Final answer:
The loop of Henle enables the formation of concentrated urine through a countercurrent multiplier system, which creates an osmotic gradient enabling water reabsorption and solute transport, thereby concentrating the urine. Without this system, animals excrete dilute urine.
Step-by-step explanation:
Animals without a loop of Henle are unable to form concentrated urine because the loop of Henle in mammalian kidneys is crucial for creating a high concentration of solutes in the urine. This is achieved through a mechanism known as the countercurrent multiplier system, which involves the interplay of the descending and ascending limbs of the loop of Henle. The descending limb is permeable to water but not to solutes, allowing water to be reabsorbed into the body, while the ascending limb pumps out sodium to create a high salt concentration in the surrounding interstitial fluid. Consequently, as the filtrate moves through the loop, there is a continuous exchange that generates an osmotic gradient, enabling the reabsorption of water from the filtrate by osmosis in the descending limb and active transport of solutes in the ascending limb. Without this mechanism, animals would not be able to reabsorb sufficient water and produce urine that is more concentrated than their body fluids, leading to the excretion of dilute urine.