Final answer:
Smooth muscles relax when intracellular calcium ion levels are reduced, which happens when calcium ions are actively transported into the sarcoplasmic reticulum. Distension or stretching can also induce relaxation in certain smooth muscle systems when the stimulus is removed or reduced.
Step-by-step explanation:
Smooth muscles relax when the calcium ion levels within the muscle fiber are reduced. This typically occurs when calcium ions are actively transported into the sarcoplasmic reticulum, a process that requires energy in the form of ATP. In systemic regulation, vasodilation occurs when vascular smooth muscle responds to a decrease in stretch due to, for example, low blood flow, leading to the relaxation of the muscle and an increase in blood flow.
Smooth muscle contraction can be induced by the autonomic nervous system, hormones, or local factors, and in some areas like the digestive system or urinary tract, through peristaltic movements. However, for relaxation to occur, there generally needs to be a removal of the initiating stimulus (like a reduction in stretching within the blood vessel), or an active process to reduce the levels of calcium ions within the muscle fibers that triggers relaxation.