In the normal animal urine is produced in the kidney. It drains through tubes from the kidneys to the bladder (the ureters). Urine is stored in the bladder until a convenient time for voiding. The bladder is an expandable bag that slowly stretches as it fills with urine. Urine leaves the bladder and drains to the outside through another tube (the urethra).

In the normal animal urine is kept in the bladder because the urethra is flattened. The urethral wall is made of elastic tissue and muscles that normally keep it flattened so that urine cannot flow. The muscles in the wall of the urethra are stronger in entire females (because of the hormones in the blood). When they have been spayed female dogs have lower oestrogen levels and because the urethra muscles are weaker they may be more prone to leaking urine.

When the bladder is full the bladder wall is stretched and the animal has a feeling that the bladder needs to be emptied. Contractions of the bladder muscle increase the pressure in the bladder and cause urine to flow out through the urethra. The impulse to void urine can be over-ridden by conscious control for some time (until the animal can reach an appropriate site for urination) but eventually the bladder must be emptied.