The bladder.
The little bag that holds urine is called a urine collection bag or a urinary drainage bag.
Urine bag
A bag like organ that holds urine is the bladder.
The bladder.
The Bladder
Detrusor muscle is found in the urinary bladder.
The bag used to collect urine during an operation is called a urinary drainage bag or a Foley catheter bag. This bag is connected to a catheter that is inserted into the bladder to allow for the continuous drainage of urine during surgery. It helps maintain a sterile environment and monitor urine output.
The Kidneys.Kidneys don't hold urine they produce urine which is then stored in the bladder.
Blatter
Bladder
The bladder holds the urine 
the urinary bladder