CUET UG Biology Animal Physiology Excretion Nephron Structure Urine Formation Osmoregulation

Must-Know (15–20 detailed bullets)

• The nephron is the structural and functional unit of the kidney; each human kidney contains approximately 1 million nephrons (verify from NCERT).
• A nephron consists of a renal corpuscle (glomerulus + Bowman’s capsule) and a renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct).
• Glomerular filtration occurs in the Bowman’s capsule; the glomerular filtration rate (GFR) in a healthy adult is about 125 mL/min or 180 L/day (verify from NCERT).
• Filtration membrane consists of three layers: endothelium of glomerular capillaries, basement membrane, and podocytes (visceral epithelium of Bowman’s capsule).
• The proximal convoluted tubule (PCT) reabsorbs ~70% of the glomerular filtrate, including glucose, amino acids, Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻, and water.
• Glucose is completely reabsorbed in the PCT via active transport; its presence in urine indicates hyperglycemia (e.g., diabetes mellitus).
• The loop of Henle has a descending limb (permeable to water, impermeable to solutes) and an ascending limb (impermeable to water, actively transports Na⁺, K⁺, Cl⁻ out).
• The countercurrent mechanism in the loop of Henle and vasa recta helps maintain an osmolarity gradient in the medulla, increasing from 300 mOsmol/L in cortex to 1200 mOsmol/L in inner medulla (verify from NCERT).
• The distal convoluted tubule (DCT) selectively reabsorbs Na⁺, water, and Ca²⁺ under hormonal regulation (e.g., aldosterone, ADH) and secretes K⁺, H⁺, and NH₃.
• The collecting duct reabsorbs water and urea under the influence of antidiuretic hormone (ADH); it also participates in acid-base balance by secreting H⁺ and HCO₃⁻.
• ADH (vasopressin) is secreted by the posterior pituitary; it increases water permeability of DCT and collecting duct by inserting aquaporins.
• Aldosterone, a mineralocorticoid from adrenal cortex, promotes Na⁺ reabsorption and K⁺ secretion in DCT and collecting duct.
• Atrial natriuretic peptide (ANP) reduces Na⁺ reabsorption and lowers blood pressure; it opposes the action of aldosterone and ADH.
• The juxtaglomerular apparatus (JGA), formed by juxtaglomerular cells, macula densa, and mesangial cells, regulates GFR via the renin-angiotensin-aldosterone system (RAAS).
• Renin is released by juxtaglomerular cells in response to low blood pressure; it converts angiotensinogen (from liver) to angiotensin I.
• Angiotensin-converting enzyme (ACE) in lungs converts angiotensin I to angiotensin II, a potent vasoconstrictor that stimulates aldosterone release.
• Urea is actively transported into the interstitium in the inner medullary collecting duct, contributing to high osmolarity in the medulla.
• Ammonia (NH₃) secreted by tubular cells helps buffer H⁺ ions in urine, forming NH₄⁺, which is excreted to maintain acid-base balance.
• Micturition is a reflex process controlled by the pontine micturition center and involves parasympathetic stimulation of the detrusor muscle.
• Hemodialysis removes nitrogenous wastes and corrects electrolyte imbalances in patients with kidney failure using a semipermeable membrane and dialyzing fluid.

Difficulty Level

Intermediate — requires understanding of physiological processes, hormone actions, and integration of nephron segments; numerical values (e.g., GFR) are testable but not overly complex.

Common CUET Traps

• Trap: Assuming glucose is reabsorbed in the loop of Henle or DCT.
  Avoid: Glucose is reabsorbed exclusively in the proximal convoluted tubule (PCT) via sodium-glucose co-transporters (SGLT).

• Trap: Thinking ADH increases Na⁺ reabsorption directly.
  Avoid: ADH primarily increases water reabsorption in collecting ducts; Na⁺ reabsorption is mainly regulated by aldosterone.

• Trap: Believing the descending limb of Henle is impermeable to water.
  Avoid: The descending limb is highly permeable to water and impermeable to solutes; the ascending limb is impermeable to water.

Practice MCQs

Q1. Which part of the nephron is responsible for the maximum reabsorption of filtered solutes?
A. Loop of Henle
B. Distal convoluted tubule
C. Proximal convoluted tubule
D. Collecting duct

Answer: C
Explanation: The PCT reabsorbs ~70% of glomerular filtrate, including glucose, amino acids, and ions.
Why others fail: The loop of Henle is key for concentration, not bulk reabsorption.

Q2. Which hormone increases water permeability of the collecting duct by promoting insertion of aquaporins?
A. Aldosterone
B. Renin
C. Atrial natriuretic peptide
D. Antidiuretic hormone

Answer: D
Explanation: ADH stimulates aquaporin-2 channel insertion in collecting duct cells.
Why others fail: Aldosterone affects Na⁺/K⁺ exchange, not water channels directly.

Q3. In the countercurrent mechanism, the primary role of the vasa recta is to:
A. Filter blood in the glomerulus
B. Secrete hydrogen ions into the tubule
C. Maintain the medullary osmotic gradient
D. Reabsorb glucose in the PCT

Answer: C
Explanation: Vasa recta act as countercurrent exchangers, preserving the osmolarity gradient in the medulla.
Why others fail: Filtration occurs in glomerulus; secretion and reabsorption are tubular functions.

Q4. Which of the following correctly represents the sequence of structures through which urine passes?
A. Collecting duct → DCT → PCT → Bowman’s capsule
B. Bowman’s capsule → PCT → Loop of Henle → DCT → Collecting duct
C. Glomerulus → Loop of Henle → PCT → DCT → Ureter
D. Collecting duct → Renal pelvis → Urethra → Bladder

Answer: B
Explanation: Filtrate forms in Bowman’s capsule, then flows through PCT, loop of Henle, DCT, and collecting duct.
Why others fail: Option D reverses bladder and urethra; flow ends in urethra, not starts there.

Q5. A patient has high levels of glucose in urine. Which of the following is the most likely cause?
A. Low GFR
B. Deficiency of ADH
C. Blood glucose exceeding renal threshold (~180 mg/dL)
D. Oversecretion of aldosterone

Answer: C
Explanation: Glucosuria occurs when blood glucose exceeds the renal threshold (~180 mg/dL), overwhelming PCT reabsorption capacity.
Why others fail: ADH deficiency causes diabetes insipidus (not glucosuria); aldosterone affects electrolytes, not glucose.

Last‑Minute Revision

• ⚠️ Nephron = functional unit of kidney; ~1 million per kidney (verify from NCERT).
• ⚠️ GFR = 125 mL/min or 180 L/day (verify from NCERT).
• ⚠️ PCT reabsorbs 70% of filtrate — glucose, amino acids, ions, water.
• ⚠️ Glucose reabsorption: only in PCT, via SGLT; threshold ~180 mg/dL.
• ⚠️ Descending limb of Henle: permeable to water, not solutes.
• ⚠️ Ascending limb of Henle: impermeable to water, reabsorbs Na⁺, K⁺, Cl⁻.
• ⚠️ Countercurrent mechanism: involves loop of Henle and vasa recta.
• ⚠️ Medullary osmolarity: ranges from 300 (cortex) to 1200 mOsmol/L (inner medulla) (verify from NCERT).
• ⚠️ ADH = antidiuretic hormone = vasopressin; from posterior pituitary.
• ⚠️ ADH → aquaporins in collecting duct → ↑ water reabsorption.
• ⚠️ Aldosterone → adrenal cortex → ↑ Na⁺ reabsorption, ↑ K⁺ secretion in DCT/collecting duct.
• ⚠️ ANP → from atria → ↓ blood pressure, opposes RAAS.
• ⚠️ JGA = juxtaglomerular apparatus → releases renin.
• ⚠️ Renin → angiotensin I → ACE → angiotensin II → vasoconstriction + aldosterone.
• ⚠️ Urea recycling: contributes to medullary osmolarity.
• ⚠️ NH₃ secretion in tubules buffers H⁺ → forms NH₄⁺ excreted.
• ⚠️ Micturition: parasympathetic → detrusor contraction.
• ⚠️ Hemodialysis: artificial filtration using semipermeable membrane.
• ⚠️ Filtration membrane: endothelium + basement membrane + podocytes.
• ⚠️ Mnemonic: "PC-LoD" = PCT → Loop of Henle → DCT → Collecting duct (urine flow).