The cardiovascular, gastrointestinal, genitourinary, and respiratory systems are composed mostly of hollow organs (tubular or sacular), which transport and/or store fluids (either liquids or gases) within the body. The walls of these organs contain smooth muscle, a type of tissue which enables them to constrict or dilate, in this way retarding or facilitating fluid movement as required. This is accomplished by the shortening or lengthening of the individual smooth muscle cells, which occurs in a co-ordinated fashion because the cells are electrically coupled by intercellular connections, known as gap junctions. Other structures in the body that contain smooth muscle include the myometrium - the muscular wall of the uterus - which is responsible for the rhythmic contractions of labour; the piloerector muscles, which cause skin hair to stand up; and the irises, which control the diameter of the pupils.
Found within the "walls" of hollow organs and elsewhere like the bladder and abdominal cavity, the uterus, male and female reproductive tracts, the gastrointestinal tract, the respiratory tract, the vasculature, the skin and the ciliary muscle and iris of the eye. The glomeruli of the kidneys contain a smooth muscle like cell called the mesangial cell. Smooth muscle is fundamentally different from skeletal muscle and cardiac muscle in terms of structure and function.