Respiratory System Mechanics and Transport

1. Mechanics of Quiet Inhalation

The diaphragm is the primary muscle responsible for quiet (resting) inhalation. When the diaphragm contracts, it moves downward, expanding the thoracic cavity. This increase in volume reduces the intrapulmonary (alveolar) pressure relative to atmospheric pressure, creating a pressure gradient that draws air into the lungs.

• Key point: Diaphragmatic contraction → ↑ thoracic volume → ↓ intrapulmonary pressure → air inflow.

Understanding this mechanism is essential for recognizing how respiratory disorders that impair diaphragmatic function (e.g., neuromuscular disease) affect ventilation.

2. Forced Exhalation and Airway Pressure

During forced exhalation, the body recruits the internal intercostal muscles (and accessory muscles such as the abdominal muscles) to actively decrease thoracic volume. This reduction raises intrathoracic and airway pressure, forcing air out of the lungs.

• Primary muscle group: Internal intercostals.
• Effect: ↓ thoracic volume → ↑ airway pressure → rapid expulsion of air.

Clinically, increased airway pressure during forced exhalation is observed in spirometry maneuvers such as forced vital capacity (FVC) tests.

3. Principles of Alveolar Gas Exchange

Oxygen moves from alveolar air into pulmonary capillary blood primarily because of a partial pressure gradient. The alveolar partial pressure of O₂ (~100 mmHg) is higher than that in deoxygenated blood (~40 mmHg), driving diffusion across the respiratory membrane.

• Driving force: Partial pressure gradient of O₂.
• Other factors (less dominant): Membrane thickness, surface area, and solubility.

Remember that diffusion follows Fick’s law: diffusion rate ∝ (surface area × pressure gradient) / thickness.

4. Why CO₂ Diffuses Faster Than O₂

Carbon dioxide diffuses more rapidly from blood to alveolar air because it has both a larger partial pressure gradient and higher solubility in plasma compared to oxygen. These properties increase its diffusion coefficient, allowing CO₂ to equilibrate quickly.

• Partial pressure gradient: CO₂ in venous blood (~45 mmHg) vs. alveolar air (~40 mmHg).
• Solubility: CO₂ is ~20 times more soluble than O₂ in plasma.

This rapid diffusion is why CO₂ is an excellent indicator of ventilation efficiency.

5. Tissue (Systemic) Gas Exchange

Oxygen leaves the pulmonary capillaries and enters the interstitial fluid because the partial pressure of O₂ is lower in metabolically active tissues. Active cells consume O₂, lowering its local partial pressure and creating a gradient that drives diffusion from blood to tissue.

• Driving factor: Lower tissue PO₂ due to cellular metabolism.

Understanding this gradient is crucial for interpreting conditions such as hypoxia and for designing oxygen therapy protocols.

6. Carbon Dioxide Transport from Tissues to Lungs

The majority of CO₂ produced by metabolism is transported in the blood as bicarbonate ions (HCO₃⁻). Inside red blood cells, CO₂ combines with water under the action of carbonic anhydrase to form carbonic acid, which quickly dissociates into bicarbonate and a hydrogen ion.

• Reaction: CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻.
• Transport: ~70% as bicarbonate, ~20% dissolved CO₂, ~10% bound to hemoglobin (carbaminohemoglobin).

This conversion is essential for maintaining acid‑base balance and for efficient CO₂ removal during exhalation.

7. Oxygen Carriage in Arterial Blood

Oxygen is primarily carried bound to the iron atom of hemoglobin within red blood cells. Each hemoglobin molecule can bind up to four O₂ molecules, forming oxyhemoglobin. Only a small fraction (

• Primary form: Hemoglobin‑bound O₂.
• Clinical relevance: Hemoglobin concentration and affinity (e.g., 2,3‑BPG levels) critically affect arterial oxygen content.

8. Exercise‑Induced Changes in Tidal Volume

During physical activity, tidal volume increases mainly due to enhanced contraction of the diaphragm and external intercostal muscles. This stronger inspiratory effort expands the thoracic cavity more rapidly, allowing greater air intake per breath.

• Key muscles: Diaphragm and external intercostals.
• Result: ↑ tidal volume → improved oxygen delivery and CO₂ removal.

Understanding this response helps in evaluating exercise tolerance and designing respiratory rehabilitation programs.

Summary of Core Concepts

By mastering the mechanics of breathing and the transport of gases, healthcare professionals can better assess respiratory function, interpret arterial blood gases, and manage conditions such as COPD, asthma, and metabolic acidosis.

• Diaphragm contraction drives quiet inhalation.
• Internal intercostals are essential for forced exhalation.
• Partial pressure gradients are the primary drivers of O₂ and CO₂ diffusion.
• CO₂ is mainly transported as bicarbonate; O₂ is carried bound to hemoglobin.
• Exercise increases tidal volume via stronger diaphragmatic and external intercostal activity.