What type of gaps exist in systemic capillaries versus the blood-brain barrier?

What type of gaps exist in systemic capillaries versus the blood-brain barrier?

• A. Systemic capillaries have large intercellular gaps allowing fluid and protein leak, while the BBB has minimal gaps that are sealed.
• B. Systemic capillaries have no gaps; BBB has large gaps.
• C. Both have large gaps that are not sealed.
• D. Gaps are irrelevant.

Answer: (A)

The key idea here is which barriers determine leakage between the bloodstream and surrounding tissue. The blood-brain barrier keeps the brain environment stable by making the endothelial cells of brain capillaries adhere tightly with numerous tight junctions, so paracellular movement of water and proteins is severely limited. Exchange across the BBB must go through the cell (transcellular) or via very selective transport, not through gaps between cells. Systemic capillaries, on the other hand, vary in their permeability. Some have small intercellular clefts, but others—especially sinusoidal capillaries—have large gaps between endothelial cells that allow fluid and even larger plasma components to leak into the surrounding tissue. This contrasts with the BBB, where those gaps are minimal and effectively sealed by tight junctions. So the statement that systemic capillaries can present larger intercellular gaps that permit leakage, while the BBB has minimal, tightly sealed gaps, captures the essential difference in permeability. The other ideas—no gaps in systemic capillaries, or large gaps in the BBB, or gaps being irrelevant—don’t align with how capillary permeability is organized in the body.

The key idea here is which barriers determine leakage between the bloodstream and surrounding tissue. The blood-brain barrier keeps the brain environment stable by making the endothelial cells of brain capillaries adhere tightly with numerous tight junctions, so paracellular movement of water and proteins is severely limited. Exchange across the BBB must go through the cell (transcellular) or via very selective transport, not through gaps between cells.

Systemic capillaries, on the other hand, vary in their permeability. Some have small intercellular clefts, but others—especially sinusoidal capillaries—have large gaps between endothelial cells that allow fluid and even larger plasma components to leak into the surrounding tissue. This contrasts with the BBB, where those gaps are minimal and effectively sealed by tight junctions.

So the statement that systemic capillaries can present larger intercellular gaps that permit leakage, while the BBB has minimal, tightly sealed gaps, captures the essential difference in permeability. The other ideas—no gaps in systemic capillaries, or large gaps in the BBB, or gaps being irrelevant—don’t align with how capillary permeability is organized in the body.