Ontario Grade 11 University Biology Practice Test 2025 - Free Biology Practice Questions and Study Guide

The diaphragm is a crucial muscular structure in the process of breathing. It is a dome-shaped muscle located beneath the rib cage that separates the thoracic cavity from the abdominal cavity. When you inhale, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity and decreasing the pressure inside the lungs. This pressure change allows air to flow into the lungs. Upon exhalation, the diaphragm relaxes and moves back to its original position, which decreases the volume of the thoracic cavity and increases the pressure in the lungs, forcing air out. This cyclic contraction and relaxation of the diaphragm is fundamental to the mechanics of breathing.

In contrast, the lungs are the organs where gas exchange occurs but do not have muscular properties to control airflow. The sternum is a bone that serves as an attachment point for ribs but does not contribute directly to the action of breathing. The trachea is a rigid airway that conducts air to and from the lungs but does not play an active role in the breathing process itself like the diaphragm does.