Renal Physiology Practice Questions

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: 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: 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: 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.

Q: Which one of the following substances, actively transported by tubular cells, has the highest tubular transport maximum?

Glucose. ***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.

Q: Least reabsorption out of the following in the nephron is

Urea. ***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.

Q: According to the myogenic hypothesis of renal autoregulation, the afferent arterioles contract in response to stretch induced by what?

Opening of Ca2+ channels. ***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.

Q: Which substance has the least renal clearance?

Glucose. ***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.

Q: Which substance is not absorbed in the loop of Henle?

Urea. ***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 1

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 2

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 3

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 4

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 5

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

Question 6

Which one of the following substances, actively transported by tubular cells, has the highest tubular transport maximum?

Accepted Answer

Glucose

Answer

Large plasma proteins

Answer

Myoglobin

Answer

Sodium

Question 7

Least reabsorption out of the following in the nephron is

Accepted Answer

Urea

Answer

Glucose

Answer

Na +

Answer

HCO3-

Question 8

According to the myogenic hypothesis of renal autoregulation, the afferent arterioles contract in response to stretch induced by what?

Accepted Answer

Opening of Ca2+ channels

Answer

No release

Answer

Adenosine release

Answer

Noradrenaline release

Question 9

Which substance has the least renal clearance?

Accepted Answer

Glucose

Answer

Inulin

Answer

Urea

Answer

Creatinine

Question 10

Which substance is not absorbed in the loop of Henle?

Accepted Answer

Urea

Answer

Cl-

Answer

K+

Answer

Na+

Renal Physiology — MCQs

Renal Physiology — MCQs

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