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: 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: Urine osmolality in Diabetes insipidus is:

<150 mmol/L. **Explanation:** **Diabetes Insipidus (DI)** is characterized by the inability to concentrate urine due to either a deficiency of Antidiuretic Hormone (ADH/Vasopressin) from the posterior pituitary (**Central DI**) or a lack of renal response to it (**Nephrogenic DI**). 1. **Why Option A is Correct:** In a healthy individual, ADH acts on the V2 receptors of the collecting ducts to reabsorb water via aquaporin-2 channels. In DI, this mechanism fails, leading to the excretion of large volumes of highly dilute urine (polyuria). The hallmark of DI is a **low urine osmolality**, typically ** 295 mOsm/kg). 2. **Why Other Options are Incorrect:** * **<300 mmol/L:** While urine in DI is indeed less than 300 (which is roughly isotonic to plasma), this range is too broad and includes "isosthenuria" (seen in chronic renal failure), rather than the profound dilution characteristic of DI. * **<600 and <900 mmol/L:** These values represent concentrated urine. A normal kidney can concentrate urine up to 1200 mOsm/kg. Values above 600 indicate intact ADH function and concentrated urine, which contradicts a diagnosis of DI. **High-Yield Clinical Pearls for NEET-PG:** * **Water Deprivation Test:** The gold standard for diagnosis. If urine osmolality remains low despite dehydration, DI is confirmed. * **Desmopressin (DDAVP) Challenge:** Used to differentiate Central from Nephrogenic DI. Central DI shows a >50% increase in urine osmolality; Nephrogenic DI shows little to no response. * **Urine Specific Gravity:** In DI, it is typically very low (**≤ 1.005**). * **Plasma Osmolality:** In DI, plasma is hypertonic (>295 mOsm/kg) with hypernatremia, unlike Psychogenic Polydipsia where plasma is dilute.

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.

Q: Which segment of the nephron has the lowest osmolality under the influence of antidiuretic hormone (ADH)?

Early distal tubule. ***Early distal tubule*** - This segment is known as the **cortical diluting segment** because it actively reabsorbs solutes (Na+ and Cl-) via the **Na+-Cl- cotransporter** while being impermeable to water. - This mandatory solute removal ensures the tubular fluid is maximally diluted (hypoosmolar, often around 100 mOsm/L), a process that is independent of **ADH** levels. *Descending limb of the loop of Henle* - This segment is highly permeable to **water** but largely impermeable to solutes, causing water to flow out into the surrounding hypertonic medulla. - Consequently, the osmolality of the tubular fluid **increases** significantly as it moves down toward the loop hairpin turn, making it highly concentrated, not dilute. *Collecting duct* - The influence of **ADH** is to insert **aquaporin 2** channels into the apical membrane, making this segment highly permeable to water. - This allows massive water reabsorption out of the tubule, concentrating the urine and leading to a **high osmolality** within the tubule lumen, especially near the end. *Late distal tubule* - While the fluid here is dilute, some water reabsorption is possible in the presence of **ADH** due to ADH-sensitive aquaporins, similar to the collecting duct. - This water movement slightly increases the osmolality compared to the maximally dilute fluid produced earlier in the **early distal tubule**.

Question 1

All of the following statements about the control of micturition are true except?

Accepted Answer

An individual with destruction of the sacral dorsal roots can still have a micturition reflex.

Answer

An individual with a spinal cord injury at L1 can still have a micturition reflex.

Answer

The micturition reflex can occur without voiding any urine.

Answer

Bladder volume can be more than double that present when the first urge to void occurs.

Question 2

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

Accepted Answer

Urea

Answer

K+

Answer

Cl-

Answer

Na+

Question 3

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 4

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 5

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 6

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 7

Urine osmolality in Diabetes insipidus is:

Accepted Answer

<150 mmol/L

Answer

<300 mmol/L

Answer

<600 mmol/L

Answer

<900 mmol/L

Question 8

Which of the following conditions leads to an increase in the secretion of antidiuretic hormone (ADH)?

Accepted Answer

Hypovolemia

Answer

Hypervolemia

Answer

Hypertension

Answer

Decreased osmolarity

Question 9

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

Question 10

Which segment of the nephron has the lowest osmolality under the influence of antidiuretic hormone (ADH)?

Accepted Answer

Early distal tubule

Answer

Descending limb of the loop of Henle

Answer

Collecting duct

Answer

Late distal tubule

Renal Physiology — MCQs

Renal Physiology — MCQs

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