The diaphragm is generally referred to as the main “respiratory muscle”—which is to some extent a misnomer. A better description would be that the diaphragm is the primary muscle of ventilation. Its critical physiologic role is to serve as the main muscle that moves air into the lungs, where this air, of course, oxygenates the blood. Since delivery of oxygenated blood to all parts of the body is critical to life, and since intact cardiac and respiratory systems are the essential elements to assure that this oxygenated blood is manufactured and delivered, failure of any of the components that makes up either one of these systems results in death or severe disability. One could thus make a very strong argument that along with the heart and lungs, the diaphragm is one of the three most important organs in the body.
Just as the heart is the blood pump, the diaphragm is the air pump. Brief consideration will convince the reader that the heart and diaphragm are the only two muscles in the body that are continuously active throughout the life of an individual. If either one of these muscles fails, the individual fails. As the only skeletal muscle that is continuously active (the heart is made up of cardiac muscle), the diaphragm presents a number of interesting physiologic adaptations. Diaphragm muscle fibers demonstrate unique characteristics that permit it to function adequately given this unusual role of constant activity. Further, its special role seems to influence the diaphragm muscle’s response to various disease states (e.g., emphysema) and medical interventions (e.g., mechanical ventilation [MV]), such that it often does not respond in the same manner as skeletal muscles in the limbs do under similar circumstances.
Beyond its physiologic role, the diaphragm also serves an important anatomic role: separating the mobile abdominal contents from the compressible lung. Defects in this anatomic barrier result in herniation of abdominal contents into the chest, which can lead to both malfunction/failure of the abdominal organs and dysfunction of the lung. This anatomic role of the diaphragm, like its physiologic role in ventilation, serves at bottom to preserve a fully functioning respiratory system such that oxygenated blood can efficiently reach the heart.
Basic Anatomy and Embryology
The diaphragm is made up of two muscular parts and a central tendon. The muscular portions are the costal diaphragm and the crural diaphragm. The costal diaphragm is its main functional component. It is formed by muscle fibers that originate on the lower six ribs and the xiphoid process and attach on the central tendon (Fig. 149-1). The crural diaphragm consists of bundles of muscle fibers that arise from the first three lumbar vertebral bodies and the medial and lateral arcuate ligaments on each side. Although some mistakenly think of the costal diaphragm as being made up of two separate muscles—right ...