A minimal amount of pleural fluid present in the pleural cavity:
| 1. | provides a medium for the exchange of respiratory gases |
| 2. | reduces friction between the visceral and parietal pleurae |
| 3. | provides lubrication for movements of the diaphragm |
| 4. | permits the exchange of electrolytes during respiration |

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The alveoli are lined with:
1. Simple cuboidal epithelium
2. Pseudo-stratified columnar ciliated epithelium
3. Transitional epithelium
4. Simple squamous epithelium

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| I: | P is an incomplete cartilaginous ring seen only in trachea and principal bronchus. |
| II: | Q is the point where the trachea divides into a right and left primary bronchus and corresponds to the level of 5th thoracic vertebra. |
| III: | R shows the double-layered pleura where the outer pleural membrane is in close contact with the thoracic lining. |
| IV: | S is pleural cavity with minimal amount of pleural fluid which reduces friction on the lung surface. |

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