The diaphragm is made of skeletal muscle tissue. Any cell that has the ability to contract is considered a muscle cell. Skeletal muscles are typically voluntary muscles, which means that people can control when they contract and relax.
The diaphragm divides a person's thoracic cavity, which contains the lungs, from their abdominal cavity. Only three structures traverse the diaphragm: the inferior vena cava, the esophagus and the aorta. When a person breathes in, the diaphragm contracts and moves down, which opens up more space in the thoracic cavity. This allows the lungs to expand as air is inhaled. When a person exhales, the diaphragm relaxes and moves back up.