Final answer:
The total fluid flow rate coming out of the distal tubules should be lower than that coming out of the ascending limb due to water reabsorption and differences in NaCl transport in these regions of the nephron.
Step-by-step explanation:
The total fluid flow rate coming out of the distal tubules should be lower compared to the flow rate coming out of the ascending limb. During the journey of the filtrate through the nephron, reabsorption and secretion occur. In the ascending limb of the loop of Henle, NaCl is actively transported out, which does not happen in the distal tubules. Absorption of water from the filtrate will decrease, leading to less water volume being present by the time it exits the distal tubules. The descending loop of Henle is primarily involved in water reabsorption, influenced by the osmolarity of the surrounding interstitial fluid, which further explains why the flow rate is reduced in the distal tubules due to water reabsorption.
If NaCl active transport stops in the ascending limb, the filtrate osmolarity would decrease. This is because the removal of NaCl from the ascending limb is responsible for maintaining the high osmolarity in the interstitial fluid, which in turn promotes water reabsorption from the descending limb and collecting duct. Therefore, when active transport stops, less NaCl is reabsorbed, keeping more NaCl and water in the filtrate, hence reducing its osmolarity.