notes | Study Biology Class 11 - NEET is a part of the NEET Course Biology Class 11.
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