Capillaries, a crucial constituent of the circulatory system, are diminutive vessels with a diameter between 5–10 micrometers, accommodating perfusion to the tissues through the phenomenon known as microcirculation. Through their permeable walls, consisting of an endothelial layer ensconced by a basement membrane and sporadically dispersed smooth muscle fibers, the exchange of substances between the blood and the interstitial fluid becomes plausible. Variance in wall composition exists, with larger capillaries possessing several adjoining endothelial cells, whilst the smaller ones merely display a single cell layer that circles around to self-contact.

Three principal types of capillaries exist, classified based on the extent of their permeability: continuous, fenestrated, and sinusoid capillaries.

Continuous capillaries, the most prevalent among the three, are found ubiquitously in vascularized tissues. Their distinct feature is the complete endothelial lining with tight junctions between endothelial cells. Albeit typically impermeable, these tight junctions in capillaries often present intercellular clefts, facilitating the exchange of minute molecules. Notably, continuous capillaries in the brain are part of the blood-brain barrier with their tight junctions, lack of intercellular clefts, thick basement membrane, and astrocyte extensions known as end feet, synergistically inhibiting the movement of nearly all substances.

Fenestrated capillaries are characterized by their pores (fenestrations) within the endothelial lining alongside the tight junctions, rendering them permeable to larger molecules. Their permeability and fenestration frequency, however, vary based on their location. They are commonly found in the small intestine, kidneys, the choroid plexus of the brain, and numerous endocrine structures such as the hypothalamus, pituitary, pineal, and thyroid glands.

The sinusoid capillary, the rarest capillary type, exhibits large intercellular gaps, fenestrations, and incomplete basement membranes, giving them a Swiss cheese-like appearance. The large pores of sinusoid capillaries permit the passage of the largest molecules and even cells. These capillaries are predominantly located in the liver, spleen, bone marrow, lymph nodes, and certain endocrine glands, including the pituitary and adrenal glands. Their high permeability enables the passage of newly formed blood cells from the bone marrow into the blood supply and allows the liver to process materials from the digestive tract and spleen efficiently.