By John Brennan
Updated Mar 24, 2022
Staying alive requires continuous cellular renewal and energy extraction from fuels like sugar and fat. These metabolic processes generate waste products—mainly urea and carbon dioxide—that must be removed promptly to preserve cell function. The body accomplishes this through coordinated actions of the lungs, liver, skin, and kidneys.
The liver and kidneys are primary detoxifiers, while the lungs and skin also play vital roles in expelling cellular waste.
During aerobic respiration, each glucose molecule broken down yields six molecules of carbon dioxide (CO₂) and six molecules of water. CO₂ diffuses into the bloodstream and, within red blood cells, reacts with water to form carbonic acid (H₂CO₃). This acid dissociates into hydrogen and bicarbonate ions (HCO₃⁻), transporting most of the CO₂ back to the lungs. In the pulmonary capillaries, CO₂ diffuses out, lowering blood CO₂ concentration. This shift drives bicarbonate ions to recombine into CO₂, which is then exhaled, effectively removing metabolic CO₂ from the body.
The liver is central to detoxification. Proteins and nucleic acids contain nitrogen, and their catabolism releases ammonia (NH₃), a toxic compound. The liver conjugates ammonia with CO₂ to form urea, a less harmful substance that can be safely transported in the bloodstream and later eliminated by the kidneys.
Perspiration is primarily a thermoregulatory mechanism, but sweat also carries trace amounts of salts, amino acids, and lipids. By excreting these substances with moisture, the skin contributes to the overall removal of metabolic waste.
Kidneys filter the blood through the glomerulus, removing urea, excess salts, vitamins, amino acids, and other dissolved molecules. While many essential substances are reabsorbed back into circulation, the remaining water and waste products are collected in the bladder via the ureters and expelled as urine, completing the excretion process.
