The pCO2 (partial pressure of carbon dioxide) is highest in the tissues.
Explanation: In the human body, carbon dioxide is produced as a waste product during cellular metabolism. Tissues actively metabolize nutrients and produce CO2, which diffuses out of the cells into the interstitial fluid and then into the bloodstream. This results in a higher concentration of CO2 in the tissues compared to other areas such as the alveoli or oxygenated blood, where CO2 has been expelled or is at lower levels. In the alveoli, CO2 is exchanged for oxygen, leading to a lower pCO2. Therefore, among the options given, the tissues have the highest level of pCO2.