FUNCTIONAL ANATOMY OF RESPIRATORY TRACT & RESPIRATORY UNIT

 

By LadyofHats, Jmarchn - Own work using:Sobotta, Johannes (1982) Atlas der Anatomie des Menschen / 2 Brust, Bauch, Becken, untere Extremitäten, Haut. (18th ed.), Munich: Urban & Schwarzenberg ISBN: 3-541-02828-9. OCLC: 260005032.Gray, Henry (1980) Gray's Anatomy (36th ed.), Edinburgh: Churchill Livingstone ISBN: 0-443-01505-8. OCLC: 7775214.Yokochi, Chihiro (1991) Atlas fotográfico de anatomía del cuerpo humano (3rd ed.), Mexico: Interamericana/McGraw-Hill ISBN: 968-25-1677-3. OCLC: 33318149.Also used several online diagrams like:[1][2], Public Domain, https://commons.wikimedia.org/w/index.php?curid=3945169

FUNCTIONAL ANATOMY OF RESPIRATORY TRACT

Respiratory tract is the anatomical structure through which air moves in and out. It includes nose, pharynx, larynx, trachea, bronchi and lungs.

Pleura

Each lung is enclosed by a BI layered serous membrane called pleura or pleural sac. Pleura has two layers namely inner visceral and outer parietal layers. Visceral layer is attached firmly to the surface of the lungs. At hilum, it is continuous with parietal layer, which is attached to the wall of thoracic cavity.

Intrapleural Space or Pleural Cavity

Intrapleural space or pleural cavity is the narrow space in between the two layers of pleura.

Intrapleural Fluid

Intrapleural space contains a thin film of serous fluid called intrapleural fluid, which is secreted by the visceral layer of the pleura.

Functions of intrapleural fluid

1. It functions as the lubricant to prevent friction between two layers of pleura

2. It is involved in creating the negative pressure called intrapleural pressure within intrapleural space.

Pleural Cavity in Abnormal Conditions

In some pathological conditions, the pleural cavity expands with accumulation of air (pneumothorax), water (hydrothorax), blood (hemothorax) or pus (pyothorax).

Tracheobronchial Tree

Trachea and bronchi are together called tracheobronchial tree. It forms a part of air passage.

Components of tracheobronchial tree

1. Trachea bifurcates into two main or primary bronchi called right and left bronchi

2. Each primary bronchus enters the lungs and divides into secondary bronchi

3. Secondary bronchi divide into tertiary bronchi. In right lung, there are 10 tertiary bronchi and in left lung, there are eight tertiary bronchi

4. Tertiary bronchi divide several times with reduction in length and diameter into many generations of

bronchioles

5. When the diameter of bronchiole becomes 1 mm or less, it is called terminal bronchiole

6. Terminal bronchiole continues or divides into respiratory bronchioles, which have a diameter of 0.5 mm.

Upper and Lower Respiratory Tracts

Generally, respiratory tract is divided into two parts:

1. Upper respiratory tract that includes all the structures from nose up to vocal cords; vocal cords are the folds of mucous membrane within larynx that vibrates to produce the voice

2. Lower respiratory tract, which includes trachea, bronchi and lungs.

RESPIRATORY UNIT

Parenchyma of lungs is formed by respiratory unit that forms the terminal portion of respiratory tract. Respiratory unit is defined as the structural and functional unit of lung. Exchange of gases occurs only in this part of the respiratory tract.

Structure of Respiratory Unit

Respiratory unit starts from the respiratory bronchioles. Each respiratory bronchiole divides into alveolar ducts. Each alveolar duct enters an enlarged structure called the alveolar sac. Space inside the alveolar sac is called antrum. Alveolar sac consists of a cluster of alveoli. Few alveoli are present in the wall of alveolar duct also. Thus, respiratory unit includes:

1. Respiratory bronchioles

2. Alveolar ducts

3. Alveolar sacs

4. Antrum

5. Alveoli.

Each alveolus is like a pouch with the diameter of about 0.2 to 0.5 mm. It is lined by epithelial cells.

Alveolar Cells or Pneumocytes

Alveolar epithelium consists of alveolar cells or pneumocytes, which are of two types namely type I alveolar cells and type II alveolar cells.

Type I alveolar cells

Type I alveolar cells are the squamous epithelial cells forming about 95% of the total number of cells. These cells form the site of gaseous exchange between the alveolus and blood.

Type II alveolar cells

Type II alveolar cells are cuboidal in nature and form about 5% of alveolar cells. These cells are also called granular pneumocytes. Type II alveolar cells secrete alveolar fluid and surfactant.