We measure bladder emptying by filling the bladder retrogradely with 300 mL of saline through the catheter already in place, then withdrawing the catheter and allowing the patient to urinate ("retrograde-fill" technique). This allows us to assess the rate at which the patient is able to empty his or her bladder. The normal human bladder has an average capacity of 500-600 mL. It is generally filled by the venous system through elastic valves located in the lower extremities. The urinary tract is protected from pressure changes when the body stands up because the kidneys adjust the amount of urine produced to match the amount drained via the ureters. If the bladder becomes full, it signals to the brain that there is no room for more urine and the patient must go to the bathroom immediately. In healthy individuals, the normal voiding process involves three steps: storage, elimination, and recovery. Storage starts as soon as the bladder fills up and continues until it is completely empty. Elimination occurs when the patient passes urine. Recovery begins as soon as the patient has finished passing urine and continues until the bladder is again full. Most people can hold their breath for about five minutes before they start to suffer adverse effects due to oxygen deprivation. The time it takes for the blood oxygen level to drop below 95% depends on how long the individual can stay underwater.