Carbon dioxide (CO2) is a byproduct of cellular metabolism. This waste product must be transported from tissues to the lungs for exhalation. The body uses mechanisms to ensure CO2 removal and maintain a stable internal environment.
Three Forms of Carbon Dioxide Transport
Carbon dioxide travels through the bloodstream in three primary ways. Approximately 5% to 7% is dissolved directly in the blood plasma. Another 10% to 30% binds to hemoglobin within red blood cells, forming carbaminohemoglobin. The most significant amount, 70% to 85%, occurs as bicarbonate ions in the plasma.
The Bicarbonate System: The Primary Pathway
The majority of carbon dioxide is transported as bicarbonate ions, primarily within red blood cells and then in the blood plasma. As CO2 diffuses from tissues into red blood cells, it rapidly combines with water to form carbonic acid (H2CO3). This reaction is accelerated by carbonic anhydrase, an enzyme abundantly present inside red blood cells.
Carbonic acid is unstable and quickly dissociates into a hydrogen ion (H+) and a bicarbonate ion (HCO3-). Hemoglobin buffers the hydrogen ions, preventing a drastic drop in blood pH within the red blood cell. Deoxygenated hemoglobin is particularly effective at this, having a greater affinity for hydrogen ions.
Bicarbonate ions accumulate inside red blood cells. To prevent an electrical imbalance, these negatively charged ions are transported out into the blood plasma. A specialized protein transporter on the red blood cell membrane exchanges a bicarbonate ion for a chloride ion (Cl-) from the plasma. This exchange, known as the “chloride shift” or Hamburger phenomenon, maintains electrical neutrality.
In the lungs, this process reverses. As oxygen binds to hemoglobin, it causes the release of hydrogen ions. These hydrogen ions combine with bicarbonate ions that re-enter the red blood cells via the chloride shift. Carbonic anhydrase converts carbonic acid back into carbon dioxide and water. The CO2 then diffuses out of the red blood cells and into the alveoli for exhalation.
Other Transport Mechanisms
Other mechanisms also contribute to CO2 transport. A small amount of carbon dioxide remains dissolved directly in the blood plasma. This dissolved CO2 contributes to the partial pressure of carbon dioxide, influencing its diffusion across membranes. CO2 is more soluble in water than oxygen, allowing for this direct dissolution.
Another portion is transported bound to hemoglobin and other plasma proteins, forming carbamino compounds. When CO2 binds to hemoglobin, it attaches to the amino groups of the globin chains, not the heme groups where oxygen binds. This means CO2 transport does not directly compete with oxygen binding. This binding is reversible, allowing for CO2 release in the lungs. Deoxygenated hemoglobin has a higher affinity for CO2, enhancing its ability to pick up CO2 in tissues.
Maintaining Body Balance
CO2 transport is linked to maintaining the body’s acid-base balance. The bicarbonate buffering system is a primary regulator of blood pH. The reversible reaction between CO2, water, carbonic acid, and bicarbonate allows the body to manage acidity. When CO2 levels rise, more carbonic acid forms, increasing hydrogen ion concentration and making the blood more acidic. Conversely, exhaling CO2 shifts the reaction, reducing acidity and maintaining blood pH within a narrow, healthy range.