Canonical Question
ii. Pulm Gas Volumes – Factors
Master answer
Functional residual capacity
- Volume of air in lungs at the end of normal tidal expiration
- Occurs at equilibrium point between the recoil of chest/diaphragm and lungs to collapse inwards
- Sum of residual volume and expiratory reserve volume
- Normal FRC 30ml/kg (approx 2.2l in 70kg male)
Function of FRC
- Oxygen reservoir
- Normally 270mls O2 → can be expanded to 1.8l if breathing FiO2 100%
- PaO2 buffer
- Ensures no fluctuation in PaO2 between inspiration and expiration
- Prevents atelectasis
- Normally FRC above closing capacity
- Decreases V/Q mismatch
- Minimizes work of breathing
- Minimizes airway collapse → resistance decreased
- Minimizes alveolar atelectasis → decreased work required to re-inflate alveoli
- Lung kept at most compliant segment of compliance curve
- Decreases pulmonary vascular resistance
- Extra-alveolar and alveolar capillaries kept open
- Minimizes hypoxic vasoconstriction
Factors Affecting FRC
| Factors which increase FRC | Factors which reduce FRC |
|---|---|
| Age | Impaired Lung and chest wall compliance |
| Acute asthma | Increased Intra-abdominal pressure |
| Emphysema | Supine position (by 20%) |
| PEEP Extrinsic Intrinsic (gas trapping) | Anaesthesia (by 20) |
| Upright and Prone position | Obesity |
Sakurai / Gladwin / JC 2020
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2020B Q02 | Define functional residual capacity (10% marks). Outline the functions of the functional residual capacity (70% marks) and the factors affecting it (20% marks). | historical_member | — |