Which muscle type lines the walls of blood vessels and hollow organs and is not under voluntary control?

• A. Tendon
• B. Skeletal Muscle
• C. Cardiac Muscle
• D. Smooth Muscle

Answer: (D)

Smooth muscle is the type that lines the walls of blood vessels and hollow organs and operates without conscious control. Its fibers are spindle-shaped and arranged in sheets, which gives it a non-striated appearance and the ability to contract smoothly and steadily. This design is perfect for controlling the diameter of vessels through vasoconstriction and vasodilation, and for moving contents through hollow organs via peristaltic motions. The activity is governed by the autonomic nervous system and local chemical signals, not by voluntary effort. By contrast, tendons are connective tissue, skeletal muscle is voluntary and striated, and cardiac muscle makes up the heart and is a different kind of involuntary muscle specialized for pumping.