Human respiratory system| parts and mechanism

Human Respiratory system

☆What is Human respiratory system ?

Human respiratory system is crucial to every human being . Without it ,we would cease to live outside of the womb. During  inhalation or exhalation air is pulled towards or away from the lungs , by several cavities , tubes and opening . The organs of respiratory system make sure that oxygen enters our bodies and carbon dioxide leaves our bodies.

In human respiratory system may be divided into two major components:

 1- Respiratory tract

 2- Respiratory organs

   1- Respiratory tract

    Respiratory tract serves as  a passageway for the fresh air to flow from outside to the lungs and for the foul air to return from the lungs to the exterior. Gas  exchange does not occur here.

    ” A passage  which allows movement of exhale and inhale air in and out of the lungs is called as   Respiratory tract. 

          In respiratory tract in human respiratory system is divided into two parts 
a. Upper Respiratory tract -:The upper respiratory tract start with nose(nostrils) from the pharynx.
b. Lower Respiratory tract -: The lower respiratory tract starts with the larynx from the trachea, the two bronchi that branch from the trachea , and lungs themselves.

a. Upper Respiratory tract

 External Nares(Nostrils) -:

The two nostrils are openings of the nasal cavity and lie above the mouth. They are separated by the septum.


Nasal chamber-:

The nasal chambers or nasal passage are a pair of passages in  the head above the palate.The two chambers are separated from each other by a median partitions, the nasal septum.
Each chambers shows three regions -:

  a- Vestibular Region -:

It lies just within the external naris.It is very short and lined with skin. It bears hair and contains some sweat and oil glands.The hairs act as filters and prevent the entry of dust particles.

b- Respiratory Region-: 

It is middle region of nasal chamber.It lined with “Respiratory Epithelium(It is highly vascular and appears pink and reddish and act as an air conditioning and filtering unit in breathing).

  • This is ciliated, pseudostratified columnar epithelium rich in gland cells.
  • The latter include mucous and serous cells.
  • The mucous cells secert a viscid fluid called mucus. 
  • The serous cells produce a watery fluid.

c-Olfactory Region-:

It is the upper region of nasal chamber.It is lined with “Olfactory epithelium”(This epithelium is confined to the upper part of nasal chamber and the superior nasal concha.It looks yellowish- brown.)

The olfactory region acts as an organ of smell.It detect the odour of the inspired air.If the odour is offensive or pungent,the air is not allowed to pass in.

Nasal Conchae-: 

Arising from the wall of each nasal chamber are three shallow bony ridges called nasal conchae.It covered with mucous membrane. They greatly increase the surface of nasal chambers .

Paranasal sinuses-:

These are cavities in the frontal , maxillary,ethmoid and sphenoid bones.  Globet cells in their lining epithelium produce mucus that drains into the nasal chambers.

Paranasal sinuses diagram
Paranasal sinuses


  Internal nares (Choanae )  

The nasal chambers open behind into the nasopharynx by internal nares.


The pharynx (Plural:- Pharynges  )is the part of throat that is behind the mouth and nasal cavity and above the oesophagus and the larynx.

  • The pharynx is a short, vertical, about 12 cm long tube.
  • The food and air passages cross here.
  • It is divided into three parts-


The upper part is  called nasopharynx. The internal nares open into it.


The middle part is called oropharynx. The latter receives the buccal cavity.


The lower part is called laryngopharynx. It leads into two tubes: the one at the front is the windpipe or trachea and the one at the back is the foodpipe or oesophagus.

Pharynx , Pharynx parts
Diagram of parts of Pharynx

b.  Lower Respiratory tract

Larynx(Voice box)

  • “The larynx is often called the “Adam apple”.
  • The  larynx is the upper part of the trachea. It is a short, tubular chamber supported by a cartilaginous framework. It opens into the laryngopharynx by a slit like aperture,the glottis (always remains open except during swallowing ).
  • The glottis bears a leaf -life cartilaginous flap,the epiglottis,at its anterior margin.
  • The epiglottis projects into the pharynx opposite the uvul
  •  During swallowing, the larynx moves upward to meet the epiglottis. This close the glottis to check the entry of food into it.
  • Larynx produces sounds by vibrations of vocal cords when air is passed between them under pressure.


☆Difference between pharynx and Larynx

Trachea(Wind pipe)

  • The trachea is a thin- walled tube, about 11cm long and 2.5 cm wide. It is also called windpipe
  • Its located on downward through the neck.


Bronchi and Bronchioles

  • In the middle of the thorax,trachea divides into two tubes, the major ,or primary bronchi. One major bronchus enters the right lungs and the other , the left lung.
  • The right primary bronchus further divides into three lobar or secondary bronchii and the left primary bronchus divides into two lobar or secondary bronchii.
  • The lobar or secondary bronchi subdivide into smaller tertiary bronchi which divide into still smaller Bronchioles.
  • The smaller terminal Bronchioles give off respiratory bronchioles which divide into alveolar ducts[ It have thin, nonciliated cuboidal epithelium].The latter enter the alveolar sacs.
  • The respiratory passageway within the lungs are referred to as the bronchial tree.
  • Wall of trachea,bronchi and bronchioles ( It have smooth muscle controlled by autonomic nervous system ) is composed of fibromuscular tissue and is lined by pseudostratified ciliated columnar epithelium rich in mucus- secreting cells.
  • Cartilaginous rings, incomplete behind, support the walls of the trachea and the bronchi to prevent their collapsing. The rings gradually become thinner, and finally disappear over the bronchioles.

 ☆ Difference between Tracheoles and Bronchioles

Difference between Tracheoles and Bronchioles
Tracheoles and Branchioles

Respiratory system in human


2-Respiratory Organs

The respiratory organs in human respiratory system provide the surface for the exchange of gases. In man, the respiratory organs are a pair of lungs.


  The lungs lie in the thoracic cavity (It is air-tight chamber enclosed by the thoracic vertebrae)  on the side of the heart.Thoracic cavity is closed below by the diaphragm.

 Protective coat:-

Each lungs is enclosed in two membranes,the pleura , which actually are the layers of peritoneum of the thorax. The inner membrane is called the visceral pleuron.Its firmly  bound to the surface of the lungs.
      The outer membrane is called the parietal pleuron.Its held to the thoracic wall and diaphragm by connective tissue.

A very narrow space exist between the two pleura.It is called the pleural cavity ,and  contains a film of watery fluid,the pleura fluid, secreted by pleura.

The pleural cavity is air tight and pressure in it remains 3-4 mm Hg lower than that in the lungs.

The adhesive force of thin layer of pleural fluid.


The pleura of two sides meet to form a sort of septum named mediastinum.

External features

  • The lungs are soft , spongy, elastic organs with smooth shining surface.
  • Theyar pinkish at birth but gradually turn grey and finally black with age due to environmental pollutants, particularly cigarette smoke.
  • Theyare conical shape.
  • Thelungs are divided externally into lobes by grooves called fissures.
  • The left lung has two lobes :left superior and left inferior demarcated by an oblique fissures and right lung has three lobes: right superior, middle and inferior  demarcated by transvarse and oblique fissures is show in the diagram –

External features of Lungs , lungs diagram
External features of Lungs

 *Each lung weighs about 450 grams.

=In the lung,each alveolar duct opens into a blind chamber,the alveolar sac or infundibulum.

= The latter consists of a central passage giving off several small pouches,the alveoli or air sacs on the all sides.(The  alveoli have very thin [0.0001mm thick] wall composed of simple moist, nonciliated, squamous epithelium).

Respiratory membranes

The alveolar wall seems to be  a sheet of flowing blood and  is called Respiratory membrane. It consists of  five components namely- alveolar epithelium, epithelial basement membrane, a thin interstitial space, capillary basement  membrane and capillary endothelial membrane.
The alveolar wall is very thin (0.0001 mm thick) .It closely surround by a network  of pulmonary capillaries which arises from the pulmonary artery and rejoin to the pulmonary vein. The blood in the capillary is separated from the inspired air in the alveoli by only two layers of cells i.e. capillary endothelium and alveolar epithelium.
Structure of Respiratory membrane, Respiratory membrane
Respiratory membrane

Mechanism of human respiratory system

Mechanism of Respiratory system in human in lungs have little musculature and cannot expand or contract of their own.  Breathing is brought about by alternate expansion and contraction of the thoracic cavity wherein the lungs lie .  This leads to intake of fresh air called inspiration (inhalation, breathing in) and elimination of foul air called expiration (exhalation, breathing out) respectively.  Inspiration and expiration are together referred to us respiratory movements.
                 Lungs  can be expanded or contracted in two ways:
  1.  By downward and upward movement of the diaphragm to expand or contract the chest cavity.
  2. By raising or lowering of the ribs to increase or decrease the diameter of the chest cavity.
  3.    In large and heavy animals, movement of diaphragm plays a greater role than the movement plasma of ribs e.g.  elephant.


               Inspiration is an active process.  It is brought about by diaphragm muscles and external intercostal muscles.  These muscles are, therefore, called inspiratory muscles.  The abdominal muscles play a passive role in this process.  
  • Diaphragm
The diaphragm is convex upward and has a peripheral muscle attached to the ribs and vertebral column.  This muscle contracts and lowers the diaphragm, making it flat.  This pushes the abdominal viscera downward and enlarges the thoracic cavity vertically.  
  • External Intercostal Muscles

These muscles slant forward and downward between the ribs .  Their contraction pulls the ribs and sternum upward and outward.  This enlarges the thoracic cavity from front – to – back as well as from side-to-side.

  •  Abdominal muscles

  These muscles relax and allow compression of abdominal organs by the diaphragm .

  • Movement of Fresh Air into the Lungs

  As the lungs are held tightly against thoracic wall, enlargement of the thoracic cavity results in expansion of the lungs.  This reduces the pressure of air in the lungs below atmospheric pressure by a few (-2 to -6) mm Hg.  

         In other words, lungs come to have a negative pressure.  Air always moves from a place of higher pressure to a place of lower pressure.  Hence, the fresh air from outside, where pressure is higher, rushes into the lungs through the respiratory passage till the pressure of air in the lungs becomes equal to the atmospheric pressure.  On reaching the lungs, the fresh air is distributed by the bronchi, bronchioles and alveolar ducts to the alveoli.  The fresh air follows the following route –

External Nares———->  Nasal Chambers ———-> Internal Nares———-> Pharynx ———–>Glottis————> Larynx————> Trachea————>Bronchi ————-> Bronchioles————> alveolar ducts ————->Alveoli.

  With the expansion of the thoracic cavity and the lungs, additional blood is drawn into the blood vessels of the lungs and the thorax.

Conditioning and Filtration of Air

The air passing through the nasal chambers is subjected to four important process –
  • Warming
  • Moistening
  • Sterilization
  • Cleaning


Expiration is normally a passive as it simply involves relaxation of the inspiratory muscles,viz., Peripheral muscle of diaphragm and external intercostal muscles.
  • Diaphragm

The peripheral muscle of the diaphragm relaxes.  With this, the abdominal viscera, compressed during inspiration, push the diaphragm upward, making it convex.

  • External Intercostal muscles

These muscles also relax.  This brings the ribs and the sternum to their original position.  This is aided by the elastic recoil (contraction) of the lungs and thoracic wall which are stretched during inspiration. With the above  two events, the thoracic cavity becomes smaller.

  • Abdominal and Internal Intercostal Muscles

In forced breathing,as during exercise, abdominal muscles and internal intercostal muscles come  into action, making expiration also an active, energy – consuming process. These muscles contract and decrease the thoracic cavity further. The contraction of abdominal muscles presses the abdominal viscera against the diaphragm, bulging it further upward. This shortens the thoracic cavity  vertically. The abdominal muscles are practically inactive during normal breathing, but become active during coughing and sneezing. Contraction of internal intercostal muscles moves the ribs downward and inward. This reduces the thoracic cavity from front – to – back and also from side – to  – side. The abdominal and internal intercostal muscles are called expiratory muscles.

  • Movement of Foul Air out the Lungs

 All round decrease in the thoracic cavity, reduces the lungs and raises the pressure of air in the lungs above atmospheric pressure by +3 to +4 mm Hg.  This positive pressure in the lungs pushes out the foul air from the lungs until the air pressure in the lungs falls to that of the atmosphere.  On its return journey, the air again passes through the respiratory passage.  Much air stays in the alveoli after expiration.  This air keeps the walls of the alveoli from sticking together and collapsing. 

 The foul air follows the following routes –

Alveoli ———> Alveolar ducts ————-> Bronchioles ———–> Bronchi ———–> Trachea ————-> Larynx ————-> Glottis ———-> Pharynx ———-> Internal nares ———> Nasal chamber ————> External Nares ————>  Atmospheres .

Respiratory Rate

Each respiration consists of one inspiration and one expiration. At rest, breathing occurs about 14 – 18 times per minute in a normal man.

Related Topics –

Respiratory system human – part 1

Respiratory system diseases

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