Three Forms of Carbon Dioxide Transport
The transport process begins after carbon dioxide leaves tissue cells. Within the bloodstream, it is distributed and transported in three main forms:
- Physical dissolution. A fraction of the gas exists in plasma in a free, dissolved state. This fraction is relatively small, accounting for about 3 vol %.
- Chemical compounds in plasma. This is the primary transport pathway, accounting for 50 vol % of the total volume. The gas is transported as hydrogen carbonates (bicarbonates), specifically potassium bicarbonate (KHCO3) and sodium bicarbonate (NaHCO3).
- Chemical compounds in erythrocytes. Inside red blood cells, carbon dioxide forms a complex with hemoglobin—carbaminohemoglobin. This form accounts for 5 vol %. Additionally, a portion of the gas is transported within erythrocytes as potassium bicarbonate (KHCO3).
It is worth noting that during the subsequent transformations of hemoglobinic acid (HHb), the resulting carbaminohemoglobin can transport a total of about 15% of all carbon dioxide.
Tissue Gas Exchange and the Hamburger Phenomenon
In systemic tissue capillaries, the direction of carbon dioxide movement is strictly defined: it diffuses from cells into the plasma and then enters the erythrocyte. A cascade of biochemical reactions unfolds inside the cell:
- The enzyme carbonic anhydrase catalyzes the hydration reaction: carbon dioxide binds with water, forming carbonic acid (H2CO3).
- Carbonic acid rapidly dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-).
- Concurrently, carbaminoic acid (HHbCO2) is formed—a binding form of the gas with hemoglobin.
Under these conditions, the Haldane effect is prominently displayed: oxyhemoglobin (KHbO2) in an environment with high carbon dioxide content releases oxygen to tissues more readily. Having released oxygen, it binds with carbonic acid, turning into hemoglobinic acid (HHb). Fundamentally, this mechanism mirrors the Bohr effect.
Ion dynamics are accompanied by the chloride shift (Hamburger phenomenon). HCO3- ions exit the erythrocyte into the plasma along a concentration gradient, where they combine with sodium ions to form NaHCO3. To maintain erythrocyte electroneutrality, chloride ions (Cl-) enter the cell from the plasma. Potassium ions released inside the cell bind the remaining bicarbonate.
Gas Exchange in Pulmonary Capillaries
When venous blood reaches the lungs, physiological processes run in reverse. Carbon dioxide passes from the erythrocyte into the plasma and then into the alveolar air, while oxygen from the alveoli rushes into the erythrocyte.
The process is driven by two main factors:
- Pressure gradient. The partial pressure of carbon dioxide (pCO2) in the alveoli is significantly lower than in venous blood plasma. Furthermore, an increase in pO2 in the lungs facilitates the release of carbon dioxide.
- Hemoglobin oxygenation. Upon entering the erythrocyte, oxygen converts deoxygenated hemoglobin into oxyhemoglobin (HbO2). Oxygen binding sharply enhances the acidic properties of hemoglobin. As a stronger acid, oxyhemoglobin actively displaces hydrogen ions and potassium ions from bicarbonates.
The released hydrogen ions interact with bicarbonate, and carbonic anhydrase triggers the dehydration reaction. Carbonic acid breaks down into water and carbon dioxide, which leaves the cell. Simultaneously, a reverse chloride shift occurs: bicarbonate ions return to the erythrocytes from the plasma, and chloride ions move outward. Key rule: oxygen delivery to tissues helps the blood bind carbon dioxide, whereas oxygenation in the lungs stimulates its release.
Carbon Dioxide Dissociation Curve
The dependence of blood carbon dioxide content on its partial pressure (pCO2) is graphically represented using the dissociation curve.
This graph features two lines: red for arterial blood and blue for venous blood. The line connecting the calculated arterial blood point to the venous point clearly demonstrates actual gas exchange—the real volume and dynamics of carbon dioxide transfer.