Final answer:
The endothelium of continuous capillaries is held together by tight junctions, which are particularly impermeable in the brain's blood-brain barrier, ensuring selective permeability for central nervous system function.
Step-by-step explanation:
The endothelium of continuous capillaries is held together by tight junctions. These junctions are usually impermeable and allow only for the passage of water and ions. Specifically, in the brain, where capillaries are a part of the blood-brain barrier, tight junctions with no intercellular clefts, a thick basement membrane, and astrocyte extensions called end feet combine to prevent the movement of nearly all substances.
In general capillaries, although tight junctions are often incomplete, leaving intercellular clefts that allow for the exchange of water and other very small molecules between the blood plasma and the interstitial fluid. Substances that can pass between cells include metabolic products such as glucose, water, and small hydrophobic molecules like gases and hormones, as well as various leukocytes. However, the integrity of the continuous capillaries in the brain is crucial in maintaining the selective permeability required for the proper function of the central nervous system.