Which condition slows the rate of filtration and urine formation?

• A. Dehydration
• B. Increased blood pressure
• C. High dietary water intake
• D. Overhydration

Answer: (A)

Fluid status directly influences how much blood the kidneys receive and how they handle water. When you’re dehydrated, circulating blood volume drops, which reduces renal perfusion and lowers the glomerular filtration rate. At the same time, antidiuretic hormone increases to conserve water, so the kidneys reabsorb more water in the tubules and produce less urine that is more concentrated. That combination slows both filtration and urine formation.

Increased blood pressure tends to keep or even raise filtration by increasing the driving pressure across the glomerulus (within the kidneys’ autoregulatory range), so it doesn’t slow filtration. A high dietary water intake or overhydration promotes more urine production as the body excretes the excess water, rather than slowing filtration.