Lesson 11: The Respiratory System (page 2 of 19)

A and C. The purpose of the lungs is to supply the body with oxygen and eliminate carbon dioxide.

Ribcage The ribs enclose the thoracic cavity providing bony protection for the vital organs of the thorax—the heart and lungs.Because the thorax must expand and contract to allow breathing, the ribs move slightly in a “bucket handle” type of motion with each breath. Bucket handle motion of ribs


The act of breathing is performed primarily by the diaphragm, a large muscle that separates the thoracic cavity from the abdominal cavity. During inspiration (breathing in), the diaphragm contracts, drawning downward, creating a vacuum in the thoracic cavity. This vacuum inflates the lungs by drawing air into the body through the trachea, or windpipe. During normal expiration (breathing out), the diaphragm relaxes allowing the air to flow out as the lungs deflate, similar to the way an inflated balloon deflates when released.


The diaphragm receives nerve innervation from the phrenic nerve, which is made up from branches from the 3rd, 4th and 5th cervical (neck) nerve roots. If the cervical spine is severed below the level of the 5th nerve root, paralysis may occur in the body below the shoulders, but breathing will be normal because the phrenic nerve will be intact. If the cervical spine is severed above the 3rd nerve root, the diaphragm will be paralyzed, and if the patient survives, mechanical assistance will be needed for breathing. If the cervical spine is severed between the 3rd and 5th nerve roots, partial paralysis of the diaphragm may occur and some assistance may be needed, but the patient may be able to do some breathing on his or her own. To help remember the nerve enervation to the diaphragm, medical students are taught the mnemonic phrase, “C3, 4 and 5 keep the diaphragm alive.”

Inhalation (breathing in) occurs when . . .


 the diaphragm contracts.
 the diaphragm relaxes.
 the ribs contract.
 the ribs relax.

(Select the best answer and click on the “Continue” button.)