The kidneys filter approximately 180 liters of plasma per day, yet only about 1.5 liters is excreted as urine. What happens to the remaining ~178.5 liters?
AIt is excreted as sweat and water vapor
BIt is reabsorbed into the bloodstream through the nephron tubules
CIt is stored temporarily in the bladder and released gradually
DIt is converted to metabolic water by cellular respiration
About 99% of the glomerular filtrate is reabsorbed back into the peritubular capillaries as it passes through the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. This massive reabsorption — driven by active transport and osmosis — recovers glucose, amino acids, ions, and water, leaving behind only waste products and excess ions for excretion.
Question 2 True / False
Glucose is normally detectable in urine because the kidneys cannot reabsorb most of the glucose that is filtered from the blood.
TTrue
FFalse
Answer: False
In a healthy individual, all filtered glucose is completely reabsorbed in the proximal convoluted tubule via sodium-glucose cotransporters (SGLT). Glucosuria (glucose in urine) only occurs when blood glucose exceeds the renal threshold (~180 mg/dL), saturating the transporters' capacity — a clinical sign of uncontrolled diabetes mellitus.
Question 3 Short Answer
What is the functional significance of the loop of Henle's countercurrent arrangement, and why does the kidney need it?
Think about your answer, then reveal below.
Model answer: The loop of Henle creates a steep osmotic concentration gradient in the renal medulla through countercurrent multiplication: the descending limb loses water while the ascending limb pumps out sodium without allowing water to follow. This medullary gradient is essential for the collecting duct to concentrate urine under ADH — without it, the kidney could not produce urine more concentrated than blood plasma.
The countercurrent arrangement means that the descending and ascending flows run in opposite directions, allowing a small ionic gradient at each segment to be multiplied into a large overall gradient. ADH-regulated water channels (aquaporins) in the collecting duct then exploit this gradient for osmotic reabsorption of water, enabling the kidney to conserve water and produce concentrated urine when needed.