If the bladder is functioning correctly, it should empty completely, according to MedicineNet. Urinary retention is the term associated with having trouble emptying the bladder completely, or having trouble starting to urinate.
According to WebMD, when you are unable to empty your bladder completely, it can lead to overflow incontinence, when urine leaks out of the bladder. Bacteria can also grow in urine that is left in the bladder, leading to problems such as urinary tract infections. Some causes of incomplete emptying of the bladder include blockages of the urethra, weak bladder muscles, medications and injury of the nerves around the bladder.