Arterioles, Arteries, Veins, Venules and Capillaries are the five blood vessels which make up the circulatory system.
Arterioles are tubes of endothelial cells which are enclosed by smooth muscle cells and connective tissue cells. They have a thick muscular wall, narrow lumen and are small in size being only a few millimetres long. 
Their main function is:
1. To control the flow of blood to certain regions of an organ or particular tissue
2. To distributer blood within the capillary network. 
They are able to increase resistance and therefore decrease blood flow by vasoconstriction and can also decrease resitance and therefore increase blood flow by vasodilation of their walls. 
1. Rhoades, R. A., & Bell, D. R. (2009). Physiology; Principles for Clinical Medicine; 3rd Edition. Philadelphia: Lippincott Williams and Wilkins, A Wolters Kluwer business.
2. Khurana, I. (2008). Essentials of Medical Physiology. Noida: Elsevier.