Respiratory muscles

Breathing is a process that we perform 15,000–20,000 times a day, most often without thinking. However, behind each breath there is a coordinated work of several muscle groups that change the volume of the chest and create a pressure difference.

Basic muscles of inspiration

The diaphragm is the main respiratory muscle, providing up to 70% of the volume of a quiet inhalation. When contracted, its dome flattens by 1.5–2 cm, increasing the vertical size of the thoracic cavity. The diaphragm is innervated by the phrenic nerve (C3–C5).

External intercostal muscles - 11 pairs of muscles located between the ribs. Their fibers run obliquely from top to bottom and from front to back. When they contract, they lift the ribs up and forward, increasing the anteroposterior and transverse dimensions of the chest.

Inhalation mechanism

1. The diaphragm contracts → the dome lowers
2. External intercostal spaces contract → ribs rise
3. The volume of the chest increases
4. The pressure in the pleural cavity becomes even more negative
5. The lungs expand → the pressure in the alveoli drops below atmospheric
6. Air rushes into the lungs

Exhale

Calm exhalation is a predominantly passive process: the muscles relax, the lungs and chest return to their original position due to elastic traction.

Forced exhalation requires active work:

• Internal intercostal muscles - lower the ribs
• Abdominal muscles (rectus, oblique, transverse) - increase intra-abdominal pressure and put pressure on the diaphragm from below

Accessory muscles

With increased breathing (physical activity, shortness of breath) connect:

• sternocleidomastoid muscle - raises the sternum
• scalene muscles - raises the 1st–2nd ribs
• Pectoralis major and minor - with fixed arms, they expand the chest

Use of accessory muscles is a clinical sign of respiratory failure. The doctor, seeing the retraction of the intercostal spaces and the tension of the neck muscles when breathing, understands that the patient is having difficulty breathing.

Tidal volumes

Thanks to the work of the respiratory muscles, the lungs are able to accommodate:

• Tidal volume — 500 ml (quiet inspiration)
• Vital capacity - 3,500–5,000 ml (maximum inhalation + maximum exhalation)
• Total capacity – up to 6,000 ml

Spirometry - the study of these volumes - is the main method for diagnosing obstructive and restrictive lung diseases.

In 3D atlas Humio, you can view the diaphragm and intercostal muscles in the context of the chest—rotate the model to understand their location and interaction.