Carbon dioxide transport by blood

Three forms of carbon dioxide transport

The transport process begins after carbon dioxide leaves tissue cells. In the bloodstream, it is distributed and transported in three main forms:

1. Physical dissolution. Part of the gas is in the plasma in a free, dissolved state. This fraction is relatively small and accounts for about 3 vol.%.
2. Chemical compounds in plasma. This is the main transport pathway, accounting for 50 vol.% of the total volume. The gas is transported in the form of hydrogencarbonates (bicarbonates), specifically potassium bicarbonate (KHCO3) and sodium bicarbonate (NaHCO3).
3. Chemical compounds in erythrocytes. Inside red blood cells, carbon dioxide forms a complex with hemoglobin — carbaminohemoglobin. This form accounts for 5 vol.%. Additionally, part of the gas is transported within the erythrocyte as potassium bicarbonate (KHCO3).

It is worth noting that during the subsequent transformations of hemoglobinic acid (HHb), the resulting carbaminohemoglobin is capable of transporting a total of about 15% of all carbon dioxide.

Tissue gas exchange and the chloride shift

In the systemic capillaries of tissues, the direction of carbon dioxide movement is strictly defined: it diffuses from the cells into the plasma and then penetrates into 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-).
• In parallel, carbamic acid (HHbCO2) is formed — a form of gas binding with hemoglobin.

Under these conditions, the Haldane effect is clearly manifested: oxyhemoglobin (KHbO2) in an environment with a high carbon dioxide content releases oxygen to tissues more easily. Having released oxygen, it binds with carbonic acid, turning into hemoglobinic acid (HHb). Essentially, this mechanism echoes the Bohr effect.

Ion dynamics is accompanied by the chloride shift (Hamburger phenomenon). HCO3- ions leave the erythrocyte into the plasma along the concentration gradient, where they combine with sodium ions to form NaHCO3. To maintain the electroneutrality of the erythrocyte, 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 the reverse direction. Carbon dioxide passes from the erythrocyte into the plasma and then into the alveolar air, whereas oxygen from the alveoli rushes into the erythrocyte.

The process is driven by two main factors:

1. Pressure gradient. The partial pressure of carbon dioxide (pCO2) in the alveoli is significantly lower than in venous blood plasma. In addition, the increase in pO2 in the lungs facilitates the release of carbon dioxide.
2. Hemoglobin oxygenation. Entering the erythrocyte, oxygen turns reduced hemoglobin into oxyhemoglobin (HbO2). The addition of oxygen sharply enhances the acidic properties of hemoglobin. As a stronger acid, oxyhemoglobin actively displaces hydrogen ions as well as 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, the reverse chloride shift occurs: bicarbonate ions return to the erythrocytes from the plasma, and chloride ions go outward. Important rule: the influx of oxygen into tissues helps the blood bind carbon dioxide, while oxygenation in the lungs stimulates its release.

Carbon dioxide dissociation curve

The dependence of the carbon dioxide content in the blood on its partial pressure (pCO2) is graphically displayed using the dissociation curve.

This graph shows two lines: red for arterial blood and blue for venous. The line connecting the calculated arterial blood point with the venous point clearly demonstrates actual gas exchange — the real volume and dynamics of carbon dioxide transition.

Class and exam questions

What else to study on this topic

• Detailed analysis of the Bohr effect and its physiological connection with the Haldane effect
• Properties and transformations of hemoglobinic and carbamic acids in the erythrocyte
• Mechanisms for maintaining the acid-base balance of blood plasma
• Detailed analysis of carbon dioxide dissociation curve graphs

Other topics in the "Respiration" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.