The Brainliest Answer! Capillaries have the smallest diameter. The smallest of the arteries eventually branch into arterioles... They, in turn, branch into a extremely large number of the smallest diameter vessels—thecapillaries (with an estimated 10 billion in the average human body).

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A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter, and having a wall one endothelial cell thick. They are the smallest blood vessels in the body: they convey blood between the arterioles and venules. These microvessels are the site of exchange of many substances with the interstitial fluid surrounding them. Substances which exit include water (proximal portion), oxygen, and glucose; substances which enter include water (distal portion), carbon dioxide, uric acid, lactic acid, urea and creatinine. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in the micro circulation.

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