Final answer:
When PO₂ levels decrease in the blood, PCO₂ levels usually increase due to the exchange of gases in the body tissues. Oxygen is utilized by cells, causing a drop in PO₂, while carbon dioxide is produced and enters the bloodstream, leading to an increase in PCO₂.
Step-by-step explanation:
Whenever the partial pressure of oxygen (PO₂) levels decrease below normal in the blood, the partial pressure of carbon dioxide (PCO₂) levels generally increase.
This is because as blood travels through the body tissues, oxygen diffuses out of the blood and into the cells, while carbon dioxide, a waste product of metabolism, diffuses from the cells into the blood.
Therefore, in the systemic capillaries, as oxygen is released and the PO₂ drops, the PCO₂ naturally rises due to the accumulation of carbon dioxide that needs to be carried back to the lungs for exhalation.