The kidney's ability to concentrate urine depends on the maintenance of an osmolarity gradient between the interstitial fluid of two structures: the cortex and the medulla. 2. this osmolarity gradient is maintained by both the passive transport and the active transport of filtrate. 3. the osmolarity gradient is also maintained by the passive transport of . 4. water responds to the osmolarity gradient by moving from an area of lower solute concentration to an area of higher solute concentration--a process called . 5. osmosis does not require energy. for this reason, osmosis is a form of . 6. the loss of water from the results in urine t

1. The kidney's ability to concentrate urine depends on the maintenance of an osmolarity gradient between the interstitial fluid of two structures: the cortex and the medulla.
The answer is true. To concentrate urine, the kidney has to move some substance through the medulla and cortex cells. The osmolarity gradient of both structures will determine the capability to do it work.

2. this osmolarity gradient is maintained by both the passive transport and the active transport of NaCl.
The water will follow salt. By transporting the sodium chloride(salt), it will help inducing water movement in the latter phase. This will makes the urine concentrated as the water content is decreased. This mechanism will also help to retain body fluid in case of hypovolemia. There is Na/K atpase pump that actively moves sodium and potassium ion.

3. the osmolarity gradient is also maintained by the passive transport of Urea.
Urea is a hydrophobic molecule that can move between the membrane. Urea and ammonia will be able to diffuse in and out of the filtrate. In fact, one of the main function of urine production is to dump the urea because it is toxic to the body.
Urea also has active transport called urea transporter(UT-B and UT-2A) so it is not totally passive.

4. water responds to the osmolarity gradient by moving from an area of lower solute concentration to an area of higher solute concentration--a process called osmosis.
The movement of solvent between semipermeable membrane is called osmosis. The substance that dissolved will attract water, makes the solution with higher concentration will attract water more than the solution with lower concentration. 

5. osmosis does not require energy. for this reason, osmosis is a form of passive transportation.
The direction of osmosis is not against the gradient of concentration. This movement is not using energy so it is called passive transport. The opposite would be active transport that use energy to move a substance against the gradient of concentration.

6. the loss of water from the filtrate results in urine that highly concentrated
After the water is reabsorbed from the filtrate, it will leave the kidney as a highly concentrated urine. Note that the concentration of the urine depends on the fluid level of the body. If the body has an excess fluid, the urine will be diluted instead.