2.2 - RESPIRATORY PHYSIOLOGY PART 2: GAS EXCHANGE AND REGULATION

Lecture #12: Respiratory Physiology II – Gas Exchange & Regulation

• The lecture focuses on gas exchange and regulation in the respiratory system.
• The presenter is Julia M. Hum, Ph.D.
• Office hours are Monday/Wednesday/Friday, 11:00 AM - 12:00 PM, and 2:00 PM - 2:50 PM.
• Contact information is provided for the instructor.

Learning Objectives

• Describe the physiological importance of hemoglobin (Hb) in gas exchange and its role in facilitating this process.
• Identify the changes on an oxygen (O2) dissociation curve.
• Diagram carbon dioxide (CO2) uptake by red blood cells (RBCs) and understand how the body transports CO2.
• Compare and contrast active regulation of blood flow in pulmonary (lung) versus systemic (body) resistance vessels.
• List other active regulators of pulmonary vessels.
• Describe two types of "limited" gas exchange (diffusion-limited and perfusion-limited).
• Define venous admixture and the ventilation/perfusion ratio (Va/Q).
• Identify and describe the Va/Q in various health and disease states.
• Define the respiratory control pathways.
• Understand the roles of central and peripheral chemoreceptors and predict how they respond to changes in partial pressures of oxygen (PO2) and carbon dioxide (PCO2).

Gas Pressures

• Gases move between air and blood passively through diffusion.
• The driving force for gas exchange is differences in partial pressure gradients, not concentration.

Hemoglobin

• Hemoglobin is vital for oxygen transport.
• Hemoglobin contains four heme groups that bind oxygen reversibly.
• Oxygen (O2) saturation describes the percentage of occupied oxygen binding sites on hemoglobin.
• Arterial blood is typically 100% saturated, venous blood is around 75% saturated
• Hemoglobin concentration (Men 13-18 g/dL, Women 12-16 g/dL) significantly affects oxygen-carrying capacity

O₂ Dissociation Curve: Dissociation

• Hemoglobin's affinity for oxygen increases as oxygen partial pressure increases.
• The dissociation curve shifts right with factors like temperature increase, increased CO2, or increased acidity (lower pH). This facilitates oxygen release in tissues that need it most.
• Shifting results in decreased affinity for O₂; rightward shift favors unloading.
• Leftward shift results in increased affinity for O₂; leftward shift favors loading of O₂

CO₂ Transport

• CO2 is transported in the blood through several mechanisms:
  • dissolved in plasma,
  • as bicarbonate (HCO3-) in blood plasma and RBCs (primary mechanism)
  • bound to hemoglobin as carbaminohemoglobin.

O₂ Transport (Uptake by RBCs)

• Carbon dioxide (CO2) enters red blood cells and is converted to bicarbonate (HCO3-).
• Bicarbonate is then exchanged for chloride (Cl−), maintaining ionic balance.
• This process facilitates CO2 transport in blood.

Gas Exchange

• Diffusion-limited gas exchange = amount of gas transported across the alveolar-capillary membrane limited by the diffusion process
• Perfusion-limited gas exchange = amount of gas transported across the alveolar-capillary membrane limited by blood flow

Diffusion-limited Exchange (Carbon Monoxide)

• Carbon monoxide (CO) has a significantly higher affinity for hemoglobin than oxygen.

Ventilation/Perfusion Ratio (Va/Q)

• Va/Q ratio represents the ratio of alveolar ventilation (Va) to pulmonary blood flow (Q)
• At rest, the Va/Q ratio is typically healthy (~0.8), reflecting balanced ventilation and perfusion between alveoli and pulmonary capillaries
• If there's an airway or vascular obstruction, the ratio is altered, with potentially very different consequences in gas exchange

Respiratory Regulation

• Respiratory system involves central and peripheral chemoreceptors.
• Central chemoreceptors are located in the medulla oblongata and respond to changes in carbon dioxide levels in the cerebrospinal fluid (CSF).
• Peripheral chemoreceptors are located in the carotid and aortic bodies and respond to changes in blood oxygen (PO2), carbon dioxide (PCO2), and pH levels.

Arterial Pressure Control: Sensors

• Arterial baroreceptors, cardiopulmonary receptors, and chemoreceptors monitor and convey pressure and flow information to the brainstem. .

Venous Admixture

• Venous admixture occurs when blood mixes with deoxygenated blood.
• Different degrees of venous admixture are possible due to anatomical or physiological shunts.

Other Active Regulators of Pulmonary Blood Flow

• List of pulmonary blood flow vasodilators and vasoconstrictors.

Test your knowledge on gas exchange and regulation in the respiratory system with this quiz based on Lecture #12. Explore the roles of hemoglobin, carbon dioxide transport, and blood flow regulation in both pulmonary and systemic vessels.