Gas Exchange and Oxygen Transport

Gas Exchange and Transport

• External and Internal Respiration:
  • External respiration is pulmonary gas exchange. O2 diffuses from the alveoli to blood in the pulmonary capillaries, and CO2 diffuses in the opposite direction. This converts deoxygenated blood to oxygenated blood, returning to the left side of the heart.
  • Internal respiration is systemic gas exchange, exchanging O2 and CO2 between systemic capillaries and tissue cells. Oxygenated blood becomes deoxygenated as O2 is released, and internal respiration occurs throughout the body.

Oxygen Transport

• Oxygen's Solubility: Oxygen does not dissolve easily in water. Only 1.5% of inhaled oxygen is dissolved in blood plasma; the remaining 98.5% is bound to hemoglobin in red blood cells.

• Hemoglobin and Oxygen Binding: Oxygen and hemoglobin bind in a reversible reaction to form oxyhemoglobin. This binding is easily reversible.

Relationship Between Hemoglobin and Oxygen Partial Pressure

• PO2 and Saturation: PO2 (partial pressure of oxygen) is the most important factor determining how much O2 binds to hemoglobin. Fully saturated hemoglobin is converted from reduced hemoglobin (Hb) to oxyhemoglobin (Hb-O2). Partially saturated hemoglobin is a mixture of Hb and Hb-O2.
• Oxygen-Hemoglobin Dissociation Curve: The curve illustrates the relationship between percent saturation of hemoglobin and PO2 at normal body temperature.

Other Factors Affecting Hemoglobin Affinity for Oxygen

• Acidity (pH): Lower pH (increased acidity) decreases hemoglobin's affinity for O2, causing more O2 to be released. This is known as the Bohr effect. Higher pH (alkalosis) has the opposite effect.
• Partial Pressure of Carbon Dioxide (PCO2): An increase in PCO2 also decreases hemoglobin's affinity for O2, facilitating oxygen unloading. Lower PCO2 leads to a leftward shift.
• Temperature: Increased temperature decreases hemoglobin's affinity for O2, causing more O2 to be unloaded. Higher temperatures increase the release of oxygen.

2,3-Bisphosphoglycerate (BPG)

• Effect on Oxygen Affinity: BPG is a substance in red blood cells that decreases hemoglobin's affinity for oxygen, promoting its unloading.
• Formation in Red Blood Cells: BPG is produced during glycolysis in red blood cells. Certain hormones (like thyroxine) increase BPG levels.

Carbon Dioxide Transport

• Dissolved CO2: A small amount (7%) of CO2 is dissolved in blood plasma, diffusing into alveolar air upon reaching the lungs for excretion.

• Carbamino Compounds: About 23% of CO2 combines with amino acids and proteins in blood forming carbamino compounds.

• Bicarbonate Ions: Approximately 70% of CO2 is transported as bicarbonate ions (HCO3⁻) in blood plasma. This is part of a process where CO2 reacts with water to produce carbonic acid, which dissociates into hydrogen ions and bicarbonate ions.