Which types of muscle are characterized as involuntary?

Involuntary muscles are those that function without conscious control, and the types of muscle characterized as involuntary are cardiac and smooth muscle.

Cardiac muscle, found exclusively in the heart, is responsible for pumping blood throughout the body. It has unique properties that allow it to contract rhythmically and continuously, ensuring that it maintains a steady heartbeat without requiring conscious effort. This muscle type is influenced by the autonomic nervous system, which regulates functions such as heart rate.

Smooth muscle, found in the walls of hollow organs such as the intestines, blood vessels, and the bladder, also operates involuntarily. It helps in various functions, including moving food through the digestive tract and regulating blood flow by altering the diameter of blood vessels. Just like cardiac muscle, smooth muscle contractions occur without conscious control, allowing for automatic bodily functions.

By understanding these involuntary muscle types and their roles, it is clear why the correct choice highlights cardiac and smooth muscles specifically, as both are essential for vital bodily functions that do not require voluntary movement.