pCO2

''p''CO₂, pCO₂, or $P_\ce$ is the partial pressure of carbon dioxide (CO₂), often used in reference to blood but also used in meteorology, climate science, oceanography, and limnology to describe the fractional pressure of CO₂ as a function of its concentration in gas or dissolved phases. The units of ''p''CO₂ are mmHg, atm, torr, Pa, or any other standard unit of atmospheric pressure. The ''p''CO₂ of Earth's atmosphere has risen from approximately 280 ppm ( parts-per-million) to a mean 2019 value of 409.8 ppm as a result of anthropogenic release of carbon dioxide from fossil fuel burning. This is the highest atmospheric concentration to have existed on Earth for at least the last 800,000 years.

Medicine

In medicine, the partial pressure of carbon dioxide in arterial blood is called $P_$ or PaCO₂. Measurement of $P_$ in the systemic circulation indicates the effectiveness of ventilation at the lungs' alveoli, given the diffusing capacity of the gas. It is a good indicator of respiratory function and the closely related factor of acid–base homeostasis, reflecting the amount of acid in the blood (without lactic acid). Normal values for humans are in the range of 35–45 mmHg. Values less than this may indicate hyperventilation and (if blood pH is greater than 7.45) respiratory alkalosis. Values greater than 45 mmHg may indicate hypoventilation, and (if blood pH is less than 7.35) respiratory acidosis.Leticia Godoy Dias Sanderson. Gasometria arterial - Artigo de revisão. Fevereiro 2012.

Aquatic sciences

Oceanographers and limnologists use ''p''CO₂ to measure the amount of carbon dioxide dissolved in water, as well as to parameterize its flux into (influx) and out of (efflux) the atmosphere. Carbon dioxide reacts with water to form bicarbonate and carbonate ions, such that the relative solubility of carbon dioxide in water is greater than that of other unreactive gasses (e.g. Helium). As more carbon dioxide dissolves in water, its ''p''CO₂ rises until it equals the ''p''CO₂ of the overlying atmosphere. Conversely, a body of water with a ''p''CO₂ greater than that of the atmosphere effluxes carbon dioxide.

''p''CO₂ is additionally affected by water temperature and salinity. Carbon dioxide is less soluble in warmer water than cooler water, so hot water will exhibit a larger ''p''CO₂ than cold water with the same concentration of carbon dioxide. ''p''CO₂ can be used to describe the inorganic carbon system of a body of water, together with other parameters such as pH, dissolved inorganic carbon, and alkalinity. Together, these parameters describe the concentration and speciation of inorganic carbon species (CO_{2 (aq)}, HCO₃⁻, CO₃^2-) in water.

Biological processes such as respiration and photosynthesis affect and can be affected by aquatic ''p''CO₂. Respiration degrades organic matter, releasing CO₂ into the water column and increasing ''p''CO₂. Photosynthesis assimilates inorganic carbon, thereby decreasing aquatic ''p''CO₂.

See also

* Acidosis
* Alkalosis
* Arterial blood gas
* Blood gas tension
* Chemical equilibrium
* Hypercapnia
* pH
*Carbon Cycle
* xCO₂

References

Acid–base chemistry
Analytical chemistry
Equilibrium chemistry
Units of measurement
Chemical oceanography
Limnology

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