Biological Systems MCAT Practice Exam

What muscle primarily aids in inspiration by contracting and moving downward?

1. Intercostal muscles

2. Diaphragm

3. Abdominal muscles

4. Pectoral muscles

The correct answer is: Diaphragm

The diaphragm is the primary muscle involved in the process of inspiration. When it contracts, it moves downward into the abdominal cavity, increasing the volume of the thoracic cavity. This decrease in pressure within the thoracic cavity relative to the atmospheric pressure allows air to flow into the lungs. During normal, quiet breathing, the diaphragm is responsible for a significant portion of the airflow into the lungs. Its dome-shaped structure flattens when it contracts, creating a vacuum effect that draws air in. This mechanism is essential for the process of gas exchange that supports cellular respiration. Other muscles, while they play supportive roles, are not primarily responsible for the initial phases of inhalation. The intercostal muscles assist by elevating the rib cage, which further aids in expanding the thoracic cavity but are secondary to the diaphragm's primary action during regular breathing. Abdominal muscles are more involved in forced expiration rather than during inspiration, and pectoral muscles do not have a significant role in normal breathing patterns. Thus, the diaphragm’s unique function in actively increasing lung capacity makes it the key muscle in the process of inspiration.