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Which of the following substances is NOT selectively reabsorbed as the urine flows along the tubules?
Glucose
Amino acids
Urea
Salts
Urea
In the human nephron, substances like glucose, amino acids, and essential salts (like Na┬аCl) are actively or passively reabsorbed from the glomerular filtrate back into the blood to maintain homeostasis. Urea, being a nitrogenous waste product that the body needs to excrete, is not selectively reabsorbed; instead, it is primarily concentrated in the urine.
In the human nephron, substances like glucose, amino acids, and essential salts (like Na┬аCl) are actively or passively reabsorbed from the glomerular filtrate back into the blood to maintain homeostasis. Urea, being a nitrogenous waste product that the body needs to excrete, is not selectively reabsorbed; instead, it is primarily concentrated in the urine.
Think of a water filter that saves the 'good' minerals for the water we drink but allows the 'dirt' (urea) to be flushed away as waste.
G.A.S. (Glucose, Amino acids, Salts) are Saved; Urea is Unwanted.
GFR=Kf├Ч(PGCтАЛтИТPBSтАЛтИТ╧АGCтАЛ) тАФ The Glomerular Filtration Rate equation, where PGCтАЛ is glomerular hydrostatic pressure.
UreaclearanceтАЛ=PU├ЧVтАЛ тАФ The renal clearance formula for urea, where U is concentration in urine, V is urine flow rate, and P is plasma concentration.
The nephron functions via three main processes: glomerular filtration, tubular reabsorption, and tubular secretion. While the proximal convoluted tubule (PCT) reabsorbs most nutrients (glucose, amino acids) and water, urea remains largely in the tubule because the tubular epithelial cells are relatively impermeable to it, allowing it to be excreted as a component of urine.
Glucose and amino acids are completely reabsorbed in the proximal convoluted tubule under normal physiological conditions.
Urea is a metabolic waste product generated from protein catabolism.
Small amounts of urea may undergo recycling in the loop of Henle to maintain the medullary concentration gradient, but it is effectively 'not reabsorbed' in the sense of nutritional recovery.
Selective reabsorption is a highly regulated mechanism mediated by transport proteins.
Maintenance of blood plasma osmolarity.
Elimination of nitrogenous metabolic wastes.
Threshold substance: Glucose has a renal threshold of approximately 180mg/dL; above this, it appears in urine (glycosuria).
Option A (Glucose), B (Amino acids), and D (Salts) are all vital nutrients/electrolytes that the body reclaims during filtration.
C is correct тАФ Urea is a waste product meant for excretion and is not selectively reabsorbed like nutrients such as glucose or amino acids.
Remember the acronym 'G-A-S' for substances that are reabsorbed (Glucose, Amino acids, Salts) to quickly eliminate them in similar multiple-choice questions.
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