PCO2

Carbon dioxide molecule.

pCO₂, pCO₂, or $P_{{\ce {CO2}}}$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 pCO₂ are mmHg, atm, torr, Pa, or any other standard unit of atmospheric pressure. The pCO₂ 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

Main articles: Hypercapnia and Hypocapnia

In medicine, the partial pressure of carbon dioxide in arterial blood is called $P_{a_{{\ce {CO2}}}}$ or PaCO₂. Measurement of $P_{a_{{\ce {CO2}}}}$ 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 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.

With normal lung function, a stimulation to take another breath occurs when a patient has a slight rise in PaCO₂. The slight rise in PaCO₂ stimulates the respiratory centre in the brain, creating the impulse to take another breath. In some patients with a chronically high level of PaCO₂, such as those with chronic obstructive pulmonary disease (COPD), the stimulus and drive to breathe is caused by a decrease in PaCO₂. This is called a hypoxic drive. When oxygen is administered to patients with known CO₂ retention, patients need to be watched for signs of hypoventilation, a decreased level of consciousness, and apnea.

Aquatic Sciences

See also: Ocean acidification and Freshwater acidification

Oceanographers and limnologists use pCO₂ to measure the amount of carbon dioxide dissolved in water, as well as its flux into and out of 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 pCO₂ rises until it equals the pCO₂ of the overlying atmosphere. Conversely, a body of water with a pCO₂ greater than that of the atmosphere effluxes carbon dioxide.

pCO2 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 pCO₂ than cold water with the same concentration of carbon dioxide. pCO₂ 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 pCO₂. Respiration degrades organic matter, releasing CO₂ into the water column and increasing pCO₂. Photosynthesis assimilates inorganic carbon, thereby decreasing aquatic pCO₂.

See also

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

References

 This article incorporates text by Glynda Rees Doyle and Jodie Anita McCutcheon available under the CC BY 4.0 license.