Lung Volume and Compliance Quiz

Lung Volume Changes During Breathing

• Inspiration: Thoracic cavity expands, lung volume increases, alveolar pressure decreases, pleural pressure decreases, transpulmonary pressure increases
• Expiration: Thoracic cavity contracts, lung volume decreases, alveolar pressure increases, pleural pressure increases, transpulmonary pressure decreases

Pulmonary Compliance

• Compliance: Measures lung and chest recoil
• It is the slope of the pressure-volume curve
• During inspiration, lung volume at a given pressure is less than the volume at the same pressure during expiration (hysteresis loop)
• Normal compliance: 0.1 Liter/cmH2O
• Low compliance: Needs more inflation pressure
• High compliance: Needs less inflation pressure
• Compliance curve shifts downwards and to the right in interstitial pulmonary fibrosis and pulmonary congestion
• Compliance curve shifts upwards and to the left in emphysema

Factors Affecting Compliance

• Compliance decreases (curve shifts downward and to the right): Pulmonary congestion, interstitial pulmonary fibrosis, supine position, restrictive lung disease, pneumothorax, hydrothorax, asthma
• Compliance increases (curve shifts upward and to the left): Emphysema, old age

Elastic Properties of Lung and Thoracic Cage

• Lung: Tissue elastic forces (elastin and collagen fibres of lung parenchyma), elastic forces caused by surface tension of the fluid lining alveoli
• Thoracic cage: Elastic nature of ribs, muscles and tendons

Lung Compliance

• Extent to which the lungs expand for each unit increase in transpulmonary pressure.
• Total compliance of both lungs together in normal adult: 200 ml of air per cm of water transpulmonary pressure.

Surface Tension and Alveolar Surfactant

• Alveoli lined with fluid, creating air-water interface
• Surface tension of water tends to collapse alveoli
• Surfactant reduces surface tension, counteracting collapse
• Surfactant concentration increases as alveoli shrink, stabilizing them.

Functions of Alveolar Surfactant

• Stabilization: Allows alveoli of different radii to remain at equilibrium
• Prevention of collapse: Surfactant minimizes collapse during expiration
• Prevention of pulmonary edema: Prevents fluid from entering the alveoli
• Increases lung compliance: Reduces surface tension, making lungs more stretchable
• Immune function: Regulates lung inflammation and promotes phagocytosis

Surfactant and Laplace’s Law

• Laplace’s Law: Pressure = 2T/r (pressure is proportional to surface tension and inversely proportional to radius)
• Without surfactant, small alveoli would collapse into larger ones
• Surfactant lowers surface tension in small alveoli, allowing them to co-exist with larger ones

Formation and Metabolism of Surfactant

• Surfactant components synthesized in the endoplasmic reticulum, transported through Golgi apparatus, and packaged in lamellar bodies
• Secreted into alveoli, forming a monolayer that lowers surface tension
• Surfactant components recycled by type II cells and alveolar macrophages

Infant Respiratory Distress Syndrome (IRDS)

• Premature babies often lack surfactant, leading to lung collapse
• IRDS can be treated with antenatal steroids to enhance pulmonary maturity, continuous positive airway pressure (CPAP) and surfactant replacement therapy.

Pleural Layers

• Parietal Pleura: Lines the thoracic cavity, subdivided into costal, diaphragmatic, cervical, and mediastinal pleurae
• Visceral Pleura: Closely invests the lung
• Both layers are continuous around lung roots and pulmonary ligaments.

Pleural Recesses

• Spaces formed by folds of parietal pleura; act as reserve space for lung expansion
• Costo-mediastinal recess: Between costal and mediastinal pleurae
• Costo-diaphragmatic recess: Between lower limit of pleural sac and lower border of lung; most dependent part of pleural sac
• Functions of Costo-diaphragmatic Recess: Allows lung expansion in full inspiration; collects fluid if pleural effusion occurs.

Pleura

• A closed serous sac
• Contains two layers: Parietal and Visceral
• Both layers are continuous around lung roots and pulmonary ligaments

Visceral Pleura

• Closely invests lung (except hilum and attachment of lung root) and is inseparable from it

Parietal Pleura

• Named according to the structures it lines
• Subdivided into Costal, Diaphragmatic, Cervical, and Mediastinal pleurae

Costal Pleura

• Lines the inner surface of the sternum, ribs, costal cartilages, intercostal spaces, and sides of the vertebral bodies

Diaphragmatic Pleura

• Covers the thoracic surface of the diaphragm

Cervical Pleura

• Covers the apex of the lung and passes downwards and medially to continue with the mediastinal pleura

Mediastinal Pleura

• Forms the lateral boundary of the mediastinum and covers the mediastinal surface of the lungs

Recesses of Pleura

• Widening of the pleural cavity
• Folds of the parietal pleura which act as reserve spaces for lungs to expand during deep inspiration

Costo-Mediastinal Recess

• Between costal and mediastinal pleurae

Costo-Diaphragmatic Recess

• Potential space between lower limit of pleural sac and lower border of corresponding lung

Costo-Diaphragmatic Recess - Lower Limit of Pleura and Lung

• At the mid-clavicular, mid-axillary, and scapular lines: 8th, 10th, and 12th ribs respectively
• Lower border of lung: 6th, 8th, and 10th ribs

Costo-Diaphragmatic Recess - Functions

• Allows expansion of lungs in full inspiration
• Most dependent part of the pleural sac
• If fluid appears in the pleural sac, it will collect first in the Costo-diaphragmatic recess

Determinants of Lung Compliance

• Both lungs and the thoracic cage are viscoelastic structures which can be expanded (stretched)
• The measure of expansibility/distensibility is known as compliance
• Lung compliance: 200 ml of air per cm of water transpulmonary pressure in a normal adult
• Tissue elastic forces represent 1/3 of total lung elasticity
• Fluid-air surface tension elastic forces in alveoli represent 2/3 of total lung elasticity

Pulmonary Compliance - Factors Affecting Compliance

• Decreased compliance (curve shifted downward and to the right): Pulmonary congestion, interstitial pulmonary fibrosis, supine position, restrictive lung disease, pneumothorax, hydrothorax, asthma
• Increased compliance (curve shifted upward and to the left): Emphysema, old age

Surface Tension

• In each alveolus there is an interface between air and water
• The water molecules at the superficial layer of alveolar fluid are attracted laterally and inwardly
• This causes the fluid layer to shrink or minimize the liquid-air interface (surface tension)
• The alveolus tends to collapse
• Surfactant resists the collapse of the lung

Action of Surfactant

• Presence of surfactant reduces surface tension
• Surfactant content of each alveolus is relatively constant
• When the alveolus size decreases, the alveolar surfactant concentration increases, which lowers surface tension

Functions of Alveolar Surfactant

• Alveolar stabilization: Enables alveoli of different radius to remain at equilibrium
• Prevention of collapse of alveoli: Alveoli become smaller during expiration
• Prevent pulmonary edema: Surfactant deficiency causes high surface tension of alveoli, drawing fluids from capillaries leading to pulmonary edema
• Increases lung compliance: Reduces the centripetal force of surface tension in alveoli and increases stretch ability of lungs
• Immune function: Regulates lung inflammation, facilitates phagocytosis

Components of Surfactants

• Surfactant is a lipid surface-tension-lowering agent
• It is a mixture of dipalmitoylphosphatidylcholine (DPPC), other lipids, and proteins
• Law of Laplace: P = 2 T/r (distending pressure equals two times the tension divided by the radius)
• If alveoli have half the normal radius, the pressures are doubled

Use of Surfactant

• Prevents collapse during expiration
• Preventions pulmonary edema

Laplace’s Law

• Relates pressure to (surface) tension and radius: Pressure = 2 T/r
• Small radius = High pressure
• Without surfactant small alveoli would empty into bigger alveoli and collapse
• In presence of surfactant, surface tension is no longer a constant

Formation of Surfactant

• Components of surfactant are synthesized from precursors in the endoplasmic reticulum and transported through the Golgi apparatus by multi-vesicular bodies.
• Components are packaged in lamellar bodies, intracellular storage granules.
• Secretion (exocytosis) into the liquid lining of the alveolus, surfactant phospholipids are organized into a complex lattice called tubular myelin
• Tubular myelin is believed to generate the phospholipid that provides material for a monolayer at the air-liquid interface in the alveolus, lowering surface tension

Metabolism of Surfactant

• Surfactant phospholipids and proteins are subsequently taken back into type II cells in the form of small vesicles
• Transported for storage into lamellar bodies for recycling
• Alveolar macrophages also take up some surfactant in the liquid layer.
• Phospholipid components of surfactant remain in the alveolar lumen for few hours and taken back into type II cells and are reused 10 times before being degraded

IRDS (Infant Respiratory Distress Syndrome)

• Respiratory distress syndrome of the newborn/infant/hyaline membrane disease
• Surfactant is not usually secreted into the alveoli until 6-7 months (or even later) of gestation
• Many premature babies have little or no surfactant and their lungs have an extreme tendency to collapse
• Improvement in morbidity and mortality in infants with IRDS by:
  • Antenatal steroids to enhance pulmonary maturity
  • Appropriate resuscitation - immediate use of continuous positive airway pressure (CPAP)