The diaphragm is the primary muscle used in the process of inspiration or inhalation. It is a dome-shaped sheet of muscle that is inserted into the lower ribs. Lying at the base of the thorax (chest), it separates the abdominal cavity from the thoracic cavity. It is a thin, skeletal muscle that can contract voluntarily.
During inhalation, the diaphragm contracts and moves in the inferior direction, enlarging the volume of the thoracic cavity and reducing intra-thoracic pressure (the external intercostal muscles also participate in this enlargement), forcing the lungs to expand. In other words, the diaphragm's movement downwards creates a partial vacuum in the thoracic cavity, which forces the lungs to expand to fill the void, drawing air in the process.
The diaphragm is also involved in non-respiratory functions, helping to expel vomit, feces, and urine from the body by increasing intra-abdominal pressure, aiding in childbirth, and preventing acid reflux by exerting pressure on the esophagus as it passes through the esophageal hiatus.