Humans have the most complex muscle system in the entire animal kingdom. The muscular system of the homo sapiens are divided into three categories: smooth, skeletal, and cardiac. We have voluntary and involuntary muscles. Our muscles are responsible for most of our processes. Skeletal muscles are attached to your bones. These skeletal muscles allow us to move our bones. Cardiac muscles are very important because they regulate the heartbeat. Smooth muscles are found within the walls of blood vessels. It also is found in the bladder, uterus, reproductive tracts, respiratory tracts, and the iris of the eye.