Which of the following substances crosses capillary walls primarily through water-filled clefts between the endothelial cells?

Because O2, CO2 and CO are lipophilic, they cross capillary walls primarily by diffusion through the endothelial cell membranes. Glucose is water soluble; it cannot cross through the lipid component of the cell membrane and is restricted to the water-filled clefts, or pores, between the cells.