In renal glycosuria, the renal threshold for glucose
A i.e., LowTranspo Maximum (T.)For most substance that are actively reabsorbed or secreted there is a maximum rate at which the solute can be transpoedQ ofter k/a transpo maximum. This limit is d/t saturation of specific transpo system.Tm for kidney in practice is less than calculated value (i.e.,threshold of substance is less than its Tm) because, not all nephrons have the same Tm and some of the nephron excrete substance before others have reached their TmQ. It is impoant to know that overall Tm of substance for kidney is reached when all nephrons have reached their capacity to reabsorb or secrete that substance.Tm for glucose in normal adult is - 375 mg/minQ, whereas its filtered load is only 125mg/min (GFR x Plasma glucose = 125 ml/min x 1 mg/ml). With large increase in plasma glucose &/or GFR, which increase the filtered load above 375 mg/min, the excess glucose filtered is not reabsorbed & passes into urine. However, even when the plasma glucose concentration rises above 200 mg/100m1 (2mg/m1) (so filtered load is 250 mg/min, which is less than Tm), a small amount of glucose appears in urine. This is known as threshold of glucose. So renal threshold of substance is the plasma level at which it first appears in urine in more than the normal minute volumes.Substances that do not exhibit Tm Substances that are passively reabsorbed do not demonstrate Tm, because their rate is determined by electrochemical gradient, permeability, & time that solute remains within tubule eg. are urea, chlorideQ, water Some actively transpoed substance also have electrochemical gradient- time transpo. e.g. is Na reabsorption in PCTQ. However in DCT (where epithelium has much tighter junction & so transpoing only small amounts of Na+) sodium reabsorption exhibits Tm.
Low
High
Same
Greatly increased