Capillaries are tiny blood vessels that play a crucial role in transporting blood, nutrients, and oxygen to cells in the body. They are the smallest blood vessels in the body and are only about 5 to 10 micrometers in diameter. Capillaries are part of the microcirculation system and connect arteries to veins. They are composed of only the tunica intima, which is the innermost layer of an artery or vein, consisting of a thin wall of simple squamous endothelial cells. Capillaries are located inside all tissues and have a very thin wall, which is only one cell in thickness. They carry both oxygenated and deoxygenated blood and do not have muscle tissue or valves. There are three types of capillaries: continuous, fenestrated, and sinusoid capillaries. Continuous capillaries are the most common type and contain small gaps in between their endothelial cells that allow for things like gases, water, sugar, and some hormones to pass through. Fenestrated capillaries are "leakier" than continuous capillaries and contain small pores in addition to small gaps between cells in their walls that allow for the exchange of larger molecules. Sinusoid capillaries are the rarest and "leakiest" type of capillary and allow for the exchange of large molecules, even cells. They are found in certain tissues, including the liver, bone marrow, anterior pituitary gland, and brain circumventricular organs.