Final answer:
The capillary least likely to allow substances to enter or exit the blood due to tight junctions that join the endothelial cells is the closed capillary.
Step-by-step explanation:
The capillary least likely to allow substances to enter or exit the blood due to tight junctions that join the endothelial cells is the closed capillary. Closed capillaries have complete tight junctions, which make them impermeable to substances. Fenestrated capillaries have pores that allow for the passage of larger molecules, continuous capillaries have intercellular clefts that allow for small molecules to pass through, and sinusoidal capillaries have extensive intercellular gaps that allow for the passage of even larger molecules.