The three different types of capillaries (400X)

There are three different types of capillaries: continuous, fenestrated and discontinuous capillaries. The latter are often open and meandering and are then called sinusoids.

- Continuous Capillaries: These are the most common type of capillaries found in the body. They have a continuous endothelium and basal lamina (the innermost layer of cells in blood vessels) with small gaps between cells called intercellular clefts. These gaps allow for the diffusion of small molecules like oxygen and carbon dioxide between the blood and surrounding tissues. Continuous capillaries are found in muscle, skin, and the central nervous system.

- Fenestrated Capillaries: Fenestrated capillaries have small pores or fenestrations in their endothelium that allow for the rapid exchange of fluids and small molecules. These capillaries are found in organs that require a high degree of exchange, such as the kidneys, intestines, and endocrine glands such as the pituitary- and thyroid glands.

- Discontinuous Capillaries: Also known as sinusoidal capillaries, these capillaries have large gaps in their endothelium and an irregular shape. They adapt in the spaces between epithelial cells in the organs where they are found. Therefore, they often have a curved course, which has given them the name sinusoid (lat. bending). They allow for the passage of large molecules such as proteins and cells, making them important for the exchange of nutrients and waste products in organs such as the liver, spleen, and bone marrow. The basal lamina is missing.

Each type of capillary is adapted to the unique requirements of the tissues it serves, and their structure reflects their specific function.