Which capillary would be least likely to allow substances to enter or exit the blood due to tight junctions that join the endothelial cells

Those capillaries that have endothelial cells tightly attached have an increase in cell-to-cell junctions, such as desmosomal junctions that are poorly permeable to substances.

These capillaries generally function as conductors or collectors and not as permeabilizers like the pulmonary capillaries that are fenestrated and allow a high passage of substances and gases.

The capillaries are waterproof and prevent the passage of substances or fluids such as in the blood-brain barrier that is very selective

Explanation:

The blood-brain barrier is too selective since it is not favorable for the organism that many fluids, drugs, substances or chemical compounds enter the brain space.

This is the reason why the vessels are waterproof and why pharmacology was challenged to invent drugs that could cross this barrier since it was considered impossible.

An example is dopamine, which in parkynson's disease the levo group is added to dopamine, thus forming levodopamine as a product, this is the only way that the drug crosses the blood-brain barrier and can pass the impermeable barrier generated by capillaries with endothelial cells that are close to each other or closely linked