12.2 Mechanics of Breathing

Inspiration and Expiration

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The Process of Inspiration (Inhalation)

Inspiration is an active process, meaning it requires energy and muscle contraction. Here's what happens step by step:

1. The diaphragm and external intercostal muscles contract.
2. Their contraction increases the volume of the thoracic cavity.
3. As thoracic volume increases, pressure inside the lungs drops below atmospheric pressure.
4. Because air moves from high pressure to low pressure, air flows into the lungs.

This entire process is regulated by the respiratory center in the medulla oblongata of the brainstem. The respiratory center receives input from chemoreceptors (which monitor blood $CO_2$, $O_2$, and pH levels) and mechanoreceptors (which detect stretch in the lungs and chest wall). Together, these signals adjust the rate and depth of breathing to match the body's needs.

The Process of Expiration (Exhalation)

Quiet expiration is a passive process, which means no muscle contraction is needed. It works like this:

1. The diaphragm and external intercostal muscles relax.
2. The thoracic cavity decreases in volume as the elastic tissue of the lungs and chest wall recoils.
3. Lung pressure rises above atmospheric pressure.
4. Air flows out of the lungs down the pressure gradient.

Forced expiration is different. It's an active process that recruits additional muscles:

• The internal intercostal muscles contract, pulling the ribs downward and inward.
• The abdominal muscles contract, pushing abdominal organs upward against the diaphragm.
• These combined actions further reduce thoracic volume, increasing lung pressure and expelling air more forcefully. Think of coughing, sneezing, or blowing out birthday candles.

Diaphragm and Intercostal Muscles in Breathing

The Diaphragm

The diaphragm is the primary muscle of respiration. It's a dome-shaped sheet of skeletal muscle that separates the thoracic cavity from the abdominal cavity, sitting right at the base of the lungs.

• During contraction, the diaphragm flattens and moves downward, increasing the vertical dimension of the thoracic cavity. This lowers intrapulmonary pressure and draws air in.
• During relaxation, the diaphragm returns to its dome shape, decreasing thoracic volume and pushing air out.

The diaphragm alone accounts for roughly 75% of the air movement during quiet breathing. That's why diaphragm injuries or paralysis (such as from a spinal cord injury at C3–C5) can be life-threatening.

The Intercostal Muscles

Two sets of intercostal muscles sit between the ribs, and they have opposing actions:

• External intercostals assist in inspiration. When they contract, they pull the ribs upward and outward (think of a bucket handle swinging up). This increases the anterior-posterior and lateral dimensions of the thoracic cavity.
• Internal intercostals assist in forced expiration. When they contract, they pull the ribs downward and inward, decreasing thoracic volume.

The abdominal muscles (rectus abdominis, external obliques, internal obliques, and transversus abdominis) also assist in forced expiration. They contract and push abdominal contents upward against the diaphragm, further reducing thoracic volume and forcing air out.

Lung Volumes and Capacities

Lung volumes and capacities are measured using a spirometer and are essential for assessing respiratory function. Volumes are individual measurements, while capacities are combinations of two or more volumes.

Lung Volumes

• Tidal volume (TV): The volume of air inhaled or exhaled during a single, normal breath. Typically around 500 mL in adults.
• Inspiratory reserve volume (IRV): The maximum additional volume of air you can inhale beyond a normal tidal breath. Typically around 3,000 mL.
• Expiratory reserve volume (ERV): The maximum additional volume of air you can forcefully exhale beyond a normal tidal breath. Typically around 1,100 mL.
• Residual volume (RV): The volume of air that remains in the lungs even after the most forceful expiration. Typically around 1,200 mL. This air keeps the alveoli from completely collapsing.

Lung Capacities

Each capacity is calculated by adding specific volumes together:

• Total lung capacity (TLC): The maximum volume of air the lungs can hold.
  • $TLC = TV + IRV + ERV + RV$
  • Typically around 6,000 mL
• Vital capacity (VC): The maximum volume of air that can be exhaled after a maximal inhalation. This is the single most useful measurement for assessing respiratory function.
  • $VC = TV + IRV + ERV$
  • Typically around 4,600 mL
• Functional residual capacity (FRC): The volume of air remaining in the lungs at the end of a normal, quiet expiration.
  • $FRC = ERV + RV$
  • Typically around 2,300 mL

Notice that any capacity involving residual volume (TLC and FRC) cannot be measured directly by spirometry, since you can never fully empty your lungs. These require indirect methods like helium dilution or body plethysmography.

Lung Compliance and Elasticity

Lung compliance refers to how easily the lungs can be stretched and expanded. Elasticity refers to the lungs' ability to recoil back to their original shape after being stretched. These two properties work together: compliance allows the lungs to inflate during inspiration, and elasticity drives passive expiration.

Factors Affecting Lung Compliance

Two main factors determine compliance:

1. Elastic fibers in lung tissue (primarily elastin) allow the lungs to stretch during inspiration and recoil passively during expiration. Conditions that damage these fibers, like emphysema, reduce elastic recoil. The lungs become overly compliant (too easy to inflate) but lose their ability to deflate efficiently, trapping air inside.

2. Surface tension of alveolar fluid naturally resists lung expansion. Pulmonary surfactant, a mixture of phospholipids and proteins secreted by type II alveolar cells, reduces this surface tension. Without surfactant, the alveoli would collapse (atelectasis), and the work of breathing would increase dramatically.

Age-related changes also affect compliance. Over time, elastic fibers break down and collagen becomes more cross-linked, making the lungs stiffer. This leads to decreased vital capacity and increased residual volume in older adults.

Disorders Affecting Lung Compliance

• Respiratory distress syndrome (RDS): Seen in premature infants whose type II alveolar cells haven't matured enough to produce adequate surfactant. Without surfactant, surface tension is too high, alveoli collapse, and each breath requires enormous effort.
• Pleural disorders: Pleural effusion (fluid in the pleural space) and pneumothorax (air in the pleural space) both limit lung expansion by disrupting the negative intrapleural pressure that normally keeps the lungs inflated against the chest wall.
• Pulmonary fibrosis: Scar tissue accumulates in the lung interstitium, making the lungs stiff and resistant to expansion. Compliance drops, and patients struggle to take deep breaths. This is the opposite of emphysema: the lungs are too rigid rather than too floppy.