Structure and Function of the Pulmonary System

Clinical Note: V/Q Mismatch

• Low V/Q ratio: Occurs when there is low ventilation relative to perfusion (e.g., airway obstruction). This leads to low $PaO_2$ and high $PaCO_2$.
• High V/Q ratio: Occurs when there is high ventilation relative to perfusion (e.g., pulmonary embolism). This leads to higher $PaO_2$ and lower $PaCO_2$.

Anatomical Structure: From Nose to Alveoli

The respiratory tract is divided into upper and lower airways, serving different roles in preparing air for the delicate gas-exchange surfaces.

Upper Airways and Conducting Airways

The upper airways function primarily to warm and humidify inhaled air. This pathway includes the nasopharynx, the oropharynx, and the larynx, which acts as the bridge to the lower airways.

The lower airways begin with the trachea, which descends into the chest and divides at a ridge called the carina into the right and left bronchi. These bronchi enter the lungs at the hila, the area where the airways, blood vessels, and lymph vessels enter the lung tissue.

The airway hierarchy continues through a series of branching tubes: the bronchi, then the smaller bronchioles, and finally the terminal bronchioles. These constitute the conducting airways, which move air but do not participate in gas exchange.

The Acinus and Gas-Exchange Airways

The functional unit of gas exchange is the acinus, which consists of the respiratory bronchioles, alveolar ducts, and the alveoli.

The alveoli are the primary sites where