Why are different muscles required for inspiration and expiration, and how does muscle contraction contribute to each process?

Sourav PanNovember 8, 2024

Answered

Different muscles are required for inspiration and expiration due to the distinct physiological processes involved in each phase of breathing. Here’s an overview of how muscle contractions contribute to these processes:

Inspiration (Inhalation)

1. Primary Muscles Involved

• Diaphragm: The diaphragm is the primary muscle responsible for inspiration. When it contracts, it moves downward, increasing the vertical dimension of the thoracic cavity.
• External Intercostal Muscles: These muscles are located between the ribs and assist by contracting to lift the rib cage upward and outward, further expanding the thoracic cavity.

2. Mechanism

• Volume Increase: The contraction of the diaphragm and external intercostal muscles increases the volume of the thoracic cavity. According to Boyle’s Law, as volume increases, pressure decreases within the lungs (intrapulmonary pressure becomes lower than atmospheric pressure).
• Airflow: This pressure gradient causes air to flow into the lungs from the atmosphere, filling the alveoli with fresh air.

3. Active Process

• Inspiration is generally considered an active process because it requires energy for muscle contraction. During vigorous activities or exercise, additional accessory muscles (such as scalenes and sternocleidomastoid) may also be recruited to enhance lung expansion.

Expiration (Exhalation)

1. Primary Muscles Involved

• Relaxation of Diaphragm and External Intercostals: During normal expiration, these muscles relax after contraction, allowing passive recoil of lung tissue.
• Internal Intercostal Muscles: These muscles may be activated during forced expiration to pull the ribs downwards and inwards, decreasing thoracic volume more rapidly.

2. Mechanism

• Volume Decrease: As the diaphragm relaxes and returns to its dome shape, and as external intercostals relax, the volume of the thoracic cavity decreases.
• Pressure Increase: The decrease in volume leads to an increase in intrapulmonary pressure (higher than atmospheric pressure), which forces air out of the lungs.

3. Passive vs. Active Process

• Normal expiration is primarily a passive process that relies on elastic recoil of lung tissues and surface tension within the alveoli. However, during vigorous activities or when rapid exhalation is needed (like during exercise), forced expiration becomes an active process that recruits internal intercostal muscles and abdominal muscles to push air out more forcefully.