Final answer:
Rings of smooth muscle called sphincters regulate the movement of materials in the digestive and urinary systems through processes like peristalsis. These involuntary muscles are present in hollow organs and around various passageways in the body.
Step-by-step explanation:
In the digestive and urinary systems, rings of smooth muscle called sphincters regulate the movement of materials along internal passageways. These smooth muscles play a crucial role in processes such as peristalsis—a rapid, involuntary, wave-like series of contractions that moves food through the gastrointestinal tract and urine through the urinary tract.
Smooth muscle tissue, which is not under voluntary control and has a single nucleus per cell, forms the contractile component of the digestive, urinary, and other systems. Its contractions are responsible for involuntary movements within the internal organs. For example, in the digestive system, smooth muscles contract in sequence, much like how you might squeeze toothpaste from a tube, to move food through the body.
In addition to their role in peristalsis, smooth muscles are also involved in regulating the flow of blood through precapillary sphincters, which are circular rings of smooth muscle that control blood entry into the capillaries. These involuntary muscles are present in various locations throughout the body, including the hollow organs such as intestines, stomach, and urinary bladder, as well as around the respiratory tract and blood vessels.