PY6.1-13 | Respiratory Physiology — Glossary

Airways from the nose to terminal bronchioles that warm, humidify, and filter air but perform no gas exchange; constitutes anatomical dead space (~150 mL).

Respiratory bronchioles, alveolar ducts, and alveoli where gas exchange occurs; contains ~300 million alveoli with ~70 m2 surface area.

The five-layer barrier (0.2-0.5 micrometres thick) between alveolar air and capillary blood across which O2 and CO2 diffuse.

Flat alveolar epithelial cells covering 95% of the alveolar surface area; form the thin gas exchange surface.

Cuboidal alveolar cells that produce surfactant (DPPC); mature by week 35 of gestation.

Phospholipid mixture (mainly DPPC) that reduces alveolar surface tension, prevents collapse, and maintains alveolar stability according to LaPlace's Law.

At constant temperature, pressure and volume are inversely proportional (P1V1 = P2V2); the physical basis for airflow during breathing.

The ease with which the lung expands; measured as change in volume per unit change in pressure (C = delta-V/delta-P); increased in emphysema, decreased in fibrosis.

The tendency of the lung to recoil to its resting volume; the inverse of compliance.

Volume of air breathed in or out during a normal quiet breath, approximately 500 mL.

Maximum volume of air that can be exhaled after a maximal inspiration; equals IRV + TV + ERV (~4,600 mL).

Volume of air remaining in the lungs after maximal expiration (~1,200 mL); cannot be measured by spirometry.

Ratio of forced expiratory volume in 1 second to forced vital capacity; reduced (<0.7) in obstructive disease, normal or increased in restrictive disease.

Volume of the conducting airways (~150 mL) where no gas exchange occurs.

Sum of anatomical and alveolar dead space; in healthy individuals nearly equals anatomical dead space.

Volume of fresh air reaching the alveoli per minute; VA = (TV - VD) x RR; the effective ventilation for gas exchange.

Rate of gas diffusion is proportional to surface area, diffusion coefficient, and pressure gradient, and inversely proportional to membrane thickness.

Sigmoid curve relating PO2 to haemoglobin oxygen saturation; the S-shape reflects cooperative binding of O2 to Hb's four haem groups.

Increased CO2 and H+ (decreased pH) shift the oxyhaemoglobin dissociation curve rightward, promoting O2 release to metabolically active tissues.

Deoxygenated haemoglobin carries more CO2 than oxyhaemoglobin; facilitates CO2 loading at tissues and CO2 unloading at lungs.

Exchange of Cl- for HCO3- across the RBC membrane (via Band 3 protein) to maintain electrical neutrality during CO2 transport.

Ventilation-perfusion ratio; ideally ~0.8 overall; V/Q = 0 is shunt, V/Q = infinity is dead space.

Unique pulmonary response where low alveolar PO2 causes local arteriolar constriction, diverting blood to better-ventilated regions.

Sensors on the ventrolateral medulla that respond to H+ in CSF (derived from CO2); responsible for ~80% of the chemical drive to breathe.

Carotid body (CN IX) and aortic body (CN X) sensors that detect low PaO2, high PaCO2, and low pH; the only sensors of blood oxygen levels.