In what ways is carbon dioxide carried in the blood, and how does it bind to hemoglobin?

Sourav PanNovember 9, 2024

Answered

Carbon dioxide (CO2) is transported in the blood through three primary mechanisms, each playing a crucial role in maintaining acid-base balance and facilitating gas exchange. Here’s a detailed overview of how CO2 is carried in the blood and how it binds to hemoglobin.

Mechanisms of Carbon Dioxide Transport

1. Dissolved CO2:
   • Approximately 7-10% of carbon dioxide is transported dissolved directly in the plasma. This dissolved CO2 establishes a partial pressure of CO2 in the blood, which is important for driving diffusion between tissues and the lungs. The partial pressure of CO2 in venous blood is typically around 45 mmHg, compared to about 40 mmHg in arterial blood.
2. Bicarbonate Ions (HCO3-):
   • The majority of CO2, about 70-90%, is converted to bicarbonate ions through a process facilitated by the enzyme carbonic anhydrase within red blood cells. The reaction proceeds as follows:
     CO2+H2O↔H2CO3↔H++HCO3−​
   • Carbonic acid (H2CO3) dissociates into bicarbonate (HCO3-) and hydrogen ions (H+). The bicarbonate ions then diffuse out of the red blood cells into the plasma, where they play a significant role in buffering blood pH and maintaining acid-base homeostasis .
3. Carbamino Compounds:
   • About 20-30% of CO2 binds to hemoglobin and other proteins to form carbaminohemoglobin. This occurs when CO2 binds to the amino groups of hemoglobin’s globin chains, particularly at the N-terminal ends and side chains of amino acids like arginine and lysine. The formation of carbaminohemoglobin can be summarized as follows:
     Hb+CO2↔Hb CO2​
   • The binding of CO2 to hemoglobin reduces its affinity for oxygen, a phenomenon known as the Bohr effect, which facilitates oxygen unloading in tissues where CO2 concentration is high .

Binding Mechanism and Effects on Hemoglobin

Carbamino Formation

• When CO2 binds to hemoglobin, it forms carbaminohemoglobin, which stabilizes hemoglobin in a form that favors the release of oxygen. This interaction occurs primarily in tissues where CO2 levels are elevated due to cellular metabolism .

Bohr Effect

• The Bohr effect describes how increased levels of CO2 (and associated H+ ions) lower hemoglobin’s affinity for oxygen. As hemoglobin releases oxygen in metabolically active tissues, it becomes more capable of binding CO2. This mechanism ensures that areas with high metabolic activity receive adequate oxygen while facilitating efficient removal of CO2 .

Haldane Effect

• The Haldane effect complements the Bohr effect by describing how deoxygenated hemoglobin has a higher affinity for CO2 than oxygenated hemoglobin. As oxygen binds to hemoglobin in the lungs, it promotes the release of bound CO2, thus enhancing gas exchange efficiency during respiration