- Dead space: The volume of air in the conducting airways can’t participate in gas exchange, so it’s called anatomic “dead space.” It includes air from the nasal passages to the terminal bronchioles. The average anatomic dead space is 150 ml.
Physiologic dead space is the total volume of air that does not participate in gas exchange. It includes:
- Anatomic dead space
- Functional (alveolar) dead space from unperfused alveoli
Normally, alveolar dead space is close to zero, so physiologic dead space is close to anatomic dead space. Physiologic dead space increases in V/Q defects. The ratio of physiologic dead space to TV is ⅓.
2. Bronchopulmonary segments: These are lung segments supplied by their own segmental (tertiary) bronchus and a branch of the pulmonary artery. This arrangement allows individual segments to be resected separately. There are 10 segments in the right lung and 8 segments in the left lung.
3. TLC corrected for age remains almost constant throughout life. However, the gradual increase in FRC and RV with age results in simultaneous decreases in IC and VC.
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Factors that increase FRC
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Factors that decrease FRC
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| PEEP, CPAP, tall stature, male sex, obstructive lung disease.
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Supine position; Induction of anesthesia, female sex, pregnancy, ascites, post-op period, restrictive lung disease,
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- Closing capacity and volume: Closing capacity (CC) is the volume of air remaining in the lungs when small airways start to close. In contrast, residual volume (RV) is the volume remaining when all small airways are closed. The volume of air expired between CC and RV is called the closing volume.
- Hysteresis of the lung: Hysteresis means the dynamic pressure-volume loop during inspiration differs from the loop during expiration. This occurs because surface tension and small airway resistance differ between inspiration and expiration.
Surfactant reduces surface tension in the alveoli. During inspiration, as alveoli expand, surfactant is dispersed over a larger surface area, which increases surface tension. The opposite occurs during expiration.
Hysteresis is also influenced by airway recruitment and closure:
- During inspiration, progressively larger numbers of smaller airways are recruited.
- During expiration, progressively larger numbers of smaller airways close.
- Hering-Breuer reflex: Stretch receptors in airway smooth muscle are activated by distension of the lungs and airways. This triggers a reflex decrease in respiratory rate. Expiration is prolonged, allowing more air to be breathed out.
- Oxygen content of blood: Oxygen content is the total amount of O2 present per volume of blood. It depends mainly on hemoglobin concentration and hemoglobin’s O2 binding capacity. Variants of hemoglobin have different O2 binding capacities. Oxygen content determines O2 delivery to tissues: lower O2 content results in lower O2 delivery, and higher O2 content results in higher O2 delivery.
