Final answer:
The kidneys retain water and increase blood pressure by producing angiotensin, a protein that constricts blood vessels and signals the body to retain sodium and water.
Step-by-step explanation:
The kidneys adjust the water level in the body by retaining water when water intake decreases. This is done to maintain the body's water balance. In addition, the kidneys increase blood pressure by producing angiotensin, a protein that constricts blood vessels and signals the body to retain sodium and water. This helps restore normal blood pressure.