Renal Physiology Practice Questions

Q: Efferent arteriole constriction causes which change in Glomerular Filtration Rate (GFR)?

GFR to initially increase then decrease. ### Explanation The effect of efferent arteriole constriction on GFR is **biphasic**, depending on the degree of constriction. **1. Why Option C is Correct:** * **Initial Increase:** Mild to moderate constriction of the efferent arteriole increases resistance to outflow from the glomerulus. This raises the **Glomerular Capillary Hydrostatic Pressure ($P_{GC}$)**, which is the primary driving force for filtration, thereby increasing GFR. * **Subsequent Decrease:** With severe or prolonged constriction, the renal plasma flow (RPF) decreases significantly. As blood stays longer in the capillary and more fluid is filtered, the **Capillary Oncotic Pressure ($\pi_{GC}$)** rises sharply. Eventually, this oncotic pressure (which opposes filtration) increases enough to outweigh the hydrostatic pressure, causing the GFR to fall. **2. Why Other Options are Incorrect:** * **Option A:** This is only true for mild/moderate constriction. It ignores the effect of rising oncotic pressure and reduced plasma flow seen in severe constriction. * **Option B:** This only occurs during severe constriction or if the afferent arteriole were constricted. * **Option D:** This sequence is physiologically incorrect; the hydrostatic pressure rise always precedes the limiting effect of oncotic pressure. **3. High-Yield Clinical Pearls for NEET-PG:** * **Afferent Arteriole Constriction:** Always **decreases** both RPF and GFR. * **ACE Inhibitors/ARBs:** These drugs dilate the efferent arteriole. This reduces $P_{GC}$, which is why they are **renoprotective** in diabetic nephropathy (reducing hyperfiltration) but can cause a transient drop in GFR. * **Filtration Fraction (FF):** Since Efferent constriction increases GFR (initially) and decreases RPF, the **FF (GFR/RPF) always increases.**

Q: Which of the following aquaporin water channels is located on the apical membrane of principal cells of the collecting duct?

Aquaporin 2. **Explanation:** The correct answer is **Aquaporin 2 (AQP2)**. **1. Why Aquaporin 2 is correct:** Aquaporin 2 is the only water channel in the collecting duct that is **hormonally regulated**. It is located on the **apical (luminal) membrane** of the principal cells. When Antidiuretic Hormone (ADH/Vasopressin) binds to V2 receptors on the basolateral membrane, it triggers a cAMP-mediated signaling pathway that causes the insertion of AQP2-containing vesicles into the apical membrane. This increases the water permeability of the collecting duct, allowing for concentrated urine production. **2. Why other options are incorrect:** * **Aquaporin 1 (AQP1):** This is found in the **Proximal Convoluted Tubule (PCT)** and the descending limb of the Loop of Henle. It is responsible for constitutive (always active) water reabsorption and is not regulated by ADH. * **Aquaporin 3 & 4 (AQP3 & AQP4):** These are located on the **basolateral membrane** of the principal cells in the collecting duct. Unlike AQP2, they are constitutively expressed and provide the exit pathway for water to move from the cell into the medullary interstitium. **3. High-Yield Clinical Pearls for NEET-PG:** * **Nephrogenic Diabetes Insipidus:** Can be caused by mutations in the AQP2 gene or the V2 receptor, rendering the kidney unable to concentrate urine despite high ADH levels. * **Lithium Toxicity:** A common cause of acquired nephrogenic diabetes insipidus because lithium downregulates the expression of AQP2. * **Location Summary:** Apical = AQP2; Basolateral = AQP3, AQP4; PCT = AQP1.

Q: How does glomerular capillary pressure differ from other capillaries of the body?

Higher filtration pressure. **Explanation:** The glomerular capillary pressure is significantly higher than that of other systemic capillaries, a feature essential for the kidney's primary function of ultrafiltration. **Why Option A is Correct:** In most systemic capillaries, the hydrostatic pressure is approximately **15–30 mmHg**. In contrast, the glomerular capillary hydrostatic pressure ($P_{GC}$) is maintained at a much higher level, roughly **60 mmHg**. This high pressure is achieved due to two unique anatomical features: 1. **The "High-Resistance" Arrangement:** The glomerular capillaries are situated between two arterioles—the **afferent** and the **efferent**. The high resistance of the efferent arteriole acts as a "dam," backing up blood and elevating the pressure within the glomerular tuft. 2. **Short, Wide Afferent Arteriole:** The afferent arteriole is relatively short and wide, offering low resistance to incoming blood flow directly from the renal artery. **Why Options B, C, and D are Incorrect:** * **Option B:** Lower filtration pressure would result in a failure to overcome the opposing oncotic pressure, leading to a cessation of glomerular filtration and subsequent renal failure. * **Options C & D:** These are logically excluded as the physiological mechanism is unidirectional and specific to high-pressure filtration. **High-Yield Clinical Pearls for NEET-PG:** * **Starling Forces:** Net Filtration Pressure (NFP) = $(P_{GC} + \pi_{BS}) - (P_{BS} + \pi_{GC})$. In the glomerulus, $\pi_{BS}$ (oncotic pressure in Bowman's space) is effectively zero. * **Autoregulation:** The Myogenic mechanism and Tubuloglomerular Feedback (TGF) work to keep $P_{GC}$ constant despite fluctuations in systemic blood pressure. * **Effect of Drugs:** **ACE Inhibitors** dilate the efferent arteriole more than the afferent, thereby *decreasing* glomerular capillary pressure—a key mechanism in their nephroprotective effect in diabetic patients.

Q: In a healthy individual on a normal diet, which of the following is significantly reabsorbed in the renal tubules, except?

Urea. **Explanation:** The renal tubules are highly efficient at conserving essential electrolytes and water while selectively excreting metabolic waste. The distinction lies in the **percentage of the filtered load** that is reabsorbed. **Why Urea is the Correct Answer:** In a healthy individual, urea is a metabolic end-product of protein metabolism. While it is partially reabsorbed (approximately **40-50%**) to maintain the medullary osmotic gradient, a significant portion (about 50%) is **excreted** in the urine. Compared to the other options, urea has the lowest fractional reabsorption rate. It is considered a waste product that the kidney is designed to eliminate, rather than conserve. **Why the other options are incorrect:** * **Sodium (Na+):** Over **99%** of filtered sodium is reabsorbed (65% in the PCT) to maintain extracellular fluid volume and blood pressure. * **Chloride (Cl-):** Follows sodium passively and actively; approximately **99%** is reabsorbed to maintain electrical neutrality. * **Potassium (K+):** Under normal conditions, about **85-95%** of filtered potassium is reabsorbed before reaching the distal tubule. While the distal tubule can secrete K+ based on aldosterone levels, the bulk of the filtered load is recovered. **High-Yield NEET-PG Pearls:** 1. **PCT Reabsorption:** 100% of Glucose and Amino Acids are reabsorbed in the Proximal Convoluted Tubule (PCT) via secondary active transport. 2. **Urea Recycling:** Urea reabsorption occurs mainly in the PCT and the Medullary Collecting Ducts (via UT-A1/A3 transporters), contributing to the **corticopapillary osmotic gradient**. 3. **ADH Influence:** Antidiuretic Hormone (ADH) increases urea reabsorption in the medullary collecting ducts, further concentrating the medullary interstitium.

Q: The highest percentage of glomerular filtrate reabsorption occurs in which part of the nephron?

Proximal tubule. **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for the reabsorption of glomerular filtrate. Approximately **65-70%** of the total filtered water and sodium, and nearly 100% of glucose and amino acids, are reabsorbed here. This high capacity is due to the presence of a dense "brush border" of microvilli, which significantly increases the surface area for transport, and a high density of mitochondria to provide energy for active transport. **Analysis of Options:** * **A. Bowman’s Capsule:** This is the site of **ultrafiltration**, not reabsorption. It collects the filtrate from the glomerulus and directs it into the PCT. * **C. Thick Ascending Limb (TAL):** This segment reabsorbs about **15-25%** of filtered sodium. It is known as the "diluting segment" because it is impermeable to water but actively transports solutes. * **D. Distal Tubule:** The distal convoluted tubule and collecting ducts are responsible for "fine-tuning" the urine composition, reabsorbing only about **5-10%** of the filtrate, largely under hormonal control (Aldosterone and ADH). **High-Yield NEET-PG Pearls:** 1. **Isotonic Reabsorption:** Reabsorption in the PCT is **iso-osmotic**; the fluid remaining at the end of the PCT has the same osmolarity as plasma (300 mOsm/L). 2. **Obligatory Water Reabsorption:** Water reabsorption in the PCT follows solutes passively and is independent of ADH. 3. **Carbonic Anhydrase:** This enzyme is highly active in the PCT, making it the site of action for **Acetazolamide**. 4. **SGLT-2:** Located in the PCT, these transporters are responsible for glucose reabsorption and are the target of "Gliflozin" drugs used in diabetes.

Q: What is the important role of the distal tubule of the kidney in acid-base balance?

Secretion of Ammonia. The kidney plays a vital role in maintaining acid-base homeostasis through the excretion of hydrogen ions ($H^+$). However, the minimum urinary pH is approximately 4.5; to excrete more acid, $H^+$ must be buffered. ### **Why "Secretion of Ammonia" is Correct** In the distal tubule (specifically the late distal tubule and collecting ducts), ammonia ($NH_3$) is the most important adaptive buffer. $NH_3$ is lipid-soluble and diffuses from the tubular cells into the lumen. There, it combines with secreted $H^+$ ions to form ammonium ($NH_4^+$). Because $NH_4^+$ is water-soluble and membrane-impermeable (**"ion trapping"**), it is excreted in the urine. This process allows for the excretion of large amounts of metabolic acid without further lowering the urinary pH. ### **Analysis of Incorrect Options** * **B. Secretion of bicarbonate:** The kidney primarily **reabsorbs** filtered bicarbonate (mostly in the proximal tubule) and generates "new" bicarbonate in the distal tubule to replenish the body's alkaline reserve. Secretion only occurs in specific states (via Type B intercalated cells) but is not the primary role in acid-base balance. * **C. Secretion of HCl:** The kidney does not secrete hydrochloric acid directly; it secretes $H^+$ ions which then pair with anions like $Cl^-$ or buffers. * **D. Absorption of Ammonia:** Ammonia is produced and **secreted** (not absorbed) to facilitate acid excretion. ### **NEET-PG High-Yield Pearls** * **Site of Production:** Most ammonia is produced in the **Proximal Convoluted Tubule (PCT)** from the amino acid **Glutamine**. * **Titratable Acidity:** Refers to $H^+$ buffered by phosphates ($HPO_4^{2-}$). Unlike ammonia, phosphate levels are relatively fixed and cannot increase significantly during chronic acidosis. * **Clinical Correlation:** In **Distal Renal Tubular Acidosis (Type 1 RTA)**, the distal tubule fails to secrete $H^+$, leading to an inability to acidify urine (pH remains > 5.5).

Q: Which substance acts as a vasodilator in most parts of the body but causes vasoconstriction specifically in the afferent arteriole of the kidney?

Adenosine. **Explanation:** The correct answer is **Adenosine**. This substance exhibits a unique "paradoxical" effect in the kidney compared to the rest of the systemic circulation. **1. Why Adenosine is Correct:** In most systemic tissues (like the heart or skeletal muscle), adenosine acts as a potent vasodilator to increase blood flow during metabolic demand. However, in the kidney, adenosine is the primary mediator of **Tubuloglomerular Feedback (TGF)**. When there is an increase in NaCl delivery to the *macula densa*, adenosine is released. It binds to **A1 receptors** on the afferent arteriole, causing **vasoconstriction**. This reduces the Glomerular Filtration Rate (GFR) to prevent excessive fluid loss. **2. Why the Other Options are Incorrect:** * **Nitric Oxide (NO):** A potent vasodilator in both systemic and renal circulations. It counteracts the effects of Angiotensin II in the kidney. * **Bradykinin:** A vasodilator that acts by stimulating NO release and prostaglandins. It dilates both afferent and efferent arterioles. * **Endothelin:** A powerful vasoconstrictor in almost all vascular beds, including the kidney. It does not show the site-specific reversal seen with adenosine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Receptor Specificity:** Adenosine causes vasoconstriction via **A1 receptors** (afferent arteriole) but can cause vasodilation in the efferent arteriole via **A2 receptors**. * **TGF Mechanism:** Increased GFR → Increased NaCl at Macula Densa → Adenosine release → Afferent vasoconstriction → Decreased GFR. * **Theophylline/Caffeine:** These are adenosine receptor antagonists; they can cause diuresis partly by inhibiting TGF-mediated afferent vasoconstriction.

Question 1

Efferent arteriole constriction causes which change in Glomerular Filtration Rate (GFR)?

Accepted Answer

GFR to initially increase then decrease

Answer

Increased GFR

Answer

Decreased GFR

Answer

GFR to initially decrease then increase

Question 2

Renal medullary hyperosmolarity is due to which of the following?

Accepted Answer

Increased Na+ concentration

Answer

Increased Na+ content

Answer

Increased Glucose concentration

Answer

All of the above

Question 3

Which of the following statements about renal circulation is FALSE?

Accepted Answer

Glomerular capillary hydrostatic pressure is approximately 30 mmHg.

Answer

It accounts for about 25% of the cardiac output.

Answer

Renal cortical blood flow is greater than renal medullary blood flow.

Answer

Autoregulation of renal blood flow (RBF) is primarily mediated by the myogenic mechanism.

Question 4

Which of the following aquaporin water channels is located on the apical membrane of principal cells of the collecting duct?

Accepted Answer

Aquaporin 2

Answer

Aquaporin 1

Answer

Aquaporin 3

Answer

Aquaporin 4

Question 5

How does glomerular capillary pressure differ from other capillaries of the body?

Accepted Answer

Higher filtration pressure

Answer

Lower filtration pressure

Answer

Both of the above

Answer

None of the above

Question 6

In a healthy individual on a normal diet, which of the following is significantly reabsorbed in the renal tubules, except?

Accepted Answer

Urea

Answer

Sodium (Na+)

Answer

Potassium (K+)

Answer

Chloride (Cl-)

Question 7

Formation of a small volume of concentrated urine in dehydration is associated with which of the following?

Accepted Answer

All of the above

Answer

Increased reabsorption in the proximal convoluted tubule

Answer

Increased concentration in the loop of Henle

Answer

Increased reabsorption in the distal convoluted tubule and collecting ducts

Question 8

The highest percentage of glomerular filtrate reabsorption occurs in which part of the nephron?

Accepted Answer

Proximal tubule

Answer

Bowman's capsule

Answer

Thick ascending limb of loop of Henle

Answer

Distal tubule

Question 9

What is the important role of the distal tubule of the kidney in acid-base balance?

Accepted Answer

Secretion of Ammonia

Answer

Secretion of bicarbonate

Answer

Secretion of HCl

Answer

Absorption of Ammonia

Question 10

Which substance acts as a vasodilator in most parts of the body but causes vasoconstriction specifically in the afferent arteriole of the kidney?

Accepted Answer

Adenosine

Answer

Nitric oxide

Answer

Bradykinin

Answer

Endothelin

Renal Physiology — MCQs

Renal Physiology — MCQs

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