Final answer:
An increase in PCO₂ to 45 mmHg would most likely stimulate an increase in ventilation due to the respiratory center's sensitivity to carbon dioxide levels in the blood.
Step-by-step explanation:
The question, "Which of the following would most likely stimulate an increase in ventilation?" pertains to the respiratory system's response to changes in arterial blood gases. An increase in PCO₂ would be the most likely to stimulate an increase in ventilation. Therefore, the correct answer is d) an increase in PCO₂ to 45 mmHg. The respiratory center in the brain is sensitive to the levels of carbon dioxide in the blood. A higher PCO₂ indicates a higher level of carbon dioxide, which would trigger an increase in breathing to expel CO₂ and maintain homeostasis.
When blood passes from the arteries to the veins within body tissues, PO₂ drops and PCO₂ increases. This exchange of gases is due to the pressure gradients that drive the diffusion of oxygen from capillaries to tissue cells and carbon dioxide from tissue cells to the capillaries.