In order from the smallest in diameter to the largest in diameter, blood vessels are composed of capillaries, veins, and arteries.
Capillaries are extremely small in diameter, noting that their size is similar to that of a human hair. Major arteries in the body are connected to the veins through the capillaries. Gas exchanges, waste disposal, and nutrient absorbance which occurs in and out of the blood that flows in the walls of the capillaries.
The veins are primarily responsible for directing the flow of the body’s blood towards the heart; this blood is usually oxygen poor blood, which the heart filters by placing oxygen into the oxygen poor blood. By converting oxygen poor blood to oxygen rich blood, the arteries can then transport this blood throughout the body for use.
The arteries are responsible for transporting oxygen rich blood away from the heart; oxygen rich blood is distributed throughout the body and used as a fuel source to maintain the body and homeostasis.