Lung Volumes and Capacities
| Volume / Capacity | Normal Value | Definition |
|---|---|---|
| Tidal volume (TV) | 500 mL | Volume with normal breath |
| IRV | 3000 mL | Extra air inhaled after normal inspiration |
| ERV | 1200 mL | Extra air exhaled after normal expiration |
| RV | 1200 mL | Air remaining after maximal exhalation (cannot measure with spirometry) |
| VC (IRV+TV+ERV) | 4700 mL | Maximum air moved in one breath |
| FRC (ERV+RV) | 2400 mL | Volume at end-expiration (lung at rest) |
| TLC (all volumes) | 6000 mL | Total lung capacity |
Breathing Mechanics
Inspiration is active (diaphragm + external intercostals contract). Normal expiration is passive (elastic recoil). Forced expiration uses internal intercostals and abdominals.
- Compliance = change in volume / change in pressure. High compliance = easy to stretch (emphysema). Low compliance = stiff lung (pulmonary fibrosis, pulmonary edema).
- Surfactant (produced by type II pneumocytes) reduces surface tension → increases compliance → prevents alveolar collapse. Deficient in RDS of newborn (premature infants).
- Airway resistance — increases in small airways (asthma, mucus, edema). Bronchodilators (beta-2 agonists) decrease resistance.
Gas Exchange
Gas exchange occurs at the alveolar-capillary membrane by passive diffusion (Fick's law: rate proportional to area, pressure gradient, and diffusion coefficient; inversely proportional to membrane thickness).
| Gas | Alveolar Level | Arterial Level | Venous Level |
|---|---|---|---|
| PO2 | PAO2 ~100 mmHg | PaO2 ~95 mmHg | PvO2 ~40 mmHg |
| PCO2 | PACO2 ~40 mmHg | PaCO2 ~40 mmHg | PvCO2 ~46 mmHg |
A-a gradient = PAO2 - PaO2 (normal: less than 10-15 mmHg on room air). Elevated in V/Q mismatch, diffusion impairment, R-to-L shunt. Normal in hypoventilation (PaCO2 elevated, PaO2 decreased).
V/Q Matching
Ventilation-perfusion (V/Q) ratio determines gas exchange efficiency. Normal V/Q ~0.8.
| V/Q State | V/Q Ratio | Example | Effect |
|---|---|---|---|
| Normal | ~0.8 | Healthy lung base (better perfusion) | Efficient gas exchange |
| V/Q = 0 (shunt) | 0 | Pneumonia, atelectasis, ARDS | Blood passes without gas exchange; hypoxia does NOT improve with O2 |
| V/Q = infinity (dead space) | Infinity | Pulmonary embolism | Ventilated but not perfused; CO2 retention |
| V/Q mismatch | Between 0 and infinity | COPD, asthma | Most common cause of hypoxia; responds to supplemental O2 |
O2 and CO2 Transport
Oxygen transport: 97% bound to hemoglobin (1 Hgb carries 4 O2); 3% dissolved in plasma. Oxyhemoglobin dissociation curve: S-shaped (cooperative binding). Right shift (decreased affinity — offloads O2 to tissues): increased temperature, increased CO2, increased 2,3-DPG, acidosis.
CO2 transport: 70% as bicarbonate (CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-; catalyzed by carbonic anhydrase in RBCs). 20% as carbaminohemoglobin. 10% dissolved.
Respiratory Control
The respiratory center is in the medulla (pre-Botzinger complex for rhythm) and pons (apneustic and pneumotaxic centers for rate modification).
- Central chemoreceptors (medulla): respond primarily to PaCO2 (via CO2 crossing BBB → H+ production). Main driver of respiratory drive in healthy individuals.
- Peripheral chemoreceptors (carotid bodies, aortic arch): respond to PaO2 less than 60 mmHg, PaCO2, pH. Primary driver in COPD patients (hypoxic drive).