Has a greater partial pressure in the pulmonary capillaries than in the alveoli

CO2

Explanation:

Diffusion is a simple process of molecular movement, from their higher concentration to the area with their lower concentration. The process of CO2 movement from the pulmonary capillaries into the alveoli is part of the gas exchange (together with oxygen movenent in opposite direction) and it is actually simple diffusion. Alveoli have huge surface area, thin cell walls and a lot of blood vessels around them. All of these are adaptations that facilitate gas exchange. There is a gradient in partial pressure of the CO2:  it is much higher in the pulmonary capillaries than in the alveoili so it moves down its gradient-from the blood  to the alveoli (exhale).

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