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Answer:
Capillaries are very tiny blood vessels — so small that a single red blood cell can barely fit through them. They help to connect your arteries and veins in addition to facilitating the exchange of certain elements between your blood and tissues. This is why tissues that are very active, such as your muscles, liver, and kidneys, have an abundance of capillaries. Less metabolically active tissues, such as certain types of connective tissue, don’t have as many.
What are the functions of capillaries?
Capillaries connect the arterial system — which includes the blood vessels that carry blood away from your heart — to your venous system. Your venous system includes the blood vessels that carry blood back to your heart. The exchange of oxygen, nutrients, and waste between your blood and tissues also happens in your capillaries. This happens through two processes:
Passive diffusion: This is the movement of a substance from an area of higher concentration to an area of lower concentration.
Pinocytosis: This refers to the process through which your body’s cells actively take in small molecules, such as fats and proteins.
The walls of capillaries are made up of a thin cell layer called endothelium that’s surrounded by another thin layer called a basement membrane.
Their single-layer endothelium composition, which varies among the different types of capillaries, and surrounding basement membrane makes capillaries a bit “leakier” than other types of blood vessels. This allows oxygen and other molecules to reach your body’s cells with greater ease.
Additionally, white blood cells from your immune system can use capillaries to reach sites of infection or other inflammatory damage.