Carbon dioxide (CO2) is a natural waste product generated within the human body. Its removal is important to maintain the body’s internal balance. The body has efficient mechanisms to produce, transport, and eliminate this gas.
How Carbon Dioxide is Produced
The human body generates carbon dioxide primarily through cellular respiration. This metabolic pathway occurs within cells, particularly in the mitochondria.
During cellular respiration, cells break down nutrient molecules, such as glucose, fats, and proteins, in the presence of oxygen. This breakdown releases energy, which is captured in the form of adenosine triphosphate (ATP), the body’s main energy currency.
Carbon dioxide is an inevitable byproduct of these energy-producing reactions. As glucose is oxidized, its carbon atoms combine with oxygen to form CO2.
Transporting Carbon Dioxide Through the Bloodstream
Once produced, carbon dioxide must be efficiently transported to the lungs for removal. This journey involves its movement from the cells, through the interstitial fluid, and into the bloodstream. The blood then carries CO2 in three primary forms.
Approximately 7-10% of the total carbon dioxide dissolves directly into the blood plasma. CO2 is significantly more soluble in water than oxygen, allowing it to be carried in this dissolved gaseous state. This dissolved CO2 contributes directly to the partial pressure of carbon dioxide in the blood, which is a key factor in gas exchange.
Another method involves carbon dioxide binding to hemoglobin within red blood cells, forming a compound called carbaminohemoglobin. This accounts for about 20-30% of CO2 transport. Notably, CO2 binds to the globin (protein) part of the hemoglobin molecule, not the iron-containing heme group where oxygen binds. The formation of carbaminohemoglobin is more favorable when hemoglobin has released oxygen, a phenomenon known as the Haldane effect.
The largest proportion of carbon dioxide, accounting for 60-70% or sometimes up to 85%, is transported in the form of bicarbonate ions (HCO3-). This process primarily occurs within the red blood cells.
As CO2 diffuses into these cells, it quickly reacts with water to form carbonic acid (H2CO3), a reaction rapidly catalyzed by the enzyme carbonic anhydrase. Carbonic acid is unstable and immediately dissociates into a bicarbonate ion (HCO3-) and a hydrogen ion (H+).
The hydrogen ions produced are buffered by hemoglobin, preventing significant changes in blood pH. To maintain electrical neutrality, as bicarbonate ions move out of the red blood cell into the plasma, chloride ions (Cl-) move into the red blood cell; this exchange is known as the chloride shift. This efficient conversion and exchange mechanism allows for the continuous uptake of CO2 from the tissues into the blood.
Removing Carbon Dioxide Through Breathing
The final stage of carbon dioxide removal occurs in the lungs, where the gas is expelled. As the blood, rich in CO2, arrives at the tiny capillaries surrounding the alveoli (air sacs) in the lungs, gas exchange takes place. The partial pressure of carbon dioxide is higher in the blood within these capillaries than in the alveolar air. This difference creates a gradient, causing CO2 to diffuse from the blood into the alveoli. The various forms of transported CO2 then undergo a reversal of processes that occurred in the tissues.
Bicarbonate ions re-enter the red blood cells in a process called the reverse chloride shift, where chloride ions move out to maintain electrical neutrality.
Inside the red blood cells, the bicarbonate ions combine with hydrogen ions, reforming carbonic acid. The enzyme carbonic anhydrase then rapidly converts this carbonic acid back into carbon dioxide and water.
Simultaneously, carbaminohemoglobin releases its bound CO2. All these forms of carbon dioxide, now as a gas, diffuse into the alveolar space.
Once in the alveoli, CO2-rich air is expelled through exhalation. During quiet exhalation, the diaphragm and external intercostal muscles relax.
This relaxation causes the diaphragm to move upward and the rib cage to move downward and inward. The combined effect of these muscle relaxations decreases the volume of the chest cavity.
This reduction in volume increases the air pressure within the lungs, creating a pressure gradient that forces the air, now laden with carbon dioxide, out of the body through the airways.