Why is a small amount of urea retained in the kidney matrix?

Compare the statements $A$ and $B$.
$\text{Statement } A$: When the urine moves through the descending limb,it becomes hypertonic,and as it passes through the ascending limb of Henle's loop,it becomes hypotonic.
$\text{Statement } B$: The descending limb is permeable to sodium ions,while the ascending limb is impermeable to sodium ions.

Salmon have gills that are more permeable to water than to salts. Salmon hatch in fresh water streams and then migrate to the ocean. Once they reach the ocean,you would expect the rate of uptake of water into their body through the gills to:

In human kidneys,which substances are mainly responsible for creating the osmolarity gradient of the medullary interstitial fluid?

Mention the substances that exit from the tubules in order to maintain a concentration gradient in the medullary interstitium.

Medullary gradient is mainly developed due to