Final answer:
Substances can enter the renal tubules by both filtration and secretion. Filtration occurs in the glomerulus and then substances are either reabsorbed into the blood or secreted into the tubular fluid in the renal tubules.
Step-by-step explanation:
Substances can enter the renal tubules through filtration and secretion. Filtration is the first step in the urine-forming process, whereby blood is filtered in the glomerulus, resulting in almost all solutes (except proteins) being filtered into the Bowman’s capsule. The filtrate then enters the renal tubules, where it undergoes tubular reabsorption and tubular secretion.
During tubular reabsorption, substances such as salts, water, and organic solutes are reclaimed from the filtrate and returned to the blood in the peritubular capillaries. This primarily occurs in the proximal convoluted tubule and the loop of Henle. Conversely, during tubular secretion, additional solutes and wastes that do not get reabsorbed are added to the tubular fluid. This step can eliminate certain drugs and hormones through carrier proteins in the renal tubules.
Therefore, substances can enter the renal tubules by both filtration and secretion processes, which means that option 4 is the correct answer to the student’s question about renal processes.