The Kidney forms urine from blood by first, Glomerular Filtration of blood plasma. The filtrate forms in Bowman's capsule. The filtrate is free of the blood's formed elements, and almost free of protein. Much Tubular reabsorption is by active transport processes, performed by cells in the tubule wall. However water reabsorption is passive by osmosis.
Tubular secretion is by active transport.
Filtration, reabsorption and secretion are summarized in an equation. When a diabetic excretes glucose in the urine it is because the filtered glucose exceeds the rate at which the kidneys can reabsorb it. The plasma level of the drug gradually falls, until you take another dose. Many drugs are both filtered and secreted, so if the rate of clearance of such a drug is normal, then both filtration and secretion are probably normal. As secretion and reabsorption are both activities of the tubule cells, then the tubule cells are probably normal.