CO2 Transport in Blood: Dissolved, Carbaminohemoglobin, and Bicarbonate Pathways (Osmosis)

Overview

In this Osmosis video from Elsevier, learn how carbon dioxide is carried in the blood from tissues to the lungs via three main routes: dissolved in plasma, bound to hemoglobin as carbaminohemoglobin, and as bicarbonate formed inside red blood cells. The explanation links CO2 transport to partial pressures of CO2 and O2 and to physiologic pH maintenance, highlighting how these processes are reversible during expiration.

• Dissolved CO2 in plasma is calculated from PCO2 and solubility, yielding about 5 to 10 percent of total CO2 transport.
• Carbaminohemoglobin binds CO2 to hemoglobin, with Bohr effect facilitating oxygen unloading where CO2 is abundant.
• Most CO2 is carried as bicarbonate after a rapid reaction in red blood cells, balanced by chloride exchange.
• In the lungs, CO2 release is aided by pressure differences and the Haldane effect, which promotes CO2 offloading as oxygen binds hemoglobin.

Content is drawn from Osmosis from Elsevier.

CO2 Transport in Blood: Three Pathways

Osmosis from Elsevier explains that carbon dioxide produced by cells is transported in the blood through three distinct mechanisms. First, a small portion dissolves directly in the plasma; second, CO2 binds to terminal amino groups on the four globin chains of hemoglobin forming carbaminohemoglobin; and third, the majority is converted into bicarbonate within red blood cells and then released into the plasma. The video emphasizes how these processes are driven by the partial pressures of CO2 and O2, and how the body maintains acid–base balance through bicarbonate formation and the chloride shift. The system is dynamic and reversible, ensuring CO2 can be picked up in tissues and expelled in the lungs during expiration.

Dissolved CO2 in Plasma

The dissolved fraction of CO2 is determined by multiplying the partial pressure of CO2 by its solubility in blood. The solubility is described as the amount of CO2 that can dissolve per 100 mL of blood at a given CO2 pressure. Specifically, the solubility is about 0.07 mL CO2 per 100 mL of blood for each mmHg of CO2, so venous CO2 at a PCO2 of about 45 mmHg yields roughly 3.15 mL CO2 dissolved per 100 mL of blood. This dissolved portion accounts for approximately 5 percent of total CO2 transport, though it can rise to about 10 percent under certain conditions. The dissolved pool is small but crucial because it diffuses readily into the plasma and can be expired without chemical modification.

"0.07 milliliters of carbon dioxide is dissolved per millimeter of mercury per 100 milliliters of blood." - Osmosis from Elsevier

Carbaminohemoglobin and the Bohr Effect

A second transport route involves CO2 binding directly to hemoglobin on the terminal amino acids of the globin chains, forming carbaminohemoglobin. Each hemoglobin molecule can carry up to four CO2 molecules. This binding slightly alters hemoglobin’s shape and lowers its affinity for oxygen, a phenomenon known as the Bohr effect. As CO2 levels rise in tissues, more oxygen is released from hemoglobin, facilitating gas exchange in metabolically active tissues. In the lungs, the increased availability of oxygen reduces CO2 binding to hemoglobin, shifting the balance toward CO2 offloading. This rightward shift in the oxygen-hemoglobin dissociation curve helps meet the tissue’s need for oxygen while promoting carbon dioxide removal in the lungs.

"carbaminohemoglobin alters the shape of the hemoglobin molecule slightly and it decreases hemoglobin's affinity for oxygen." - Osmosis from Elsevier

Bicarbonate Formation and the Chloride Shift

The largest fraction of CO2 transport, roughly 70–80 percent, is carried as bicarbonate ions formed inside red blood cells. CO2 rapidly reacts with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate. The enzyme carbonic anhydrase accelerates this reaction inside the red blood cell, producing large amounts of bicarbonate and hydrogen ions. To maintain electrical balance as bicarbonate leaves the cell in exchange for chloride ions entering the cell (the chloride shift), bicarbonate is transported into the plasma. This reaction is reversible, so bicarbonate can re-enter red blood cells when CO2 is required for expiration. Bicarbonate plays a key role in buffering hydrogen ions and maintaining blood pH, balancing shifts according to Le Chatelier's principle. When hydrogen ion concentration rises, bicarbonate can bind to hydrogen to reform carbonic acid, which then splits into water and CO2 for elimination.

"carbonic acid easily dissociates into hydrogen ions and bicarbonate ions." - Osmosis from Elsevier

CO2 Release in the Lungs and the Haldane Effect

CO2 exchange at the lungs is driven by a lower partial pressure of CO2 and a higher partial pressure of O2 compared with tissues. More oxygen diffuses into red blood cells and binds to the heme group, slightly altering hemoglobin’s conformation and reducing its affinity for CO2 and hydrogen. This Haldane effect promotes release of CO2 from hemoglobin and from bicarbonate back into the plasma, facilitating diffusion into the alveoli where CO2 partial pressure is around 40 mmHg. CO2 diffuses out of the blood into the alveolar air much faster than oxygen diffuses into the blood because CO2 is far more soluble in blood. The bicarbonate system thus cycles CO2 through the blood, plasma, and lungs, ensuring efficient expiration of metabolic waste.

"This is called the Haldane effect, and it means that as more oxygen binds to hemoglobin, more, more carbon dioxide and hydrogen becomes unbound." - Osmosis from Elsevier

Recap and Physiologic Significance

In summary, carbon dioxide is moved around the body by three main mechanisms: dissolved CO2 in plasma, carbaminohemoglobin bound to hemoglobin, and bicarbonate formed in red blood cells and transported in plasma. All three processes are reversible in response to changes in tissue activity and lung function, enabling CO2 pickup where it is produced and expiration where it is expelled. The video emphasizes the interplay of partial pressures, solubility, and enzymatic catalysis that makes this transport system efficient and tightly regulated, helping maintain acid–base balance and metabolic homeostasis during respiration.

"The processes of carbon dioxide pick up in the tissues are reversible in the lungs." - Osmosis from Elsevier