The bladder is a hollow and muscular organ that collects and stores urine (pee). It sits in the lower part of the tummy (abdomen), called the pelvis.
The inside of the bladder is covered with a lining. This lining is called the urothelium. It stops urine from being absorbed back into the body. The cells that make up this lining are called urothelial cells or transitional cells.
Urine is made of water and waste products. It is produced by the kidneys. Urine drains from your kidneys to your bladder through long tubes called ureters.
As your bladder fills, it expands to store the urine. When it is full, it sends messages (nerve signals) to the brain. This makes you feel like you need to empty your bladder.
The bladder muscle tightens and squeezes the urine out of the body through the urethra at the bottom of the bladder.
The urethra has rings of muscle called the external and internal sphincter. These sphincters help to keep the urethra closed. This stops urine leaking out.
The bladder is supported by a group of muscles that sit at the bottom of the pelvis (the area between the hips). They are called the pelvic floor muscles.
In women, the urethra is a short tube that lies in front of the vagina. In men, the urethra is longer and passes through the prostate gland to the end of the penis.