What causes urine to become more concentrated when you don't drink enough water?

Question

A. Release of ADH from the pituitary gland

B. Active transport of sodium into the interstital fluid

C. A collecting duct with a loop of Henle

D. The loop of Henle plus the renel artery

Answer 1

the urea is less diluted!

Answer 2

The kidneys react to low blood pressure by secreting factors which, among other things, recover more water from the proto-urine so the remainder is more concentrated.

Answer 3

When you don't drink enough fluids, you become dehydrated.  The waste products are still excreted by the kidneys but there is less fluid available to dilute the urine.  The body simply draws from the available fluids in the cells and tissues until they don't function anymore, unable to regulate the body's temperature, etc....then you die.