Tm (transprot maximum) is defined as maximal
Correct Answer: Tubular reabsorption
Description: Ans. B. Tubular Secretion and absorption (Ref. Ganong 23rd/ pg. 709-712).Guyton Physiology 11th/pg. 331 & F/gure 27-4.........."The transport maximum is the maximum rate at which glucose can be reabsorbed from the tubules. The threshold for glucose refers to the filtered load of glucose at which glucose first begins to be excreted in urine".Discussion:Tmax# The Tmax represents the average reabsorptive capacity of all the renal nephrons. The Tmax for glucose is the maximum rate of glucose reabsorption from the kidney.# Typically, the Tmax is 375 mg/min.# The higher-than-normal reabsorptive capacity accounts for the lower-than-expected urinary concentration.# The Tmax is calculated by subtracting the amount of glucose excreted from the filtered load at any plasma concentration at which the amount of glucose excreted increases linearly as plasma glucose cone increases.# For example, when the plasma glucose concentration is 600 mg/dL, filtered load of glucose is 600 mg/min, the amount of glucose excreted is 100 mg/min, and the amount reabsorbed (the Tmax) is 500 mg/min.# Essentially all filtered glucose is reabsorbed, inasmuch as the transport maximum (Tmax) for glucose (320 mg/min) is not exceeded in normal per- sons. In diabetes mellitus, hyperglycemia results in a tubular filtration load that exceeds the Tmax, and glycosuria ensues.# The renal threshold for glucose is the plasma concentration at which glucose first appears in the urine. The glucose is excreted at a plasma concentration of of 200 mg/dL.Transport maximum for substances That Are Actively Reabsorbed.# For most substances that are actively reabsorbed or secreted, there is a limit to the rate at which the solute can be transported, often referred to as the transport maximum.# This limit is due to saturation of the specific transport systems involved when the amount of solute delivered to the tubule (referred to as tubular load) exceeds the capacity of the carrier proteins and specific enzymes involved in the transport process.# The glucose transport system in the proximal tubule is a good example.# In the adult human, the transport maximum for glucose averages about 375 mg/min, whereas the filtered load of glucose is only about 125 mg/min (GFR - plasma glucose = 125 ml/min - 1 mg/ml). With large increases in GFR and/or plasma glucose concentration that increase the filtered load of glucose above 375 mg/ min, the excess glucose filtered is not reabsorbed and passes into the urine.# When the plasma concentration of glucose rises above about 200 mg/100 ml, increasing the filtered load to about 250 mg/min, a small amount of glucose begins to appear in the urine. This point is termed the threshold for glucose. Note that this appearance of glucose in the urine (at the threshold) occurs before the transport maximum is reached.# One reason for the difference between threshold and transport maximum is that not all nephrons have the same transport maximum for glucose, and some of the nephrons excrete glucose before others have reached their transport maximum.# The overall transport maximum for the kidneys, which is normally about 375 mg/min, is reached when all nephrons have reached their maximal capacity to reabsorb glucose.Transport Maximums for Substances That Are Actively Secreted:SubstanceTransport MaximumCreatinine16 mg/minPara-aminohippuric acid80 mg/min# Some of the important transport maximums for substances actively reabsorbed by the tubules are as follows:SubstanceTransport MazimumGlucose375 mg/minPhosphate0.10 mM/minSulfate0.06 mM/minAmino acids1.5 mM/minUrate15 mg/minLactate75 mg/minPlasma protein30 mg/min
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