Ventilation of lung segments is heterogeneous

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Ventilation of lung segments is heterogeneous
Gravity
Time constant
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Dead space

• Physiological dead space
• Respiratory units that are ventilated but not
  perfused.
• Relate to V/Q ratios

Anatomical dead space
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J Kfc (Piv Pis) s (piv pis)
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• Control of pulmonary resistance
• Recruitment of capillaries
• Distention of capillaries
• 40 of total resistance is in the capillaries!
• Also vasoactive substances in the arterioles

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Heterogeneity of perfusion
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Most ventilation and blood flow go to alveoli
with well matched ventilation and perfusion
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Oxygen and carbon dioxide transport
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Perfusion vs diffusion limitation
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Importance of Oxygen delivery ? Oxygen content
NOT PO2 DO2 Qt x CaO2
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Oxygen consumption

• Importance of microcirculation
• Oxygen extraction ratio- highly variable

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