What are muscles arranged in circles that are able to decrease the diameter of tubes called?

Muscles that are arranged in circular formations and have the ability to reduce the diameter of tubes are known as sphincters. These specialized circular muscles function by contracting and relaxing to control the passage of substances through openings in the body, such as the digestive tract and the urinary system. Sphincters play a crucial role in maintaining appropriate flow and pressure within these systems, allowing for controlled movements and preventing leakage.

In contrast, striated muscles refer to skeletal muscles that are responsible for voluntary movements and are characterized by a banded appearance under a microscope. Spinal muscles is not a recognized category of muscle in this context, as it refers more generally to muscles associated with the spine, rather than a specific functional type. Cardiac muscles are involuntary and striated muscles located in the heart; they are responsible for pumping blood but do not serve the same function as sphincters. Thus, the defining characteristic of sphincters is their ability to encircle tubes and regulate their openings, making them the correct answer.