Smooth muscle is found in the walls of blood vessels, vessels of the lymphatic system, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, most ducts, and a number of other places such as the iris.
The smooth muscle looks different from the skeletal muscle, and works differently. It can squeeze slowly, and move in waves along a duct. Most types of smooth muscle squeeze gently for a long time, and use little energy. Some smooth muscle squeezes quickly and relaxes in phases.