The diaphragm is the dome-shaped sheet of muscle and tendon that serves as the main muscle of respiration and plays a vital role in the breathing process. Also known as the thoracic diaphragm, it serves as an important anatomical landmark that separates the thorax, or chest, from the abdomen. The origins of the diaphragm are found along the lumbar vertebrae of the spine and the inferior border of the ribs and sternum. Openings in the diaphragm allow the esophagus, phrenic and vagus nerves, descending aorta, and inferior vena cava to pass between the thoracic and abdominal cavities.
The lungs are enclosed in the thoracic cavity by the rib cage on the front, back, and sides with the diaphragm forming the floor of the cavity. When we inhale, the diaphragm contracts and is drawn inferiorly into the abdominal cavity until it is flat. At the same time, the external intercostal muscles between the ribs elevate the anterior rib cage like the handle of a bucket. The thoracic cavity becomes deeper and larger, drawing in air from the atmosphere. During exhalation, the rib cage drops to its resting position while the diaphragm relaxes and elevates to its dome-shaped position in the thorax. Air within the lungs is forced out of the body as the size of the thoracic cavity decreases.
Structurally, the diaphragm consists of two parts: the peripheral muscle and central tendon. The peripheral muscle is made up of many radial muscle fibers — originating on the ribs, sternum, and spine — that converge on the central tendon. The central tendon — a flat aponeurosis made of dense collagen fibers — acts as the tough insertion point of the muscles. When air is drawn into the lungs, the muscles in the diaphragm contract, and pull the central tendon inferiorly into the abdominal cavity. This enlarges the thorax and allows air to inflate the lungs.
The peripheral muscle can be further divided by its origins into the sternal, costal, and lumbar regions. The sternal region is made up of two small muscular segments that attach to the posterior aspect of the xiphoid process. The costal region is made up of several wide muscle segments whose origins are found on the internal surface of the inferior six ribs and costal cartilages. The lumbar region has its origins on the lumbar vertebra by way of two pillars of tendon called the musculotendinous crura. These pillars wrap around the aorta as it passes through the diaphragm to form the aortic hiatus.
The diaphragm sometimes contracts involuntarily due to certain irritations; these contractions can happen because we eat too quickly, drink carbonated beverages, experience some acid indigestion, or are dealing with a stressful day. If air is inhaled at these times of contraction, the space between the vocal cords at the back of the throat closes suddenly, producing the noise we call hiccups. Short-lived hiccuping episodes are very common. Longer-term hiccups (lasting for days) can occur as well and are usually caused by irritated nerves, though medical attention would be needed in order to rule out other health concerns.