Renal Physiology Practice Questions

Q: Which substance is not significantly absorbed in the loop of Henle?

Urea. **Explanation:** The Loop of Henle (LoH) plays a critical role in the countercurrent multiplier system, primarily focusing on the reabsorption of electrolytes to create a hypertonic medullary interstitium. **Why Urea is the correct answer:** Urea is unique because it is primarily **secreted** into the thin descending limb of the Loop of Henle rather than being absorbed. While urea is heavily reabsorbed in the Proximal Convoluted Tubule (50%) and the Medullary Collecting Ducts (via UT-A1/A3 transporters under ADH influence), the Loop of Henle itself is not a site of significant urea absorption. Instead, urea "recycles" from the medullary interstitium back into the tubular lumen of the LoH to maintain the osmotic gradient. **Why the other options are incorrect:** * **Na+, Cl-, and K+:** These electrolytes are significantly reabsorbed in the **Thick Ascending Limb (TAL)** of the Loop of Henle. This occurs via the **NKCC2 transporter** (Sodium-Potassium-2 Chloride symporter). Approximately 25% of the filtered load of these ions is reclaimed here. This segment is known as the "diluting segment" because it is impermeable to water but highly active in solute reabsorption. **High-Yield Clinical Pearls for NEET-PG:** * **NKCC2 Transporter:** This is the target of **Loop Diuretics** (e.g., Furosemide). Inhibition leads to powerful diuresis and potential hypokalemia. * **Bartter Syndrome:** A genetic defect in the NKCC2 transporter (or related channels in the TAL) that mimics chronic loop diuretic use. * **Countercurrent Multiplier:** The TAL reabsorbs NaCl without water, while the Descending Limb is permeable to water but not solutes. This separation is essential for concentrating urine. * **Urea Recycling:** This process is essential for maintaining the high osmolarity of the inner medulla, which allows for water reabsorption in the collecting ducts.

Q: Hypertonic urine is excreted due to water absorption in which part of the nephron?

Collecting ducts. ### Explanation The final concentration of urine (hypertonicity) is primarily determined in the **collecting ducts** under the influence of **Antidiuretic Hormone (ADH/Vasopressin)**. **1. Why Option A is Correct:** While water is reabsorbed in various segments of the nephron, the collecting duct is the **final site** where the urine concentration is adjusted. In the presence of ADH, **Aquaporin-2 channels** are inserted into the apical membrane of the collecting duct cells. This allows water to move out of the tubule into the highly medullary interstitium via osmosis, resulting in the excretion of concentrated (hypertonic) urine. **2. Why the Other Options are Incorrect:** * **Distal Convoluted Tubule (DCT):** The early DCT is part of the "diluting segment." It is relatively impermeable to water but actively reabsorbs solutes, making the tubular fluid more dilute (hypotonic), not hypertonic. * **Ascending Limb of the Loop of Henle:** This segment is **impermeable to water** but actively transports NaCl into the interstitium (via the NKCC2 transporter). It is known as the "diluting segment" because it removes solute without water, decreasing tubular tonicity. * **Descending Limb of the Loop of Henle:** While water is reabsorbed here, this segment contributes to the **Countercurrent Multiplier** system to create a medullary gradient. It does not determine the final tonicity of the excreted urine. **Clinical Pearls for NEET-PG:** * **V2 Receptors:** ADH acts on V2 receptors in the collecting duct to increase cAMP, leading to Aquaporin-2 insertion. * **Obligatory Water Reabsorption:** Occurs in the PCT (65%) and is independent of ADH. * **Facultative Water Reabsorption:** Occurs in the collecting ducts and is **ADH-dependent**. * **Diabetes Insipidus:** A deficiency of ADH (Central) or resistance to it (Nephrogenic) prevents water reabsorption in the collecting ducts, leading to large volumes of dilute (hypotonic) urine.

Q: What is the normal glomerular filtration rate?

125 ml/min. **Explanation:** **Glomerular Filtration Rate (GFR)** is the volume of fluid filtered from the renal glomerular capillaries into the Bowman’s capsule per unit of time. In a healthy adult male of average size (1.73 m² body surface area), the normal GFR is approximately **125 ml/min**. This equates to roughly **180 Liters per day**, meaning the entire plasma volume is filtered and processed by the kidneys about 60 times daily. * **Why 125 ml/min is correct:** This value represents the physiological norm. It is determined by the Net Filtration Pressure (NFP) and the Capillary Filtration Coefficient ($K_f$). * **Why other options are incorrect:** * **50 ml/min:** This indicates significant renal impairment (Stage 3 Chronic Kidney Disease). * **250 ml/min & 500 ml/min:** These values are pathologically high and exceed the physiological capacity of the human renal system. **High-Yield Clinical Pearls for NEET-PG:** * **Gender Difference:** GFR is slightly lower in females (approx. 110 ml/min) compared to males. * **Gold Standard Marker:** **Inulin clearance** is the gold standard for measuring GFR because it is freely filtered but neither reabsorbed nor secreted. * **Clinical Marker:** **Creatinine clearance** is the most common clinical method used to estimate GFR, though it slightly overestimates it due to minor tubular secretion. * **Filtration Fraction:** This is the ratio of GFR to Renal Plasma Flow (RPF). Normal value is ~0.2 (20%). * **Autoregulation:** GFR remains constant between mean arterial pressures of **75 to 160 mmHg** due to myogenic mechanisms and tubuloglomerular feedback.

Q: Which of the following substances is NOT significantly absorbed in the proximal convoluted tubule (PCT)?

H+. The **Proximal Convoluted Tubule (PCT)** is the "workhorse" of the nephron, responsible for the bulk reabsorption of essential solutes. ### **Why H+ is the Correct Answer** Hydrogen ions ($H^+$) are **secreted**, not absorbed, in the PCT. This occurs primarily via the **$Na^+$-$H^+$ Exchanger 3 (NHE3)** on the apical membrane. This secretion is a vital step in acid-base balance, as the secreted $H^+$ reacts with filtered bicarbonate ($HCO_3^-$) to facilitate its indirect reabsorption. Therefore, $H^+$ moves from the peritubular capillaries/tubular cells into the lumen. ### **Why the Other Options are Incorrect** * **Bicarbonate ($HCO_3^-$):** Approximately **80-90%** of filtered bicarbonate is reabsorbed in the PCT. This is mediated by carbonic anhydrase (CA), which converts luminal $HCO_3^-$ to $CO_2$ and $H_2O$ for cellular entry. * **Sodium ($Na^+$):** About **65%** of filtered sodium is reabsorbed here via various symporters (with glucose/amino acids) and antiporters. It is the primary driving force for the reabsorption of water and other solutes. * **Phosphate ($PO_4^{3-}$):** Roughly **80%** of filtered phosphate is reabsorbed in the PCT via $Na^+$-$PO_4^{3-}$ cotransporters (NaPi-IIa). This process is clinically significant as it is inhibited by **Parathyroid Hormone (PTH)**. ### **High-Yield NEET-PG Pearls** * **Obligatory Water Reabsorption:** 65% of water is reabsorbed in the PCT iso-osmotically. * **Glucose & Amino Acids:** 100% are reabsorbed in the early PCT (S1 segment). * **Carbonic Anhydrase Inhibitors (Acetazolamide):** Act specifically on the PCT, leading to bicarbonate loss and metabolic acidosis. * **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, urate, and phosphate in the urine.

Q: What is the effective renal blood flow in humans?

625 mL/min. ### Explanation **1. Why Option C is Correct:** The effective renal plasma flow (ERPF) is typically measured using **Para-aminohippuric acid (PAH) clearance**, as PAH is both filtered and secreted, resulting in almost complete extraction from the blood. * **The Calculation:** Total Renal Blood Flow (RBF) in a healthy 70 kg adult is approximately **1100–1200 mL/min** (about 20-25% of cardiac output). * Since the average hematocrit is 45%, the Renal Plasma Flow (RPF) is roughly 55% of the RBF. * $1100 \text{ mL/min} \times 0.55 = 605 \text{ mL/min}$. * In standard medical texts (like Guyton and Ganong), the average value for **Effective Renal Plasma Flow** is cited as **625 mL/min**. **2. Why Other Options are Incorrect:** * **Option A (425 mL/min) & B (525 mL/min):** These values are too low for a healthy adult. Such values would indicate significant renal vasoconstriction or a state of shock/hypovolemia. * **Option D (725 mL/min):** This value is higher than the physiological average for plasma flow. While RBF can increase during pregnancy or high protein intake, 625 mL/min remains the standard "textbook" value for exams. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Filtration Fraction (FF):** Calculated as GFR/RPF. Normal FF is approximately **20%** ($125/625 = 0.2$). * **PAH Clearance:** It underestimates true RPF by about 10% because some blood supplies non-secreting portions of the kidney (like the medulla and perirenal fat). Therefore, **True RPF = ERPF / 0.9**. * **Gold Standard:** While PAH measures ERPF, **Inulin clearance** is the gold standard for measuring GFR. * **Renal Oxygen Consumption:** The kidneys have the highest oxygen consumption per gram of tissue after the heart, primarily to fuel the $Na^+/K^+$ ATPase pump for sodium reabsorption.

Q: Erythropoietin is inhibited by which hormone?

Estrogen. **Explanation:** The production of **Erythropoietin (EPO)**, the primary regulator of erythropoiesis, is primarily stimulated by renal hypoxia. However, sex steroids and other hormones significantly modulate its secretion and action. **Why Estrogen is the Correct Answer:** **Estrogen** is known to **inhibit** erythropoiesis. It acts by suppressing the production of erythropoietin in the kidneys and directly inhibiting the proliferation of erythroid stem cells in the bone marrow. This inhibitory effect is one of the physiological reasons why women of reproductive age typically have lower hemoglobin and hematocrit levels compared to men. **Analysis of Incorrect Options:** * **Testosterone (Option D):** This is a potent **stimulator** of erythropoietin. Testosterone increases EPO production from the kidneys and enhances the sensitivity of erythroid progenitors to EPO. This explains the higher hemoglobin levels in males. * **Thyroxine (Option C):** Thyroid hormones **stimulate** erythropoiesis by increasing the metabolic rate and oxygen consumption of tissues, which creates a state of relative hypoxia that triggers EPO release. * **Progesterone (Option B):** Unlike estrogen, progesterone does not have a significant inhibitory effect on erythropoietin; in some contexts, it may even mildly stimulate ventilation, indirectly affecting oxygenation. **High-Yield Clinical Pearls for NEET-PG:** * **Site of EPO Production:** 85% from the **Peritubular interstitial cells** of the renal cortex; 15% from the liver (Kupffer cells/hepatocytes). * **Stimulants of EPO:** Hypoxia (most potent), Testosterone, Thyroxine, ACTH, Catecholamines (via β-receptors), and Prostaglandins (PGE2). * **Inhibitors of EPO:** Estrogen and Chronic Kidney Disease (due to loss of peritubular cells). * **Mechanism:** EPO acts via the **JAK2/STAT pathway** to prevent apoptosis of erythroid precursor cells (CFU-E).

Q: Which of the following substances are freely filtered by the kidney across glomerular capillaries?

Creatinine, HCO3-, and Glucose. ### Explanation The glomerular filtration barrier consists of the fenestrated endothelium, the glomerular basement membrane (GBM), and the podocyte slit diaphragms. Filtration is determined by two primary factors: **molecular size** and **electrical charge**. **1. Why Option A is Correct:** * **Creatinine (113 Da):** A small metabolic waste product that is freely filtered. * **Bicarbonate (HCO3-) (61 Da):** A small ion that passes easily into the Bowman’s space. * **Glucose (180 Da):** A small organic molecule that is freely filtered (and normally 100% reabsorbed in the proximal tubule). All three substances have a molecular weight well below the threshold of **70,000 Daltons** and are not significantly hindered by the negative charge of the basement membrane. **2. Why Other Options are Incorrect:** * **Options B & D (Albumin):** Albumin has a molecular weight of ~69,000 Da. While it is near the size cutoff, it is **negatively charged**. The GBM contains heparan sulfate (polyanionic), which electrostatically repels albumin, preventing its filtration. * **Option C (Globulin):** Globulins are much larger proteins (90,000 to 150,000+ Da) than albumin. Their large size and charge prevent them from crossing the filtration barrier. **High-Yield Clinical Pearls for NEET-PG:** * **Filtration Fraction:** The ratio of GFR to Renal Plasma Flow (Normal ≈ 0.2 or 20%). * **Neutral vs. Charged:** For the same molecular radius, **cationic** (positive) substances are filtered most easily, followed by neutral substances, while **anionic** (negative) substances are filtered the least. * **Minimal Change Disease:** Characterized by the loss of the negative charge on the GBM (loss of polyanions), leading to selective proteinuria (albuminuria) despite no visible structural change on light microscopy.

Question 1

Which substance is not significantly absorbed in the loop of Henle?

Accepted Answer

Urea

Answer

K+

Answer

Cl-

Answer

Na+

Question 2

Hypertonic urine is excreted due to water absorption in which part of the nephron?

Accepted Answer

Collecting ducts

Answer

Distal convoluted tubule (DCT)

Answer

Ascending limb of the loop of Henle

Answer

Descending limb of the loop of Henle

Question 3

Which of the following statements regarding nephron function is true?

Accepted Answer

The osmolality of intratubular content in the proximal convoluted tubule (PCT) is isotonic to the surrounding interstitium.

Answer

The ascending thick limb is permeable to water.

Answer

The descending thin limb is impermeable to water.

Answer

The osmolality of intra-tubular content in the distal convoluted tubule (DCT) is greater than the surrounding interstitium.

Question 4

What is the normal glomerular filtration rate?

Accepted Answer

125 ml/min

Answer

50 ml/min

Answer

250 ml/min

Answer

500 ml/min

Question 5

The hyperosmolarity of the renal medulla is due to increased content of what?

Accepted Answer

Sodium (Na+)

Answer

Potassium (K+)

Answer

Glucose

Answer

Divalent cations (e.g., Ca2+, Mg2+)

Question 6

Which of the following substances is NOT significantly absorbed in the proximal convoluted tubule (PCT)?

Accepted Answer

H+

Answer

Bicarbonate

Answer

Sodium

Answer

Phosphate

Question 7

What is the effective renal blood flow in humans?

Accepted Answer

625 mL/min

Answer

425 mL/min

Answer

525 mL/min

Answer

725 mL/min

Question 8

Which of the following substances is least filtered by the glomerulus?

Accepted Answer

Haemoglobin

Answer

Albumin

Answer

Myoglobin

Answer

Inulin

Question 9

Erythropoietin is inhibited by which hormone?

Accepted Answer

Estrogen

Answer

Progesterone

Answer

Thyroxine

Answer

Testosterone

Question 10

Which of the following substances are freely filtered by the kidney across glomerular capillaries?

Accepted Answer

Creatinine, HCO3-, and Glucose

Answer

Albumin, Creatinine, and HCO3-

Answer

Globulin, HCO3-, and Glucose

Answer

Albumin, HCO3-, and Glucose

Renal Physiology — MCQs

Renal Physiology — MCQs

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