13.4 : 呼吸机制一：吸气

The respiratory system comprises the conducting and the respiratory zones.

The conducting zone, including the nasal cavity, trachea, and bronchi, helps filter, clean, warm, and humidify the air as it passes to the lungs.

The respiratory zone consists of the bronchioles, alveolar ducts, and alveoli, which enable the gaseous exchange between air and blood.

The breathing process involves inspiration or inhalation of air through the mouth or nose into the lungs, followed by exhalation of air.

Inspiration occurs when inspiratory muscles—the diaphragm and the external intercostal muscles contract.

The contraction of the diaphragm expands the thoracic and pleural cavities. In contrast, the contraction of the external intercostal muscle elevates the ribs and sternum.

During labored inhalation, the accessory inspiratory muscles, like the sternocleidomastoid, the serratus anterior,  the latissimus dorsi, the pectoralis minor and major, and the serratus posterior superior muscles, may also contract.

The forces of inspiratory muscles and the elasticity of the lung tissue cause the lungs to increase their volume.

As a result, the air pressure within alveolar spaces decreases below atmospheric pressure, causing airflow into the lungs.