Smooth muscle is a type of muscle tissue that operates without conscious control, making it an involuntary muscle. Unlike skeletal muscle, it does not have a striated, or striped, appearance when viewed under a microscope. This muscle tissue plays a role in regulating the functions of many internal organs and is responsible for moving substances throughout the body. It enables various automatic processes that are essential for daily functioning.
In the Digestive and Urinary Systems
Smooth muscle is found throughout the digestive tract, where it moves food. In the esophagus, stomach, and intestines, these muscles contract in wave-like motions (peristalsis), pushing food along. Smooth muscle also assists in the gallbladder and bile ducts, helping to release bile.
Within the urinary system, smooth muscle is important for waste removal. It lines the ureters, connecting kidneys to the bladder, propelling urine towards it. The bladder itself contains smooth muscle in its walls, allowing it to store and expel urine. It is also in the urethra, controlling urine flow.
In the Respiratory and Reproductive Systems
The respiratory system uses smooth muscle to regulate airflow. Smooth muscle is found in the walls of the trachea, bronchi, and smaller bronchioles. Its constriction or dilation controls air entering and leaving the lungs, adapting to oxygen needs. This regulation is important for maintaining optimal respiratory function.
In the reproductive system, smooth muscle has several roles. In females, the uterus contains smooth muscle, which contracts during menstruation, pregnancy, and childbirth. These contractions expel the uterine lining, support fetal development, and facilitate labor. Smooth muscle also lines the fallopian tubes, aiding egg transport. In males, smooth muscle in structures like the vas deferens and epididymis transports sperm.
In Blood Vessels and the Eyes
Smooth muscle is a key component of the circulatory system, found within blood vessel walls. It is present in arteries, arterioles, veins, and venules. By contracting or relaxing, this muscle tissue regulates blood pressure and controls blood flow, adapting to physiological demands. This constant adjustment is important for maintaining overall cardiovascular health.
The eyes also contain smooth muscle, controlling visual functions. The iris contains smooth muscle that adjusts the pupil size. This regulates light entering the eye, allowing clear vision in varying conditions. Smooth muscle in the ciliary body changes the lens shape, enabling focus at different distances.
In the Skin and Other Areas
Smooth muscle is also found in the skin. Arrector pili muscles are attached to hair follicles. When these muscles contract, they cause the hair to stand erect, creating “goosebumps”.
Smooth muscle is also present in the ducts of various glands. For instance, in sweat and salivary glands. Here, its contractions help expel secretions from these glands.