Answer: decrease
Step-by-step explanation:
Urine volume and urine concentration is regulated by kidneys, pituitary gland, and hypothalamus.
Vasopressin (aldosterone and the antidiuretic hormone) controls the concentration and volume of urine on the basis of osmotic reabsorption of solute free water (Plasma osmolality). Vasopressin promotes osmotic reabsorption of solute‐free water in the collecting tubules which results in decreases urine production and increases its concentration or vice-versa.
Hence, in the absence of Vasopressin urine volume increases and urine concentration decreases.