Capillaries are the smallest blood vessels that connect arterioles and venules in order to facilitate the exchange of nutrients and waste materials between blood and tissues. The capillaries are part of the circulation system. From the heart, blood is pumped to the arteries branching into arterioles and then into capillaries. These capillaries service the various organs of the body. From these organs, capillaries widen to venules and then veins which bring the blood back to the heart.
There are two types of capillaries based on their location in the body and the particular function that each perform. These are the continuous capillaries and the fenestrated capillaries. The continuous capillaries are made up of cells with continuous lining with very tight junctions inside. These capillaries permit only the smallest molecules like water molecules to pass through. The central nervous system, skeletal muscles, gonads, lungs and the skin have continuous capillaries.
The fenestrated capillaries have very small openings called fenestra and allow the passage of small molecules and protein. The renal glomerulus, endocrine glands, intestines, pancreas and kidney have fenestrated capillaries. Included with the fenestrated capillaries are sinusoidal or discontinuous capillaries. The endothelium of the sinusoidal capillaries have larger openings and can accommodate red blood cells, white blood cells, and different serum proteins to pass through. These are considered special fenestrated capillaries and do not have the typical cylindrical shape of the other capillaries. These are found in the liver, spleen, bone marrow, lymph nodes and the adrenal gland.
Sinusoidal capillaries can be found in the spleen, liver, and bone marrow.