When the diaphragm and external intercostal muscles contract during quiet inspiration, what happens to the volume of the thoracic cavity and the lungs?

Introduction / Context:
Breathing involves coordinated movements of the diaphragm and intercostal muscles that change the volume of the thoracic cavity. These volume changes create pressure differences that drive air into and out of the lungs. Understanding how muscle contraction affects thoracic volume is fundamental to respiratory physiology and is commonly tested in general biology and medical entrance exams. This question focuses on what happens when the diaphragm and external intercostal muscles contract during normal inspiration.

Given Data / Assumptions:

• The muscles mentioned are the diaphragm and external intercostals.

• The phase considered is contraction of these muscles during quiet inspiration.

• You must identify the correct change in thoracic and lung volume as a result of this contraction.

• Basic knowledge of pressure volume relationships in respiration is assumed.

Concept / Approach:
During quiet inspiration, the diaphragm contracts and moves downward, flattening from its relaxed dome shape. At the same time, external intercostal muscles contract to lift and expand the rib cage. Both actions increase the volume of the thoracic cavity. Because the lungs are attached to the thoracic wall via the pleural membranes, they expand as the thoracic cavity enlarges. An increase in lung volume causes the pressure inside the alveoli to fall slightly below atmospheric pressure, so air flows in. Therefore, any statement that says thoracic or lung volume decreases during contraction of these muscles is incorrect.

Step-by-Step Solution:
Step 1: Recall that the diaphragm is a dome shaped muscle separating the thoracic and abdominal cavities. Step 2: When the diaphragm contracts, it moves downward, increasing the vertical dimension of the thoracic cavity. Step 3: External intercostals run between the ribs; their contraction lifts the ribs upward and outward, increasing the anteroposterior and lateral diameters of the thorax. Step 4: Together, these actions increase the total volume of the thoracic cavity. Step 5: Because the lungs adhere to the thoracic wall via pleural fluid, they are pulled outward, and their volume increases with the thoracic cavity. Step 6: An increase in lung volume causes a drop in intrapulmonary pressure relative to atmospheric pressure, resulting in airflow into the lungs. Step 7: Thus, the correct description is that thoracic volume increases, allowing the lungs to expand and air to flow in.

Verification / Alternative check:
Respiratory physiology diagrams show the sequence of events during inspiration and expiration. In the inspiration panel, arrows indicate the diaphragm moving downward and ribs moving outward, with the thoracic cavity volume labeled as increased. Text explanations state that external intercostal and diaphragm contraction leads to lowered intrathoracic pressure and air entry. None of these descriptions indicate a decrease in volume during this phase, which confirms that options stating decreased volume or collapse are wrong.

Why Other Options Are Wrong:

The volume of the thoracic cavity decreases: This describes expiration, not inspiration; it is opposite to what happens when these muscles contract.

The volume of the lungs decreases as air is forced out: This is also expiration, when air leaves the lungs, not inspiration.

The lungs shrink and collapse during this muscular contraction: Collapse would occur in pathological conditions, not during normal inspiration.

No change in thoracic volume because only abdominal muscles are involved: Abdominal muscles are mainly active in forced expiration, while the diaphragm and external intercostals are the main inspiratory muscles.

Common Pitfalls:
One frequent error is reversing the direction of change and thinking that muscle contraction always squeezes or decreases volume. In the case of the thorax, certain muscle contractions actually increase the space. Another pitfall is forgetting which intercostals are active in inspiration versus expiration. To avoid confusion, remember that inspiration is an active process driven mainly by diaphragmatic contraction and external intercostal activity, which enlarge the thoracic cavity and draw air in.

Final Answer:
When these muscles contract, the volume of the thoracic cavity increases, allowing the lungs to expand and air to flow into the lungs.