Which structure is primarily involved in the act of breathing?

The diaphragm is the primary structure involved in the act of breathing because it is a dome-shaped muscle located at the base of the thoracic cavity. When the diaphragm contracts, it moves downward and increases the volume of the thoracic cavity, which causes a decrease in internal air pressure. This pressure difference allows air to be drawn into the lungs.

As the diaphragm relaxes, it moves back to its original position, which decreases the thoracic cavity's volume and helps to expel air from the lungs. This process of inhalation and exhalation is essential for gas exchange, allowing oxygen to be taken into the body and carbon dioxide to be expelled.

While alveoli are crucial for the exchange of gases in the lungs, they are not the primary structure responsible for the mechanics of breathing itself. The spinal cord plays a vital role in transmitting signals to the diaphragm and other respiratory muscles, but it is not directly involved in the act of breathing. The heart is responsible for pumping blood but does not perform any function related to the physical act of breathing.