Osmolalities of intracellular and extracellular fluid is the same, but the composition of fluid is different.

Water is freely permeable between intracellular extracellular compartments.

2/3 intracellular fluid  (28 liters) and 1/3 extracellular fluid (14 liters)

Extracellular fluid is interstitial and vascular space

Extracellular fluid is predominantly salt solution.

Renal homeostatic functions

Waste/toxin excretion

Extracellular fluid volume 

Operationally kidney regulates NaCl absorption as the primary adjustment to ECF volume

Osmolar regulation

Osmolar balance is regulated by ECF water

Blood pressure regulation

Acid-base regulation

Mineral regulation

Renal metabolic and Hormonal regulation