Renal Physiology Practice Questions

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.

Q: Inactivation of cortisol into cortisone occurs mainly in which organ?

Kidney. ***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.

Question 1

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 2

Least reabsorption out of the following in the nephron is

Accepted Answer

Urea

Answer

Glucose

Answer

Na +

Answer

HCO3-

Question 3

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 4

Which substance has the least renal clearance?

Accepted Answer

Glucose

Answer

Inulin

Answer

Urea

Answer

Creatinine

Question 5

Which substance is not absorbed in the loop of Henle?

Accepted Answer

Urea

Answer

Cl-

Answer

K+

Answer

Na+

Question 6

Inactivation of cortisol into cortisone occurs mainly in which organ?

Accepted Answer

Kidney

Answer

Lungs

Answer

Adrenals

Answer

Liver

Renal Physiology — MCQs

Renal Physiology — MCQs

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