Respiratory System: Lung Compliance and Airway Resistance

Trachea Functional Anatomy

• Respiratory zone: site of gas exchange, consisting of microscopic structures (respiratory bronchioles, alveolar ducts, and alveoli)
• Conducting zone: conduits to gas exchange sites, includes all other respiratory structures
• Respiratory muscles: diaphragm and other muscles that promote ventilation

The Upper Respiratory Tract

• Bronchi and subdivisions: 23 orders of branching, forming the bronchial (respiratory) tree
• Conducting zone structures:
  • Trachea → right and left main (primary) bronchi
  • Each main bronchus enters the hilum of one lung
  • Right main bronchus is wider, shorter, and more vertical than the left
• Each main bronchus branches into lobar (secondary) bronchi (three right, two left)
• Each lobar bronchus supplies one lobe

Respiratory Zone

• Respiratory bronchioles, alveolar ducts, alveolar sacs (clusters of alveoli)
• ~300 million alveoli account for most of the lungs’ volume and are the main site for gas exchange
• Alveoli are surrounded by fine elastic fibers and contain open pores that connect adjacent alveoli and allow air pressure throughout the lung to be equalized

Respiratory Membrane

• ~0.5-μm-thick air-blood barrier where alveolar and capillary walls meet
• Alveolar walls consist of a single layer of squamous epithelium (type I cells) and scattered type II cells that secrete surfactant and antimicrobial proteins

Pleura of the Lungs

• Thin, double-layered serosa
• Parietal pleura on thoracic wall and superior face of diaphragm
• Visceral pleura on external lung surface
• Pleural fluid fills the slitlike pleural cavity, providing lubrication and surface tension

Blood Supply

• Pulmonary circulation (low pressure, high volume):
  • Pulmonary arteries deliver systemic venous blood
  • Pulmonary veins carry oxygenated blood from respiratory zones to the heart
• Systemic circulation (high pressure, low volume):
  • Bronchial arteries provide oxygenated blood to lung tissue
  • Bronchial veins carry most venous blood back to the heart

Mechanics of Breathing

• Pulmonary ventilation consists of two phases:
  1. Inspiration: gases flow into the lungs
  2. Expiration: gases exit the lungs
• Pressure relationships in the thoracic cavity:
  • Atmospheric pressure: 760 mm Hg at sea level
  • Respiratory pressures are described relative to 760 mm Hg
  • Negative respiratory pressure is less than 760 mm Hg
  • Positive respiratory pressure is greater than 760 mm Hg

Intrapulmonary Pressure and Intrapleural Pressure

• Intrapulmonary (intra-alveolar) pressure: pressure in the alveoli, fluctuates with breathing
• Intrapleural pressure: pressure in the pleural cavity, fluctuates with breathing, and is always negative

Airway Resistance and Lung Compliance

• Airway resistance: as resistance rises, breathing movements become more strenuous
• Lung compliance: a measure of the change in lung volume that occurs with a given change in transpulmonary pressure
• Normally high due to distensibility of lung tissue and alveolar surface tension
• Diminished by nonelastic scar tissue (fibrosis), reduced production of surfactant, and decreased flexibility of the thoracic cage

Gas Exchanges

• External respiration: exchange of O2 and CO2 between the lungs and the atmosphere
• Internal respiration: exchange of O2 and CO2 between the blood and tissues

Dalton's Law of Partial Pressures

• Total pressure exerted by a mixture of gases is the sum of the pressures exerted by each gas
• The partial pressure of each gas is directly proportional to its percentage in the mixture

Henry's Law

• When a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion to its partial pressure
• At equilibrium, the partial pressures in the two phases will be equal
• The amount of gas that will dissolve in a liquid also depends on its solubility