Problem 17 - Entrance Test

In uncontrolled diabetes mellitus, blood glucose levels are consistently very high (hyperglycemia). Let's analyze the physiological basis for polyuria: * Filtered Glucose: In the kidneys, glucose is freely filtered from the blood into Bowman's capsule. Normally, all filtered glucose is reabsorbed in the proximal convoluted tubule (PCT) by SGLT transporters. * Renal Threshold: There's a maximum capacity for these transporters, known as the renal threshold for glucose (approximately 180-200 mg/dL). If blood glucose levels exceed this threshold, the SGLT transporters become saturated, and glucose starts to appear in the urine (glucosuria). * Osmotic Diuresis: Glucose is an osmotically active solute. When it remains in the tubular fluid (because it's not reabsorbed), it draws water with it by osmosis. This increased solute concentration in the tubular fluid prevents water from being reabsorbed effectively, particularly in the collecting ducts, even if ADH is present. This leads to an increased volume of urine output, a condition called osmotic diuresis. * Polydipsia: The excessive water loss through urination leads to dehydration and increased plasma osmolarity, which stimulates osmoreceptors and triggers the sensation of thirst (polydipsia). Let's evaluate the options: A. Increased ADH secretion: High blood glucose (and potentially increased osmolarity) might initially stimulate ADH, but the overwhelming osmotic effect of glucose in the tubules overrides ADH's ability to reabsorb water, leading to diuresis. Furthermore, increased ADH would cause decreased urination, not polyuria. B. Glucose exceeding the renal threshold for reabsorption, leading to osmotic diuresis in the nephron: This is the most accurate and direct explanation. When glucose transporters are saturated, glucose remains in the filtrate, acts as an osmotic agent, and prevents water reabsorption, leading to polyuria. C. Damage to the juxtaglomerular apparatus, impairing the renin-angiotensin-aldosterone system: While kidney complications can occur in diabetes, damage to the JGA is not the primary cause of polyuria in uncontrolled diabetes. RAAS primarily regulates blood pressure and sodium/water balance, not directly the osmotic effect of glucose. D. Reduced permeability of the descending limb of the loop of Henle to water: The descending limb is always highly permeable to water. A reduction in its permeability would impair the concentration of filtrate in the medulla, but it is not the primary mechanism of polyuria in diabetes mellitus. Also, osmotic diuresis is mainly exerted downstream in the collecting ducts.