Nephron and filtration

Filtration path through the nephron (in order):

• Blood enters the glomerulus, where high pressure pushes water and small solutes into the tubule (filtration).
• Filtrate moves into the proximal tubule, where glucose, most water, and salts are reabsorbed into the blood.
• The loop of Henle concentrates the filtrate by pulling out water and salt.
• The distal tubule fine-tunes salt and water balance.
• The collecting duct sends the finished urine toward the bladder.

Key terms in my own words:

• Urinalysis: a test that reads what is in the urine (protein, glucose, blood, pH) to check how the kidney is filtering.
• Gel electrophoresis: a method that sorts DNA fragments by size in a gel so PKD genetic markers show up as bands.

Enzyme action in digestion:

• The enzyme amylase acts on starch and breaks it into smaller sugars; the enzyme is reused, the starch is the substrate.

My question: if the glomerulus is damaged, why does protein start showing up in the urine when it normally stays in the blood?

(Tip: filtration is passive and driven by pressure at the glomerulus, while reabsorption is selective and happens down the tubule; keep those two steps separate.)