Regarding the control of water balance by the kidneys:
1. The renal medulla has an osmotic gradient that decreases from the border with the cortex to the renal papilla.
2. ADHG is secreted by anterior pitutary in response to a decrease in the osmolality of the blood.
3. A normal person cannot produce urine with an osmolality greater than 300 mOsmil.kg-1.
4. ADH acts on the cells of the collecting ducts to increase their permeability to water.