Excretory Products and Their Elimination MCQs for NEET — Zoology Questions with Answers

The context explicitly states: 'HL primarily helps to maintain osmolar gradient (300 mOsmolL –1 -1200 mOsmolL –1 ) within the kidney interstitium.' It also mentions, 'A counter current mechanism operates between the two limbs of the loop of Henle and those of vasa recta...The Henle’s loop and vasa recta play a significant role in this.' This mechanism is key to producing concentrated urine.

The text states: 'The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a counter current pattern.' This opposing flow is the definition of a countercurrent mechanism.

The NCERT text mentions: 'Electrolytes and urea are retained in the interstitium by this arrangement (counter current mechanism).'

Though not explicitly stated in a single sentence, the text implies this: 'The filtrate gets concentrated as it moves down the descending limb but is diluted by the ascending limb.' And it emphasizes the role of 'electrolytes' in the interstitium maintained by the countercurrent. The general knowledge (and implied by context) of nephron function is that the descending limb is permeable to water, and the ascending limb is impermeable to water but actively transports solutes.

The text states: 'This segment (Collecting Duct) allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity.'

The NCERT text clearly states: 'DCT and collecting duct concentrate the filtrate about four times, i.e., from 300 mOsmolL –1 to 1200 mOsmolL –1, an excellent mechanism of conservation of water.'

The countercurrent mechanism is essential for establishing and maintaining the medullary osmolar gradient (300-1200 mOsmolL-1). This gradient is crucial for the reabsorption of water from the collecting ducts, thus allowing the production of concentrated urine. Without it, the ability to concentrate urine would be severely impaired, leading to the excretion of a large volume of dilute urine and significant water loss.

The vasa recta, by mimicking the countercurrent flow of the Henle's loop, minimizes the washout of solutes (electrolytes and urea) from the medullary interstitium. This 'countercurrent exchange' property allows the blood in the vasa recta to pick up water and lose solutes in a way that preserves the high osmolarity of the medulla, which is vital for concentrating urine.

The text explicitly states: 'HL primarily helps to maintain osmolar gradient (300 mOsmolL –1 -1200 mOsmolL –1 ) within the kidney interstitium.' and 'DCT and collecting duct concentrate the filtrate about four times, i.e., from 300 mOsmolL –1 to 1200 mOsmolL –1, an excellent mechanism of conservation of water.' This concentrating ability relies entirely on the countercurrent mechanism.

The text states: 'The filtrate gets concentrated as it moves down the descending limb'. This concentration occurs due to the passive diffusion of water out of the descending limb, indicating its permeability to water. Conversely, the ascending limb is described as diluting the filtrate, implying the active transport of solutes out without water following, making the descending limb largely impermeable to electrolytes.