Overview:
The renal tubule is a part of the nephron, which is the functional unit of the kidney. It is responsible for reabsorbing certain substances from the filtrate back into the bloodstream. The reabsorption process helps maintain the body's homeostasis by selectively retaining essential substances while eliminating waste products.

Explanation:
Under normal conditions, the renal tubule completely reabsorbs glucose. This is because glucose is an essential nutrient that needs to be conserved by the body.

Reabsorption of Glucose:
1. Proximal Convoluted Tubule (PCT): The majority of glucose reabsorption occurs in the PCT. Glucose is reabsorbed via secondary active transport using sodium-glucose cotransporters (SGLT). SGLT1 is responsible for the reabsorption of glucose from the filtrate into the epithelial cells lining the PCT, while SGLT2 reabsorbs it from the epithelial cells back into the bloodstream.

2. Loop of Henle: The loop of Henle is responsible for reabsorbing water and salts, but it does not reabsorb glucose.

3. Distal Convoluted Tubule (DCT) and Collecting Duct: In the DCT and collecting duct, glucose is not reabsorbed under normal conditions. Instead, it remains in the filtrate and is excreted in the urine. This occurs because the reabsorption of glucose in the PCT is highly efficient, and there is usually no glucose left to be reabsorbed in the DCT and collecting duct.

Reabsorption of Other Substances:
- Urea: Urea is partially reabsorbed in the renal tubule. Approximately 50% of filtered urea is reabsorbed, while the remaining 50% is excreted in the urine.
- Uric Acid: Uric acid is both filtered and secreted in the renal tubule. A small amount may be reabsorbed, but the majority is excreted in the urine.
- Salts: Salts, such as sodium and chloride, are reabsorbed in varying amounts depending on the body's needs. The reabsorption of salts helps maintain electrolyte balance in the body.

In conclusion, under normal conditions, glucose is completely reabsorbed in the renal tubule, while urea, uric acid, and salts are only partially reabsorbed or excreted in the urine.