Question

Describe the mechanism of urine formation. How is urine prevented from flowing back into the
ureters?

Answer 1

 The ureter is always filled with liquid. No chance urine can flow backward. It is full can't fit anymore in there. 
Also, back-flow of urine is prevented by pressure exerted by the nephron column. It is also prevented by the use of ATP as an active transport mechanism. Hormones, such as rennin and epinephrine constricts the back-flow path.