What is hydronephrosis? Hydronephrosis is an abnormal condition of the kidney. The kidneys are vital internal organs located in the upper abdomen. Normally, people have two bean-shaped kidneys, which form a part of the urinary tract in the genitourinary system. Hydronephrosis occurs when an underlying disease, disorder or condition creates a partial or total obstruction or blockage of the ureter, the tube that carries urine from the kidney to the bladder. This results in pooling of urine in the kidneys accompanied by kidney swelling and distention. This problem more often involves just one kidney but it can affect both (bilateral). This is why affected individuals often have no early symptoms since the other kidney continues to function. Hydronephrosis can also result from a condition that causes the abnormal reverse flow of urine from the bladder back up through the ureter into the kidney. Hydronephrosis can occur at any age, including in newborn infants born with the condition. Hydronephrosis is a serious medical sign of a variety of conditions. Both hydronephrosis and its underlying causes can lead to kidney damage and deterioration of a kidney’s ability to perform functions that are critical to life and your overall health including: Filtering waste products and excess water and salts from the blood, which are then eliminated from the body through the ureters, bladder and urethra in the form of urine Producing certain hormones, such as renin, which helps regulate blood pressure Producing the active form of vitamin D (calcitriol) Regulating electrolytes and other vital substances, such as sodium, calcium and potassium Regulating the level and quality of fluid in the body Stimulating red blood cell production Hydronephrosis can occur in one or both kidneys. It is especially serious if it occurs in both kidneys because of the potential for permanent damage to both vital organs and the risk of kidney failure. Hydronephrosis is often caused by a serious underlying condition of the urinary tract. Rapid diagnosis and treatment of many underlying causes of hydronephrosis may prevent hydronephrosis altogether or prevent or slow the progression of serious kidney damage. Seek prompt medical care if you have unexplained symptoms, such as painful or frequent urination and nausea or vomiting. Seek immediate medical (call 911) care if you, or someone you are with, have serious symptoms, such as shortness of breath, bloody or pink urine, decrease in urinating or lack of urine, severe flank or abdominal pain, or a change in consciousness or alertness.