The Respiratory System



The muscles employed to ventilate the lungs of humans vary with the ventilation demands at the time. It is convenient to consider the minimal ventilation requirements and labored ventilation requirements to review how the muscles used.

In relaxed breathing the ribs are lifted by external intercostal muscles. The angle of the external intercostals cause each rib to be lifted up toward the rib above when they contract. Also, the diaphragm contracts to lower the floor of the thoracic cavity. Lifting the ribs expands the thoracic cavity walls, and lowering the floor also increases the thoracic cavity space.

Exhalation in the relaxed state involves the recoil of the tissues to their pre-inhalation position. No additional muscular work is required to exhale in the relaxed state.


Forced or labored breathing involves the sternocleidomastoid and scalene muscles to lift the upper rib cage even more than in normal breathing. By lifting the upper portion of the rib cage the action of the intercostals is magnified.

Forced exhalation employs the internal intercostals and the abdominal muscles. The internal intercostals are positioned at such an angle to pull the ribs downward when they contract, thereby reducing the volume of the thoracic cavity. The abdominal muscles (rectus abdominis, external oblique, internal oblique, and transversus abdominis) contract to push inward on the abdominal organs and force the diaphragm back up.






























Overview of Respiratory System

Human Nasal Passages

Human Pharynx Regions

Human Larynx and Trachea

Human Bronchi and Lungs

Muscles of Human Respiration

Lower Respiratory Development

Fish Lungs/Air Bladders

Tetrapod Lungs

Lung Ventilation Systems

Gills of Vertebrate Animals

Things I Should Know