Oxygen and Carbon Dioxide Transport

Epithelial Structure and Functions

• Alveolar epithelium:
  • Composed of a single layer of flat cells that line the alveoli in the lungs.
  • Aids in gas exchange through diffusion.
• Basement membrane:
  • Thin, fibrous layer providing structural support and anchoring the epithelial cells.
• Surfactant layer:
  • A fluid layer that reduces surface tension in the alveoli, preventing collapse during exhalation.
• Alveolus:
  • The small air sac where gas exchange occurs.
• Capillary:
  • Small blood vessels surrounding alveoli, facilitating the exchange of oxygen and carbon dioxide.

Diffusion

• Key concept for gas exchange:
  • Oxygen (O₂) moves from areas of higher concentration (alveoli) to lower concentration (capillaries) via diffusion.
  • Conversely, carbon dioxide (CO₂) diffuses from blood into alveoli to be expelled.

Introduction to Oxygen Transport

• Importance of understanding oxygen and carbon dioxide transport:
  • Essential for the study of pulmonary physiology and clinical interpretation of arterial and venous blood gases.
  • Methods:
  • Radial-arterial blood gas stick for blood sampling.

Normal Blood Gas Values

• Note:
  • Only Pao₂ and Pco₂ are direct blood gas values.
  • pH and HCO3 measures are calculated indirectly.

Key Learning Outcomes

• Comprehensive understanding required of:
  1. Oxygen transport from lungs to tissues.
  2. Oxyhemoglobin dissociation curve and clinical significance.
  3. Oxygen transport calculations to assess cardiac and ventilatory status.
  4. Major forms of tissue hypoxia.
  5. Carbon dioxide transport from tissues to lungs.

Oxygen Transport Mechanisms

• Oxygen is transported in blood in two primary forms:
  1. Dissolved oxygen in blood plasma.
  2. Chemically bound to hemoglobin (Hb) within erythrocytes (RBCs).
  • Both forms are necessary for calculating total oxygen content of blood.

Henry's Law and Oxygen Dissolved in Plasma

• Definition:
  • Relates the amount of gas that dissolves in a liquid at a specific temperature to the partial pressure of the gas in that liquid.
  • For O₂ dissolved in plasma:
  • O₂ (mL) = PaO₂ (torr) \times 0.003
  • Quantity of oxygen that dissolves is small relative to transportation bound to hemoglobin.

Total O₂ Content Calculation

Step 1 - Oxygen Dissolved in Plasma

• Approximately 0.003 mL of O₂ dissolves in 100 mL of blood for every 1 torr of Pao₂.
• Converted into volume percent (vol%):
  • Example:
  • 10 vol% indicates 10 mL of O₂ in 100 mL of blood.

Step 2 - Oxygen Bound to Hemoglobin

• Most O₂ binds to hemoglobin molecules in RBCs.
• Each RBC:
  • Contains approximately 280 million Hb molecules.
• Normal adult hemoglobin (Hb A):
  • Composed of four heme groups and four amino acid chains (globin).

Oxygen Binding Dynamics

• Definitions:
  • Oxyhemoglobin
  • Hemoglobin bound with oxygen.
  • Reduced hemoglobin
  • Also known as deoxyhemoglobin, not bound with oxygen.

Hemoglobin Concentration Values

• Normal adult male Hb concentration:
  • 14-16 g percent.
• Normal adult female Hb concentration:
  • 12-15 g percent.

Total O₂ Content Calculation

Oxygen Bound to Hemoglobin Step 1

• Each gram percent (g% Hb) can carry 1.34 mL of O₂.
  • Example: If Hb level is 15 g%, fully saturated Hb carries approximately 20.1 vol% of O₂.

Oxygen Saturation Adjustment

• At normal Pao₂ (100 torr), typical Hb saturation is about 97%.
• Adjust calculations based on physiological shunts:
  1. Thebesian venous drainage.
  2. Bronchial venous drainage.
  3. Under-ventilated alveoli.

Total O₂ Content Calculation Step 3

• Combine both dissolved and bound O₂ amounts to obtain total O₂ content.
  • Need practice with case studies:

Case Study: Anemic Patient

• 27-year-old female with history of anemia, respiratory distress signs:
  • Respiratory rate: 36 breaths/min
  • Heart rate: 130 bpm
  • Blood pressure: 155/90 mmHg
  • Hb concentration: 6 g percent.
  • Pao₂: 80 torr, SaO₂: 90%.

Total O₂ Content Calculation for the Patient

1. Dissolved O₂:
   • 0.24 ext{ vol% O}2 = 80 ext{ Pao}2 \times 0.003
2. Oxygen bound to hemoglobin:
   • 8.04 ext{ vol% O}_2 = 6 ext{ g% Hb} \times 1.34
   • Adjusted for saturation:
   • 7.236 ext{ vol% O}_2 = 8.04 imes 0.90
3. Total O₂:
   • 7.476 ext{ vol% O}_2 = 7.236 + 0.24

Analysis of Anemic Patient Results

• Patient’s total oxygen content is less than 50% of normal (19.5 vol% O₂).
• Low hemoglobin concentration identified as primary mechanism for oxygen transport.

Oxygen Transport Important Equations

• CaO₂: Oxygen content of arterial blood.
• CvO₂: Oxygen content of mixed venous blood.
• CcO₂: Oxygen content of pulmonary capillary blood.
• SvO₂: Oxygen content returning to right side heart (venous).

Clinical Connection: Polycythemia

• Definition: Condition characterized by elevated hemoglobin levels:
  • Men: > 18.5 g percent (normal 14-16 g percent)
  • Women: > 16.5 g percent (normal 12-15 g percent).

Case Study: COPD Patient with Polycythemia

• Patient shows significant symptoms:
  • Barrel chest, cyanosis, clubbing fingers, respiratory distress.
  • Vital signs:
  • BP: 135/90, HR: 85 bpm, RR: 10/min.
  • Pao₂: 45 torr, SaO₂: 75%, Hb level: 19 g percent.

Total Calculation for COPD Patient with Polycythemia

1. Dissolved O₂:
   • 1.35 ext{ vol% O}2 = 45 ext{ Pao}2 \times 0.003
2. Oxygen bound to hemoglobin:
   • 25.46 ext{ vol% O}_2 = 19 ext{ g% Hb} \times 1.34
   • Adjusted saturation:
   • 19.095 ext{ vol% O}_2 = 25.46 \times 0.75
3. Total O₂:
   • 19.23 ext{ vol% O}_2 = 19.095 + 0.135

Notes on Total Arterial Oxygen Content

• Despite low Pao₂, total oxygen content is normal.
• Increased viscosity of blood can offset benefits from polycythemia, especially in cases with hematocrit > 55-60%.

Oxyhemoglobin Dissociation Curve

• Purpose: Illustrates the percentage of hemoglobin that binds with oxygen at varying partial pressures (Pao₂).
• Key Insights:
  • Sharp increases in binding up to 60 torr; then a plateau (safety zone).
  • Small decreases in PO₂ facilitate release of oxygen to tissues.

Clinical Implications of Dissociation Curve

• Above normal Pao₂ (80-100 torr):
  • Shift does not significantly impact hemoglobin oxygen transport.
• Below normal Pao₂:
  • Shifts can greatly affect oxygen delivery due to steep curve portion.

Factors Affecting Oxygen Dissociation Curve

• Shifts due to:
  • pH changes
  • Temperature variations
  • Concentrations of carbon dioxide
  • Presence of 2,3-bisphosphoglycerate (BPG)
  • Different hemoglobin types (e.g., fetal hemoglobin, carboxyhemoglobin).

Right Shift Effects

• Indicate decreased pH, increased temperature, increased CO₂, elevated BPG.
• Results in hemoglobin releasing oxygen more readily (decreased affinity).

Left Shift Effects

• Indicate increased pH, decreased temperature, diminished CO₂.
• Results in hemoglobin holding onto oxygen more tightly (increased affinity).

Clinical Significance of Hypoxia

• Hypoxemia: Abnormally low Pao₂ often correlating with inadequate tissue oxygenation.
• Hypoxia Types:
  1. Hypoxic hypoxia: Low Pao₂ affecting oxygen delivery.
  2. Anemic hypoxia: Normal Pao₂ but inability of blood to carry adequate oxygen (e.g., low hemoglobin or CO poisoning).
  3. Circulatory hypoxia: Normal O₂ levels, but insufficient blood flow (i.e., stagnation or shunting).
  4. Histotoxic hypoxia: Impaired cellular oxygen use (e.g., cyanide poisoning).

Carbon Dioxide Transport Mechanisms

• CO₂ transported from tissues to lungs through various forms:
  1. As dissolved gas in plasma.
  2. As carbamino compounds (bound to proteins).
  3. As bicarbonate ion.

Carbon Dioxide Conversion and Elimination

1. CO₂ forms bicarbonate in the tissues.
2. Bicarbonate reconverted to CO₂ for elimination in the alveoli.

Summary of CO₂ Transport Mechanisms

Clinical Connection: Pulse Oximeter

• Device to monitor arterial oxyhemoglobin saturation indirectly:
  • Reports oxygen saturation as Spo₂ versus direct measurement (SaO₂) from arterial blood samples.
• Operates on light reflection principles to gauge Hb saturation.
• Excludes capillary/venous blood measurements.

General Pa/SaO₂ Relationship Rule

• Approximate PaO₂ levels correlate with SaO₂ readings:
  • PaO₂ of 40 torr corresponds to SaO₂ of 70%.
  • PaO₂ of 50 torr corresponds to SaO₂ of 80%.
  • PaO₂ of 60 torr corresponds to SaO₂ of 90%.