Urinary System Anatomy

Normal urination

Urination is a complex bodily function that involves nerves, muscles, the spinal cord and the brain. The bladder is the muscular sac that stores urine, which is composed of water and waste products produced by the body. The bladder releases urine though the urethra, which is a tube that connects the bladder to the outside of our body. For the process of urination to occur correctly, all the systems need to work together to ensure that voiding occurs in a voluntary manner.

Muscles Related to the Urinary System

The bladder consists of two muscles - the detrusor muscle and the sphincter. The detrusor is a muscular balloon like sac that stores urine. When this muscle squeezes, it forces urine toward the base or neck of the bladder. The sphincter is a circular or cuff like muscle that lies at the base of the bladder. The sphincter muscles stay contracted most of the time to hold urine inside of the bladder. When the detrusor muscle squeezes, this triggers the sphincter muscles to relax, allowing urine to pass into the urethra. The brain and spinal cord transmit the signal of when the bladder is full and it is time to urinate.

The pelvic floor muscles sit below the bladder and are a sling like group of muscles in the lower part of the pelvis to provide support to the pelvic organs (bladder, rectum and uterus). These muscles keep your organs in their proper position. The pelvic floor muscles can also hold urine back when you are not near a bathroom. Any weakness of the pelvic floor muscles can cause incontinence or organ prolapse (dropping of the organs down out of their proper position).