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Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
Which of the following substances crosses capillary walls primarily through water-filled clefts between the endothelial cells?
assistant
The correct answer is  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.