Concept:As the glomerular filtrate moves through the urinary tubule, water is continuously reabsorbed into the blood, so the remaining substances become more concentrated.
Explanation:The part labelled I receives the filtrate at the beginning of the tubule.
The part labelled IV lies further along the tubule, where a large amount of water has already been reabsorbed.
Water leaves the tubule, but most urea stays inside, so urea becomes more concentrated at part IV than at part I.
Glucose is completely reabsorbed in a healthy mammal, so its concentration does not rise.
Lipids are not normally present in the filtrate, and uric acid is not the main nitrogenous waste in mammals.
Therefore, urea is greater in concentration at part IV compared with part I.
Answer:Urea
