Urinary Tract Anatomy
The urinary tract comprises the kidneys, the ureters, the bladder, the prostate (in men only) and the urethra.
The kidneys are a paired organ, situated in the flank, one on either side. The kidneys are bean-shaped (although some beans are described as kidney shaped), and approximately 3x6x12 cm of size. The kidneys' main function is to filter waste products from the blood, and thus form urine. The urine is collected in the kidney pelvis, which continues to form the ureters - two long and narrow tubes leading the urine from the kidneys into the bladder.
The bladder serves as a reservoir for the urine, and urine is held back until voiding is necessary and practical. Voiding is initiated by the voiding reflex, which in turn is initiated by the filling of the bladder, usually when the bladder contains approximately 500mL of urine. The bladder is a muscular organ, shaped much like a balloon. During voiding, the passage upwards through the ureter is closed and the passage outwards is opened. Then, the muscles of the bladder contract, and the urine is expelled. If there is a bladder outlet obstruction, voluntary use of the abdominal muscles may be needed to empty the bladder.
Finally, the urine is lead out through the urethra. In men the urethra is much longer than in women.
The prostate is a wallnut sized gland in men, positioned just below the bladder, and around the urethra. The prostate gland secretes fluid that is part of the seminal fluid. The prostate is prone to enlargement with increasing age, and is thus one of the causes of bladder outlet obstruction.