An arteriole is a blood vessel that extends and branchs out from an artery and leads to capillaries.
Arterioles have thick muscularized walls and are the primary site of vascular resistance. The mean blood pressure in the arteries supplying the body is a result of the interaction between the cardiac output (the volume of blood the heart is pumping per minute) and the vascular resistance, usually termed total peripheral resistance by physicians and researchers.
The up and down fluctuation of the arterial blood pressure is due to the pulsatile nature of the cardiac output and determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries.