Respi 2

• Mechanics of Breathing


  Factors that participate in moving of the lungs and chest wall along the respiratory cycle

  
• Breathing


  Pulmonary ventilation, consisting of inspiration (air flows into the lungs) and expiration (gases exit the lungs)
• Pulmonary Ventilation


  A mechanical process that depends on volume changes in the thoracic cavity, which lead to pressure changes and flow of gases to equalize pressure
• Pressure changes and airflow

  • Flow of air into and out of the lungs is governed by Boyle's Law (changes in volume result in changes in pressure) and air flows from areas of higher to lower pressure

  
• Boyle's Law


  Relationship between the pressure and volume of gases, where volume is inversely proportional to pressure (if temperature is constant)
• Pulmonary Pressures


  • Atmospheric pressure (Patm)
  • Intrapulmonary pressure (Palv)
  • Intrapleural pressure (Pip)
  • Transpulmonary pressure

  
• Atmospheric Pressure (Patm)


  760 mmHg at sea level, decreases as altitude increases, increases under water, for convenience set to 0 mmHg
• Intra-alveolar pressure (Palv)


  Also known as intrapulmonary pressure, pressure in the alveoli, at rest equalizes with Patm, varies with phase of respiration

  
• Intrapleural Pressure (Pip)


  Pressure inside the pleural space, always less than Palv and Patm, at rest -4 mmHg, varies with phase of respiration

  
• Transpulmonary Pressure


  Palv - Pip, distending pressure across the lung wall that keeps the lungs against the chest wall and prevents collapse
• Driving force for airflow

  Pressure gradient drives flow, air moves from high to low pressure, inspiration has negative pressure, expiration has positive pressure
• Resting phase of respiration

  1. Inspiratory muscles contract, thoracic cavity volume increases, lungs stretched, intrapulmonary pressure drops, air flows into lungs

  2. Inspiratory muscles relax, thoracic cavity volume decreases, elastic lungs recoil passively, intrapulmonary pressure rises, air flows out of lungs

  
• Inspiratory Phase


  Intrapulmonary volume increases, intrapulmonary pressure drops from 0 mmHg to -1 mmHg, air rushes into lungs along pressure gradient until Ppul = Patm

  
• Expiratory Phase


  Both thoracic and intrapulmonary volume decrease, compression of alveoli increases Ppul from 0 mmHg to +1 mmHg, forces gases to flow out of lungs

  
• Intrapulmonary pressure and intrapleural pressure fluctuate with breathing phases, intrapulmonary pressure eventually equalizes with atmospheric pressure, intrapleural pressure is always less than intrapulmonary and atmospheric pressure
• Gas exchange of the lungs

  Diffusion of O2 and CO2 in the lungs and peripheral tissues
• Dalton's law of partial pressure

  Each gas in a mixture of gases will exert a pressure independent of other gases present, the total pressure of a gas mixture is equal to the sum of the individual gas pressures
• In sea level atmosphere, Patm = 760 mmHg = Pn2 + Po2 + Pco2 + Ph2o

  
• In humidified tracheal air at 37°C, total atmospheric pressure = 760 - 47 = 713 mmHg, Po2 = 713 x 0.21 = 150 mmHg, Pn2 = 713 x 0.79 = 563 mmHg

  
• Charles' Law


  If pressure is constant, the volume of a gas and its temperature vary proportionately

  
• Henry's Law (Dissolved gases)


  The solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid, the amount of gas dissolved depends on its solubility
• Po2 in alveolar air is approximately equal to Po2 in blood, CO2 is the most soluble gas, O2 is 1/20th as soluble as CO2, N2 is practically insoluble in plasma, dissolved [O2] = Po2 x solubility of O2 in blood
• Partial pressures of gases in blood

  Depend on solubility of gas in fluid, temperature of fluid, and partial pressure of gas

  
• Structural Characteristics of respiratory membrane

  • Alveolar fluid with surfactant
  • Alveolar epithelium
  • Epithelial basement membrane
  • Interstitial space
  • Capillary basement membrane
  • Capillary endothelial membrane
• Diffusion


  Transfer of gas across the blood-gas barrier, net movement from high to low partial pressure until no partial pressure difference exists
• Factors affecting rate of gas diffusion

  • Partial pressure difference
  • Surface area of membrane
  • Thickness of membrane
  • Diffusion coefficient (solubility and molecular weight)
• Fick's Law of Diffusion

  Rate of diffusion is proportional to area and partial pressure difference, but inversely proportional to thickness and square root of molecular weight
• Gas diffusion through respiratory membrane

  Depends on membrane thickness, diffusion coefficient, surface area, and partial pressure differences
• Diffusion capacity of lung

  Volume of gas that will diffuse through the membrane per minute for a pressure difference of 1 mmHg, expresses overall ability of respiratory membrane for gas exchange

  
• In emphysema, diffusion capacity decreases due to decreased alveolar surface area
• Membrane Thickness


  • In pulmonary fibrosis or pulmonary edema
• Diffusion coefficient of gas


  Affected by solubility and molecular weight
• Surface Area


  • In emphysema, TB, lung Ca
• Partial pressure differences (pressure gradient)

  1. Gas moves from area of higher partial pressure to area of lower partial pressure (Fick's Law)

  2. Normally, partial pressure of oxygen is higher in alveoli than in blood
• Diffusion capacity of lung

  • Volume of gas that will diffuse through the membrane each minute for a pressure difference of 1 mmHg
  • Expresses the overall ability of respiratory membrane for gas exchange between alveoli and pulmonary blood
  • Diffusing capacity of O2 under resting condition averages 21 ml/min/mmHg
  • CO2= 400 – 450 ml / min/ mmHg
  • Combines the diffusion coefficient of the gas, the surface area of the membrane, and the thickness of the membrane
• In emphysema


  DL decreases because of alveolar destruction → decreased surface area
• In fibrosis of pulmonary edema

  DL decreases because membrane thickness increases
• In anemia


  DL decreases
• During exercise


  DL increases
• Both oxygen and carbon dioxide diffuse down their partial pressure gradient