Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 541: Which of the following nephron segments are involved in the active reabsorption of sodium ions?

A. Distal tubule
B. Collecting duct
C. Proximal tubule
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - The **proximal tubule**, **distal tubule**, and **collecting duct** all play crucial roles in the active reabsorption of **sodium ions** to maintain electrolyte balance. - While the proximal tubule reabsorbs the bulk of filtered sodium, the distal tubule and collecting duct fine-tune sodium reabsorption under **hormonal control**. *Proximal tubule* - The **proximal tubule** reabsorbs approximately **65-70%** of filtered sodium, primarily through the **Na+/K+-ATPase pump** on the basolateral membrane. - This process is largely **unregulated** and driven by the large amount of filtered sodium and the energy demands of **secondary active transport**. *Distal tubule* - The **distal tubule** reabsorbs about **5-8%** of filtered sodium, a process that is highly regulated by **aldosterone** and other hormones. - Sodium reabsorption here occurs mainly via the **Na-Cl cotransporter** and subsequent exit via the **Na+/K+-ATPase pump**. *Collecting duct* - The **collecting duct** reabsorbs about **1-5%** of filtered sodium, also under the influence of **aldosterone**, which increases the activity of **epithelial sodium channels (ENaC)**. - Sodium reabsorption in the collecting duct is critical for establishing the **medullary osmotic gradient** and for final adjustments of sodium and water balance.

Question 542: Which of the following factors primarily activates thirst?

A. Extracellular hyperosmolarity (Correct Answer)
B. Increased ANP levels
C. Increased renin levels
D. Increased angiotensin II levels

Explanation: ***Extracellular hyperosmolarity*** - **Increased osmolality** of the extracellular fluid is the most potent stimulus for thirst. - This is detected by **osmoreceptors** in the hypothalamus, leading to the sensation of thirst and the release of ADH. *Increased ANP levels* - **Atrial natriuretic peptide (ANP)** is released in response to atrial stretch, indicating increased blood volume. - ANP typically **inhibits thirst** and ADH release to reduce total body water and sodium. *Increased renin levels* - **Renin** initiates the **renin-angiotensin-aldosterone system (RAAS)**, which primarily regulates blood pressure and volume. - While ultimately leading to angiotensin II, increased renin itself is not the primary direct activator of thirst. *Increased angiotensin II levels* - **Angiotensin II** is a potent dipsogen (thirst-stimulating agent), but it is a secondary messenger. - This systemic hormone is activated by the RAAS in response to **decreased blood volume** and **blood pressure**, rather than being the primary direct trigger like hyperosmolarity.

Question 543: Which of the following conditions leads to increased secretion of renin?

A. Decreased amount of Na+ in DCT leading to secondary effects (Correct Answer)
B. Decreased amount of Na+ in PCT
C. Renal ischemia leading to perceived renal hypoperfusion
D. Narrowing of afferent arterioles leading to renal hypoperfusion

Explanation: ***Decreased amount of Na+ in DCT leading to secondary effects*** - The **macula densa** in the distal convoluted tubule (DCT) senses the amount of sodium and chloride in the filtrate. - A decrease in sodium delivery to the macula densa triggers **juxtaglomerular cells** to release **renin**. - This is the **most specific and direct mechanism** mentioned in the options, representing the tubular-mediated pathway of renin regulation. - This mechanism is particularly important in states of **volume depletion, diuretic use, and sodium restriction**. *Renal ischemia leading to perceived renal hypoperfusion* - This **IS a valid stimulus** for renin release through **baroreceptors** in the afferent arteriole wall. - Decreased renal perfusion pressure is one of the three major mechanisms stimulating renin secretion. - However, "perceived" makes this option less precise, and the macula densa mechanism is more directly stated in the correct option. *Decreased amount of Na+ in PCT* - The **proximal convoluted tubule (PCT)** is responsible for bulk reabsorption of sodium (60-70% of filtered sodium). - However, the PCT does **not have macula densa cells** and does not directly regulate renin release. - The primary sensor for sodium-mediated renin regulation is located in the **DCT at the macula densa**. *Narrowing of afferent arterioles leading to renal hypoperfusion* - Narrowing (vasoconstriction) of afferent arterioles would **decrease** glomerular capillary pressure and reduce renin release via the baroreceptor mechanism. - **Dilation** of afferent arterioles or **constriction of efferent arterioles** would be expected to maintain or increase renin release. - This option represents a scenario that would actually **inhibit** rather than stimulate renin secretion.

Question 544: Glucose is reabsorbed at:

A. PCT (Correct Answer)
B. DCT
C. Collecting duct
D. All of the options

Explanation: ***PCT*** - The **proximal convoluted tubule (PCT)** is responsible for the reabsorption of nearly all filtered glucose, amino acids, and bicarbonate. - This reabsorption is mediated by **sodium-glucose co-transporters (SGLTs)**, primarily SGLT2, located on the apical membrane of PCT cells. *DCT* - The **distal convoluted tubule (DCT)** primarily reabsorbs calcium, sodium, and chloride, and is permeable to water only in the presence of antidiuretic hormone (ADH). - It plays a role in **fine-tuning electrolyte balance** and is not involved in glucose reabsorption. *Collecting duct* - The **collecting duct** is mainly involved in water and urea reabsorption, and acid-base balance, under hormonal control. - It does not participate in the reabsorption of glucose. *All of the options* - This option is incorrect because glucose reabsorption is a specific function of the PCT and does not occur in the DCT or collecting duct.

Question 545: Which part of the nephron reabsorbs the greatest fraction of filtered water, irrespective of ADH?

A. Loop of Henle
B. Distal Convoluted Tubule (DCT)
C. Collecting Duct
D. Proximal Convoluted Tubule (PCT) (Correct Answer)

Explanation: ***Proximal Convoluted Tubule (PCT)*** - The **PCT** reabsorbs approximately **65-70%** of filtered water and solutes, making it the primary site of reabsorption in the nephron. - Water reabsorption in the PCT is **obligatory**, meaning it occurs along with solute reabsorption via osmosis and is not regulated by ADH. *Loop of Henle* - The loop of Henle reabsorbs about **15-20%** of filtered water, primarily in its **descending limb**. - While significant, this fraction is less than that reabsorbed by the PCT, and its primary role is to establish the **osmotic gradient** in the medulla. *Distal Convoluted Tubule (DCT)* - The DCT reabsorbs a relatively small percentage of filtered water, typically around **5-10%**. - Water reabsorption in the DCT is **facultative** and is primarily regulated by the presence of **ADH**. *Collecting Duct* - The collecting duct reabsorbs between **0-19%** of filtered water, depending on the body's hydration status. - Water reabsorption in the collecting duct is entirely **facultative** and is critically dependent on **ADH** to increase its permeability to water.

Question 546: Activation of the renin system stimulates

A. Sodium retention (Correct Answer)
B. Magnesium excretion
C. Potassium excretion
D. Water retention

Explanation: ***Sodium retention*** - The **renin-angiotensin-aldosterone system (RAAS)**, when activated, leads to the release of **aldosterone** from the adrenal cortex. - **Aldosterone** acts on the **principal cells of the collecting duct** to increase **sodium reabsorption**, which is the **primary and defining function** of RAAS activation. - The main physiological purpose of RAAS activation is **volume expansion and blood pressure regulation** through **sodium retention**, making this the **most appropriate answer**. *Potassium excretion* - **Aldosterone** does simultaneously stimulate **potassium excretion** at the same site (principal cells of collecting duct) where it promotes sodium reabsorption. - However, **potassium excretion** is mechanistically **coupled to sodium reabsorption** rather than being the primary goal of RAAS activation. - The **primary stimulus** for RAAS activation is to restore blood volume/pressure via **sodium (and water) retention**, not to eliminate potassium. - In the context of what RAAS "primarily stimulates," **sodium retention** is the correct answer as it represents the system's main physiological objective. *Water retention* - **Water retention** follows **sodium retention** passively due to osmotic forces, making it a **secondary consequence** rather than a direct stimulation. - **Antidiuretic hormone (ADH/vasopressin)** is the primary hormone directly regulating **water reabsorption** via aquaporin-2 channels. - Angiotensin II does stimulate ADH release, but water retention is not considered the primary direct effect of RAAS activation. *Magnesium excretion* - The **renin-angiotensin-aldosterone system** does not primarily regulate **magnesium excretion**. - **Magnesium balance** is mainly controlled by other factors acting on the **thick ascending limb of the loop of Henle** and the **distal convoluted tubule**.

Question 547: Which of the following does not form a filtration barrier in nephrons?

A. Endothelial cell
B. Basement membrane
C. Mesangium (Correct Answer)
D. Podocytes

Explanation: ***Mesangium*** - The **mesangium** consists of **mesangial cells** and **mesangial matrix**, which provide structural support to the glomerular capillaries and play a role in regulating glomerular blood flow, but do not directly participate in the filtration barrier. - While mesangial cells have phagocytic properties and secrete various substances, they are located between the glomerular capillaries and are not part of the selective filtration layers. *Podocytes* - **Podocytes** are specialized epithelial cells that form the **visceral layer** of Bowman's capsule and are a critical component of the filtration barrier. - Their **foot processes (pedicels)** interdigitate, forming **slit diaphragms** that restrict the passage of proteins and large molecules. *Endothelial cell* - The **fenestrated endothelial cells** of the glomerular capillaries form the innermost layer of the filtration barrier. - Their **fenestrations (pores)** allow the passage of plasma components while restricting blood cells. *Basement membrane* - The **glomerular basement membrane (GBM)** is a crucial component of the filtration barrier, lying between the endothelial cells and the podocytes. - This **negatively charged** layer acts as a physical and electrostatic barrier, preventing the filtration of large proteins.

Question 548: Which hormone is the primary regulator of short-term blood pressure changes?

A. Aldosterone
B. Angiotensin II
C. ADH
D. Epinephrine (Correct Answer)

Explanation: ***Epinephrine*** - **Epinephrine** (adrenaline) is released from the **adrenal medulla** during acute stress and acts within **seconds to minutes** to rapidly increase heart rate, cardiac contractility, and cause vasoconstriction in many vascular beds. - It is part of the **sympathetic "fight-or-flight" response** and provides the most immediate hormonal response to acute blood pressure changes. - While short-term BP regulation is primarily **neural** (baroreceptor reflex), epinephrine provides the fastest **hormonal** contribution to acute BP control. *Angiotensin II* - **Angiotensin II** is a potent vasoconstrictor that acts within minutes and is a key component of the **renin-angiotensin-aldosterone system (RAAS)**. - While it has rapid vasoconstrictor effects, it is classically considered more important for **medium-to-long-term blood pressure regulation** through sustained vasoconstriction and effects on sodium retention. - The RAAS system takes longer to activate compared to the immediate catecholamine release. *ADH (Vasopressin)* - **Antidiuretic Hormone (ADH)** primarily regulates **water reabsorption** in the kidney collecting ducts for osmotic balance. - At high concentrations, it can cause vasoconstriction (hence called "vasopressin"), but this is not its primary physiological role in short-term BP regulation. - Its main effect on BP is through **long-term fluid balance** rather than acute vascular changes. *Aldosterone* - **Aldosterone** acts on the kidneys to increase **sodium and water reabsorption** and potassium excretion, which increases blood volume over hours to days. - Its effects are the **slowest to manifest** among these options, making it primarily a **long-term blood pressure regulator** rather than contributing to immediate adjustments.

Question 549: All of the following are functions of angiotensin II, except:

A. Aldosterone secretion
B. Increased ADH secretion
C. Vasodilation (Correct Answer)
D. Stimulation of thirst

Explanation: ***Vasodilation*** - **Angiotensin II** is a potent **vasoconstrictor**, not a vasodilator, playing a crucial role in increasing systemic vascular resistance and blood pressure. - Its primary actions are aimed at raising blood pressure and blood volume, which are counteracted by vasodilation. *Stimulation of thirst* - **Angiotensin II** directly acts on the **hypothalamus** to stimulate thirst, promoting fluid intake to increase blood volume. - This is a key mechanism by which the **renin-angiotensin-aldosterone system (RAAS)** contributes to fluid balance. *Aldosterone secretion* - **Angiotensin II** binds to receptors in the **adrenal cortex** to stimulate the synthesis and secretion of **aldosterone**. - **Aldosterone** promotes sodium reabsorption and potassium excretion in the kidneys, leading to increased blood volume and pressure. *Increased ADH secretion* - **Angiotensin II** stimulates the release of **antidiuretic hormone (ADH)** from the posterior pituitary gland. - **ADH** increases water reabsorption in the kidneys, which helps conserve fluid and further increases blood volume and pressure.

Question 550: Which protein in the glomerular basement membrane is primarily responsible for charge-dependent filtration?

A. Collagen type IV
B. Fibronectin
C. Proteoglycan (Correct Answer)
D. Albumin

Explanation: ***Proteoglycan*** - Proteoglycans in the **glomerular basement membrane** play a crucial role in the **charge-dependent filtration** of proteins, allowing selective permeability based on size and charge [1]. - They interact with collagen and other matrix components, forming a matrix that maintains the filtration barrier's structure. *Collagen type IV* - While collagen type IV contributes to the structural integrity of the **glomerular basement membrane**, it does not have a primary role in **charge-dependent filtration**. - Its mainly structural role does not involve the selectivity of filtration based on charge characteristics. *Fibronectin* - Fibronectin acts mainly as a **cell adhesion molecule** and is not primarily responsible for the regulation of filtration charges in the glomerulus. - This protein is more involved in wound healing and tissue repair rather than in the filtration barrier of the kidneys. *Albumin* - Albumin is a **plasma protein** that is typically filtered by the glomerulus but is not a component of the **glomerular basement membrane**. - Its primary role is as a marker of proteinuria when filtration is compromised, not in the mechanism of charge-dependent filtration itself [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 905-907.

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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