Final answer:
The fluid inside the nephron loop is known as filtrate, which is processed in the nephron to form urine.
Step-by-step explanation:
The fluid contained within the lumen of a nephron loop is called filtrate. This fluid consists of water, salts, and other solutes such as sugars and urea that have been filtered from the blood. It undergoes various processes of reabsorption and secretion throughout the nephron, ultimately becoming concentrated into urine by the time it reaches the collecting duct, where some water may be reabsorbed into the blood. The blood that leaves the kidney through the renal vein is cleansed of much of its waste due to these processes, and the urine is then collected and stored in the urinary bladder.