Question

Describe the respiratory zone and explain how external respiration (transfer of gasses between the air and pulmonary circuit) occurs.

Answer 1

Answer

Respiraotry tract is divides into 2 zones :

1. Conducting zone
2. Respiratory zone

Respiratory zone

There are three structures involved in respiratory zone :

• Respiratory bronchioles
  - Terminal bronchioles continue as respiratory bronchioles.
  - Size : 0.5 mm
  - They are made up of simple cuboidal epithelium but also contain some smooth muscle fibers and eleastic fibers.
  - The epithelium of respiratory bronchioles too permit some gaseous exchange.
  
  
• Alveolar duct
  - Alveolar duct are the continuation of respiratory bronchioles.
  - They enter alveolar sac and supply air to alveoli.
  - Alveolar duct too contain simple cuboidal epithelium with terminal portion of duct composed of simple squamous epithelium.
  
  
• Alveoli
  - They are arranged in clusters called alveolar sacs.
  - They are functional unit of lungs that serves as primary site for gaseous exchange.
  - There are three types of cells in alveoli : Type I alveolar cells, Type II alveolar cells and alveolar macrophages.

External respiration :

• The gaseous exchange of oxygen and carbon dioxide at alveolar surface in the lungs is known as external repiration.
• Pulmonary capillaries surround alveoli or air sacs.
• The gaseous exchange occurs across the respiratory membrane.
• The respiratory membrane is formed by alveolar epithelial cells (simple squamous epithelium), basement membrane and capillary endothelium.
• The partial pressure of oxygen is high in the alveoli as compared to the pulmonary capillaries.
• As a result, oxygen moves along the pressure gradient into the pulmonary capillaries.
• The partial pressure of carbon dioxide is high in the pulmonary capillaries as compared to the alveoli.
• As a result, carbon dioxide moves along the pressure gradient into the alveoli.

After external respiration, the oxygenated blood from pulmonary capillaries is transported back to the heart for supply to all the body parts.