Mechanics of Respiration and Diaphragm

Breathing is driven by pressure changes generated by the respiratory muscles acting on the thoracic cage. Understanding the anatomy of the thoracic wall and the mechanics of rib movement underpins the clinical assessment of respiratory effort.

The diaphragm is the principal muscle of inspiration. It is a dome-shaped musculotendinous sheet that separates the thoracic and abdominal cavities. Its peripheral muscular fibres arise from the xiphoid process of the sternum, the inner surfaces of the lower six costal cartilages, and the lumbar vertebrae via the left and right crura. These fibres insert into the central tendon. When the diaphragm contracts, the dome descends, increasing thoracic volume vertically and reducing intrathoracic pressure. This drives air into the lungs.

Rib movements augment the volume change produced by the diaphragm. Upper ribs move in a pump-handle motion: the anterior end of the rib swings upward and forward, increasing the anteroposterior diameter of the thorax. Lower ribs move in a bucket-handle motion: the lateral shaft of the rib swings outward, increasing the transverse diameter of the thorax. Both movements increase thoracic volume during inspiration.

The intercostal muscles occupy the spaces between adjacent ribs. The external intercostals run obliquely downward and forward; they are active during inspiration and elevate the ribs. The internal intercostals run downward and backward, perpendicular to the external intercostals; they are active during forced expiration and depress the ribs. The innermost intercostals are the deepest layer and run in the same direction as the internal intercostals. The intercostal neurovascular bundle (vein, artery, nerve from superior to inferior) runs in the costal groove under the lower border of each rib; procedures such as pleural aspiration are performed just above the upper border of the rib below to avoid this bundle.

Normal expiration is passive, relying on the elastic recoil of the lungs and chest wall as the diaphragm and external intercostals relax. Forced expiration recruits the internal intercostals and the muscles of the anterior abdominal wall (rectus abdominis, external and internal obliques, transversus abdominis), which increase intra-abdominal pressure and push the diaphragm upward.

Accessory muscles of inspiration include the scalenes, which elevate the first two ribs, and the sternocleidomastoids, which elevate the sternum. Their use at rest indicates significant respiratory distress and increased work of breathing.