One substance has no membrane transporters to move it but can diffuse freely through open leak channels if there is a concentration gradient. Initially, this substance's concentrations in the filtrate and extracellular fluid are equal. Later, however, the active transport of Na+ and other solutes creates a gradient by removing water from the lumen of the tubule where it is located. What substance is this?

Urea

Explanation:

Urea is a small nitrogenous compound which is the main end product of protein catabolism in mammals

Urea is a nitrogen-containing substance normally cleared from the blood by the kidney into the urineIt is made predominantly in the liver from ammonia and bicarbonate and is one of the main components of urineThe rate of synthesis varies from 300 to 600 mmol/day depending on the protein intakeAll of this urea eventually finds its way into the urineBecause urea makes up a large part of the obligatory solute excretion, its osmotic pressure requires significant volumes of water to carry the ureaUrea passively crosses biological membranes, but its permeability is low because of its low solubility in the lipid bilayerSome cells speed up this process through urea transporters, which move urea by facilitated diffusionUrea is passively reabsorbed in the proximal tubule, but its route of transport is not clearUrea transporters have not yet been identified for the proximal tubule