Final answer:
The urinary bladder has three openings: two for the ureters and one for the urethra. The trigone is the area at the base of the bladder where these openings are located. Transitional epithelium lines the bladder, and muscle contractions are involved in urination.
Step-by-step explanation:
There are three openings in the urinary bladder. The urinary system includes the ureters, urinary bladder, and urethra. The urinary bladder is an important organ for storing urine before it is excreted from the body. The bladder has three openings; two for the ureters, which are the tubes that bring urine from the kidneys, and one for the urethra, through which urine leaves the body. The area at the base of the bladder where the urethral opening is located is called the trigone, a triangular area central to the two ureteral openings. Transitional epithelium lines the bladder. In terms of urination, urine does not travel through the urinary system solely due to the force of gravity; it also involves muscle contractions. Also, urination specifically refers to the process where urine is expelled from the body, not its formation to elimination. Lastly, the smooth muscle in the walls of the urethra relaxes when the bladder muscle contracts to allow urine to pass through.