Hydronephrosis 40mm is considered more serious, hydronephrosis is a common symptom of urology, which can have different degrees of impact on kidney function. When blood flows through the kidneys, the kidneys will produce urine, through the fallopian tubes to drain him to the cystitis, temporarily stored in the bladder, and discharged out of the body through the urethra, so when the fallopian tubes bladder or urethra of any part of the obstruction, will lead to poor urine drainage, can not be discharged, the phenomenon of hydronephrosis will occur. The degree of hydronephrosis varies according to the different degrees of obstruction, and with more seriousness, the degree of hydronephrosis increases, the renal cortex becomes thinner gradually, and the damage of renal function increases gradually. If the obstruction is detected and relieved in time, the renal function is likely to recover, otherwise it will cause research damage and renal failure, etc. If hydronephrosis occurs, you should go to the hospital immediately to avoid delay.