Breathing is the mechanical process that moves air into and out of the lungs so the body can exchange oxygen and carbon dioxide. It depends on the coordinated action of the diaphragm, intercostal muscles, rib cage, airways, and elastic lung tissue. Understanding this process explains how pressure changes, not just muscle force, drive airflow.
It also helps students connect anatomy to everyday events such as exercise, coughing, and shortness of breath.
During inhalation, muscles enlarge the thoracic cavity, causing lung volume to increase and air pressure inside the lungs to fall below atmospheric pressure. Air then flows through the trachea and bronchi into the lungs until pressures equalize. During exhalation at rest, the diaphragm relaxes and the lungs recoil, decreasing thoracic volume and raising pressure inside the lungs.
Lung volumes such as tidal volume and vital capacity describe how much air moves during normal and maximum breathing.
Key Facts
- Inhalation occurs when thoracic volume increases and lung pressure decreases below atmospheric pressure.
- Exhalation at rest is mostly passive because elastic recoil of the lungs and chest wall pushes air out.
- Boyle's law explains breathing mechanics: P1V1 = P2V2, so pressure decreases when volume increases.
- The diaphragm contracts and flattens during inhalation, increasing the vertical size of the thoracic cavity.
- External intercostal muscles lift the rib cage during inhalation, increasing chest volume.
- Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume.
Vocabulary
- Diaphragm
- A dome-shaped muscle below the lungs that contracts to increase chest volume during inhalation.
- Intercostal muscles
- Muscles between the ribs that help move the rib cage during breathing.
- Thoracic cavity
- The chest space that contains the lungs, heart, trachea, bronchi, and related structures.
- Tidal volume
- The amount of air moved into or out of the lungs during one normal resting breath.
- Elastic recoil
- The tendency of stretched lung tissue to return to its resting size during exhalation.
Common Mistakes to Avoid
- Thinking air is pulled in by suction from the lungs themselves. Air enters because muscle action increases thoracic volume, lowering lung pressure below atmospheric pressure.
- Saying the diaphragm moves upward during inhalation. During inhalation, the diaphragm contracts and flattens downward, which increases the space in the chest.
- Confusing the trachea with the bronchi. The trachea is the main airway from the throat, while the bronchi are the two major branches that enter the lungs.
- Assuming exhalation always requires active muscle contraction. Quiet exhalation is usually passive because relaxed muscles and elastic recoil reduce lung volume.
Practice Questions
- 1 A student inhales 500 mL of air with each resting breath and breathes 14 times per minute. What is the student's minute ventilation in mL/min and L/min?
- 2 A person's tidal volume is 0.50 L, inspiratory reserve volume is 2.8 L, and expiratory reserve volume is 1.2 L. Calculate the person's vital capacity.
- 3 Explain why air flows into the lungs during inhalation using diaphragm movement, thoracic volume, and pressure difference.