Final answer:
The filtrate enters the proximal convoluted tubule during glomerular filtration, which is the first stage of urine formation.
Step-by-step explanation:
The filtrate flows into the proximal convoluted tubule during the process of glomerular filtration. This is the first step in urine formation, where the nephrons filter blood via the capillary network in the glomerulus, excluding major solutes such as proteins. Following glomerular filtration, the filtrate collected in the renal tubules undergoes tubular reabsorption, where most solutes and water are reabsorbed into the bloodstream. Tubular secretion is the subsequent step, where additional solutes and wastes are secreted back into the tubules if they are not initially reabsorbed.