AP BIOLOGY

Detailed explanation-1: -Contraction of these muscles raises the rib cage upward and outward, thereby contributing to the expansion of the thoracic cavity and reduction of pressure within the lung, causing air to move from the air into the lungs (see Figure 6.1.

Detailed explanation-2: -During inspiration, the diaphragm and external intercostal muscles contract, causing the rib cage to expand and move outward, and expanding the thoracic cavity and lung volume. This creates a lower pressure within the lung than that of the atmosphere, causing air to be drawn into the lungs.

Detailed explanation-3: -The innermost intercostal muscles relax, while the external intercostal muscles contract, causing the chest cavity to expand. This expansion allows the lungs to fill with air, due to the negative pressure created by the extra space. Air fills the lungs, gases are exchanged, and it is time to exhale.

Detailed explanation-4: -Contraction of the diaphragm (the skeletal muscle below the lungs) causes an increase in the size of the thoracic cavity, while contraction of the external intercostal muscles elevates the ribs and sternum. Thus, both muscles cause the lungs to expand, increasing the volume of their internal air passages.

Detailed explanation-5: -At the same time, the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside. As a result, air rushes in and fills the lungs. The second phase is called expiration, or exhaling.