Which capillary type is found in tissues such as the kidney and endocrine glands, enabling rapid exchange?

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

Answer: (C)

Capillary permeability varies with endothelium structure to suit tissue needs. Fenestrated capillaries have pores in the endothelial lining that allow rapid movement of water and small solutes between blood and surrounding tissue. This makes them ideal for tissues that require quick exchange, such as the kidney—where plasma components are filtered and reabsorbed—and endocrine glands—where hormones must rapidly enter the bloodstream. In contrast, continuous capillaries have an intact lining with tight junctions and only small gaps, limiting exchange to small molecules and making rapid transfer less efficient. Sinusoidal capillaries have larger gaps and a discontinuous basement membrane, allowing transfer of larger substances but not optimized for the high-rate exchange seen in the kidney and endocrine glands. Lymphatic capillaries drain interstitial fluid rather than facilitate rapid exchange with blood. Thus, fenestrated capillaries best support rapid exchange in these tissues.