Smooth muscle is an involuntary muscle tissue. It is found throughout the body, forming the walls of most hollow internal organs, blood vessels, and other internal structures. Unlike voluntary skeletal muscles, smooth muscle performs its functions automatically to maintain bodily processes.
Regulating Internal Passages and Flow
Smooth muscle controls the diameter of various internal tubes and orifices, regulating the flow of substances. This control is evident in the cardiovascular and respiratory systems, and in specialized muscle structures.
In the walls of arteries and arterioles, smooth muscle cells are arranged circularly, allowing them to contract (vasoconstriction) or relax (vasodilation). This influences blood pressure and redirects blood flow. For example, during exercise, smooth muscles in arterioles supplying skeletal muscles relax to increase blood flow, while those supplying digestive organs constrict.
Smooth muscle also lines the walls of the bronchioles, smaller airways. Their contraction can narrow these passages, restricting airflow. Conversely, their relaxation widens the bronchioles, facilitating greater airflow. This regulates air reaching the alveoli.
Specialized rings of smooth muscle, known as sphincters, function as biological gates, controlling substance movement. For example, the pyloric sphincter regulates the passage of partially digested food from the stomach to the small intestine. Similarly, the internal anal and urethral sphincters remain contracted to prevent involuntary defecation and urination.
Transporting Substances Through the Body
Smooth muscle is responsible for the propulsive movement of substances through various organ systems. This action often involves wave-like contractions known as peristalsis.
In the digestive system, smooth muscle layers in the esophagus, stomach, and intestines perform peristalsis. After food is swallowed, esophageal smooth muscles contract sequentially, pushing it towards the stomach. Within the stomach, smooth muscle mixes food with digestive juices and propels the chyme into the small intestine, continuing through the large intestine for waste elimination.
The urinary system also relies on smooth muscle for transport. The ureters contain smooth muscle that rhythmically propels urine from the kidneys to the bladder. These waves prevent backflow and accumulation.
Smooth muscle also facilitates transport within the reproductive systems. In males, smooth muscle in the vas deferens contracts during ejaculation, propelling sperm. In females, smooth muscle in the fallopian tubes generates contractions to move the ovum from the ovary to the uterus after ovulation.
Specialized Roles in Various Organs
Smooth muscle performs specific functions in various organs.
In the eyes, smooth muscles contribute to vision by controlling focusing and light regulation. The ciliary muscles, around the lens, adjust its shape to focus on objects at varying distances. The smooth muscles of the iris control pupil size, regulating light entry.
In the skin, arrector pili muscles are attached to hair follicles. When these muscles contract, they pull hair follicles upright, causing hair to stand on end. This action results in goosebumps.
The uterus contains layers of smooth muscle for powerful contractions. During childbirth, these uterine contractions push the baby through the birth canal. Rhythmic contractions also shed the uterine lining during menstruation.