Which muscles account for approximately one-third of the air that ventilates the lungs during quiet respiration?

The intercostal muscles are primarily responsible for the movement of the rib cage during breathing, playing a significant role in ventilation, especially during quiet respiration. They are located between the ribs and facilitate inhalation by expanding the thoracic cavity, which helps draw air into the lungs.

During quiet respiration, contraction of the external intercostal muscles elevates the ribs, allowing for an increase in thoracic volume and a decrease in intrathoracic pressure, thereby allowing air to flow into the lungs. These muscles, therefore, contribute around one-third of the air that ventilates the lungs during this relaxed state of breathing.

While other muscles like the sternocleidomastoid and scalene muscles are involved more during forced or deep breathing, and abdominal muscles primarily assist with exhalation, their contribution during quiet respiration is considerably less significant than that of the intercostal muscles. This delineates the intercostal muscles as the key players in regular, restful breathing.