After catheterization the patient simply opens and closes the drainage bag to allow the trapped urine to drain. Catheterization is a basic operation commonly used in clinical practice, which involves inserting a catheter through the urethra into the bladder to draw out urine in order to discharge the patient’s retained urine. It can also be used to measure the amount of urine remaining in the bladder or to inject contrast or medication to aid in the diagnosis and treatment of disease, among other things. The catheter used in catheterization requires sterility. The perineum and urethral opening are sterilized, and the catheter is slowly inserted into the bladder through the penis or female perineum until urine flows from the catheter. The catheter can be used to help patients with limited mobility to carry out normal physiologic elimination, or to resolve acute urinary retention or difficulty in urination. The use of catheters should be carried out correctly under the guidance of a medical professional, and if the patient experiences serious discomfort, he or she should report it to the doctor in time to avoid infections and other negative consequences.