Salt concentrations fall in all direct ions from this zone. The counter-current action in the blood vessels ceases by the fanning out or spreading of the blood vessels in the cortex of the kidney. The urine ducts loop back to the outer medulla thereby lowering the ionic pressure and then return to the concentrated zone increasing the ion pressure and exiting from the kidney into the ureter at the concave inner surface of the kidney. The length of the Henle's loops determines the maximum concentration that can be achieved. Desert animals have long loops.
Blood travels at near constant speed through the vessels.
The speed of water flow in the urine ducts is a variable and regulates the degree of uric acid concentration. The kidney is a marvelous organ and, as I said before, much more complicated than I describe here. However, we can learn a lesson from its shape. The shape is an interesting form at various geographic sales. We are often concerned with the transfer of elements from one travel domain to another.