Carbon monoxide (CO) is a colorless, odorless, and tasteless gas, making it undetectable by human senses. This characteristic contributes to its danger, as individuals can be exposed without awareness. Understanding its journey through the body and environment is important.
Entry into the Body
Carbon monoxide primarily enters the human body through inhalation. Once breathed in, the gas quickly moves from the lungs into the bloodstream. This occurs efficiently across the thin membranes of the alveoli, the tiny air sacs in the lungs where gas exchange normally takes place.
Once in the bloodstream, carbon monoxide circulates throughout the body. Its rapid absorption is a consequence of the large surface area of the lungs available for gas exchange and the efficient transport system of the circulatory network. CO quickly reaches various tissues and organs.
Physiological Impact
Upon entering the bloodstream, carbon monoxide exerts its primary toxic effect by binding to hemoglobin (Hb), the protein in red blood cells responsible for oxygen transport. Carbon monoxide has an affinity for hemoglobin that is approximately 200 to 300 times greater than that of oxygen. This strong binding forms a stable compound called carboxyhemoglobin (COHb).
The formation of carboxyhemoglobin significantly reduces the blood’s capacity to carry oxygen to tissues and organs. Not only does CO occupy oxygen-binding sites on hemoglobin, but its binding also increases the affinity of the remaining hemoglobin sites for oxygen, causing a “left shift” in the oxygen dissociation curve. This shift means that any oxygen still bound to hemoglobin is less readily released to the body’s cells, leading to cellular oxygen deprivation, or hypoxia.
Organs with high oxygen demands, such as the brain and heart, are particularly susceptible to this oxygen deprivation. Beyond its effect on hemoglobin, carbon monoxide can also directly interfere with cellular respiration by binding to other heme-containing proteins, including mitochondrial cytochrome oxidase. This inhibition can disrupt the cell’s ability to produce energy, further contributing to tissue damage and dysfunction.
Elimination from the Body
The body removes carbon monoxide primarily by reversing its binding to hemoglobin and exhaling it through the lungs. As fresh, oxygen-rich air is breathed in, the higher concentration of oxygen helps displace the carbon monoxide from the hemoglobin molecules.
However, this elimination process is considerably slower than the body’s uptake of carbon monoxide. The biological half-life of COHb in a sedentary healthy adult breathing room air can range from approximately 4 to 6 hours. This extended half-life means that even after exposure ceases, carbon monoxide can persist in the bloodstream for several hours, continuing to impair oxygen delivery.
Factors such as increased ventilation, like during physical activity, can accelerate the removal of CO from the body. The body gradually restores normal oxygen-carrying capacity as CO is exhaled.
Environmental Pathways
Once released into the atmosphere, carbon monoxide undergoes chemical transformations. The primary mechanism for its removal from the air is through a reaction with hydroxyl radicals (OH). These highly reactive molecules oxidize carbon monoxide, converting it into carbon dioxide (CO2).
This reaction is important for maintaining the atmosphere’s oxidative capacity, influencing the fate of other atmospheric gases. The residence time of carbon monoxide in the atmosphere is relatively short, ranging from a few weeks to about three months, due to this efficient removal process.
Beyond atmospheric oxidation, some natural processes also contribute to CO removal. Microorganisms in soils can absorb and metabolize carbon monoxide, acting as a minor natural sink. This soil absorption is influenced by factors such as temperature and moisture, with optimal conditions enhancing microbial activity.