AP BIOLOGY

Detailed explanation-1: -The external intercostals contract during inhalation after the scalene muscles have raised the first rib. They contract sequentially from a rostral to caudal position, lifting the ribs, increasing the intrathoracic space and creating a negative intrathoracic pressure.

Detailed explanation-2: -The intercostal muscles are the muscles between the ribs. During breathing, these muscles normally tighten and pull the rib cage up. Your chest expands and the lungs fill with air. Intercostal retractions are due to reduced air pressure inside your chest.

Detailed explanation-3: -The intercostal muscles also stabilize the chest wall; in full inspiration they contract to prevent the intercostal spaces being sucked inwards by the negative intrathoracic pressure generated by contraction of the diaphragm.

Detailed explanation-4: -During normal inspiration, the diaphragm and the external intercostal muscles contract. The diaphragm moves downwards increasing the volume of the thoracic (chest) cavity, and the intercostal muscles pull the ribs up expanding the rib cage and further increasing its volume.

Detailed explanation-5: -Expiration is a process in which air is drawn out of the lungs. During expiration, the external intercostal muscles relax, and the ribs move downwards and inwards.