The process of cellular respiration in tissues creates carbon dioxide (\(\text{CO}_2\)), a metabolic waste product that must be removed from the body. \(\text{CO}_2\) directly influences the blood’s acidity (pH), and its buildup can lead to a dangerous imbalance. The circulatory system picks up \(\text{CO}_2\) from the tissues and transports it to the lungs, where it is expelled during exhalation. The body employs three distinct mechanisms for this high-volume transport.
The Bicarbonate System
The most significant method for moving carbon dioxide involves its conversion into bicarbonate ions, accounting for approximately 70% of the total \(\text{CO}_2\) transported. This conversion primarily takes place inside the red blood cells (RBCs), which contain the enzyme carbonic anhydrase.
As \(\text{CO}_2\) diffuses from the tissues into the blood, it enters the red blood cell. Inside the cell, carbonic anhydrase rapidly catalyzes the reaction between carbon dioxide and water (\(\text{H}_2\text{O}\)) to form carbonic acid (\(\text{H}_2\text{CO}_3\)). This chemical transformation prevents a backlog of \(\text{CO}_2\) that would impede its continued uptake.
Carbonic acid is unstable and dissociates into a hydrogen ion (\(\text{H}^+\)) and a bicarbonate ion (\(\text{HCO}_3^-\)). The bicarbonate ion is then transported out of the red blood cell and into the plasma, where it is carried back to the lungs. This movement of the negatively charged bicarbonate ion leaves the inside of the red blood cell more positive, requiring immediate balance to maintain electrical neutrality.
To counteract the charge imbalance, a protein transports a negatively charged chloride ion (\(\text{Cl}^-\)) from the plasma into the red blood cell in exchange for the outgoing bicarbonate ion. This mechanism is known as the Chloride Shift. The hydrogen ions produced are buffered by hemoglobin, preventing the blood from becoming too acidic.
\(\text{CO}_2\) Bound to Hemoglobin
Transport involving binding directly to the hemoglobin protein carries 20-23% of the total \(\text{CO}_2\). Unlike oxygen, which binds to the iron-containing heme groups, carbon dioxide attaches to the amino groups found on the globin protein chains. This compound is known as carbaminohemoglobin.
This binding is enhanced in the tissues where oxygen levels are low. When hemoglobin releases oxygen, its structure changes, increasing its affinity for carbon dioxide and hydrogen ions. This effect promotes \(\text{CO}_2\) uptake in low-oxygen environments.
The formation of carbaminohemoglobin occurs at a site distinct from the oxygen-binding site. This non-competitive binding allows the red blood cell to efficiently pick up \(\text{CO}_2\) while simultaneously dropping off oxygen at the tissue level.
\(\text{CO}_2\) Dissolved in Plasma
The simplest method accounts for the smallest fraction, with approximately 7-10% of \(\text{CO}_2\) remaining dissolved directly in the blood plasma. Carbon dioxide is soluble in water-based solutions like plasma, allowing a small amount to be carried without chemical modification.
The partial pressure of this dissolved gas (\(\text{P}\text{CO}_2\)) creates the concentration gradient that drives \(\text{CO}_2\) movement out of the tissue cells and into the bloodstream. This fraction is responsible for initiating the uptake of all \(\text{CO}_2\) into the blood for transport.
Releasing \(\text{CO}_2\) at the Lungs
When the blood reaches the pulmonary capillaries, the process of \(\text{CO}_2\) uptake is reversed. The low partial pressure of \(\text{CO}_2\) in the alveoli provides the necessary concentration gradient for the gas to diffuse out of the blood. The binding of oxygen to hemoglobin in the lungs triggers the reversal of the chemical reactions.
As oxygen binds to hemoglobin, the protein’s affinity for hydrogen ions decreases, causing the release of \(\text{H}^+\) into the red blood cell—a phenomenon known as the Haldane effect. These released hydrogen ions quickly combine with the bicarbonate ions (\(\text{HCO}_3^-\)) that re-enter the red blood cell from the plasma, reversing the Chloride Shift.
This combination of \(\text{H}^+\) and \(\text{HCO}_3^-\) reforms carbonic acid (\(\text{H}_2\text{CO}_3\)). Carbonic anhydrase then rapidly converts the carbonic acid back into water and gaseous \(\text{CO}_2\). This \(\text{CO}_2\), along with the gas released from carbaminohemoglobin and the amount dissolved in the plasma, diffuses into the lungs, ready to be exhaled.