The primary function of smooth muscle cells is to help hollow organs contract. These organs include the bladder and uterus as well as organs in the gastrointestinal tract. Smooth muscle cells also help the eyes maintain normal focus.
Smooth muscle cells are responsible for helping food pass through the digestive system and for pushing food up into the esophagus when vomiting occurs. In the urinary system, smooth muscle cells contract to push urine into the urethra and out of the body. When a woman gives birth, the smooth muscle cells found in the uterus contract to push the baby out of the birth canal. Smooth muscle cells also affect the diameter of the blood vessels. When the smooth muscle inside a blood vessel contracts, the diameter of the blood vessel decreases. If the smooth muscle expands, the diameter of the blood vessel increases.