Question 1

What is the glomerular filtration rate (GFR) in a healthy human?

Accepted Answer

GFR is approximately 125 mL per minute, which amounts to about 180 litres of filtrate formed per day. Since only about 1.5 litres leave as urine daily, roughly 99% of the filtrate is reabsorbed along the tubule.

Question 2

Why is the loop of Henle described as a counter-current multiplier?

Accepted Answer

The descending and ascending limbs run parallel to each other with fluid flowing in opposite directions. Because the limbs differ in permeability (water leaves the descending limb; solutes leave the water-impermeable ascending limb), each pass amplifies the osmotic gradient in the medullary interstitium from 300 to 1200 mOsm/L — hence 'multiplier'. The vasa recta acts as the counter-current exchanger that preserves this gradient.

Question 3

Which part of the nephron is impermeable to water?

Accepted Answer

The ascending limb of the loop of Henle is impermeable to water while transporting sodium and chloride out (this transport may be active or passive, as stated in NCERT). This is why the filtrate becomes dilute, falling to about 100 mOsm/L by the end of the ascending limb.

Question 4

What does ADH (vasopressin) do?

Accepted Answer

ADH, released from the posterior pituitary, makes the latter part of the distal convoluted tubule and the collecting duct permeable to water, allowing water to be reabsorbed along the medullary osmotic gradient. This produces concentrated urine (up to about 1200 mOsm/L, roughly 4× plasma). Absence of ADH causes diabetes insipidus, with large volumes of dilute urine.

Question 5

Why does glucose appear in the urine of a diabetic patient?

Accepted Answer

The proximal convoluted tubule normally reabsorbs 100% of filtered glucose via active transport, which has a maximum capacity (transport maximum/Tm). In diabetes mellitus, blood glucose exceeds this threshold, so the excess glucose cannot be reabsorbed and spills into the urine.

Nephron Lab — Follow One Drop of Filtrate Through the Kidney

Practice

Learned it? Now prove it under the clock.