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Ventilation-Perfusion Relationships and Their Impact on Alveolar Gases and Blood

This chapter explores the normal ventilation-perfusion ratio (V/Q ratio) and how it affects alveolar oxygen and carbon dioxide pressures. It also discusses how changes in the V/Q ratio impact end-capillary gases and respiratory exchange. The chapter concludes with a discussion on how respiratory disorders can affect the V/Q ratio.

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Ventilation-Perfusion Relationships and Their Impact on Alveolar Gases and Blood

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  1. CHAPTER 8 Ventilation-PerfusionRelationships

  2. Normal Ventilation-Perfusion Ratio (V/Q Ratio) • Fig. 8-1. The normal ventilation-perfusion ratio (V/Q ratio) is about 0.8.

  3. HOW THE VENTILATION-PERFUSION RATIO AFFECTS THE ALVEOLAR GASES

  4. Upright Lung and V/Q Ratio • Fig. 8-2. In the upright lung, the V/Q ratio progressively decreases from the apex to the base.

  5. INCREASED V/Q RATIO

  6. When the V/Q Ratio Is High • Fig. 8-3. When the V/Q ratio is high, the alveolar oxygen pressure (PAO2) increases and the alveolar carbon dioxide pressure (PACO2) decreases.

  7. DECREASEDV/Q RATIO

  8. When the V/Q Ratio Is Low • Fig. 8-4. When the V/Q ratio is low, the alveolar oxygen pressure (PAO2) decreases and the alveolar carbon dioxide pressure (PACO2) increases.

  9. The O2-CO2 Diagram • Fig. 8-5. The O2-CO2 diagram.

  10. HOW VENTILATION-PERFUSION RATIO AFFECTS END-CAPILLARY GASES

  11. Pulmonary Capillary Blood Gases (PcO2 and PcCO2) • Fig. 8-6. The mixing of pulmonary capillary blood gases (PcO2 and PcCO2) from the upper and lower lung regions.

  12. V/Q Ratio and PAO2 and the PcO2, the PACO2 • Fig. 8-7. How changes in the V/Q ratio affect the PAO2 and the PcO2, the PACO2, and the PcCO2, and the pH of pulmonary blood.

  13. Respiratory Quotient • Internal respiration • Gas exchange between systemic capillaries and cells • Normally, 250 mL of O2 consumed by tissue cells in one minute • In exchange, cells produce about 200 mL of CO2

  14. Respiratory Quotient (RQ) • Clinically, it is the ratio between the volume of O2 consumed (VO2) and the volume of CO2 produced (VCO2)

  15. Respiratory Quotient

  16. Respiratory Exchange Ratio • External respiration • Gas exchange between the pulmonary capillaries and the alveoli • Respiratory exchange ratio (RR) • Quantity of O2 and CO2 exchanged during a period of one minute • Normally, the RR and RQ are equal

  17. HOW RESPIRATORY DISORDERS AFFECT THE V/Q RATIO

  18. Alveolar O2 and CO2 Pressure Changes • Fig. 8-8. Alveolar O2 and CO2 pressure changes that occur as a result of V/Q ratio changes caused by respiratory disorder. (A) shunt unit; (B) normal unit; (C) dead space unit.

  19. Clinical Application 1 Discussion • How did this case illustrate … • An increased ventilation-perfusion ratio caused by an excessive amount of blood lost as a result of trauma?

  20. Clinical Application 1 • Clinical Application 1: A 34-year-old male impaled by a steel enforcement rod.

  21. Clinical Application 2 Discussion • How did this case illustrate … • A decreased V/Q ratio caused by an upper airway obstruction?

  22. Clinical Application 2 • Clinical Application 2: A 4-year-old boy who aspirated a quarter.

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