What are the functions of smooth muscle cells?

In the circulatory or vascular system, smooth muscle cells are responsible for regulating blood pressure and flow of oxygen to the various parts of the body. 

In the digestive system, the smooth muscles in the stomach and intestines play a key role in nutrient collection and digestion through peristalsis, which involves rhythmic wave-like contractions that help move the food through the convoluted digestive tract.   

In the urinary system, smooth muscle cells help to get rid of toxins from the body. They also help maintain electrolyte balance. 

Contraction in the irises and in the many sphincters in the body are also controlled by smooth muscle cells.