Carbon dioxide (CO2) is a byproduct generated by the body’s cells during metabolism. Its efficient removal from the body is necessary to maintain physiological balance. The bloodstream transports this waste product from tissues to the lungs for exhalation.
Transport as Dissolved Gas
A small portion of carbon dioxide, about 5% to 10%, is transported directly dissolved in plasma. The amount of gas that can dissolve depends on its solubility and the partial pressure. Carbon dioxide is considerably more soluble in blood than oxygen, by a factor of about 20.
Transport Bound to Hemoglobin
Another way carbon dioxide travels in the blood is by binding to hemoglobin, a red blood cell protein. Approximately 10% to 20% of CO2 is transported in this manner, forming a compound called carbaminohemoglobin. Carbon dioxide attaches to the amino groups of the globin protein part of hemoglobin. This binding is a reversible process, influenced by the partial pressure of CO2 in the surrounding environment.
Transport as Bicarbonate Ions
The most significant mechanism for carbon dioxide transport, accounting for about 70% to 85% of the total, involves its conversion into bicarbonate ions. This process primarily occurs inside red blood cells. When CO2 enters a red blood cell, it rapidly combines with water to form carbonic acid (H2CO3), a reaction greatly accelerated by the enzyme carbonic anhydrase.
Carbonic acid is unstable and quickly dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). To maintain electrical neutrality, bicarbonate ions moving out of the red blood cell into the plasma are exchanged for chloride ions (Cl-). This exchange, known as the “chloride shift,” is facilitated by a specific transporter protein. Hydrogen ions produced are largely buffered by hemoglobin, preventing significant changes in blood pH.
Releasing Carbon Dioxide in the Lungs
When the blood reaches the lungs, the processes that captured carbon dioxide in the tissues reverse, allowing for its release. The partial pressure of CO2 in the alveoli is lower (around 40 mmHg) than in the venous blood arriving from the body (around 45 mmHg), creating a gradient for CO2 to diffuse out.
Bicarbonate ions re-enter the red blood cells in exchange for chloride ions, reversing the chloride shift. Inside the red blood cells, bicarbonate ions combine with hydrogen ions, reforming carbonic acid. Carbonic anhydrase then catalyzes the conversion of carbonic acid back into carbon dioxide and water. Concurrently, carbaminohemoglobin releases its bound CO2. Dissolved CO2 in the plasma and CO2 released from other forms then diffuse out of the blood, across the capillary walls, and into the alveoli for exhalation.