Final answer:
Glucose is not regularly eliminated by the kidneys via secretion because it is reabsorbed back into the bloodstream as a valuable source of energy. Urea and creatinine, however, are typically excreted as waste products.
Step-by-step explanation:
The substance that is not regularly eliminated by the kidneys via secretion is glucose. The kidneys are responsible for maintaining homeostasis of blood plasma, which includes the reabsorption of glucose. Most of the glucose filtered by the kidneys is reabsorbed back into the bloodstream to prevent its loss. This is because glucose is a valuable source of energy that the body aims to retain.
In a healthy individual, the appearance of glucose in urine usually indicates a pathological condition, such as diabetes mellitus, where the renal threshold for glucose reabsorption is exceeded. On the other hand, substances like urea, creatinine, and various ions are regular components of urine, as they are either waste products or excess substances that the body needs to eliminate.
Urea, creatinine, and other metabolic waste components are typically secreted into the filtrate as waste products to be excreted in urine. Meanwhile, glucose is reabsorbed via carrier proteins and if the capacity of these proteins is exceeded, as in the case of diabetes, glucose may then appear in the urine.