Renal Physiology — MCQs
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Which hormone primarily controls the reabsorption of sodium chloride in the kidneys?
Which of the following nephron segments contributes most significantly to the establishment of the medullary concentration gradient through sodium reabsorption?
All of the following results in increased renin secretion except which one?
In a normally functioning kidney, which part of the nephron has the lowest permeability to water during antidiuresis?
Which one of the following substances, actively transported by tubular cells, has the highest tubular transport maximum?
Least reabsorption out of the following in the nephron is
According to the myogenic hypothesis of renal autoregulation, the afferent arterioles contract in response to stretch induced by what?
Which substance has the least renal clearance?
Which substance is not absorbed in the loop of Henle?
Inactivation of cortisol into cortisone occurs mainly in which organ?
Renal Physiology Indian Medical PG Practice Questions and MCQs
Question 561: Which hormone primarily controls the reabsorption of sodium chloride in the kidneys?
- A. Atrial Natriuretic Peptide (ANP)
- B. Adrenaline
- C. Aldosterone (Correct Answer)
- D. Antidiuretic hormone (ADH)
Explanation: ***Aldosterone*** - **Aldosterone** is a steroid hormone produced by the adrenal cortex that primarily acts on the **distal tubules** and **collecting ducts** of the kidneys to increase **sodium chloride reabsorption** and potassium secretion. - Its main role is to regulate **blood pressure** and **electrolyte balance**. *Adrenaline* - **Adrenaline (epinephrine)** is a hormone and neurotransmitter involved in the **fight-or-flight response**. - While it can indirectly influence renal blood flow, it does not directly control tubular **sodium reabsorption**. *Antidiuretic hormone (ADH)* - **Antidiuretic hormone (ADH)**, also known as **vasopressin**, primarily regulates **water reabsorption** in the collecting ducts by increasing their permeability to water. - It has a minimal direct effect on **sodium chloride reabsorption**. *Atrial Natriuretic Peptide (ANP)* - **ANP** is released by the atria in response to increased blood volume and promotes **sodium excretion** (natriuresis) rather than reabsorption. - It has the opposite effect of aldosterone, acting to **decrease blood pressure** by increasing sodium and water loss.
Question 562: Which of the following nephron segments contributes most significantly to the establishment of the medullary concentration gradient through sodium reabsorption?
- A. Ascending limb of Henle (Correct Answer)
- B. Collecting duct
- C. Distal tubule
- D. Proximal tubule
Explanation: ***Ascending limb of Henle*** - The **thick ascending limb of Henle (TALH)** is impermeable to water but actively reabsorbs large amounts of **sodium, potassium, and chloride** via the **Na-K-2Cl cotransporter (NKCC2)**. - This active solute reabsorption without water movement into the interstitium is crucial for establishing and maintaining the **medullary osmotic gradient**, which allows for subsequent water reabsorption in the collecting duct. *Collecting duct* - The collecting duct participates in the final adjustment of urine concentration by reabsorbing water (under ADH influence) and some urea, which contributes to the medullary gradient, but it does not primarily establish it. - Its main role is to facilitate **water reabsorption** from the medullary interstitium, which is already concentrated by the loop of Henle. *Distal tubule* - The distal tubule reabsorbs a small percentage of filtered sodium and chloride, primarily through the **Na-Cl cotransporter (NCC)**. - While it contributes to fine-tuning electrolyte balance, its contribution to the overall **medullary concentration gradient** is less significant compared to the ascending limb. *Proximal tubule* - The proximal tubule reabsorbs about two-thirds of filtered sodium and water in an **isosmotic fashion**, meaning water follows solutes. - Therefore, it does not create a significant osmotic gradient with the interstitium and does not directly contribute to the medullary concentration gradient.
Question 563: All of the following results in increased renin secretion except which one?
- A. Renal ischaemia
- B. Decreased amount of Na+ in DCT
- C. Decreased afferent arteriolar pressure
- D. Increased amount of Na+ in DCT (Correct Answer)
Explanation: ***Increased amount of Na+ in DCT*** - An **increased amount of Na+ in the distal convoluted tubule (DCT)** is detected by the **macula densa** cells, which then **inhibit renin secretion**. - This is part of the **tubuloglomerular feedback mechanism**, which aims to decrease glomerular filtration rate (GFR) when tubular fluid flow is too high. *Renal ischaemia* - **Renal ischaemia** (reduced blood flow to the kidneys) leads to decreased **renal perfusion pressure**. - This stimulates the **juxtaglomerular cells** to release **renin** to increase blood pressure and improve renal blood flow. *Decreased amount of Na+ in DCT* - A **decreased amount of Na+ in the distal convoluted tubule (DCT)** is sensed by the **macula densa** cells. - This low Na+ delivery to the macula densa signals the **juxtaglomerular apparatus** to **increase renin secretion**, aiming to raise GFR and improve renal perfusion. *Decreased afferent arteriolar pressure* - **Decreased afferent arteriolar pressure** (lower blood pressure in the arteries supplying the glomerulus) directly stimulates the **juxtaglomerular cells** to release **renin**. - This is a direct response of the **intrarenal baroreceptors** to maintain glomerular filtration despite reduced systemic blood pressure.
Question 564: In a normally functioning kidney, which part of the nephron has the lowest permeability to water during antidiuresis?
- A. Distal Convoluted Tubule
- B. Proximal Convoluted Tubule
- C. Thick Ascending Limb of Loop of Henle (Correct Answer)
- D. Collecting Duct
Explanation: ***Thick Ascending Limb of Loop of Henle*** - This segment is **completely impermeable to water** regardless of the presence of ADH, making it the segment with the lowest water permeability in the nephron. - Its primary function is to actively reabsorb solutes like **Na+, K+, and Cl-** via the Na-K-2Cl cotransporter, diluting the tubular fluid without water following. - This impermeability is critical for establishing and maintaining the **medullary osmotic gradient**. *Proximal Convoluted Tubule* - The **proximal convoluted tubule** is highly permeable to water, responsible for reabsorbing about **65% of filtered water** through constitutively expressed aquaporin-1 (AQP-1) channels. - Water reabsorption here is obligatory and **not regulated by ADH**. *Distal Convoluted Tubule* - The **distal convoluted tubule** has low water permeability in the absence of ADH but can be increased when ADH is present (though less responsive than the collecting duct). - Its primary role is in fine-tuning electrolyte reabsorption, particularly **sodium and calcium**. *Collecting Duct* - The **collecting duct** has variable water permeability that is highly **ADH-dependent**. - During antidiuresis (high ADH), aquaporin-2 channels are inserted into the apical membrane, making it highly permeable to water for final urine concentration. - Without ADH, it has low permeability, but it's never as impermeable as the thick ascending limb.
Question 565: Which one of the following substances, actively transported by tubular cells, has the highest tubular transport maximum?
- A. Large plasma proteins
- B. Myoglobin
- C. Glucose (Correct Answer)
- D. Sodium
Explanation: ***Glucose*** - The **tubular transport maximum (Tm)** for glucose is exceptionally high (around 375 mg/min normally) because its reabsorption is vital for energy and occurs in large quantities. - Efficient reabsorption of **glucose** prevents its loss in urine, reflecting its physiological importance. *Large plasma proteins* - While plasma proteins are reabsorbed, their **filtration normally is minimal**, and their reabsorption is largely through **endocytosis**, not a simple active transport system with a high Tm like glucose. - The amount handled by tubular cells is very small under normal physiological conditions. *Myoglobin* - Myoglobin is typically not present in plasma in significant amounts and is **not a normal substance actively transported** by tubular cells in the quantity that glucose is. - Its presence in urine usually indicates muscle damage, and its reabsorption would be negligible compared to essential filtered substances. *Sodium (regulated by physiological mechanisms)* - Sodium reabsorption is widespread across the nephron segments and highly regulated, but its transport is typically **not described by a simple Tm** in the same manner as glucose. - Its reabsorption capacity is vast and adjusted based on fluid and electrolyte balance, not a fixed maximum like a carrier-mediated transport system that determines Tm for substances like glucose.
Question 566: Least reabsorption out of the following in the nephron is
- A. Glucose
- B. Urea (Correct Answer)
- C. Na +
- D. HCO3-
Explanation: ***Urea*** - Urea is a **waste product** and while some reabsorption occurs (primarily in the **collecting duct**), a significant portion (around 50%) is excreted to facilitate water reabsorption and maintain osmotic balance. - Its reabsorption rate is the lowest among essential solutes, reflecting its role as a molecule to be eliminated from the body. *Glucose* - **Glucose** is almost entirely reabsorbed in the **proximal convoluted tubule** under normal physiological conditions, with nearly 100% recovery. - This high reabsorption ensures that this vital energy source is conserved and not lost in the urine. *Na +* - **Sodium (Na+)** is extensively reabsorbed throughout most segments of the nephron, with over **99% of filtered sodium** being recovered. - This active transport is crucial for maintaining **extracellular fluid volume**, blood pressure, and acid-base balance. *HCO3-* - **Bicarbonate (HCO3-)** is meticulously reabsorbed, primarily in the **proximal convoluted tubule**, to prevent its loss in the urine, with approximately 80-90% reabsorbed. - This process is fundamental in **maintaining the body's acid-base balance** and buffering capacity.
Question 567: According to the myogenic hypothesis of renal autoregulation, the afferent arterioles contract in response to stretch induced by what?
- A. No release
- B. Opening of Ca2+ channels (Correct Answer)
- C. Adenosine release
- D. Noradrenaline release
Explanation: ***Opening of Ca2+ channels*** - The myogenic hypothesis states that increased stretch on the afferent arteriolar wall, due to elevated blood pressure, directly activates **mechanosensitive ion channels**. - This activation leads to the **influx of Ca2+ ions** into the vascular smooth muscle cells, causing vasoconstriction and maintaining constant renal blood flow. - This is a direct mechanotransduction mechanism where mechanical stretch is translated into depolarization via Ca2+ entry. *No release* - This option is incorrect and misleading because the myogenic response **does involve an active cellular mechanism** - specifically, the opening of mechanosensitive Ca2+ channels and calcium influx. - While no extracellular chemical mediator needs to be released, the response is not simply "no release" but rather involves **intracellular calcium signaling** triggered by stretch. - The contraction is mediated by increased intracellular Ca2+ concentration, not by absence of any mechanism. *Noradrenaline release* - While **noradrenaline** can cause vasoconstriction, it is primarily associated with **sympathetic nervous system activation**, not the intrinsic myogenic autoregulation of renal blood flow. - The myogenic mechanism functions independently of neural control and does not require catecholamine release. *Adenosine release* - **Adenosine** is involved in the **tubuloglomerular feedback (TGF)** mechanism, another component of renal autoregulation, where it mediates afferent arteriolar constriction in response to high distal tubule NaCl concentration detected by the macula densa. - It is not the direct mediator of the myogenic response to stretch, which is an intrinsic vascular smooth muscle property.
Question 568: Which substance has the least renal clearance?
- A. Glucose (Correct Answer)
- B. Inulin
- C. Urea
- D. Creatinine
Explanation: ***Glucose (Correct Answer)*** - Under normal physiological conditions, **glucose is almost completely reabsorbed** in the proximal tubule of the nephron, leading to a **renal clearance of nearly zero**. - While glucose is freely filtered by the glomerulus, the extensive reabsorption mechanisms (via SGLT2 and SGLT1 transporters) ensure that virtually no glucose appears in the urine under normal circumstances. - This makes glucose the substance with the **least renal clearance** among the given options. *Inulin (Incorrect)* - **Inulin** is freely filtered by the glomerulus but is neither reabsorbed nor secreted by the renal tubules. - Its renal clearance equals the **glomerular filtration rate (GFR)** (~125 mL/min), making it an ideal marker for GFR measurement. - Inulin has a **much higher clearance than glucose**. *Urea (Incorrect)* - **Urea** is filtered by the glomerulus, and approximately **50% of the filtered urea** is reabsorbed in the renal tubules, primarily in the proximal tubule and medullary collecting duct. - Its clearance (~60-70 mL/min) is lower than GFR but still **significantly higher than glucose clearance**. *Creatinine (Incorrect)* - **Creatinine** is freely filtered by the glomerulus and is also **secreted by the renal tubules** (approximately 10-20% secreted). - This secretion means its renal clearance (~130-140 mL/min) is slightly **higher than the actual GFR**. - Despite this, creatinine is commonly used as an estimate of GFR due to its relatively stable production and ease of measurement.
Question 569: Which substance is not absorbed in the loop of Henle?
- A. Urea (Correct Answer)
- B. Cl-
- C. K+
- D. Na+
Explanation: ***Urea*** - Urea undergoes **minimal passive reabsorption** in the loop of Henle (thin ascending limb), unlike the active transport of electrolytes. - While urea contributes to the **medullary osmotic gradient**, it is **not actively transported** in the loop of Henle; significant reabsorption occurs later in the **inner medullary collecting ducts**. - Among the given options, urea has the **least significant reabsorption** in the loop of Henle compared to the electrolytes. *K+* - **Potassium (K+)** is actively reabsorbed in the **thick ascending limb** via the Na+-K+-2Cl- cotransporter. - Approximately **20-25%** of filtered K+ is reabsorbed here, contributing to the **medullary osmotic gradient**. *Cl-* - **Chloride (Cl-)** is actively reabsorbed in the **thick ascending limb** via the Na+-K+-2Cl- cotransporter. - About **25%** of filtered Cl- is reabsorbed here, crucial for creating the **medullary osmotic gradient**. *Na+* - **Sodium (Na+)** is extensively reabsorbed in the **thick ascending limb** via the Na+-K+-2Cl- cotransporter. - Approximately **25%** of filtered Na+ is reabsorbed here, essential for the **countercurrent multiplication** mechanism and urine concentration.
Question 570: Inactivation of cortisol into cortisone occurs mainly in which organ?
- A. Lungs
- B. Adrenals
- C. Kidney (Correct Answer)
- D. Liver
Explanation: ***Correct: Kidney*** - The **kidney** is the primary site for the conversion of active **cortisol to inactive cortisone** via the enzyme **11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2)**. - **11β-HSD2 is highly expressed in the kidney**, particularly in mineralocorticoid target tissues like the **distal tubules and collecting ducts**. - This conversion protects the **mineralocorticoid receptor** from inappropriate activation by cortisol, as both cortisol and aldosterone have similar affinity for this receptor. - By inactivating cortisol locally, 11β-HSD2 ensures that aldosterone can selectively activate the mineralocorticoid receptor. *Incorrect: Liver* - The liver is crucial for **overall steroid metabolism** (conjugation, reduction, and excretion), but it primarily expresses **11β-HSD1**, not 11β-HSD2. - **11β-HSD1 predominantly catalyzes the REVERSE reaction** (cortisone → cortisol), regenerating active cortisol. - While the liver metabolizes steroids extensively, the specific enzymatic inactivation of cortisol to cortisone occurs mainly in the kidney. *Incorrect: Lungs* - The lungs are involved in some drug metabolism but are not a significant site for cortisol-to-cortisone conversion. - The lungs are known for converting **angiotensin I to angiotensin II** via ACE (angiotensin-converting enzyme). *Incorrect: Adrenals* - The **adrenal cortex** synthesizes and secretes cortisol from cholesterol in response to **ACTH stimulation**. - Adrenals produce cortisol but are not the primary site for its inactivation to cortisone.
Practice by Chapter
Renal Blood Flow and Glomerular Filtration
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Tubular Reabsorption and Secretion
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Concentration and Dilution of Urine
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Acid-Base Regulation by the Kidneys
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Sodium and Water Balance
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Potassium Regulation
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Calcium and Phosphate Handling
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Micturition Physiology
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Renal Function Tests
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Integrative Responses to Fluid Challenges
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