Capillaries connect arterioles and venules. They are the site of nutrient and gas exchange. Fluids flow out of capillaries on the arterial side (filtration) and return on the venous side (reabsorption).
A capillary consists of a single endothelial cell. They are distinguished by their leakiness. Continuous capillaries have endothelial cells that are tightly joined, limiting the materials that can pass into or out of them. Fenestrated capillaries have endothelial cells with pores and are more leaky. Sinusoidal capillaries have large openings in their endothelium. They allow white blood cells to pass through.
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