Renal Physiology Practice Questions

Q: Where does potassium reabsorption primarily occur in the kidney?

Primarily in the PCT. ***Primarily in the PCT*** - The **proximal convoluted tubule (PCT)** reabsorbs the majority (65-70%) of filtered potassium, making it the **primary site** of potassium reabsorption. - This reabsorption occurs primarily through **paracellular routes** along with water and sodium reabsorption (solvent drag). - The PCT plays a crucial role in maintaining overall **potassium homeostasis** by retrieving most of the filtered load before the filtrate reaches distal segments. *In the glomerulus* - The **glomerulus** is responsible for **filtration** of blood, not reabsorption of substances. - All small solutes, including potassium, are freely filtered across the **glomerular capillaries** into Bowman's capsule. - Filtration is the first step; reabsorption occurs in tubular segments. *In the collecting duct through secretion* - The **collecting duct** is primarily involved in **potassium secretion** into the urine, regulated by aldosterone, to fine-tune potassium balance. - While some reabsorption can occur via intercalated cells, the net effect in the collecting duct is **secretion**, not reabsorption. - This is critical for regulating final urinary K+ excretion. *Limited to the descending limb of Henle* - The **descending limb of the loop of Henle** is primarily permeable to **water** but relatively impermeable to solutes. - **No significant potassium reabsorption** occurs in the descending limb. - Significant K+ reabsorption also occurs in the thick ascending limb (20-25% of filtered load), but the PCT remains the primary site.

Q: Minimum urinary output required to excrete the end products of protein metabolism is

400 mL/24 hours. ***400 mL/24 hours*** - This volume is considered the **obligatory urine output** needed to excrete the daily solute load, primarily derived from **protein metabolism**, preventing the accumulation of toxic waste products. - This minimum ensures that the kidneys can adequately perform their excretory function without causing **azotemia** or other metabolic imbalances. *200 mL/24 hours* - This volume is generally too low to effectively excrete the daily metabolic waste products from protein breakdown, often leading to **oliguria** and potential **uremic toxicity**. - A persistent urine output this low would suggest **acute kidney injury** or severe dehydration, where metabolic waste accumulation is likely. *300 mL/24 hours* - While better than 200 mL, 300 mL per 24 hours is still generally insufficient to excrete the typical daily solute load produced by protein metabolism for an average adult, risking **waste product accumulation**. - This level is closer to the definition of **oliguria**, indicating inadequate renal function or significant fluid deficit. *500 mL/24 hours* - This volume is generally sufficient for excreting metabolic waste products and is often considered a normal lower limit for fluid balance, but **400 mL/24 hours** is the established physiological minimum for solute excretion. - While 500 mL/24 hours is adequate, the question asks for the *minimum* required, and 400 mL represents the **absolute lowest functional threshold**.

Q: Glomerular filtrate contains all except what?

Albumin. ***Albumin*** - **Albumin** is a large protein that normally does not pass through the **glomerular filtration barrier** due to its size and negative charge. - Its presence in the urine (**albuminuria**) indicates **glomerular damage**. *HCO3* - **Bicarbonate ions (HCO3-)** are small enough to be freely filtered at the glomerulus. - They are crucial for **acid-base balance** and are extensively reabsorbed in the renal tubules. *Glucose* - **Glucose** is a small molecule that is freely filtered by the glomerulus. - Under normal conditions, almost all filtered glucose is reabsorbed in the **proximal tubule**. *Sodium* - **Sodium ions (Na+)** are small and are readily filtered across the glomerulus. - They play a vital role in **fluid balance** and are actively reabsorbed along the entire tubule.

Q: What is the percentage of Na+ reabsorbed from the ascending loop of Henle via the Na+–K+–2Cl– symporter?

20% Na+ – K+ – 2Cl– symporter. ***20% Na+ – K+ – 2Cl– symporter*** - Approximately **20-25%** of filtered Na+ is reabsorbed in the **thick ascending limb of the loop of Henle** primarily via the **Na+-K+-2Cl- symporter (NKCC2)**. - This symporter is crucial for concentrating the urine and creating the **medullary osmotic gradient**. *10% Na+ – H+ exchange* - **Na+-H+ exchange** (NHE3) is the primary mechanism for Na+ reabsorption in the **proximal convoluted tubule**, where about **65-70%** of filtered Na+ is reabsorbed. - This exchange is also vital for **bicarbonate reabsorption** and **pH regulation**, not the primary Na+ reabsorption in the ascending loop of Henle. *50% Na+ - K+ - 2Cl– symporter* - Reabsorption of **50% of Na+** by the Na+-K+-2Cl- symporter would be an **overestimation** of its role in the thick ascending limb. - While it's a significant route, the total Na+ reabsorption in this segment is closer to **20-25%**. *30% Na+ – Cl– symporter* - The **Na+-Cl- symporter** is the main mechanism for Na+ reabsorption in the **distal convoluted tubule**, where approximately **5-10%** of filtered Na+ is reabsorbed. - This transport mechanism is responsible for the reabsorption of Na+ and Cl- without significant water reabsorption, making this segment important for **further diluting the filtrate**.

Q: The site of action of vasopressin is

Distal tubule and collecting duct. ***Distal tubule*** - Vasopressin, also known as **antidiuretic hormone (ADH)**, primarily acts on the collecting ducts and the late distal convoluted tubule. - Its main function is to increase the **reabsorption of water** by inserting **aquaporin-2 channels** into the apical membrane of principal cells. *Ascending limb of the loop of Henle* - This segment is largely **impermeable to water** regardless of hormonal influence, focusing on active reabsorption of solutes. - Its primary role is to dilute the tubular fluid and reabsorb ions like **Na+, K+, and Cl-**. *Descending limb of the loop of Henle* - This limb is freely permeable to water but largely impermeable to solutes, allowing water to exit the tubule due to the **medullary osmotic gradient**. - Vasopressin does not significantly influence water permeability in this segment. *Proximal tubule* - The proximal tubule reabsorbs a large percentage (about 65-70%) of filtered water and solutes in a relatively unregulated manner. - Its function is largely independent of vasopressin, which acts further down the nephron to fine-tune water reabsorption.

Q: Which hormone primarily controls the reabsorption of sodium chloride in the kidneys?

Aldosterone. ***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.

Q: All of the following results in increased renin secretion except which one?

Increased amount of Na+ in DCT. ***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.

Q: Which of the following nephron segments contributes most significantly to the establishment of the medullary concentration gradient through sodium reabsorption?

Ascending limb of Henle. ***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.

Q: In a normally functioning kidney, which part of the nephron has the lowest permeability to water during antidiuresis?

Thick Ascending Limb of Loop of Henle. ***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 1

Where does potassium reabsorption primarily occur in the kidney?

Accepted Answer

Primarily in the PCT

Answer

In the glomerulus

Answer

Limited to the descending limb of Henle

Answer

In the collecting duct through secretion

Question 2

Minimum urinary output required to excrete the end products of protein metabolism is

Accepted Answer

400 mL/24 hours

Answer

200 mL/24 hours

Answer

300 mL/24 hours

Answer

500 mL/24 hours

Question 3

Which of the following statements is MOST true regarding water reabsorption in the nephron?

Accepted Answer

The bulk of water reabsorption occurs in the proximal tubule secondary to Na+ reabsorption.

Answer

Facultative reabsorption primarily occurs in the collecting ducts.

Answer

Obligatory reabsorption occurs regardless of hydration state.

Answer

Water reabsorption can vary significantly depending on the body's hydration needs.

Question 4

Glomerular filtrate contains all except what?

Accepted Answer

Albumin

Answer

Glucose

Answer

Sodium

Answer

HCO3

Question 5

What is the percentage of Na+ reabsorbed from the ascending loop of Henle via the Na+–K+–2Cl– symporter?

Accepted Answer

20% Na+ – K+ – 2Cl– symporter

Answer

10% Na+ – H+ exchange

Answer

50% Na+ - K+ - 2Cl– symporter

Answer

30% Na+ – Cl– symporter

Question 6

The site of action of vasopressin is

Accepted Answer

Distal tubule and collecting duct

Answer

Descending limb of the loop of Henle

Answer

Proximal tubule

Answer

Ascending limb of the loop of Henle

Question 7

Which hormone primarily controls the reabsorption of sodium chloride in the kidneys?

Accepted Answer

Aldosterone

Answer

Atrial Natriuretic Peptide (ANP)

Answer

Adrenaline

Answer

Antidiuretic hormone (ADH)

Question 8

All of the following results in increased renin secretion except which one?

Accepted Answer

Increased amount of Na+ in DCT

Answer

Renal ischaemia

Answer

Decreased amount of Na+ in DCT

Answer

Decreased afferent arteriolar pressure

Question 9

Which of the following nephron segments contributes most significantly to the establishment of the medullary concentration gradient through sodium reabsorption?

Accepted Answer

Ascending limb of Henle

Answer

Collecting duct

Answer

Distal tubule

Answer

Proximal tubule

Question 10

In a normally functioning kidney, which part of the nephron has the lowest permeability to water during antidiuresis?

Accepted Answer

Thick Ascending Limb of Loop of Henle

Answer

Distal Convoluted Tubule

Answer

Proximal Convoluted Tubule

Answer

Collecting Duct

Renal Physiology — MCQs

Renal Physiology — MCQs

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