Human respiratory system

Human respiratory system:

Definition: A system of organs functioning in respiration is called respiratory system.

Explanation: Human respiratory system consists of lungs and air passage to carry air to and from lungs.

  1. A) Air passage: The respiratory tract /tube in which gases are passed toward lungs for exchange is called air passage.

Parts: The air passage way consists of the following parts.

  • Nose: The nose is only external part of respiratory system which links the respiratory system with environment for exchange of respiratory gases.

Structure: Nose is external visible part of the respiratory system which is slightly different.

in each individual depending upon the ethmoid bone and the cartilages.

But each nose structure consists of bones, cartilage and fatty tissue.

Nostrils: It is the openings of nose in which enters to respiratory tract.

Nasal cavities: Each nostril opens to cavities called nasal cavity, which then open into pharynx. →Each nasal cavity is lined by ciliated epithelial cells which secrete which secret mucous and also bear hair.

             Functions: a) Through nose respiratory gases is enter to respiratory tube.

  1. b) The hair of nose acts as a filter to trap large dust particles.
  2. Nose hair also serve as a defense mechanisms against the harmful pathogens and solid particles present in the air.
  3. The mucous and cilia filter the air and prevent the entry of foreign particles such as microorganisms, dust, etc.
  4. The mucous helps in moistening the air.
  5. Underneath the mucous membrane, there are blood capillaries that help to warm the air.

2) Pharynx: The nasal cavities open into cone shaped muscular passage called pharynx.

→Pharynx also lined mucous membrane.

Functions: a) The mucous of pharynx moisturized the air, warm the air, and trap dusts and solid particles.

  1. b) It regulate the air and food to trachea and esophagus.

3) larynx:

Intro: It is a cartilaginous structure present on the upper end of the trachea. It is also lined with mucous membrane.

Cartilaginous: The larynx is composed of external skeleton of cartilage plates that prevents collapse of the structure.

Glottis and epiglottis: The opening of larynx  is known as glottis which is guarded by lid called epiglottis.

The epiglottis prevents entry of food or liquid into larynx.

Vocal cord: The vocal cords, also known as vocal folds, are folds of tissue in the throat that are key in creating sounds through vocalization. The size of vocal cords affects the pitch of voice. Open when breathing and vibrating for speech or singing, the folds are controlled via the recurrent laryngeal branch of the vagus nerve.

They are composed of twin infoldings of mucous membrane stretched horizontally, from back to front, across the larynx. They vibrate, modulating the flow of air being expelled from the lungs during phonation.

4) Trachea:

Intro: It is tubular cartilaginous structure which is called wind-pipe, lies below the larynx. It is internally lined by ciliated epithelium which bears mucous secreting cell.

Length: Trachea is about 10-20cm long and 2cm wide.

Cartilaginous rings: The trachea bear 16-20 ‘’C’’ or horse shoe shaped cartilage rings which prevent the trachea from collapsing and keep it open.

Function: a) The mucous of trachea moist and warmth the air.

  1. b) Trachea provide a passage for air to move toward lung.
  2. c) Mucous and cilia of trachea filter the air and protects the respiratory surface from an accumulation of foreign particles.

5) Bronchi:

Intro: The trachea Is divides into left and right bronchi which enters to the to each long.

→Bronchi are cartilaginous air-passage which with mucous membrane.

Right and left bronchi: The right bronchus has larger diameter and is shorter. While the left bronchus has smaller diameter is longer.

Function of bronchi:

  1. a) Bronchi (right + left bronchi) divided the air for each lung.
  2. b) Bronchi also lined with mucous membrane which filter, moisturized and warmth the air.

6) Bronchioles:

Intro: The bronchi divided and re-divided forming an air way network inside the lungs called bronchioles.

Non-cartilagenous: The bronchioles lack cartilage rings or plates but contain circular smooth muscles and elastic fibers.

Diameter: the bronchioles are approximately 1mm or less in diameter.

Functions: The lungs are filled with millions of microscopic alveoli to allow for a high rate of exchange of gases with the atmosphere. Each air sac provides surface area for this gas exchange. In order to get air to all of the alveoli, the bronchioles have to branch smaller and smaller. Bronchioles range in diameter from several millimeters to less than half a millimeter. The tip of each bronchiole, called a terminal bronchiole, ends at a cluster of alveoli that it feeds. The function of the bronchioles is to ensure that incoming air is supplied to each alveolus.


Intro: The bronchioles divide and subdivide and finally open into small collection of air sac where gases exchange occurs called alveoli.

Number: There are over 700-million alveoli present in lungs, representing a total area 70-90 m.

Blood capillaries: On outside of alveoli there is dense network of blood capillaries which provides blood to alveoli for the exchange of gases.

Functions:1) Alveoli from the gas exchange surface, where exchange of respiratory gases occurs.

2) The collagen of elastic fibers of alveoli allow the alveoli to expand and recoil easily during breathing.

3) Special cells on the alveolus wall secrete a detergent like chemical (surfactant) which lowers the surfaces tension of fluid layer lining the alveolus as a result reduce the amount of effort needed for breathing.

4) Surfactants also helps to kill any bacteria which may reach the alveoli.

B) lungs:

Number and location: Human have two lungs (a right and a left) which are located in the chest (thoracic) cavity.

→In the chest cavity, base of two lungs is present of two diaphragm and apexes of lungs is extended above the first rib.

Membrane: Each lung is surrounded by two tough, flexible, transparent, thin membrane called pleural membrane.

Function: a) The pleural membrane are filled with fluid which reduces friction between the lungs and the walls of thorax.

  1. b) It protects the lung from over extension or external factors.
  2. c) It stops the leakage of air from lungs two the thoracic cavity.

Spongy in nature:

The lungs are spongy because of the presence of millions of alveoli.

Right and left lung:

Right and left lungs are slightly unequal in size.

→The right lung represents 56% of the total lung volume. The right lung is composed of three section.

→while the left lung is smaller in volume because of asymmetrical position of the heart. The left lung is composed of two lobes.

Pulmonary artery and vein:

Blood passes to lungs are brought by pulmonary artery and taken away from lungs by the pulmonary vein.

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