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Alveoli

 

Alveoli are functional unit of lungs.

Each lung contains over 300 million alveoli, each surrounded by many capillaries.

Alveoli are clustered in bunches like grapes and provide enough surface area to allow extensive gas exchange.

An alveolus is cup shaped.

Out pouching lined by simple squamous epithelium and supported by thin elastic basement membrane.


Size of Alveoli

Single alveolus like tiny bubble

Diameter - 200 micrometer (0.2 mm) in diameter


Alveolar wall or alveolar septum consist of alveolar epithelial cells.


Type of Alveolar Cells

There are two types of alveolar cells present -

1. Type l pneumocytes or alveolar cells  and,

2. Type ll pneumocytes


1. Type 1 Alveolar Cells


Lining of epithelium of alveoli consist single layer of squamous cell (simple squamous pulmonary epithelial cells) called type 1 cells.

Type l  Alveolar cell are main site where gas exchange take place.


2. Type ll Cells or Septal Cells 

Increase lung surface area when gas exchange take place.

Scattered cubodial secretory cells.

Theses cell secrete detergent like phospholipid called lung surfactant.

Helps to keep alveoli inflated by reducing surface tension.

Surfactant prevent collapsing of alveolar walls.

Prevent movement of water from capillary blood into the alveoli.

Alveolar macrophages (dust cells)

Alveoli contain phagocytic alveolar macrophages.

Digest and destroy microorganisms and other foreign substances that enter the alveoli.

Foreign material move upward by ciliary action to be expelled by coughing.

Or,

It enters the lymphatics to be carried to the lymph nodes at the hilum of the lungs.


Alveolar duct 


Alveolar duct are tiny duct that connect respiratory branchioles to alveolar sac.

Alveolar duct can control air movement in acini.

Acinus- ending of tiny airways in the lungs where alveoli (air sac)are located.


Alveolar Sac

A group of several alveoli with a common opening into an alveolar duct is called an alveolar sac.

Alveolar sacs are sacs of many alveoli, which are the cells that exchange oxygen

 and CO2 in the lungs.


Function of Alveoli


1. Alveoli are sites of gas exchange.

2. Exchange of gases during respiration take place across the alveolar capillary membrane.

3. Give the lungs big surface areas.

4. Exchange O2 and CO2 molecule to and from blood stream.

5. Digest and destroy microorganisms/ debris.


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