Gas and physiology · BEGINNER LESSON
Carbon dioxide
Carbon dioxide (CO2) is a colorless gas produced by metabolism and removed by breathing, and its amount is often measured as partial pressure in air or blood. In the body, it is carried to the lungs mainly as bicarbonate before being exhaled.
What carbon dioxide is and what is measured
Carbon dioxide (CO2) is a gas made by living cells during metabolism and released when we exhale. In physiology, it is often described by partial pressure, which means the pressure contribution of one gas in a mixture. The sources note blood values around 40 mmHg on the arterial side and 45 to 48 mmHg on the venous side, while NASA discusses a 1-hour average CO2 partial pressure limit in habitable space of no more than 3 mmHg. These measurements matter because too much CO2 can build up in enclosed spaces or when the respiratory system does not remove it well.
[1] OCHMO-TB-004 Carbon Dioxide Rev D - NASA [2] Physiology, Carbon Dioxide Transport - StatPearls - NCBI Bookshelf [3] CO2 – Intrinsic Product, Essential Substrate, and Regulatory ...
How the body moves and removes carbon dioxide
Cells make CO2 during normal metabolism, especially in energy-producing reactions. CO2 diffuses out of cells into blood, where most of it is converted in red blood cells into bicarbonate (HCO3-), a dissolved form that helps carry carbon dioxide through the bloodstream. This conversion is sped up by carbonic anhydrase, an enzyme that helps reactions happen faster. In the lungs, the process reverses: bicarbonate is converted back to CO2, which moves into the alveoli, the tiny air spaces for gas exchange, and is then exhaled. A smaller fraction of CO2 stays dissolved in blood or binds to proteins such as hemoglobin.
[2] Physiology, Carbon Dioxide Transport - StatPearls - NCBI Bookshelf
Why high or low carbon dioxide matters
The body uses CO2 removal to help regulate blood pH, which is a measure of acidity or basicity. If CO2 rises, blood pH tends to fall; if CO2 falls, pH tends to rise. NASA’s material describes hypercapnia as CO2 buildup in the body and notes symptoms such as headache, fatigue, shortness of breath, and in severe cases death. In enclosed environments like spacecraft or suits, CO2 removal systems are needed because natural environmental removal is limited.
[1] OCHMO-TB-004 Carbon Dioxide Rev D - NASA [2] Physiology, Carbon Dioxide Transport - StatPearls - NCBI Bookshelf
Where carbon dioxide comes from in the environment
Outside the body, CO2 is also part of Earth’s atmosphere and is removed naturally by processes such as photosynthesis in plants and trees and by weathering. The third source also describes CO2 as a trace atmospheric gas and notes that its level can affect microbial and biochemical processes, showing that CO2 is not only waste from animals but also a substance with multiple roles in living systems.
[1] OCHMO-TB-004 Carbon Dioxide Rev D - NASA [3] CO2 – Intrinsic Product, Essential Substrate, and Regulatory ...
SEE THE IDEA
How carbon dioxide moves through the body
Metabolism produces CO2 as a waste product.
Most CO2 becomes bicarbonate (HCO3-) in red blood cells.
Bicarbonate is converted back to CO2 and exhaled.
If removal fails, CO2 builds up and can cause hypercapnia.
sequence
CO2 is produced in cells, carried in blood, converted in the lungs, and removed by exhalation; if removal is insufficient, levels rise.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.