Carbon dioxide (CO2) is a metabolic byproduct generated as tissues consume oxygen and produce energy. This gas must be efficiently removed from the body to prevent accumulation, which would lead to an increase in blood acidity, a condition known as acidosis. The circulatory system manages this task by transporting CO2 from the metabolizing cells to the lungs for exhalation. This journey involves a complex series of chemical and physical processes within the blood, utilizing three distinct mechanisms for rapid and high-volume transport.
CO2 Entry and Transport in Dissolved Form
The process begins as CO2 diffuses out of tissue cells and into the surrounding interstitial fluid, driven by the difference in partial pressure between the producing cell and the blood capillary. Because CO2 is significantly more soluble in water than oxygen, a small fraction of the gas dissolves into the liquid component of the blood, the plasma. This physically dissolved CO2 accounts for approximately 7 to 10 percent of the total CO2 being transported.
While this dissolved form is a minor contributor to the overall transport capacity, it is functionally important because only dissolved CO2 exerts a partial pressure. This partial pressure gradient dictates the direction of CO2 movement, driving the gas out of the tissues and eventually into the lungs. This maintains the concentration gradient that allows for continuous CO2 uptake from metabolically active cells.
Binding to Hemoglobin and Plasma Proteins
Carbon dioxide binds directly to blood proteins to form carbamino compounds, accounting for about 20 to 25 percent of the total CO2 carried in the blood. The gas attaches to exposed amino groups (-NH2) on the protein chains, primarily on the globin portion of the hemoglobin molecule inside red blood cells, creating carbaminohemoglobin.
CO2 does not bind to the iron atom like oxygen does, allowing both gases to be transported simultaneously. The capacity of hemoglobin to bind CO2 is enhanced when oxygen has been released, a relationship known as the Haldane Effect. Deoxygenated hemoglobin acts as a more effective acceptor for CO2, facilitating the efficient loading of CO2 into the blood for transport away from the tissues.
The Bicarbonate System: Primary CO2 Carrier
Roughly 70 percent of carbon dioxide is transported through the bicarbonate buffer system, a chemical pathway that occurs primarily within the red blood cells. When CO2 enters the red blood cell, it rapidly combines with water to form carbonic acid (H2CO3). This reaction is accelerated by the enzyme carbonic anhydrase (CA), which is present in high concentrations inside the red blood cell but is largely absent from the plasma.
Carbonic anhydrase speeds up the conversion reaction by many thousands of times, allowing the blood to continuously pick up CO2. The resulting carbonic acid is unstable and immediately dissociates into a hydrogen ion (H+) and a bicarbonate ion (HCO3-). The hydrogen ions produced in this reaction are quickly buffered by being absorbed by hemoglobin, which prevents drastic changes to the blood’s pH.
The newly formed bicarbonate ion must now leave the red blood cell to travel in the plasma. This movement is accomplished by a transport protein on the red blood cell membrane in a process called the Chloride Shift. The bicarbonate ion is exchanged for a chloride ion (Cl-) from the plasma, which moves into the red blood cell.
This exchange ensures that the electrical neutrality of both the red blood cell and the plasma is maintained. The bicarbonate ions then travel dissolved in the plasma to the lungs, functioning as the body’s primary method for transporting metabolic CO2. This system also allows bicarbonate to act as a chemical buffer, helping to stabilize the blood’s pH throughout the transport process.
Exchanging CO2 in the Lungs
The carbon dioxide-rich blood reaches the pulmonary capillaries surrounding the alveoli in the lungs. Here, the partial pressure of CO2 in the alveolar air is significantly lower than in the blood, which reverses the concentration gradient and drives the unloading process.
The bicarbonate system reverses course to release the stored CO2. Bicarbonate ions re-enter the red blood cell from the plasma in exchange for chloride ions, reversing the Chloride Shift. Inside the red blood cell, the bicarbonate ion combines with the hydrogen ion that was previously buffered by hemoglobin, forming carbonic acid.
Carbonic anhydrase then converts the carbonic acid back into water and carbon dioxide. Simultaneously, oxygen binding to hemoglobin triggers the reverse Haldane Effect. This binding decreases hemoglobin’s affinity for CO2, causing the gas to dissociate from the carbaminohemoglobin. All three forms of transport release their CO2, which then diffuses out of the blood and into the alveoli for exhalation.