Normally, almost all of the filtered glucose is reabsorbed and a negligible amount is excreted. Since the normal plasma glucose concentration is between 3.3 and 5.5 mmol/l (60-100 mg/100 ml) and GFR is 125 ml/min, 0.40.7 mmol (or 75-125 mg) glucose is reabsorbed every min. Although most glucose re-absorption occurs in the proximal tubule, more distal parts of the nephron are also capable of re-absorbing glucose. Figure RE.7 shows the relationship between filtration, re-absorption and excretion of glucose, and the plasma glucose concentration. The amount of glucose filtered is directly proportional to the plasma glucose concentration. No glucose is excreted in the urine, unless the plasma glucose concentration exceeds about 11 mmol/l (200 mg/100 ml). At this plasma glucose concentration, those nephrons with the lowest capacity for glucose re-absorption (relative to their filtration rate) reach their glucose re-absorptive rate limit, and glucose begins to be excreted. Further increases in plasma glucose concentration saturate the glucose transport process of an increasing proportion of nephrons until, when the plasma glucose concentration is about 22 mmol/l. No nephrons can absorb their entire filtered glucose load. The fact that the curves for re-absorption and excretion are 'rounded' indicates the existence of nephron heterogeneity, i.e. that all the nephrons are not identical.
The type of transport process typified by glucose reabsorption is known as Tm-limited transport. 'Tm' means tubular maximum and refers to the maximum tubular transport rate for a particular solute. From Figure RE.7, it can be seen that the maximum rate of glucose re-absorption is about 380 mg/min. This is the Tm for glucose. The most common abnormality of glucose excretion is that caused by a change in the plasma glucose concentration, so that the filtered load is altered: diabetes mellitus is caused by the relative or total absence of the pancreatic hormone, insulin, which regulates the blood glucose concentration. The filtered load of glucose can, therefore, be far in excess of the re-absorptive capacity of the nephrons, so that glucose is excreted in the urine. This excretion of osmotically active solute causes an osmotic diuresis resulting in water loss from the body and, hence, dehydration and thirst.
Figure RE.7 Glucose transport in the proximal tubule
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