Carbon Dioxide Transport in Blood: Dissolved CO2, Carbaminohemoglobin, and Bicarbonate Pathways

Overview

This Osmosis video from Elsevier explains how carbon dioxide is transported from tissues to the lungs using three main pathways: dissolved CO2 in plasma, carbaminohemoglobin, and bicarbonate formation in red blood cells. It also covers how partial pressures and solubility drive diffusion and how the system maintains pH and oxygen delivery.

Key insights

• CO2 is carried via three primary forms with distinct fractions, including a notable bicarbonate component.
• Carbonic anhydrase speeds bicarbonate formation in red blood cells and the chloride shift balances charge as bicarbonate moves into plasma.
• Bohr and Haldane effects facilitate oxygen release in tissues and CO2 unloading in the lungs.
• CO2 diffuses from blood into alveoli driven by partial pressure differences and high solubility, enabling efficient expiration.

CO2 Transport in Blood: Mechanisms and Key Players

The video explains how carbon dioxide, a waste product of cellular metabolism, is moved from tissues to the lungs for elimination. It highlights three main transport mechanisms and the biophysical and biochemical principles that govern them. This understanding is essential for grasping gas exchange and acid-base balance in physiology.

Dissolved CO2 in Plasma

A portion of CO2 remains dissolved directly in the plasma. The concentration of dissolved CO2 is calculated by multiplying the venous partial pressure of CO2 by the solubility of CO2 in blood. The video provides a specific solubility value: 0.07 milliliters of CO2 per millimeter of mercury per 100 milliliters of blood. When the venous partial pressure of CO2 is about 45 mmHg, the dissolved CO2 amount is approximately 3.15 milliliters per 100 milliliters of blood. This dissolved fraction accounts for roughly 5 percent of total CO2 transport, though it can be as high as 10 percent. This dissolved form of CO2 remains readily diffusible and participates directly in gas exchange, even when other diffusion processes are hindered by lung pathology such as edema.

Carbaminohemoglobin: CO2 Bound to Hemoglobin

In addition to dissolved CO2, about 10 to 20 percent of CO2 is transported by binding directly to hemoglobin, forming carbaminohemoglobin. CO2 attaches to terminal amino groups of the four globin chains in each hemoglobin molecule, and each hemoglobin can carry up to four CO2 molecules. Binding CO2 to hemoglobin slightly alters the protein's shape and decreases its affinity for oxygen, a phenomenon known as the Bohr effect. This effect shifts the oxygen-hemoglobin dissociation curve to the right, facilitating greater oxygen release in tissues that are rich in CO2. While CO2 binding does impact oxygen loading at tissues, it is a crucial mechanism for enhancing CO2 transport to the lungs where it will be expelled.

Bicarbonate Pathway: The major Route

The majority of CO2 transport, approximately 70 to 80 percent, occurs after CO2 reacts with water to form carbonic acid, which quickly dissociates into hydrogen ions (H+) and bicarbonate (HCO3−). In red blood cells, the enzyme carbonic anhydrase accelerates this reaction, producing large amounts of bicarbonate and protons. As bicarbonate builds up in the red blood cell, it moves down its concentration gradient into the plasma. This movement creates a charge imbalance that is corrected by an exchange with chloride ions, a process known as the chloride shift. The bicarbonate serves as a critical buffer, helping to stabilize hydrogen ion concentration and, therefore, blood pH within a narrow range.

Physiologic Balancing: Le Chatelier’s Principle and the Bohr/Haldane Effects

Le Chatelier's principle explains how the bicarbonate system maintains pH: when hydrogen ions are scarce, the system shifts to produce more bicarbonate and hydrogen ions; when hydrogen ions are abundant, bicarbonate binds with hydrogen to form carbonic acid, which can release CO2. In the lungs, the environment shifts back due to lower hydrogen ion concentration and higher oxygen partial pressure, promoting CO2 release. The Bohr effect describes how increased CO2 and H+ in tissues reduce hemoglobin's affinity for oxygen, enhancing oxygen delivery. Conversely, the Haldane effect states that as oxygen binds to hemoglobin in the lungs, more carbon dioxide and hydrogen ions are released from hemoglobin, promoting CO2 exhalation. These coordinated effects optimize gas exchange across the entire respiratory cycle.

CO2 Release in the Lungs and Diffusion Kinetics

CO2 is released into the lungs when the partial pressure of CO2 in alveolar air is around 40 mmHg, and blood CO2 partial pressure drops as oxygen is taken up. CO2 diffuses out of red blood cells into the plasma and then into the alveoli. Importantly, CO2 diffuses about 20 times faster than oxygen, largely because CO2 is roughly 23 times more soluble than oxygen. This high diffusibility enables efficient CO2 elimination even when other fluid conditions in the lungs are not ideal. In summary, the three transport pathways work in concert to move CO2 from tissues to the lungs efficiently, while the bicarbonate system buffers hydrogen ions to maintain blood pH, and hemoglobin interactions regulate oxygen delivery during the process.

Quick Recap and Takeaways

Three routes carry CO2: dissolved in plasma, bound to hemoglobin as carbaminohemoglobin, and as bicarbonate in plasma after conversion from CO2 in red blood cells. The processes are reversible and coordinated with lung gas exchange. The kidney, liver, and blood buffering systems maintain acid-base balance, enabling stable pH and optimal tissue oxygenation during respiration.