This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
Which of the following does not have a Tm value? 
Select and explain. 
A. Na+, B. Urea, C. Creatinine, D. Glucose
A: Na+
Answer: A: Na+ 
Explanation: Transpo maximum (Tm) or tubular maximum : is the maximum amount of a substance that can be transpoed across the tubule (reabsorbed or secreted) per unit time.E.g., glucose has a Tm value of 320 mg/min. Reabsorption or secretion of most substances is mediated by their respective carrier proteins in the wall of the tubules. Tm implies saturation of the carrier. Since Na+ reabsorption primarily involves the activity of Na+/K+ pump which does not exhibit saturation kinetics, Na+ reabsorption does not have a Tm value.