Smooth muscle is mainly found in tissues or organs such as in the trachea, digestive tract, blood vessels, and urogenital organs.
Smooth muscle is non-random muscle, which is an important tissue constituting the human body. According to the characteristics of functional activities and interrelationships between smooth muscle cells, smooth muscle is divided into 2 categories: single-unit smooth muscle and multi-unit smooth muscle.
In single-unit smooth muscle, all muscle cells perform contractile and diastolic activities as a whole. Single-unit smooth muscle is also known as visceral smooth muscle and is found in organs such as small blood vessels, the digestive tract, the uterus, and the ureter.
In multi-unit smooth muscle, the individual muscle cells are independent of each other and operate as a single muscle cell. Multi-unit smooth muscle is found in the iris, ciliary, and erector spinae muscles, as well as in the smooth muscle of the airways and large blood vessels.