Which capillary type is most closely associated with high permeability for large molecules such as in the liver or spleen?

• A. Continuous
• B. Fenestrated
• C. Lymphatic capillary
• D. Sinusoidal

Answer: (D)

High permeability to large molecules comes from capillaries with a highly open endothelium. Sinusoidal capillaries have discontinuous endothelium with large gaps between adjacent cells, wide fenestrae, and an incomplete basement membrane. This structure creates substantial spaces through which plasma proteins, lipoproteins, and even cells can pass, which is exactly what is needed in the liver and spleen where filtration and exchange of large molecules occur. In the liver, sinusoids line up with Kupffer cells that help filter the blood, and in the spleen, sinusoidal channels allow passage of cells during filtration of blood. The other types have tighter junctions or diaphragmed pores that restrict passage of large molecules, so they don’t match the level of permeability seen in sinusoidal capillaries.