Respiratory System 2

• Processes involved in gas exchange

  • ventilation
  • external respiration
  • gas transport
  • internal respiration
  • cellular respiration
• External respiration


  1. Movement of gases between the alveolar air and the blood

  2. Occurs in the lungs

  
• Diffusion


  The process by which gases move between the alveolar air and the blood
• Factors determining diffusion of gases

  • surface area for diffusion
  • thickness of the diffusion membrane
  • solubility of gases
  • partial pressure of gases
  • ventilation-perfusion ratio
• Surface area for diffusion

  • Normal alveolar surface area is about 70 m2
  • May be reduced by disease and necrosis
• Thickness of diffusion membrane

  • Distance from alveolar air space to lumen of capillary is less than 2 micrometers
• Henry's Law


  Solubility of gases in a liquid are inversely proportional to the temperature of the liquid
• Dalton's Law


  The total pressure exerted by a mixture of gases is equal to the sum of the individual pressures exerted independently by each gas in the mixture
• Partial pressure (P)


  Proportion of the total pressure exerted by each individual gas in a mixture of gases
• Composition of atmospheric air

  • N2 - 78%
  • O2 - 21%
  • CO2 - 0.04%
  • H2O - 0% (dry air)
• Partial pressure of gases in atmospheric air

  • PN2 = 590 mmHg
  • PO2 = 159 mmHg
  • PCO2 = 0.3 mmHg
  • PH2O = 0 mmHg
• Partial pressure of gases in alveolar air

  • PN2 = 569 mmHg
  • PO2 = 104 mmHg
  • PCO2 = 40 mmHg
  • PH2O = 47 mmHg

  
• Diffusion gradients


  • PO2 decreases from 104 mmHg in the alveolus to 40 mmHg in the capillary
  • PCO2 increases from 40 mmHg in the alveolus to 45 mmHg in the capillary
• Venous Blood Gases (VBG)

  • PVO2 = 40 mmHg
  • PVCO2 = 45 mmHg
• Arterial Blood Gases (ABG)

  • PaO2 = 104 mmHg
  • PaCO2 = 40 mmHg
• Gas Transport


  • Oxygen: 1.5% dissolved in plasma, 98.5% bound to hemoglobin in RBCs
  • Carbon dioxide: 7-8% dissolved in plasma, 20% bound to hemoglobin in RBCs, 70% as bicarbonate ion
• Internal respiration


  Gases move between the blood and the interstitial fluid through the capillary walls, depends on pressure gradient
• Oxyhemoglobin dissociation


  Regulated by PO2, temperature, blood pH, PCO2, and chemical called DPG
• The Bohr shift


  As pH decreases, the affinity of hemoglobin for oxygen decreases
• Effect of temperature


  As temperature rises, the affinity of hemoglobin for oxygen declines
• Cellular respiration


  Glucose + O2 -> CO2 + H2O + energy
• Neural regulation of respiration

  Chemoreceptors in the brainstem and peripheral chemoreceptors in the carotid sinuses and aortic bodies sense blood chemistry and stimulate the respiratory centres
• Motor control of ventilation

  1. Inspiratory and expiratory centres in the medulla oblongata send motor impulses to the muscles of inspiration and expiration

  2. Apneustic and pneumotaxic centres in the pons regulate the breathing rate
• The cerebral cortex has limited voluntary control over breathing, but emotions can influence breathing via the limbic system and hypothalamus
• Acid-base imbalance can affect respiration even if PCO2 and PO2 remain unchanged, by changing pH