In this simplified normally functioning model, water and small dissolved substances pass from blood into a nephron tubule. Blood cells and larger proteins remain in blood. Needed substances and much water return from tubule fluid to blood; additional wastes can move the other way. Remaining fluid and wastes become urine.
Worked example
Water passes the first filter into tubule fluid. Some of that water returns to blood. The first crossing alone does not prove it will leave the body in urine.
The museum has two arrow labels
Arrow P follows a small useful dissolved substance from blood through the first filter into a tubule. Arrow Q follows that substance from tubule fluid back to nearby blood. A blood cell stays in a blood vessel at the first filter. Read each arrow direction. This is a paper model, not a personal health test; no samples or measurements are needed.