What is the action of muscles that causes air to come into lungs?

Inspiration, the process by which air comes into the lungs, primarily involves the contraction of two major respiratory muscles: the diaphragm and the external intercostal muscles.

1. Diaphragm: The diaphragm is a dome-shaped muscle located at the bottom of the rib cage, separating the thoracic cavity from the abdominal cavity. During inspiration, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity. This creates negative pressure within the lungs, causing air to rush in through the nose and mouth.

2. External Intercostal Muscles: The external intercostal muscles are located between the ribs. When they contract, the rib cage is lifted upward and outward, further increasing the thoracic cavity's volume. This expansion also contributes to the decrease in air pressure within the lungs, aiding in inspiration.

As the diaphragm and external intercostal muscles relax, the elastic recoil of the lungs and the weight of the rib cage cause the chest cavity to return to its original position, pushing the air out of the lungs during expiration.