Final answer:
The first step in urine formation in a nephron is glomerular filtration, which happens in the glomerulus, followed by tubular reabsorption in the proximal convoluted tubule, and then tubular secretion into the renal tubules. The correct option is glomerular filtration.
Step-by-step explanation:
The first step in urine formation in a nephron is glomerular filtration, not tubular secretion. This process occurs in the glomerulus and involves the filtration of blood through the capillary network, where almost all solutes, except for proteins, are filtered out into the Bowman's capsule.
Following this step, tubular reabsorption occurs in the renal tubules, particularly the proximal convoluted tubule (PCT), where most of the solutes that were filtered out are reabsorbed back into the blood. The filtrate then moves down to the loop of Henle and distal tubules, where water is reabsorbed and additional solutes and wastes are added through tubular secretion. Finally, the filtered and modified liquid, now known as urine, moves to the collecting ducts and eventually out of the body via the ureters.
It's also important to note that ADH and aldosterone influence water reabsorption in the nephron, specifically in the collecting ducts. Depending on the hormonal signals, the body can adjust the volume of water reabsorbed, thus fine-tuning the urine concentration and volume. Ultimately, through these mechanisms, the kidneys fulfill their physiological role to modify plasma composition and produce urine.