Which blood vessels reach to the small spaces in human body?

Capillaries are the smallest type of blood vessels in the human body, with a diameter ranging from 5 to 10 micrometers (μm). They are sites of diffusion where the exchange of oxygen, nutrients, and waste products between the blood and the tissues takes place. 

They are found in large numbers in organs that require a constant supply of oxygen, such as the lungs and heart, as well as in tissues involved in nutrient absorption, such as the small intestine.