Question

Contraction of the external intercostal muscles causes which of the following to occur? a. The diaphragm moves downward. b. The rib cage is compressed. c. The thoracic cavity volume decreases. d. The ribs and sternum move upward.

Short answer

Answer: The ribs and sternum move upward.

Step-by-step solution

Understanding the external intercostal muscles

The external intercostal muscles are a group of muscles situated between the ribs and responsible for moving the rib cage during respiration. They are located on the outer side of each rib and extend obliquely from the lower rib to the rib above it. Their function is to stabilize and elevate the rib cage, which helps in inhalation.

Evaluating each option

a. The diaphragm moves downward - The role of the diaphragm is to separate the thoracic cavity from the abdominal cavity. When it contracts, it causes the volume of the thoracic cavity to increase, and the air enters the lungs. However, this is not directly affected by the contraction of the external intercostal muscles. b. The rib cage is compressed - This statement is incorrect, as the contraction of the external intercostal muscles elevates the ribs, increasing the volume of the thoracic cavity during inhalation. c. The thoracic cavity volume decreases - During the contraction of the external intercostal muscles, the rib cage moves upwards and outwards, which results in an increase in the thoracic cavity volume, not a decrease. This statement is incorrect. d. The ribs and sternum move upward - When the external intercostal muscles contract, they elevate the rib cage, causing the ribs and sternum to move upward, facilitating inhalation.

Identify the correct answer

Among the given options, option d (The ribs and sternum move upward) is the correct answer based on the functions of the external intercostal muscles.

Key concepts

Intercostal Muscles

The intercostal muscles are a crucial component of the respiratory system. Found nestled between the ribs, these muscles play a pivotal role in breathing. There are two primary types: external and internal intercostal muscles. The external intercostal muscles are primarily responsible for elevating the rib cage.
When these muscles contract, they pull the ribs upward and outward. This action enlarges the thoracic cavity, allowing for air to fill the lungs during inhalation. In essence, they act like small levers, lifting and expanding the rib cage around the lungs.
Understanding how these muscles function helps explain how the body creates negative pressure to draw in breath effectively.

• Provide stability to the rib cage.
• Work in harmony with the diaphragm to facilitate breathing.
• Are essential for expanding the thoracic cavity.

Thoracic Cavity

The thoracic cavity is the central chamber associated with the respiratory system. It houses the lungs and heart, surrounded by the rib cage and enclosed by the diaphragm. This cavity plays a vital role during respiration, being the space where lung expansion occurs.
As the diaphragm contracts and the intercostal muscles lift the ribs, the volume of the thoracic cavity increases. This increase reduces the pressure inside the cavity compared to the outside atmosphere, leading to air flow into the lungs.
Consider the thoracic cavity as a flexible container. When it expands, it draws air in, much like a vacuum.

• Contains essential organs like the lungs and heart.
• Protected by the rib cage.
• Volume increases during inhalation to lower internal pressure.

Inhalation Process

The inhalation process is a vital sequence where air enters the lungs. It begins with the contraction of the diaphragm, which moves downward, expanding the thoracic cavity. Coupled with this, the external intercostal muscles contract, lifting the ribs and sternum upwards and outwards.
This coordinated muscle action increases the volume of the thoracic cavity. With more space, the pressure inside decreases, allowing air to flow into the lungs to equalize the pressure.
Inhalation is a wonderful example of mechanical and biological systems working together seamlessly.

• Air flows into lungs due to negative pressure created inside.
• Diaphragm moves downward, creating space.
• Intercostal muscles elevate the rib cage.