3. Urinary bladder
It is a median, pear shaped and distensible sac. It is situated in the pelvic region on the ventral side of the rectum in the abdomen. It stores urine brought by two ureters. The storage capacity of urinary bladder is 300 - 800ml.

4. Urethra
It is a tube that takes urine from urinary bladder to outside. The opening of urinary bladder into urethra is guarded by a ring of muscles or sphincter.

Urethra is 4 cm long in females open to vestibule and in males it is about 20cm long. Its opening is separate in females but is in common with the reproductive tract in males (urino-genital duct).

Micturition

The urine is temporarily stored in the bladder. There are two sets of circular sphincter muscles in the bladder. When the bladder is filling up both these sets of muscles are constricted, so the exit is closed. However as the pressure of the urine increases the walls of the bladder are stretched and this triggers off an automatic reflex action which causes the upper sphincter to relax. But the lower sphincter is under the control of brain.

So urine can still be retained until this muscle is relaxed too. Control of urination is not possessed by very young children but is gradually learned.

Urge for micturition occurs when urinary bladder is filled with 300 - 400 ml of urine. The stretched bladder stimulates nerve endings to develop the reflex. However, urine can be retained in the urinary bladder till it gets filled up to the maximum capacity of 700 - 800ml. At this time the urge becomes painful and leads to voluntary micturition. Total amount of urine excreted per day is about 1.6-1.8 litres. It’s quantity increases with larger intake of fluids like water, fruit juices and decreases with lesser intake.

It is a transparent fluid produced by urinary system. Urine has amber color due to presence of urochrome.Composition of normal urine varies considerably depending on several factors for instance taking a protein