Extrinsic control - Autonomic reflexes

The postganglionic nerves release noradenaline at their nerve endings which acts on alpha₁ adrenoceptors on vascular smooth muscle cells in the vessel wall. When the muscle cells contract the lumen is constricted increasing resistance and reducing blood flow. Note that noradrenaline has little effect on beta₂ adrenoceptors found on vascular smooth muscle cells of some arterioles e.g. in skeletal muscle.