Renal Physiology Practice Questions

Q: Glomerular filtration rate (GFR) is increased by which of the following hormones?

Atrial Natriuretic Peptide (ANP). **Explanation:** The Glomerular Filtration Rate (GFR) is primarily determined by the net filtration pressure and the capillary filtration coefficient ($K_f$). **Why Atrial Natriuretic Peptide (ANP) is Correct:** ANP is secreted by the cardiac atria in response to stretch (volume overload). It increases GFR through a dual mechanism: 1. **Afferent/Efferent Modulation:** It causes **vasodilation of the afferent arteriole** and **vasoconstriction of the efferent arteriole**. This "push-pull" effect significantly increases the glomerular hydrostatic pressure. 2. **Surface Area:** It relaxes glomerular mesangial cells, increasing the effective surface area available for filtration ($K_f$). **Analysis of Incorrect Options:** * **Vasopressin (ADH):** Its primary role is water reabsorption in the collecting ducts via V2 receptors. While it can cause systemic vasoconstriction at high doses (V1 receptors), it generally does not increase GFR; in states of dehydration, GFR may actually decrease. * **Histamine:** While histamine is a vasodilator, in the renal microvasculature, it is not a primary physiological regulator of GFR increase compared to ANP. * **Endothelin:** This is a potent **vasoconstrictor**. It causes profound constriction of both afferent and efferent arterioles (with a preference for afferent), leading to a significant **decrease** in GFR. **NEET-PG High-Yield Pearls:** * **Vasoconstrictors that decrease GFR:** Noradrenaline, Adrenaline, Endothelin, and Angiotensin II (though Angiotensin II preferentially constricts the efferent arteriole to maintain GFR during low perfusion). * **Vasodilators that increase GFR:** ANP, Prostaglandins ($PGE_2, PGI_2$), Nitric Oxide (NO), and Bradykinin. * **Clinical Note:** NSAIDs decrease GFR by inhibiting Prostaglandins, which normally keep the afferent arteriole open.

Q: What is true about free water clearance?

Regulated by ADH. **Explanation:** **Free Water Clearance ($C_{H_2O}$)** represents the volume of blood plasma that is cleared of solute-free water per unit time. It is a measure of the kidney's ability to concentrate or dilute urine. **1. Why Option A is Correct:** The primary regulator of free water clearance is **Antidiuretic Hormone (ADH)**, also known as Vasopressin. ADH acts on the V2 receptors of the principal cells in the collecting ducts to insert **Aquaporin-2** channels. * **High ADH:** Increases water reabsorption, leading to a **negative** free water clearance (concentrated urine). * **Low ADH:** Decreases water reabsorption, leading to a **positive** free water clearance (dilute urine). **2. Why Other Options are Incorrect:** * **Option B:** **Aldosterone** primarily regulates sodium reabsorption and potassium secretion in the distal nephron. While water follows sodium osmotically, aldosterone does not independently regulate "free" (solute-free) water. * **Option C:** **Furosemide** (a loop diuretic) inhibits the Na-K-2Cl cotransporter in the Thick Ascending Limb. This abolishes the corticomedullary osmotic gradient, impairing both the dilution and concentration of urine. Consequently, furosemide brings free water clearance **closer to zero**, rather than increasing it. **3. High-Yield Clinical Pearls for NEET-PG:** * **Formula:** $C_{H_2O} = V - C_{osm}$ (where $V$ is urine flow rate and $C_{osm}$ is osmolar clearance). * **Positive $C_{H_2O}$:** Seen in Diabetes Insipidus and excessive water intake. * **Negative $C_{H_2O}$:** Seen in SIADH and states of dehydration. * **Isosthenuria:** When $C_{H_2O}$ is zero, the urine is iso-osmotic to plasma (often seen in chronic renal failure).

Q: In the nephrons, at which level does vasopressin mainly act?

Collecting tubules. **Explanation:** **Correct Answer: D. Collecting tubules** Vasopressin, also known as Antidiuretic Hormone (ADH), is the primary regulator of water reabsorption in the kidney. Its main site of action is the **principal cells of the collecting tubules** (specifically the cortical and medullary collecting ducts). **Mechanism:** ADH binds to **V2 receptors** on the basolateral membrane, triggering a cAMP-mediated signaling pathway. This leads to the insertion of **Aquaporin-2 (AQP2)** water channels into the apical (luminal) membrane. This increases the water permeability of the otherwise impermeable collecting duct, allowing water to be reabsorbed down the osmotic gradient into the hypertonic renal interstitium. **Why other options are incorrect:** * **A. Proximal Convoluted Tubules (PCT):** About 65% of water is reabsorbed here via Aquaporin-1. This process is "obligatory" and occurs isosmotically, independent of ADH. * **B. Distal Convoluted Tubules (DCT):** While the late DCT has some sensitivity to ADH, the **collecting duct** is the primary and most significant site for ADH-mediated water fine-tuning. * **C. Loop of Henle:** The descending limb is permeable to water, but this is mediated by Aquaporin-1 and is not regulated by ADH. The ascending limb is always impermeable to water. **High-Yield Clinical Pearls for NEET-PG:** * **V1 Receptors:** Located on vascular smooth muscle; cause vasoconstriction (Gq protein-coupled). * **V2 Receptors:** Located in the kidney; cause water reabsorption (Gs protein-coupled). * **Diabetes Insipidus (DI):** Central DI is a deficiency of ADH secretion, while Nephrogenic DI is due to resistance at the V2 receptor level. * **SIADH:** Characterized by excessive ADH, leading to water retention and dilutional hyponatremia.

Q: What is the maximum rate of glucose reabsorption in the renal tubules?

375 mg/min. **Explanation:** The correct answer is **375 mg/min**. This value represents the **Transport Maximum ($T_m$)** for glucose in an average adult male. **1. Why 375 mg/min is correct:** Glucose is freely filtered at the glomerulus and reabsorbed in the **Proximal Convoluted Tubule (PCT)** via secondary active transport (SGLT-2 and SGLT-1). The $T_m$ is the maximum rate at which the renal tubules can reabsorb a substance. Once all the glucose transporters (SGLT) are saturated, any additional filtered glucose cannot be reabsorbed and is excreted in the urine. For males, this limit is typically **375 mg/min**, while for females, it is approximately **303 mg/min**. **2. Analysis of incorrect options:** * **180 mg/dL (Option C):** This is the **Renal Threshold** for glucose. It refers to the plasma concentration at which glucose first begins to appear in the urine. It is different from $T_m$ (which is a rate, mg/min). * **120 mg/min & 150 mg/min (Options B & D):** These values do not correspond to standard physiological constants for glucose transport. 125 mL/min is the average Glomerular Filtration Rate (GFR), which may cause confusion. **3. Clinical Pearls for NEET-PG:** * **Splay:** The curve of glucose excretion is not a sharp angle; it curves gradually. This "splay" occurs because not all nephrons have the same $T_m$ and because the affinity of transporters varies. * **SGLT-2 Inhibitors:** Drugs like Dapagliflozin work by lowering the $T_m$ for glucose, intentionally inducing glucosuria to treat Diabetes Mellitus. * **Site of Reabsorption:** 100% of glucose is normally reabsorbed in the PCT (90% by SGLT-2 in the early part, 10% by SGLT-1 in the later part).

Q: Glomerular filtration rate (GFR) is directly proportional to which of the following pressures?

Intracapillary glomerular pressure. The Glomerular Filtration Rate (GFR) is determined by the **Starling forces** acting across the glomerular capillary membrane. The relationship is expressed by the formula: **GFR = Kf × [(Pgc – Pbc) – (πgc – πbc)]** ### 1. Why the Correct Answer is Right **Option A: Intracapillary glomerular pressure ($P_{gc}$)** This is the primary driving force for filtration. It is the hydrostatic pressure exerted by blood within the glomerular capillaries. Since it pushes fluid out of the capillary into the Bowman’s space, an increase in $P_{gc}$ leads to a direct increase in GFR. It is regulated by the resistance of the afferent and efferent arterioles. ### 2. Why the Other Options are Wrong * **Option B: Oncotic pressure ($\pi_{gc}$):** This is the osmotic pressure exerted by plasma proteins (like albumin). It acts as a "pulling" force that retains fluid inside the capillary. Therefore, GFR is **inversely proportional** to glomerular oncotic pressure; an increase in $\pi_{gc}$ decreases GFR. * **Option C: Bowman's capsular hydrostatic pressure ($P_{bc}$):** This is the pressure exerted by the fluid already present in the Bowman’s capsule. It opposes filtration. Therefore, GFR is **inversely proportional** to $P_{bc}$. Clinical conditions like kidney stones (urolithiasis) increase $P_{bc}$ and subsequently decrease GFR. ### 3. High-Yield Facts for NEET-PG * **Kf (Filtration Coefficient):** Represents the surface area and permeability. It is reduced in diseases like diabetes mellitus or chronic hypertension. * **Afferent Arteriole Constriction:** Decreases $P_{gc}$ and GFR. * **Efferent Arteriole Constriction:** Increases $P_{gc}$ and GFR (up to a point). * **Most sensitive parameter:** $P_{gc}$ is the most highly regulated variable determining GFR in healthy individuals.

Question 1

Glomerular filtration rate (GFR) is increased by which of the following hormones?

Accepted Answer

Atrial Natriuretic Peptide (ANP)

Answer

Vasopressin

Answer

Histamine

Answer

Endothelin

Question 2

What is the effect of efferent arteriole constriction and afferent arteriole dilatation on Glomerular Filtration Rate (GFR)?

Accepted Answer

Increases

Answer

Decreases

Answer

No change

Answer

First increases then decreases

Question 3

What is true about free water clearance?

Accepted Answer

Regulated by ADH

Answer

Regulated by aldosterone

Answer

Increased by furosemide

Answer

None of the above

Question 4

In the nephrons, at which level does vasopressin mainly act?

Accepted Answer

Collecting tubules

Answer

Proximal convoluted tubules

Answer

Distal convoluted tubules

Answer

Loop of Henle

Question 5

All of the following decrease the glomerular filtration rate except?

Accepted Answer

Decreased plasma protein levels

Answer

Increased tubular hydrostatic pressure

Answer

Decreased effective filtration surface area

Answer

Decreased capillary hydrostatic pressure

Question 6

Which of the following statements regarding glomerular capillaries is FALSE?

Accepted Answer

Oncotic pressure is higher in the plasma entering the glomerular capillaries than that within the glomerular capillaries themselves.

Answer

Constriction of the afferent arteriole leads to a fall in glomerular capillary blood pressure.

Answer

The concentration of glucose is equal in glomerular capillaries and the initial glomerular ultrafiltrate.

Answer

Hematocrit is elevated in the glomerular capillaries due to plasma filtration.

Question 7

Free water clearance is positive when the urine is:

Accepted Answer

Dilute

Answer

Concentrated

Answer

Isotonic

Answer

Any of the above

Question 8

The GFR of a 26-year-old man with glomerulonephritis decreases by 50% and remains at that level. For which substance would you expect to find the greatest increase in plasma concentration?

Accepted Answer

Creatinine

Answer

K+

Answer

Glucose

Answer

Na+

Question 9

What is the maximum rate of glucose reabsorption in the renal tubules?

Accepted Answer

375 mg/min

Answer

150 mg/min

Answer

180 mg/dL

Answer

120 mg/min

Question 10

Glomerular filtration rate (GFR) is directly proportional to which of the following pressures?

Accepted Answer

Intracapillary glomerular pressure

Answer

Oncotic pressure

Answer

Bowman's capsular hydrostatic pressure

Answer

All of the above

Renal Physiology — MCQs

Renal Physiology — MCQs

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