107. Transport of Gases in Blood – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Transport of gases ensures delivery of oxygen to tissues and removal of carbon dioxide, maintaining cellular metabolism and acid-base balance. Oxygen is transported mainly by hemoglobin, whereas carbon dioxide is transported predominantly as bicarbonate ions.
• Oxygen transport occurs in two forms:
  • About 98% is carried bound to hemoglobin as oxyhemoglobin.
  • About 2% is dissolved in plasma according to Henry’s law. Dissolved oxygen alone cannot meet tissue oxygen demands but becomes important during hyperbaric oxygen therapy.
• Each hemoglobin molecule contains four heme groups, each capable of binding one oxygen molecule. Oxygen binds to ferrous (Fe²⁺) iron, whereas methemoglobin (Fe³⁺) cannot carry oxygen. Oxygen binding is cooperative, producing the characteristic sigmoid oxygen-hemoglobin dissociation curve.
• Oxygen-carrying capacity of hemoglobin is 1.34 mL oxygen/g hemoglobin, and normal arterial hemoglobin saturation is 97–98%. Distinguish oxygen capacity (maximum oxygen that hemoglobin can carry) from oxygen content (actual oxygen present in blood).
• The oxygen-hemoglobin dissociation curve has:
  • Steep portion (10–60 mm Hg): Facilitates oxygen unloading in tissues.
  • Plateau portion (60–100 mm Hg): Maintains high hemoglobin saturation despite moderate falls in alveolar oxygen pressure, providing a physiological safety margin. P50 is 26–27 mm Hg and indicates hemoglobin affinity for oxygen.
• Right shift of the dissociation curve decreases hemoglobin affinity and promotes oxygen release. Causes include increased temperature, increased carbon dioxide, decreased pH (Bohr effect), increased 2,3-diphosphoglycerate (2,3-DPG), and chronic hypoxia. Left shift occurs with opposite changes, fetal hemoglobin, and carbon monoxide poisoning.
• 2,3-DPG is produced in erythrocytes and binds deoxygenated hemoglobin, shifting the curve to the right. It increases in anemia, chronic hypoxia, exercise, and high altitude, improving tissue oxygen delivery. Myoglobin has a higher oxygen affinity than hemoglobin and serves as an oxygen reserve in skeletal and cardiac muscle.
• Carbon monoxide has 200–250 times greater affinity for hemoglobin than oxygen, forming carboxyhemoglobin. It reduces oxygen-carrying capacity and shifts the curve to the left, causing severe tissue hypoxia despite normal arterial oxygen tension. Treatment includes 100% oxygen or hyperbaric oxygen therapy.
• Pulse oximetry estimates arterial oxygen saturation noninvasively but does not measure arterial oxygen tension or oxygen content. Oxygen content is especially important in anemia, where saturation may remain normal despite reduced oxygen delivery.
• Carbon dioxide transport occurs as:
  • 70% as bicarbonate ions,
  • 23% as carbamino compounds,
  • 7% dissolved in plasma.
    Conversion to bicarbonate occurs inside erythrocytes by carbonic anhydrase.
• Chloride shift (Hamburger shift) is the exchange of bicarbonate and chloride ions across the erythrocyte membrane, maintaining electrical neutrality and facilitating carbon dioxide transport. Venous hematocrit is slightly higher than arterial hematocrit because chloride entry increases red cell water content.
• The carbon dioxide dissociation curve is almost linear. The Haldane effect states that oxygenation of hemoglobin promotes carbon dioxide release in the lungs, whereas deoxygenated hemoglobin carries more carbon dioxide in tissues. The Bohr effect states that increased carbon dioxide and hydrogen ion concentration reduce hemoglobin affinity for oxygen, enhancing oxygen unloading in tissues.