Glucose appears in the urine of an untreated diabetic because
- A. the blood glucose level exceeds the capacity of the tubule carriers to reabsorb it
- B. glucose is too large to be reabsorbed
- C. the glomerulus stops filtering glucose
- D. insulin is secreted into the filtrate
Explanation
Reabsorption of glucose depends on a fixed number of carrier proteins in the proximal tubule, and once the blood concentration passes the renal threshold of roughly 180 mg per 100 cm3 the carriers are saturated and the surplus stays in the filtrate. The unabsorbed glucose then holds water osmotically, which explains the large urine volume and the constant thirst of an untreated diabetic.
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