Carbon dioxide (CO2) is a natural byproduct of the body’s metabolic activities, generated as cells convert food into energy. This gaseous waste product must be removed to maintain the body’s balance and proper physiological function. Its journey from cells, through the bloodstream, and out of the body is fundamental to both the respiratory and circulatory systems.
Cellular Origin of Carbon Dioxide
Carbon dioxide originates within the body’s cells from cellular respiration, the process converting nutrients into energy. During aerobic respiration, glucose and other fuel molecules break down in the presence of oxygen to produce adenosine triphosphate (ATP).
The Krebs cycle, a central part of aerobic respiration in the mitochondria, produces carbon dioxide as a waste product. CO2 production also occurs when pyruvate, derived from glucose, converts into acetyl coenzyme A before the Krebs cycle. The continuous generation of carbon dioxide reflects the ongoing energy demands of every cell.
CO2 Entry into the Bloodstream
Once produced, carbon dioxide moves into the bloodstream for transport to the lungs. This movement occurs through diffusion, a passive process driven by differences in partial pressure. Cellular metabolic activity leads to a high partial pressure of carbon dioxide (PCO2) within tissue cells, often 45 to 48 mmHg. Capillaries, the smallest blood vessels surrounding tissues, contain blood with a lower PCO2, typically around 40 mmHg on the arterial side.
This partial pressure gradient drives carbon dioxide from higher to lower concentration, from tissue cells and interstitial fluid into capillary blood. Carbon dioxide is notably more soluble in blood than oxygen, facilitating its entry into the plasma. As CO2 diffuses across capillary walls, it equilibrates, increasing the PCO2 in venous blood as it leaves the tissues.
Transport Mechanisms in the Blood
After entering the bloodstream, carbon dioxide is transported to the lungs through three primary mechanisms. A small portion, about 5 to 7 percent, of the carbon dioxide remains dissolved directly in the blood plasma. This dissolved CO2 contributes to the partial pressure of carbon dioxide in the blood.
Another fraction, approximately 10 to 23 percent of the total, binds to hemoglobin within red blood cells, forming a compound called carbaminohemoglobin. Carbon dioxide binds to the amino groups of the globin chains of hemoglobin, not to the iron-containing heme group where oxygen binds. The binding of carbon dioxide to hemoglobin is reversible, and this transport method is more effective when hemoglobin is deoxygenated, a phenomenon known as the Haldane effect.
The most significant proportion of carbon dioxide, accounting for about 70 to 85 percent, is transported in the form of bicarbonate ions (HCO3-). This conversion primarily occurs within red blood cells because they contain the enzyme carbonic anhydrase. This enzyme rapidly catalyzes the reaction of carbon dioxide with water to form carbonic acid (H2CO3), which then quickly dissociates into a hydrogen ion (H+) and a bicarbonate ion. The hydrogen ions produced are buffered by hemoglobin, preventing drastic changes in blood pH. The bicarbonate ions then diffuse out of the red blood cells into the plasma in exchange for chloride ions, a process known as the chloride shift, which maintains electrical neutrality across the red blood cell membrane.
Exiting the Blood and Body
Carbon dioxide is removed from the blood and expelled from the body at the lungs. As venous blood, rich in carbon dioxide, reaches the pulmonary capillaries surrounding the alveoli, the partial pressure gradient reverses. The PCO2 in the pulmonary capillaries is higher (around 45 mmHg) than in the alveolar air (around 40 mmHg). This gradient drives the diffusion of carbon dioxide from the blood into the alveoli.
The chemical reactions that occurred in the tissues to take up carbon dioxide are now reversed in the lungs. Bicarbonate ions re-enter the red blood cells from the plasma in exchange for chloride ions, reversing the chloride shift. Inside the red blood cells, bicarbonate ions combine with the hydrogen ions previously buffered by hemoglobin, reforming carbonic acid. Carbonic anhydrase then catalyzes the rapid conversion of carbonic acid back into carbon dioxide and water.
Simultaneously, carbaminohemoglobin releases its bound carbon dioxide as oxygen binds to hemoglobin in the oxygen-rich environment of the lungs. The newly formed carbon dioxide from both bicarbonate conversion and carbaminohemoglobin dissociation diffuses out of the red blood cells, across the capillary and alveolar membranes, and into the alveolar air. This carbon dioxide is then expelled from the body during exhalation.