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).

solvent

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cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!