29 - Oxygen and Carbon Dioxide Physiology

Recall Clicker Questions

• Bronchoconstriction Signals:

  • ↑ PCO₂ (increased carbon dioxide) stimulates the bronchoconstriction process to reduce airflow and enhance gas exchange efficiency.

  • ↓ PO₂ (decreased oxygen) can trigger similar responses to protect lung function.

• Vasodilation of Pulmonary Arterioles:

  • ↑ PCO₂ (increased carbon dioxide) results in vasodilation of pulmonary arterioles, increasing blood flow to areas of the lungs that are better ventilated.

  • ↓ PO₂ (decreased oxygen) in certain lung areas causes vasodilation, improving ventilation-perfusion matching.

Factors Affecting pH

• Ion Concentration and pH:

  • The concentration of H⁺ ions directly determines the pH of the blood.

  • High [H⁺] = Low pH = Acidic, affecting overall metabolic processes and hemoglobin’s oxygen binding capacity.

Causes of Decrease in PO₂

• Factors leading to decreased PO₂:

  • Issues with air supply: atmospheric problems such as high altitude or pollution that can reduce available oxygen.

  • Problems with alveoli: lung functionality issues due to conditions like pneumonia or chronic obstructive pulmonary disease (COPD).

  • Blood issues: E.g., anemia (low hemoglobin) reduces the blood's capacity to transport oxygen efficiently.

Hemoglobin Structure and Function

• Hemoglobin (Hb):

  • Hemoglobin is the main component of red blood cells, essential for oxygen transport. It is a tetramer composed of four globular protein chains (two alpha and two beta) around iron heme groups capable of binding oxygen.

  • Each heme group can bind one molecule of O₂, allowing each hemoglobin molecule to carry up to four molecules of oxygen.

• O₂ Transport:

  • Approximately 2% of oxygen is dissolved in plasma, while the remainder binds to hemoglobin. The process of forming oxyhemoglobin (HbO₂) is crucial for efficient oxygen transport.

  • The formation of HbO₂ is reversible, allowing for oxygen release at the tissue level where the partial pressure of oxygen is lower, enabling efficient oxygen delivery.

Hb Affinity for O₂

• Affinity Concept:

  • Affinity refers to the degree of attraction or how firmly hemoglobin binds to O₂:

  • Low Affinity: When hemoglobin has a low affinity for oxygen, it readily releases oxygen, which is crucial during high metabolic activity. This can be likened to children leaving a bus at their stop, where they get off easily.

  • High Affinity: Conversely, when hemoglobin has a high affinity for oxygen, it retains oxygen longer, which may be seen in conditions of lower metabolic demand. This can be compared to children staying on a bus until their final destination.

• Effects of Affinity:

  • Changes in Hb affinity can occur based on tissue demands, with increased metabolic activity in tissues prompting:

  • Low Affinity: More oxygen leaves hemoglobin to supply active tissues, which can be critical during exercise or increased activity.

  • High Affinity: Less oxygen is released, potentially leading to tissue hypoxia if not properly regulated.

Oxygen Hemoglobin Saturation Curve

• Curve Interpretation:

  • The oxygen hemoglobin saturation curve can be represented graphically. The curve illustrates how hemoglobin saturation changes with varying PO₂ levels, depicted with lines above and below the normal PO₂ range.

  • Left Shift: An increase in hemoglobin's affinity for O₂, resulting in more binding, typically occurs in conditions like alkalosis or decreased CO₂ levels.

  • Right Shift: A decrease in hemoglobin's affinity for O₂, resulting in less binding, often occurs during acidosis or increased temperature, facilitating oxygen delivery to tissues in need.

Factors Affecting Hb Affinity

• Influences on Affinity:

  • Factors such as increased H⁺, elevated temperature, and higher levels of CO₂ decrease hemoglobin's affinity for O₂, promoting oxygen release.

  • Conversely, conditions that increase affinity generally include lower concentrations of these factors, such as in the lung environment during gas exchange.

Influence of 2,3 DPG

• 2,3 DPG (2,3-Diphosphoglycerate):

  • This substance is a by-product of red blood cell metabolism, produced during hypoxia. Its levels increase in able to facilitate the release of O₂ from hemoglobin, particularly in tissues with high oxygen demand, thus enhancing oxygen delivery.

Fetal vs Adult Hemoglobin

• Fetal Hemoglobin (HbF) exhibits a higher affinity for O₂ compared to adult hemoglobin (HbA). This physiological characteristic allows the fetus to efficiently extract oxygen from maternal blood across the placenta, ensuring adequate oxygenation during development. Poor oxygenation during this stage can lead to significant developmental issues.

CO₂ in Blood

• Transport Methods:

  • CO₂ is transported in the blood through three primary methods:

  • 7% dissolved in blood plasma,

  • 23% bound to hemoglobin as carbaminohemoglobin, and

  • 70% converted to bicarbonate (HCO₃⁻), which plays a critical role in maintaining acid-base balance within the blood.

• CO₂ and pH Regulation:

  • An increase in CO₂ leads to a rise in [H⁺], which decreases pH levels in the blood, triggering compensatory mechanisms by the respiratory system.

  • The reaction facilitated by the enzyme carbonic anhydrase: H₂O + CO₂ ⇌ HCO₃⁻ + H⁺ is key to regulating acid-base balance and CO₂ removal from the body.

Respiratory Variations

• Hypoventilation:

  • Breathing at an inadequate rate results in increased CO₂ levels, leading to respiratory acidosis characterized by a decrease in pH. This condition can occur due to respiratory muscle weakness or obstructive lung disorders.

• Hyperventilation:

  • Rapid and deep respiration decreases CO₂ levels, leading to respiratory alkalosis characterized by an increase in pH. This can occur during anxiety or panic attacks, often requiring interventions to restore balance.

Summary of CO₂ and pH Effects

• Maintaining pH balance is crucial through the exhalation of CO₂. Conditions such as hypoventilation and hyperventilation substantially influence blood pH and CO₂ levels.

• It is important to consider CO₂ as a weak acid due to its role in generating H⁺ ions,