Sphincters that close off tubes are which type of muscle?

• A. Circular muscle
• B. Fusiform muscle
• C. Quadrate muscle
• D. Pennate muscle

Answer: (A)

Closing off a tube relies on a muscle arranged in a ring around the lumen. This circular arrangement allows the fibers to constrict uniformly, narrowing the diameter of the opening when they contract and opening it when they relax. That ring, or sphincter, is why many openings in the body can be tightly sealed or released as needed.

Other muscle architectures—fusiform (long and spindle-shaped), quadrate (blocky), and pennate (feather-like with fibers attaching to a central tendon)—are designed for different mechanical purposes, such as shortening force or packing more fibers, but they don’t form the continuous circular band necessary to create a sphincter around a tube. Examples of true sphincters include the rings of muscle around the mouth (orbicularis oris) and around the eyes (orbicularis oculi), as well as the circular muscle layers that thicken to form physiological sphincters in the GI tract.

So, the type of muscle that closes off tubes is the circular muscle.