Renal Physiology Practice Questions

Q: Glucose is reabsorbed at:

PCT. ***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.

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

Proximal Convoluted Tubule (PCT). ***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.

Q: Activation of the renin system stimulates

Sodium retention. ***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**.

Q: What is the normal value of the filtration fraction in a healthy adult kidney?

0.2. ***0.2*** - The **filtration fraction (FF)** is the ratio of **glomerular filtration rate (GFR)** to **renal plasma flow (RPF)**. - Normal values: **GFR ≈ 125 mL/min** and **RPF ≈ 625 mL/min** - Therefore, **FF = GFR/RPF = 125/625 = 0.2 (or 20%)** - This means that **20% of the plasma entering the glomerulus is filtered** into Bowman's capsule, while 80% continues into the peritubular capillaries. *0.45* - A filtration fraction of **0.45 (45%)** is significantly higher than normal. - This would suggest **increased glomerular filtration relative to renal plasma flow**, which can occur in conditions with **efferent arteriolar constriction** or **decreased renal plasma flow**. *0.5* - A filtration fraction of **0.5 (50%)** is markedly elevated above the normal range. - This indicates that **half of the plasma** entering the glomerulus is being filtered, which can occur with **severe efferent arteriolar constriction** or in states of **reduced renal blood flow** with preserved GFR. *0.55* - A filtration fraction of **0.55 (55%)** is abnormally high. - Such an elevated FF suggests **significant efferent arteriolar vasoconstriction** or **severe reduction in renal plasma flow**, and may be seen with conditions like **renal artery stenosis** or **angiotensin II excess**.

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

Mesangium. ***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.

Q: All of the following are functions of angiotensin II, except:

Vasodilation. ***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.

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

Proteoglycan. ***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.

Q: Which of the following is the mode of action of vasopressin?

Increases water permeability in the collecting duct. ***Increases water permeability in the collecting duct*** - **Vasopressin**, also known as **antidiuretic hormone (ADH)**, primarily acts on the **collecting ducts** of the kidney. - It binds to **V2 receptors** on the basolateral membrane, activating the **cAMP-PKA pathway**, which increases the insertion of **aquaporin-2 (AQP2)** water channels into the apical membrane, significantly enhancing water permeability and subsequent reabsorption. *Promotes water reabsorption at proximal convoluted tubule* - The **proximal convoluted tubule (PCT)** is responsible for the reabsorption of approximately **65% of filtered water**, but this process is **constitutive** and **not regulated by vasopressin**. - Water reabsorption in the PCT follows the osmotic gradient created by active sodium reabsorption, independent of hormonal control. *Stimulates water secretion at loop of Henle* - The **loop of Henle** is primarily involved in establishing an **osmotic gradient** in the renal medulla, essential for concentrating urine. - The **descending limb** is permeable to water (allowing passive reabsorption, not secretion), while the **ascending limb** is impermeable to water. - Vasopressin does not stimulate water secretion in any part of the nephron. *Decreases sodium reabsorption in the distal tubule* - Vasopressin **does not decrease sodium reabsorption**; in fact, it may indirectly enhance it. - The primary regulators of sodium reabsorption in the distal tubule are **aldosterone** (increases reabsorption) and **atrial natriuretic peptide (ANP)** (decreases reabsorption). - Vasopressin's main action is on water permeability, not direct sodium handling in the distal tubule.

Q: Which of the following statements is true about the ascending limb of the loop of Henle?

Impermeable to water. ***Impermeable to water*** - The ascending limb of the loop of Henle is **impermeable to water**, which is its defining characteristic. - It actively reabsorbs **Na+, K+, and Cl-** via the **NKCC2 transporter** (target of loop diuretics like furosemide). - This solute reabsorption without water reabsorption makes the tubular fluid **hypotonic** and contributes to the **medullary osmotic gradient**. - This is a key feature distinguishing it from the water-permeable descending limb. *Receives hypotonic saline* - The ascending limb receives fluid from the descending limb that is **hypertonic** (concentrated), not hypotonic. - The descending limb is permeable to water, so water exits and the fluid becomes concentrated. - The fluid **leaving** the ascending limb becomes hypotonic due to solute reabsorption without water. *Maximum secretion of H+ ions* - Maximum **H+ secretion** occurs in the **proximal tubule** and **collecting duct** (intercalated cells). - The ascending limb's primary function is **solute reabsorption** (Na+, K+, Cl-), not acid-base regulation. - The thick ascending limb does contribute to acid-base balance but is not the primary site. *Respond to ADH* - The ascending limb does **NOT respond to ADH** (Antidiuretic Hormone). - ADH acts primarily on the **collecting ducts** and to a lesser extent the **distal convoluted tubule**. - The ascending limb lacks ADH receptors and remains impermeable to water regardless of ADH levels.

Q: What is the filtration characteristic of neutral substances that are 6 nm in size?

Not freely filtered. ***Not freely filtered*** - At **6 nm**, a neutral substance experiences **significant size restriction** at the glomerular filtration barrier - The **effective pore size** of the glomerular basement membrane and slit diaphragms allows **partial but restricted passage** at this size - Filtration coefficient (θ) at 6 nm is approximately **0.2-0.5**, meaning only 20-50% is filtered compared to small freely filtered molecules - The term **"not freely filtered"** accurately describes this substantial size-based restriction *Filtered based on size* - While technically all filtration is based on size and charge, this term is **too vague** and doesn't convey the **degree of restriction** - At 6 nm, the key point is that filtration is **significantly impaired**, not just "size-dependent" - This option fails to distinguish between molecules that pass easily vs. those that are substantially restricted *Not filtered* - This is too absolute; **some filtration does occur** at 6 nm for neutral molecules - Complete exclusion applies to molecules **>8 nm** or large plasma proteins like albumin (~7 nm diameter with negative charge) - A 6 nm neutral substance has approximately **20-50% filterability**, so it cannot be described as "not filtered" *None of the options* - Incorrect, as **"Not freely filtered"** is the precise physiological term for a 6 nm neutral substance - This accurately reflects the **partial restriction** based on molecular size relative to the glomerular pore dimensions

Question 1

Glucose is reabsorbed at:

Accepted Answer

PCT

Answer

DCT

Answer

Collecting duct

Answer

All of the options

Question 2

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

Accepted Answer

Proximal Convoluted Tubule (PCT)

Answer

Loop of Henle

Answer

Distal Convoluted Tubule (DCT)

Answer

Collecting Duct

Question 3

Activation of the renin system stimulates

Accepted Answer

Sodium retention

Answer

Magnesium excretion

Answer

Potassium excretion

Answer

Water retention

Question 4

What is the normal value of the filtration fraction in a healthy adult kidney?

Accepted Answer

0.2

Answer

0.5

Answer

0.45

Answer

0.55

Question 5

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

Accepted Answer

Mesangium

Answer

Endothelial cell

Answer

Basement membrane

Answer

Podocytes

Question 6

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

Accepted Answer

Vasodilation

Answer

Aldosterone secretion

Answer

Increased ADH secretion

Answer

Stimulation of thirst

Question 7

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

Accepted Answer

Proteoglycan

Answer

Collagen type IV

Answer

Fibronectin

Answer

Albumin

Question 8

Which of the following is the mode of action of vasopressin?

Accepted Answer

Increases water permeability in the collecting duct

Answer

Promotes water reabsorption at proximal convoluted tubule

Answer

Stimulates water secretion at loop of Henle

Answer

Decreases sodium reabsorption in the distal tubule

Question 9

Which of the following statements is true about the ascending limb of the loop of Henle?

Accepted Answer

Impermeable to water

Answer

Receives hypotonic saline

Answer

Respond to ADH

Answer

Maximum secretion of H+ ions

Question 10

What is the filtration characteristic of neutral substances that are 6 nm in size?

Accepted Answer

Not freely filtered

Answer

Not filtered

Answer

Filtered based on size

Answer

None of the options

Renal Physiology — MCQs

Renal Physiology — MCQs

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