Gaseous Exchange and Perfusion II

Gas Exchange & Perfusion II Lecture Notes

• Writer/Reviewer Information:
  • Zainab Adel Alali
  • Zahraa Albarraqi
  • The notes were reviewed by the same team.
  • Notes are numbered 221-222-223

Academic Year, Title, and CRN

• Academic Year: 2022-2023
• Title: Gaseous Exchange and Perfusion II
• CRN: 15571 (Males) 15583 (Females)
• Block: 1.3
• Subject/Discipline: Physiology
• Expert: Dr. Arif Mohyuddin
• Block Coordinator: Dr. Shahina Khan

Vision

• To become a model in community engagement through excellence and international recognition in medical education, research, and health care.

Mission

• To promote higher standards in medical education, health care, research, and community health services

Values

• Islamic values
• Excellence
• Creativity
• Compassion
• Leadership
• Responsiveness to Community

Learning Question 2

• Define right-to-left shunt & physiological dead space (wasted ventilation).
• What effect does either have on pulmonary gas exchange?

Effect of Ventilation-Perfusion Ratio on Alveolar Gas Concentration

• Two factors determine PO₂ and PCO₂ in alveoli:
  • Rate of alveolar ventilation.
  • Rate of transfer of oxygen/carbon dioxide through the respiratory membrane.

Effect of Ventilation-Perfusion Ratio on Alveolar Gas Concentration Continued

• Normally, different areas of the lungs are ventilated/perfused.
• Some areas might be well-ventilated but have little or no blood flow.
• Other areas can have good blood flow but little or no ventilation.
• Normal lung function depends on adequate ventilation & perfusion.
• This issue appears under various conditions:
  • Good ventilation but no perfusion
  • Minimal or no ventilation and perfused well

Effect of Ventilation-Perfusion Ratio on Alveolar Gas Concentration Continued

• Ventilation-perfusion (V/Q) ratio is a quantitative concept
• It helps to understand respiratory exchange in cases of imbalance between alveolar ventilation and blood flow.
• When the ventilation and perfusion in the alveolus is normal, the V/Q is also normal.
• When ventilation in alveoli is zero and perfusion is present the V/Q is zero.
• When perfusion of the alveoli is zero and ventilation is present then V/Q is infinity.

Effect of Ventilation-Perfusion Ratio on Alveolar Gas Concentration Continued

• At either zero or infinity ratio, there is no gas exchange through the respiratory membrane in the affected alveoli.
• This explains the significance of the V/Q ratio.
• The respiratory consequences of these extremes will be explained later.

Concept of Physiological Shunt (When VA/Q is Below Normal)

• VA/Q diagram explains the normal PO2 and PCO2.

Normal PO2-PCO2, VA/Q Diagram

• 3 different points in the VA/Q diagram (V = 0, V/Q = Normal, and V/Q equals infinity) are explained with PO2 and PCO2 values

V/Q Ratio

• V/Q ratio = 0 (No Ventilation): fresh air does not enter the alveoli.
• Gas levels match venous blood (PO2 = 40 mmHg, PCO2 = 45 mmHg)
• V/Q ratio = ∞ (No Perfusion): Blood flow to the alveoli is absent.
• Gas levels match inspired air (PO2 = 149 mmHg, PCO2 = 0 mmHg)
• Normal Alveolar Air: PO2 = 104 mmHg, PCO2 = 40 mmHg

"Physiologic Shunt"

• When VA/Q is below normal, there's insufficient ventilation to fully oxygenate blood flowing through alveolar capillaries.
• A portion of venous blood passing through the pulmonary capillaries doesn't become oxygenated.
• This fraction is called shunted blood.
• Shunted blood leads to less oxygen in the blood (hypoxia).
• Some additional blood flow exists through bronchial vessels rather than through alveolar capillaries, normally about 2% of cardiac output (unoxygenated, shunted blood).

"Physiologic Dead Space"

• Explains the concept when VA/Q is above normal.
• Air in the alveoli isn't exchanged due to insufficient blood flow to carry the oxygen. This air is considered 'wasted.'
• This is due to inadequate blood flow to carry the oxygen.
• The sum of these two types of wasted ventilation is called physiological dead space.

"Physiologic Dead Space" Continued

• When physiological dead space is substantial, much of the work of ventilation is wasted.
• This is because much of the inhaled air does not reach the blood.

Abnormal VA/Q in Chronic Obstructive Lung Disease

• Smokers often develop bronchial obstruction, eventually leading to severe alveolar air trapping and emphysema.
• Emphysema damages alveolar walls.
• Obstruction of small bronchioles results in zero V/Q ratio.
• Destruction of alveolar walls results in V/Q being greater than normal.
• These conditions severely decrease the lungs' effectiveness, reducing it sometimes to less than a tenth of normal effectiveness, impairing gas exchange.

Learning Question 3

• Describe the relationship between alveolar gas exchange and the arterial blood gas values PCO2 and PO2

Normal Values

• Includes data for Alveoli, Arterial Blood, and Mixed Venous Blood. The data is displayed in a table format.
  • PA O2 (mmHg), SaO2
  • PA CO2 (mmHg)
  • HCO3 mE/L
  • pH (Values for different types of blood are displayed)

Normal PO2-PCO2, VA/Q Diagram

• Illustrative graph shows relationship of PO₂ in blood and percent hemoglobin saturation.

Oxyhemoglobin Dissociation Curve

• Describes the S-shaped oxyhemoglobin dissociation curve.
• Explains how hemoglobin binds and releases oxygen based on partial pressure.
• Describes the association and dissociation processes.
• Saturation values of hemoglobin as PO₂ changes (e.g. 50%, 97%). The graph plots percent hemoglobin saturation vs PO₂ of blood (mm Hg).