An antibiotic is circulating through the blood when it passes through the kidneys. Only a miniscule amount of it passes through the glomerular capillary and enters the glomerular filtrate, but when its presence in the urine is measured, it has been almost completely excreted! How is this possible?

Explanation: What is happening here is that the antibiotic is being actively transported into the proximal tubule during the process called tubular secretion.

Tubular secretion is when materials are transferred from peritubular capillaries to the renal tubular lumen. It is one of the many steps of the blood filtering process and the final goal is to secrete waste in form of urine.

In conclusion, the antibiotic is almost completely excreted, because it is actively transported into the proximal tubule during tubular secretion.