Respiratory Physiology Calculators

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Respiratory physiology studies how the lungs exchange oxygen and carbon dioxide between the atmosphere and blood, and how the respiratory control system regulates breathing to maintain blood gas homeostasis. Key concepts include lung volumes and capacities (tidal volume, vital capacity, FEV₁), alveolar gas exchange (governed by Fick's law and the alveolar gas equation), oxygen transport by hemoglobin (oxyhemoglobin dissociation curve, Bohr effect), CO₂ transport (bicarbonate system, carbaminohemoglobin), and central/peripheral respiratory control (medullary rhythm generators, carotid body chemoreceptors).

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Lung Volumes and Capacities

  • Tidal volume (TV): ~500 mL per breath at rest
  • Inspiratory reserve volume (IRV): ~3000 mL (maximum additional air inspired beyond TV)
  • Expiratory reserve volume (ERV): ~1200 mL (maximum air expelled beyond TV)
  • Residual volume (RV): ~1200 mL (air remaining after maximum exhalation — cannot be exhaled)
  • Vital capacity (VC) = TV + IRV + ERV ≈ 4700 mL
  • Total lung capacity (TLC) = VC + RV ≈ 5900 mL
  • FEV₁: forced expiratory volume in 1 second; FEV₁/FVC ratio < 0.70 = obstructive disease (asthma, COPD)

Alveolar Gas Exchange

Governed by Fick's law: V̇gas ∝ A × D × (P₁ − P₂)/T, where A = surface area; D = diffusion coefficient; P₁−P₂ = partial pressure difference; T = membrane thickness. Alveolar–arterial PO₂ gradient (A-a gradient) normally < 15 mmHg; elevated in V/Q mismatch, diffusion impairment.

Oxygen Transport

Hemoglobin: 4 heme groups/molecule; each binds one O₂. Oxyhemoglobin dissociation curve: sigmoidal (cooperative binding); P50 = 26.5 mmHg (O₂ pressure giving 50% saturation). Bohr effect: decreased pH or increased CO₂ shifts curve right → lower O₂ affinity → facilitates O₂ release to tissues.

Respiratory Control

Central pattern generator in medulla (pre-Bötzinger complex) drives rhythmic breathing. Central chemoreceptors (medulla): respond to CSF pH (from PCO₂). Peripheral chemoreceptors (carotid bodies): respond to PaO₂, PaCO₂, pH. Hypercapnia is the primary drive; hypoxia stimulates mainly via carotid bodies.

Glossary

Vital Capacity (VC)
Maximum volume exhaled after maximum inspiration: VC = TV + IRV + ERV ≈ 4700 mL; measured by spirometry; reduced in restrictive and obstructive lung disease.
Bohr Effect
The decrease in hemoglobin O₂ affinity caused by decreased pH or increased CO₂; shifts the oxyhemoglobin dissociation curve right; facilitates O₂ unloading in metabolically active tissues.
FEV₁/FVC Ratio
Forced expiratory volume in 1 second divided by forced vital capacity; < 0.70 indicates obstructive airflow limitation (asthma, COPD); normal ratio with reduced FVC indicates restrictive disease.

Frequently Asked Questions

Tidal volume (TV) ≈ 500 mL: air moved per normal breath. Inspiratory reserve volume (IRV) ≈ 3000 mL: extra air inspireable above TV. Expiratory reserve volume (ERV) ≈ 1200 mL: extra air expellable below TV. Residual volume (RV) ≈ 1200 mL: air remaining after maximum exhalation (prevents alveolar collapse). Vital capacity (VC) = TV + IRV + ERV ≈ 4700 mL: maximum breath volume. Total lung capacity (TLC) ≈ 5900 mL. Spirometry measures all volumes except RV (requires gas dilution or plethysmography). FEV₁/FVC < 0.70 indicates obstructive disease; reduced FVC with normal ratio indicates restrictive disease.

Each hemoglobin molecule contains four heme groups, each binding one O₂. Binding is cooperative (positive cooperativity) — binding of the first O₂ increases affinity for subsequent O₂ molecules, producing the sigmoidal oxyhemoglobin dissociation curve. At arterial PO₂ ≈ 100 mmHg: Hb is ~98% saturated. At venous PO₂ ≈ 40 mmHg: Hb is ~75% saturated. ~25% of O₂ is extracted per circulation pass at rest. Bohr effect: decreased pH or increased CO₂ (both in exercising tissues) shifts the curve right (lower affinity), facilitating O₂ unloading exactly where it is needed.

The Bohr effect describes how CO₂ and H⁺ (decreased pH) decrease hemoglobin's affinity for oxygen — shifting the oxyhemoglobin dissociation curve to the right. In exercising muscle: high CO₂ production → CO₂ enters red blood cells → carbonic anhydrase converts CO₂ to H₂CO₃ → H⁺ + HCO₃⁻ → H⁺ binds globin chains → changes Hb conformation → decreases O₂ affinity → more O₂ released to muscle. In the lungs: CO₂ is exhaled → PCO₂ falls → pH rises → Hb affinity for O₂ increases (Bohr effect reversed) → Hb fully loads with O₂. The Bohr effect is a beautiful physiological mechanism that automatically delivers more O₂ to the tissues that need it most.

Respiratory rhythm is generated by the pre-Bötzinger complex in the medulla. Rate is regulated by: (1) Central chemoreceptors (medulla): sense PCO₂ via CSF pH — hypercapnia (↑PCO₂ → ↓pH) is the strongest normal stimulus for increased ventilation. (2) Peripheral chemoreceptors (carotid bodies at carotid bifurcation; aortic bodies): sensitive to PaO₂ (hypoxic drive below PaO₂ 60 mmHg), PaCO₂, and pH. Carotid bodies mediate the hypoxic ventilatory response. At high altitude: hypoxia stimulates carotid bodies → hyperventilation → ↓PCO₂ → respiratory alkalosis → partially compensated by renal HCO₃⁻ excretion over days.