Final answer:
When the filtered amount of a substance exceeds the tubular maximum for reabsorption, it is excreted in urine. This occurs because the nephrons can only reabsorb a certain amount of substances before reaching their limit; any excess is expelled in the urine.Thus, the correct answer is: a. It is excreted in urine.
Step-by-step explanation:
When the filtered amount of a substance exceeds its tubular maximum for reabsorption, it is excreted in urine. This is because the kidney nephrons have a limit to how much of a substance they can reabsorb. Once this limit, known as the tubular maximum, is reached, any excess of the substance will not be reabsorbed into the bloodstream but will continue into the collecting ducts to be expelled in urine. An example of this process is the reabsorption of glucose in the proximal convoluted tubule; if blood glucose levels are too high, the tubular maximum for glucose gets exceeded, and the glucose remains in the urine, a condition known as glucosuria.
When considering the filtration and reabsorption process, it's important to note that filtrate first passes through the glomerulus where it is filtered. It is then collected in the renal tubules where most reabsorption occurs primarily in the proximal convoluted tubule and the loop of Henle. As the filtrate travels through the renal system, substances can also be added to the filtrate via tubular secretion, such as excess H+ ions to maintain acid-base balance, or wastes like urea and uric acid. At the distal convoluted tubule (DCT), hormones regulate additional reabsorption and secretion to fine-tune the composition of blood and urine.