RES 2

Lung Volumes and Capacities

• Pulmonary Volumes

  • Represent the specific amounts of air in each phase of the respiratory cycle.

  1) Tidal Volume (VT)

  • Volume of air inspired or expired during a normal quiet respiratory cycle

  • Approximately 500 mL

  2) Inspiratory Reserve Volume (IRV)

  • Volume of air that can be forcefully inspired after a normal quiet inspiration

  • Approximately 3,000 mL

  • Note: This is 6 times the tidal volume

  3) Expiratory Reserve Volume (ERV)

  • Volume of air that can be forcefully expired after a normal quiet expiration

  • Approximately 1,100 mL

  4) Residual Volume (RV)

  • Volume of air remaining in the respiratory passages and lungs after forceful expiration

  • Approximately 1,200 mL

  • Cannot be measured directly by spirometry

• Pulmonary Capacities

  • Combinations of volumes indicating the total functional capability of the lungs.

  1) Total Lung Capacity (TLC)

  • Sum of IRV + VT + ERV + RV

  • Approximately 5,800 mL

  • Cannot be measured directly by spirometry

  2) Inspiratory Capacity (IC)

  • IRV + VT

  • Approximately 3,500 mL

  3) Functional Residual Capacity (FRC)

  • Volume remaining in the lungs after normal quiet expiration

  • Approximately 2,300 mL

  • Cannot be measured directly by spirometry

  4) Vital Capacity (VC)

  • Volume that can be forcefully expired after maximum inhalation

  • Approximately 4,600 mL

  • Represents about 9 times the tidal volume.

Types of Ventilation

• Ventilation Frequency: Normal range is about 12 to 15 breaths/min

• Minute Ventilation:

  • Defined as VT × frequency (f)

  • For example:

  • $VT ext{ (500 mL/breath)} imes f ext{ (12 breaths/min)} = 6,000 ext{ mL/min (6 L/min)}$

• Dead Space: Regions where gas exchange does not occur.

  • Anatomical Dead Space: Conducting zone (upper respiratory tract). Approximately 150 mL in a normal adult.

  • Physiological Dead Space: Includes anatomical dead space plus poorly ventilated alveoli.

Physiological Significance of Surfactant

• Surfactant Composition:

  • Mixture of proteins (10%) and lipids (90%).

• Functions of Surfactant:
  1) Reduces surface tension, aiding lung compliance.
  2) Stabilizes alveoli, preventing collapse and ensuring even ventilation.
  3) Keeps alveolar surfaces dry, reducing excess fluid accumulation.

• Surface Tension:

  • Pure water-air interface generates approximately 70 dynes/cm. Surfactant reduces this to about 25 dynes/cm during quiet respiration.

• Laplace's Law:

  • Predicts pressure within a bubble.

  • $p = \frac{2T}{r}$

  • Where:

  • p = pressure

  • T = surface tension

  • r = radius of the bubble

• Impact on Alveolar Expansion:

  • Smaller alveoli experience greater inward pressures but surfactant allows them to expand more easily by reducing surface tension.

Factors Affecting Lung Compliance

• Pulmonary Fibrosis: Decreases compliance due to lung stiffening.

• Ageing: Alterations in elastic tissue can increase compliance.

• Diseases like Emphysema: Destroy elastic fibers, making lung inflation more difficult and reducing compliance.

Sample Questions

1) Total volume left in lungs after quiet expiration:

• a) Residual volume

• b) Expiratory reserve volume

• c) Functional residual capacity

• d) Vital capacity

2) Total volume that can be maximally inhaled after normal inspiration:

• a) Residual volume

• b) Expiratory reserve volume

• c) Inspiratory reserve volume

• d) Tidal volume

3) Effect of fibrotic lungs on compliance:

• a) Increase

• b) Remain the same

• c) Decrease

4) Application of Laplace's law:

• a) Smaller diameter alveoli empty into larger ones

• b) Larger diameter alveoli empty into smaller ones

• c) Inward pressure is consistent regardless of size

• d) Inward pressure directly proportional to size

Answers

1) c
2) c
3) c
4) a