The ureters enters the bladder wall obliquely to open into it at an angle of its trigone. Ureter Entry To BladderUreteric orifices are placed at the posterolateral trigonal angle.The ureter pierces the bladder wall obliquely, and this provides valve like action ( but not true valve), which prevents reverse flow of urine towards kidneys as bladder fills.Ref: Gray's Anatomy 40/e, Page 1240; BDC 4/e, Vol. II, Page 301.