The first step is glomerular filtration into Bowman's capsule. Filtrate is free of formed elements, and almost free of protein. Then come tubular reabsorption and secretion. Much reabsorption is by active transport, performed by cells in the tubule wall. But water reabsorption is by osmosis.
Secretion is active.
The result of all this is urine. Decide on a substance. This is also Mr. Norm's rate of reabsorption of glucose. When a diabetic excretes glucose in the urine it is because the filtered glucose is more than the tubules can reabsorb. In hospitals the equation is used but modified, as the measurement of Clearance of substances, to determine the condition of patients' kidneys.