respiratory system physiology

• Major Respiratory Organs


  • Nasal cavity
  • Oral cavity
  • Epiglottis
  • Glottis
  • Trachea
  • Right lung
  • Primary bronchi
  • Secondary bronchi
  • Tertiary bronchi
  • Bronchioles
  • Terminal bronchioles
  • Diaphragm
  • Left lung
  • Esophagus
  • Larynx
  • Pharynx
• Conducting Zone


  • Air passageway: 150 mL in volume (dead space)
  • Increases air temperature to body temperature
  • Humidifies air
  • Epithelium has goblet cells that secrete mucus
  • Epithelium has ciliated cells that move particles toward mouth
• Respiratory Zone


  • Respiratory bronchioles
  • Alveolar ducts
  • Alveolar sacs
  • Alveoli
• Alveoli


  • Site of gas exchange
  • 300 million alveoli in the lungs (size of tennis court)
  • Rich blood supply: capillaries form sheet over alveoli
  • Alveolar pores
  • Type I alveolar cells make up wall of alveoli
  • Type II alveolar cells secrete surfactant
  • Alveolar macrophages
• Respiratory Membrane


  • Epithelial cell layer of alveoli (type I)
  • Endothelial cell layer of capillaries
  • 0.2 μm thick
• Structures of the Thoracic Cavity


  • Chest wall: rib cage, sternum, thoracic vertebrae, connective tissue, intercostal muscles
  • Diaphragm
  • Pleural sac around each lung
  • Intrapleural space filled with intrapleural fluid (Volume = 15 mL)
• Chest Wall


  • Airtight, protects lungs
• Pleural Sac


  Membrane lining of lungs and chest wall
• Intrapleural Space


  Filled with intrapleural fluid
• Air Movement


  • Air moves in and out of lungs by bulk flow
  • Pressure gradient drives flow
  • Air moves from high to low pressure
• Inspiration


  Pressure in lungs less than atmospheric pressure
• Expiration


  Pressure in lungs greater than atmospheric pressure
• Atmospheric Pressure


  • 760 mm Hg at sea level
  • Decreases as altitude increases
  • Increases under water
• Intra-alveolar Pressure (Palv)


  • Pressure of air in alveoli
  • Given relative to atmospheric pressure
  • Varies with phase of respiration
  • During inspiration = negative (less than atmospheric)
  • During expiration = positive (more than atmospheric)
• Intrapleural Pressure (Pip)


  • Pressure inside pleural sac
  • Always negative under normal conditions
  • Always less than Palv
  • Varies with phase of respiration
  • At rest, –4 mm Hg
  • Negative due to elasticity in lungs and chest wall
  • Pneumothorax occurs when air enters the pleural space, causing the lung to collapse
• Transpulmonary Pressure


  • Transpulmonary pressure = Palv – Pip
  • Distending pressure across the lung wall
  • Increase in transpulmonary pressure increases distending pressure across lungs and causes lungs (alveoli) to expand, increasing volume
• Boyle's Law


  • Pressure is inversely related to volume
  • If amount of gas is the same and container size is reduced, pressure will increase
  • P1V1 = P2V2
• Determinants of Intra-alveolar Pressure


  • Quantity of air in alveoli
  • Volume of alveoli
  • Lungs expand—alveolar volume increases, Palv decreases, pressure gradient drives air into lungs
  • Lungs recoil—alveolar volume decreases, Palv increases, pressure gradient drives air out of lungs
• Respiratory Muscles


  • Inspiratory muscles: diaphragm, external intercostals
  • Expiratory muscles: internal intercostals, abdominal muscles
• Inspiration


  1. External intercostals contract

  2. Diaphragm contracts

  3. Chest wall and lungs expand
• Expiration


  1. External intercostals relax

  2. Diaphragm relaxes

  3. Chest cavity and lungs contract

  4. Internal intercostals and abdominals contract for active expiration only
• Pulmonary pressures


  Pressures related to the lungs and breathing
• Atmospheric pressure is 760 mm Hg at sea level, decreases as altitude increases, increases under water
• Mechanics of breathing


  Mechanisms for creating pressure gradients that drive air movement in and out of the lungs
• Inspiratory muscles


  • Diaphragm, external intercostals - increase volume of thoracic cavity
• Expiratory muscles


  • Internal intercostals, abdominal muscles - decrease volume of thoracic cavity
• Inspiration


  1. Neural stimulation of inspiratory muscles

  2. Diaphragm contraction, chest wall expansion

  3. Intrapleural pressure decreases, transpulmonary pressure increases

  4. Alveoli expand, Palv decreases, air flows in
• Expiration


  1. Normally passive, inspiratory muscles stop contracting

  2. Lungs and chest wall recoil, volume decreases

  3. Active expiration uses expiratory muscles for faster volume decrease
• Lung compliance


  Ease with which lungs can be stretched, larger compliance means easier inspiration
• Factors affecting lung compliance


  • Elasticity (ability to resist stretch)
  • Surface tension of lungs (surfactant decreases tension and increases compliance)
• Airway resistance


  Pressure gradient needed for air flow, normally low at ~1 mm Hg, increase makes breathing harder
• Factors affecting airway resistance


  • Contractile activity of smooth muscle (bronchoconstriction increases, bronchodilation decreases resistance)
  • Mucus secretion
• Extrinsic control of bronchiole radius


  • Autonomic nervous system (sympathetic relaxation, parasympathetic contraction of smooth muscle)
  • Hormonal control (epinephrine relaxation)
• Intrinsic control of bronchiole radius


  • Histamine (bronchoconstriction, increased mucus)
  • CO2 (bronchodilation)
• Respiratory volumes


  Measurements of the amount of air in the lungs at different stages of the breathing cycle
• Respiratory capacities


  Sums of two or more respiratory volumes
• Total lung volume is divided into a series of volumes (4) and capacities (4) useful in diagnosing problems
• Tidal volume (VT)


  Amount of air taken in during inhalation (~500 mL), with ~350 mL entering the alveoli and ~150 mL remaining in the conducting passageways (anatomic dead space)
• Respiratory rate (f)


  12-20 breaths per minute
• Inspiratory reserve volume (IRV)


  Maximum air inspired at the end of a normal inspiration (~3000 mL)