NEET Excretory Products and their Elimination Previous Year Questions with Solutions

The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. As the filtrate flows downward into the hypertonic medullary interstitium, water leaves by osmosis, concentrating the filtrate up to 1200 mOsm/L at the hairpin turn.

• • Statements (a) and (b) are both factually correct.

The ascending limb is impermeable to water but allows transport of electrolytes actively or passively. Therefore, as the concentrated filtrate passes upward, it gets diluted due to the passage of electrolytes out into the medullary fluid, dropping osmolarity to ~200 mOsm/L.

• • Statement (c) is false because it wrongly asserts ascending limb is permeable to water.
• • Statement (d) is correct because filtrate is progressively diluted upward.

PCT is lined by simple cuboidal brush border epithelium which greatly increases the surface area for reabsorption:

• • Nearly all essential nutrients (glucose, amino acids) and 70–80% of electrolytes and water are reabsorbed here (indicated by outward arrows from lumen to interstitium: $HCO_3^-$, $NaCl$, $H_2O$).
• • PCT maintains pH and ionic balance of body fluids by selective secretion of hydrogen ions ($H^+$) and ammonia ($NH_3$) into the filtrate (indicated by inward arrows into the tubule lumen).

DCT performs conditional, hormonally regulated reabsorption and fine-tuning of acid-base balance:

• • Conditional reabsorption of $Na^+$ and water takes place in DCT under the influence of aldosterone and ADH (arrows pointing outwards: $NaCl$, $H_2O$).
• • DCT is also capable of reabsorbing bicarbonate ions ($HCO_3^-$) to preserve buffer capacity (arrow pointing outwards: $HCO_3^-$).
• • DCT selectively secretes potassium ($K^+$) and hydrogen ($H^+$) ions into the tubular fluid to maintain blood pH and $Na^+$-$K^+$ balance (arrows pointing inwards into lumen: $K^+$, $H^+$).

Evaluating the arrow schematics in Option (2):

• • Diagram P has upward (outward) arrows for $HCO_3^-$, $NaCl$, and $H_2O$, and upward (inward) arrows entering from the bottom for $H^+$ and $NH_3$.
• • Diagram D has upward (outward) arrows for $NaCl$, $H_2O$, and $HCO_3^-$, and upward (inward) arrows entering from the bottom for $K^+$ and $H^+$.
• • This is a literal, 1:1 reproduction of NCERT Figure 19.5.

Diagram (2) represents the true transport vectors of the nephron.

NCERT explicitly states: "The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down." Statement I asserts it is "impermeable to water and permeable to electrolytes" (which is the property of the ascending limb). Thus, Statement I is false.

NCERT explicitly states: "Proximal Convoluted Tubule (PCT) is lined by simple cuboidal brush border epithelium which increases the surface area for reabsorption." Statement II claims it is lined by "simple columnar brush border epithelium". Thus, Statement II is false.

Both Statement I and Statement II are false, corresponding to Option (4).

In majority of nephrons (cortical nephrons), the loop of Henle is too short and extends only very little into the medulla. In some of the nephrons, the loop of Henle is very long and runs deep into the medulla. These nephrons are called juxtamedullary nephrons.

The Malpighian corpuscle, PCT and DCT of all nephrons are situated in the cortical region of the kidney. In juxtamedullary nephrons, the corpuscle lies in the deeper cortex close to the medulla (corticomedullary junction), but never inside the renal medulla itself.

Cortical nephrons constitute about 80-85% of total nephrons, whereas juxtamedullary nephrons constitute only 15-20%. Therefore, they do not outnumber cortical nephrons.

Nephrons are anatomically classified according to the depth of their renal corpuscles and extent of their loops within the kidney zones:

The length of the loop of Henle differs sharply between the two classes:

NCERT states verbatim: "In majority of nephrons, the loop of Henle is too short and extends only very little into the medulla. Such nephrons are called cortical nephrons. In some of the nephrons, the loop of Henle is very long and runs deep into the medulla. These nephrons are called juxta medullary nephrons." Reason R directly contradicts this and is unequivocally FALSE.

Analyzing the physiological mechanisms governing osmoregulation and blood volume:

Analyzing the specific contradictions planted in A and E: - Statement A: Osmoreceptors in the hypothalamus are stimulated by an *increase* in blood osmolarity (loss of fluid). Excessive fluid loss switches them ON, not off. Fluid gain switches them off. - Statement E: Because ADH increases systemic blood pressure, it elevates glomerular capillary hydrostatic pressure, thereby increasing (or maintaining) GFR, not decreasing it.

NCERT states under Regulation of Kidney Function: - "An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release ADH..." - "ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis." - "ADH has also constrictory effects on blood vessels. This causes an increase in blood pressure." - "An increase in blood pressure can increase the glomerular blood flow and thereby the GFR." - "ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure."

Diabetes mellitus is a metabolic disorder caused by deficiency of insulin or insulin resistance. As a result, blood glucose levels rise dramatically (hyperglycemia).

When blood glucose exceeds the renal threshold (normally ~180 mg/dL), proximal convoluted tubules cannot reabsorb all the filtered glucose, resulting in glucose excretion in urine (Glycosuria).

Because glucose cannot effectively enter insulin-dependent tissue cells, the body mobilizes fats for cellular energy. Accelerated beta-oxidation of fatty acids in the liver produces ketone bodies (acetoacetic acid, acetone) which spill into urine (Ketonuria).

Uremia is accumulation of urea in blood due to renal failure; Renal calculi are kidney stones composed of crystallized oxalates. Only Glycosuria and Ketonuria indicate Diabetes Mellitus.

Diuresis refers to increased production and excretion of dilute urine. Prevention of diuresis (anti-diuresis) means conserving water within the body by reabsorbing it from the nephron tubules.

Angiotensin II stimulates the adrenal cortex to release aldosterone. Aldosterone stimulates the reabsorption of Na+ and water from the distal parts of the tubule (DCT and collecting duct). This prevents water loss, restores blood volume and pressure, and directly prevents diuresis.

ADH (vasopressin) promotes water reabsorption from distal tubules. UNDERSECRETION of ADH leads to failure of water reabsorption, resulting in massive diuresis (Diabetes Insipidus). Thus, Option (1) is physiologically nonsensical.

Atrial Natriuretic Factor (ANF) is released by the heart atria in response to high blood pressure; it causes VASODILATION (not vasoconstriction) and promotes natriuresis/diuresis (antagonizing RAAS). A decrease in renin secretion would reduce aldosterone, worsening water loss. Hence, only Option (2) prevents diuresis.