Renal Physiology Practice Questions

Q: A 20-year-old male patient presents with a decrease in urinary output to about 400ml/day. Both renal blood flow (RBF) and glomerular filtration rate (GFR) were decreased. Which of the following causes a decrease in both RBF and GFR?

Constriction of the afferent arteriole. **Explanation:** The Glomerular Filtration Rate (GFR) and Renal Blood Flow (RBF) are primarily regulated by the resistance of the afferent and efferent arterioles. **Why Option C is Correct:** **Constriction of the afferent arteriole** increases resistance *before* the blood reaches the glomerulus. This leads to: 1. **Decreased RBF:** Increased total renal vascular resistance reduces blood flow into the kidney. 2. **Decreased GFR:** Reduced flow into the glomerular capillaries lowers the glomerular hydrostatic pressure ($P_{GC}$), which is the primary driving force for filtration. **Analysis of Incorrect Options:** * **Option A (Dilation of efferent arteriole):** Decreases resistance to outflow. This **increases RBF** (less resistance) but **decreases GFR** because the "back pressure" in the glomerulus drops. * **Option B (Dilation of afferent arteriole):** Decreases resistance to inflow. This **increases both RBF and GFR** (due to increased $P_{GC}$). * **Option D (Constriction of efferent arteriole):** Increases resistance to outflow. This **decreases RBF** but **increases GFR** (up to a point) because it increases the hydrostatic pressure ($P_{GC}$) within the glomerular capillaries. **High-Yield Clinical Pearls for NEET-PG:** * **NSAIDs** inhibit prostaglandins (which normally dilate the afferent arteriole), leading to **afferent constriction** and a drop in GFR. * **ACE Inhibitors** prevent the action of Angiotensin II (which preferentially constricts the efferent arteriole), leading to **efferent dilation** and a drop in GFR. * **Key Rule:** If both RBF and GFR move in the **same** direction, the primary change is at the **afferent** arteriole. If they move in **opposite** directions, the primary change is at the **efferent** arteriole.

Q: A 78-year-old man presents with nausea and vomiting. On examination, he appears dehydrated, his abdomen is soft, and the jugular venous pressure is not elevated. Laboratory tests reveal hypernatremia, and his calculated free water deficit is approximately 3 L. In which part of the normal kidney is most of the water reabsorbed?

Proximal tubule. **Explanation:** The correct answer is **B. Proximal tubule**. In a normal kidney, approximately **65-70% of the filtered water** is reabsorbed in the **Proximal Convoluted Tubule (PCT)**. This process is known as **obligatory water reabsorption**. It occurs isosmotically, meaning water follows the active reabsorption of solutes (primarily sodium via the Na+/K+ ATPase pump) through aquaporin-1 channels and paracellular pathways to maintain osmotic equilibrium. **Why other options are incorrect:** * **Collecting ducts:** While this is the site of **facultative water reabsorption** regulated by Antidiuretic Hormone (ADH), it only accounts for about 5-10% of total water reabsorption. It is crucial for final urine concentration but not for the bulk of volume recovery. * **Distal tubule:** The early distal tubule is part of the "diluting segment" and is relatively impermeable to water. * **Ascending loop of Henle:** This segment (specifically the Thick Ascending Limb) is **impermeable to water**. It actively reabsorbs solutes (Na+/K+/2Cl-) without water, which is essential for creating the medullary osmotic gradient. **High-Yield Clinical Pearls for NEET-PG:** * **PCT:** Always reabsorbs a constant fraction of filtered load (Glomerulotubular balance). * **Descending Loop of Henle:** Highly permeable to water but impermeable to solutes (concentrating segment). * **Countercurrent Multiplier:** Established by the Loop of Henle; **Countercurrent Exchanger** is the VASA RECTA. * **Clinical Correlation:** In dehydration (as seen in this patient), ADH levels rise to increase water permeability in the collecting ducts, but the PCT still performs the "heavy lifting" of bulk reabsorption.

Q: A patient is given a drug that dilates the afferent arteriole and contracts the efferent arteriole, leaving total renal vascular resistance unchanged. Which set of changes is likely to occur in the following table? (I = increased, D = decreased, NC = no change)

Table option C. ***Table option C*** - **Afferent dilation** and **efferent constriction** increases **glomerular capillary pressure**, leading to **increased GFR** while maintaining constant **RPF** since total **renal vascular resistance** remains unchanged. - **Filtration fraction** (FF = GFR/RPF) increases because GFR rises while RPF stays constant, and **glomerular capillary pressure** increases due to enhanced blood flow into and restricted flow out of glomerular capillaries. *Table option A* - This option likely shows **decreased GFR**, which contradicts the expected effect of increased **glomerular capillary pressure** from the described vascular changes. - The combination of **afferent dilation** and **efferent constriction** should enhance **filtration pressure**, not reduce it. *Table option B* - This option probably indicates **decreased filtration fraction**, which is incorrect since GFR increases more than any potential RPF changes. - **Glomerular capillary pressure** changes are likely incorrectly predicted, not accounting for the **Starling forces** modification from vascular resistance alterations. *Table option D* - This option may show **decreased glomerular capillary pressure**, which contradicts the hemodynamic effects of **afferent vasodilation** and **efferent vasoconstriction**. - The **RPF changes** are likely incorrectly predicted, not considering that total **renal vascular resistance** compensation maintains overall renal blood flow.

Q: Which of the following does not cause relaxation of the urinary bladder?

Muscarinic receptors. To understand the physiology of micturition, one must distinguish between the actions of the sympathetic and parasympathetic nervous systems on the detrusor muscle and the internal sphincter. ### **Explanation** The **detrusor muscle** (the main muscle of the bladder wall) is responsible for bladder contraction during voiding. * **Muscarinic Receptors (M3):** These are the primary receptors of the **parasympathetic** nervous system (via the Pelvic nerve). Stimulation of M3 receptors leads to **contraction** of the detrusor muscle to facilitate emptying. Therefore, muscarinic stimulation causes contraction, not relaxation. ### **Analysis of Other Options** * **Sympathetic Stimulation:** The sympathetic nervous system (via the Hypogastric nerve) promotes **bladder filling** (continence). It achieves this by relaxing the detrusor muscle and contracting the internal urethral sphincter. * **Beta Receptors ($\beta_3$):** These are the specific sympathetic receptors located in the detrusor muscle. Activation of $\beta_3$ receptors leads to **relaxation** of the bladder wall, allowing it to expand and store urine. ### **NEET-PG High-Yield Pearls** 1. **Mnemonic for Bladder Control:** * **P**arasympathetic = **P**ee (Contracts detrusor, relaxes sphincter). * **S**ympathetic = **S**tore (Relaxes detrusor via $\beta_3$, contracts internal sphincter via $\alpha_1$). 2. **Clinical Correlation:** **Mirabegron** is a $\beta_3$ agonist used to treat overactive bladder by promoting detrusor relaxation. 3. **Anticholinergics:** Drugs like Oxybutynin block muscarinic receptors to prevent involuntary bladder contractions in urge incontinence. 4. **Internal vs. External Sphincter:** The internal sphincter is involuntary (autonomic), while the external sphincter is voluntary (Somatic - Pudendal nerve).

Q: If Renal Blood Flow is 1250 ml/min, calculate Renal Plasma Flow.

690 ml/min. ### Explanation **1. Understanding the Concept** Renal Plasma Flow (RPF) is the volume of plasma that reaches the kidneys per unit time. Blood consists of two main components: cellular elements (primarily Red Blood Cells) and plasma. The proportion of blood occupied by RBCs is the **Hematocrit (Hct)**. To calculate RPF from Renal Blood Flow (RBF), we use the formula: **RPF = RBF × (1 – Hematocrit)** In standard physiological calculations (unless specified otherwise), the average human hematocrit is taken as **45% (0.45)**. * Calculation: $1250 \times (1 - 0.45)$ * $1250 \times 0.55 = \mathbf{687.5 \text{ ml/min}}$ * Rounding to the nearest option gives **690 ml/min**. **2. Analysis of Options** * **Option A (690 ml/min):** Correct. This aligns with the standard calculation using a normal hematocrit of 45%. * **Option B (650 ml/min):** Incorrect. This would imply a hematocrit of 48%, which is higher than the standard physiological average used for such questions. * **Option C (710 ml/min):** Incorrect. This would imply a hematocrit of approximately 43%. * **Option D (670 ml/min):** Incorrect. This would imply a hematocrit of approximately 46.4%. **3. High-Yield Facts for NEET-PG** * **Filtration Fraction (FF):** The ratio of GFR to RPF ($FF = GFR / RPF$). Normal value is ~20% (0.2). * **Gold Standard for RPF:** Para-aminohippuric acid (PAH) clearance is used to estimate Effective Renal Plasma Flow (ERPF) because it is both filtered and secreted. * **RBF Distribution:** Although the kidneys account for only 0.5% of body weight, they receive ~20-25% of the Cardiac Output. * **Autoregulation:** RBF and GFR remain constant between a Mean Arterial Pressure (MAP) of 80–180 mmHg due to the myogenic mechanism and tubuloglomerular feedback.

Q: What percentage of cardiac output is supplied to the kidneys?

20%. **Explanation** The kidneys are highly vascular organs that receive a disproportionately large share of the cardiac output (CO) relative to their weight. In a healthy adult, the total renal blood flow (RBF) is approximately **1100–1200 mL/min**, which constitutes roughly **20% to 25%** of the total cardiac output (assuming a CO of 5–6 L/min). **Why 20% is Correct:** This high blood flow is not required to meet the metabolic (nutritional) demands of the renal tissue itself, but rather to ensure a high **Glomerular Filtration Rate (GFR)**. This allows the kidneys to precisely regulate volume status, electrolyte balance, and the excretion of waste products by processing the entire plasma volume multiple times a day. **Analysis of Incorrect Options:** * **10% (Option A):** This is too low. While the kidneys represent only about 0.5% of total body weight, they require significantly more than 10% of CO to maintain effective filtration pressures. * **30% (Option B):** This is too high for resting physiology. While RBF can fluctuate, 30% exceeds the standard physiological range for a healthy individual at rest. **High-Yield NEET-PG Pearls:** * **Oxygen Consumption:** Despite receiving 20% of CO, the kidneys only consume about 7-10% of the body's total oxygen. Most of this oxygen is used for the active reabsorption of sodium in the proximal tubules. * **Regional Distribution:** Blood flow is not uniform; the **Renal Cortex** receives the vast majority (~90%), while the **Medulla** receives very little (~1-2% via the vasa recta). This low medullary flow is essential for maintaining the osmotic gradient required for urine concentration. * **Autoregulation:** Renal blood flow is kept constant between a Mean Arterial Pressure (MAP) of **80 to 180 mmHg** via the myogenic mechanism and tubuloglomerular feedback.

Q: Which hormone exerts short-term regulation of blood pressure through the kidney?

Antidiuretic Hormone (ADH). **Explanation:** The regulation of blood pressure by the kidney involves both short-term (neural/hormonal) and long-term (hemodynamic) mechanisms. **Antidiuretic Hormone (ADH)**, also known as Vasopressin, acts as a short-term regulator through two primary mechanisms: 1. **V1 Receptors:** Located on vascular smooth muscle, causing rapid vasoconstriction to increase systemic vascular resistance. 2. **V2 Receptors:** Located in the late distal tubule and collecting ducts, increasing water reabsorption via Aquaporin-2 channels. While its volume effect is more gradual, its potent vasoconstrictive action provides immediate BP support during acute states like hemorrhage. **Analysis of Incorrect Options:** * **Angiotensin II:** While a potent vasoconstrictor, it is part of the Renin-Angiotensin-Aldosterone System (RAAS), which is primarily considered an intermediate to long-term regulator of blood pressure and fluid balance. * **Epinephrine:** Though it acts rapidly on the heart and blood vessels (short-term), it is a catecholamine released from the adrenal medulla, not a hormone that regulates BP specifically *through the kidney*. * **Aldosterone:** This is a classic long-term regulator. It acts via gene transcription to increase sodium and water reabsorption, a process that takes hours to days to significantly impact blood pressure. **NEET-PG High-Yield Pearls:** * **ADH Stimulus:** The most potent stimulus for ADH release is increased plasma osmolarity (detected by osmoreceptors), but the most "urgent" stimulus is a decrease in ECF volume (detected by baroreceptors). * **V1 vs. V2:** Remember **V1 = Vascular** (Constriction) and **V2 = Volume** (Water reabsorption). * **Atrial Natriuretic Peptide (ANP):** The "counter-regulatory" hormone to RAAS; it lowers BP by promoting natriuresis and vasodilation.

Q: What is the normal daily urinary protein excretion in a healthy adult?

Approximately 100 mg. **Explanation:** In a healthy adult, the glomerular filtration barrier (composed of fenestrated endothelium, basement membrane, and podocytes) is highly selective [1]. It restricts the passage of large proteins like albumin based on both size and negative charge [1]. Consequently, the normal daily urinary protein excretion is **less than 150 mg/day**, with an average of **approximately 100 mg/day** [1]. Of this total protein: * **40% is Albumin:** Only a tiny fraction (approx. 30 mg) escapes the filter and remains unabsorbed [2]. * **60% is Globulins:** This includes **Tamm-Horsfall protein** (uromodulin), which is secreted by the thick ascending limb of the Loop of Henle and constitutes the bulk of non-albumin protein. **Analysis of Incorrect Options:** * **Option B (500 mg):** This level indicates **pathological proteinuria**. Values between 150 mg and 3.5 g/day are seen in various renal diseases (nephritic range). * **Options C & D (1 gm & 2 gm):** These represent significant proteinuria. Persistent excretion at these levels suggests glomerular damage or tubular dysfunction. Excretion exceeding **3.5 g/day** is the hallmark of **Nephrotic Syndrome**. **High-Yield Clinical Pearls for NEET-PG:** * **Microalbuminuria:** Defined as 30–300 mg/day of albumin. It is the earliest clinical sign of diabetic nephropathy. * **Tamm-Horsfall Protein:** The most abundant protein in normal urine; it forms the matrix of urinary casts. * **Selectivity:** The glomerular basement membrane contains **Heparan Sulfate**, which provides a negative charge that repels albumin [1]. Loss of this charge leads to "minimal change disease."

Question 1

In a cystometrogram, where does LaPlace's law have its primary application?

Accepted Answer

Phase Ib (bladder volume between 100-400 mL)

Answer

Phase Ia (bladder volume less than 100 mL)

Answer

Phase II (bladder volume more than 450 mL)

Answer

All phases of the cystometrogram

Question 2

A 20-year-old male patient presents with a decrease in urinary output to about 400ml/day. Both renal blood flow (RBF) and glomerular filtration rate (GFR) were decreased. Which of the following causes a decrease in both RBF and GFR?

Accepted Answer

Constriction of the afferent arteriole

Answer

Dilation of the efferent arteriole

Answer

Dilation of the afferent arteriole

Answer

Constriction of the efferent arteriole

Question 3

Which of the following can reduce the glomerular filtration rate (GFR)?

Accepted Answer

Endothelium-derived nitric oxide

Answer

Norepinephrine

Answer

Epinephrine

Answer

Endothelin

Question 4

A 78-year-old man presents with nausea and vomiting. On examination, he appears dehydrated, his abdomen is soft, and the jugular venous pressure is not elevated. Laboratory tests reveal hypernatremia, and his calculated free water deficit is approximately 3 L. In which part of the normal kidney is most of the water reabsorbed?

Accepted Answer

Proximal tubule

Answer

Collecting ducts

Answer

Distal tubule

Answer

Ascending loop of Henle

Question 5

A patient is given a drug that dilates the afferent arteriole and contracts the efferent arteriole, leaving total renal vascular resistance unchanged. Which set of changes is likely to occur in the following table? (I = increased, D = decreased, NC = no change)

Accepted Answer

Table option C

Answer

Table option A

Answer

Table option B

Answer

Table option D

Question 6

Which of the following does not cause relaxation of the urinary bladder?

Accepted Answer

Muscarinic receptors

Answer

Sympathetic Stimulation

Answer

Beta receptors

Answer

None of the above

Question 7

If Renal Blood Flow is 1250 ml/min, calculate Renal Plasma Flow.

Accepted Answer

690 ml/min

Answer

650 ml/min

Answer

710 ml/min

Answer

670 ml/min

Question 8

What percentage of cardiac output is supplied to the kidneys?

Accepted Answer

20%

Answer

10%

Answer

30%

Answer

None of the above

Question 9

Which hormone exerts short-term regulation of blood pressure through the kidney?

Accepted Answer

Antidiuretic Hormone (ADH)

Answer

Angiotensin II

Answer

Epinephrine

Answer

Aldosterone

Question 10

What is the normal daily urinary protein excretion in a healthy adult?

Accepted Answer

Approximately 100 mg

Answer

Approximately 500 mg

Answer

Approximately 1 gm

Answer

Approximately 2 gm

Renal Physiology — MCQs

Renal Physiology — MCQs

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