When the inspiratory muscles contract:
a. The size of the thoracic cavity increases in diameter
b. The size of the thoracic cavity increases in length
c. The volume of the thoracic cavity decreases
d. The size of the thoracic cavity increases in both length and diameter

Verified step by step guidance

1. Understand the role of inspiratory muscles, primarily the diaphragm and external intercostal muscles, in the process of inspiration (inhalation).
2. Recall that when the diaphragm contracts, it moves downward, increasing the vertical length of the thoracic cavity.
3. Recognize that contraction of the external intercostal muscles elevates the ribs, which increases the diameter of the thoracic cavity.
4. Combine these effects to see that both the length and diameter of the thoracic cavity increase during inspiration.
5. Conclude that an increase in both length and diameter leads to an increase in the overall volume of the thoracic cavity, facilitating lung expansion.

Key Concepts

Thoracic Cavity Anatomy

The thoracic cavity is the chamber within the chest that houses the lungs and heart. Its size can change in different dimensions—length and diameter—due to the movement of the rib cage and diaphragm during breathing.

Role of Inspiratory Muscles

Inspiratory muscles, primarily the diaphragm and external intercostal muscles, contract to expand the thoracic cavity. This expansion increases the cavity's volume, facilitating air intake into the lungs.

Mechanics of Breathing and Volume Changes

When inspiratory muscles contract, the thoracic cavity enlarges in both length (due to diaphragm contraction) and diameter (due to rib cage elevation), increasing its volume. This volume increase lowers pressure inside the lungs, causing air to flow in.