The diaphragm adopts a curved shape when it is relaxed, and when someone inhales it flattens and allows the lungs to expand. During expiration it gradually curves again, allowing for expiration.
The diaphragm is a naturally curved muscle that sits between the thoracic and abdominal cavities. When it is at rest, it is slightly tense. During inhalation the diaphragm begins to move downwards and flattens, which allows the lungs to expand and accommodate air. This process lowers the air pressure in the lungs, causing lower air pressure. When the pressure in the lungs is lower than atmospheric pressure, air naturally moves inwards.
During expiration, the diaphragm moves to its original position and gradually moves the air outwards. This is a controlled process that increases the air in the lungs to reach a higher pressure than the atmospheric pressure outside, which encourages it to move out.
Both inhalation and expiration are mediated by voluntary and involuntary controls. This means a person can actively choose to relax and contract their diaphragm. The diaphragm also works alongside the intercostal muscles between the ribs, which elevate the ribs and lower them to lower and raise air pressure. When someone wants to take a deep breath, they also use the pectoralis minor muscles and the sternocleidomastoid muscles.