There are an estimated 10 billion capillaries, measuring approximately 25,000 miles, in the average human body. Each capillary has a length of about 1.1 millimeter. Most capillaries are little more than a single cell layer thick.
Capillaries are the smallest and most numerous vessels in the body through which blood flows. The minimal wall thickness of the capillary facilitates its primary function, which is to permit the exchange of materials between cells in the tissue and the blood. Capillaries are classified into two main groups, continuous and fenestrated. While a person is resting, approximately 5% of the blood circulating is in the capillaries,