Help with Bladder Physiology - Human Anatomy and Physiology

When holding urine, there are two primary sphincters that help control the release of urine. The internal urethral sphincter is composed of smooth muscle, and is therefore not consciously controlled. The external urethral sphincter is composed of skeletal muscle, which means that it can be consciously controlled.

The detrusor muscle is the smooth muscle found in the walls of the bladder. When contracted, this muscle leads to urination. The rectus abdominis is a region of the abdominal wall. When contracted, this muscle can indirectly assist in urination by reducing the volume of the abdominal cavity and pressuring the bladder.