LEC 18: LUNGS

• Gross structure of lungs

  • Occupy entire thoracic cavity except mediastinum
  • Each lung suspended within own pleural cavity
  • Connected to mediastinum by lung root
  • Left lung smaller due to heart position
  • Divided into lobes separated by fissures
  • Lung tissue primarily air spaces
  • Lung stroma = elastic connective tissue
• Pleurae


  • Parietal pleura covers thoracic wall, diaphragm, heart surroundings
  • Visceral pleura covers external lung surface
  • Pleural fluid lubricates space between pleurae to reduce friction
  • Divides thoracic cavity into 3 chambers to prevent organs interference & limit infection spread
• Innervation & circulation


  • Lungs innervated by parasympathetic and sympathetic motor fibers along with visceral sensory fibers
  • Two circulations: Pulmonary network for systemic blood oxygenation, Bronchial arteries for lung tissue systemic blood supply
• Boyle's law


  Pressure of gas varies inversely with its volume
• Inspiration


  1. Diaphragm and intercostal muscles contract

  2. Thoracic volume increases

  3. Intrapulmonary pressure drops below atmospheric pressure

  4. Air flows INTO the lungs
• Expiration


  1. Diaphragm and intercostal muscles relax

  2. Thoracic volume decreases

  3. Intrapulmonary pressure rises above atmospheric pressure

  4. Air flows OUT of lungs
• Respiratory muscles


  • Contraction of diaphragm and intercostal muscles during inspiration increases thoracic volume, decreases intrapulmonary pressure, creates pressure gradient allowing air flow into lungs
  • Forced inspiration: accessory muscles of neck and thorax increase thoracic volume, facilitating greater air intake
• Lung elasticity


  • Lung tissue contains elastic fibers that recoil during expiration
  • Relaxation of respiratory muscles = lungs passively recoil to original size, decreasing lung volume, increasing intrapulmonary pressure, causing air to flow OUT of lungs
  • Aids in passive expiration process
• Functional importance of partial vacuum in intrapleural space

  Creates negative intrapleural pressure, holds lungs to thoracic wall, maintains lung inflation, prevents lung collapse, promotes lung expansion during inspiration
• Physical factors influencing pulmonary ventilation

  • Airway resistance
  • Alveolar surface tension
  • Lung compliance
  • Lung elasticity
  • Surface tension of the pleural fluid
  • Strength of respiratory muscles
  • Size and shape of the thoracic cavity
  • Presence of any obstructions in the airways
  • Compliance of the chest wall
  • Lung volume and capacity
• Respiratory volumes


  • Tidal volume
  • Inspiratory reserve volume
  • Expiratory reserve volume
• Vital and total lung capacities

  • Vital capacity
  • Residual volume