Renal Physiology Practice Questions

Q: Which transport mechanism is primarily responsible for calcium reabsorption in the proximal tubule?

Paracellular transport. ***Paracellular transport*** - In the **proximal tubule**, approximately 60-70% of filtered calcium is reabsorbed primarily through the **paracellular pathway**, driven by the electrochemical gradient and solvent drag. - This transport occurs between cells, moving through the **tight junctions**, and is passive, following the reabsorption of water. *Facilitated diffusion* - While a type of passive transport, **facilitated diffusion** typically involves membrane proteins and occurs across the cell membrane, not primarily between cells in the proximal tubule for calcium. - This mechanism is prominent for calcium reabsorption in other nephron segments like the **distal convoluted tubule** via **TRPV5/6 channels**, but not the main route in the proximal tubule. *Active transport* - **Active transport** of calcium, mainly via **calcium ATPase** and the **Na+/Ca2+ exchanger**, occurs across the luminal and basolateral membranes, respectively, in specific nephron segments. - However, in the **proximal tubule**, the bulk of calcium reabsorption is passive and paracellular, not ATP-dependent active transport across cell membranes. *Antiport with sodium* - The **Na+/Ca2+ exchanger (NCX)** is an antiport mechanism that plays a crucial role in extruding calcium from the cell into the interstitium, particularly in the basolateral membrane of the distal tubule. - However, it is not the primary mechanism for overall tubular reabsorption of calcium in the **proximal tubule**, where paracellular movement dominates.

Q: Which factor most strongly influences protein filtration at the glomerulus?

Molecular size. ***Molecular size*** - The glomerular filtration barrier, particularly the **slit diaphragms** between podocytes, acts as a size-selective filter, restricting the passage of larger molecules. - Proteins like **albumin** (molecular radius ~36 Å, molecular weight ~69 kDa) are significantly large, making them difficult to pass through the filtration barrier. - Size selectivity is the **primary and most important** factor in protein filtration. *Electrical charge* - The glomerular basement membrane contains **negatively charged proteoglycans** (heparan sulfate), which repel negatively charged proteins like albumin, contributing to their retention. - While important, the role of electrical charge is **secondary** to molecular size in preventing the bulk passage of most proteins. *Shape* - While abnormal protein shapes (e.g., **amyloid fibrils**) can impact filtration in specific disease states, the typical physiological filtration of most proteins is primarily governed by size and charge. - The inherent shape of normal globular proteins plays a less direct role compared to their overall size. *Temperature* - **Physiological temperature** is relatively constant in the body and does not directly influence the molecular interactions and physical properties of the glomerular filtration barrier in a way that significantly alters protein filtration. - Temperature changes would lead to denaturation or aggregation, which are not the primary determinants of normal protein filtration.

Q: Which hormone's secretion is primarily stimulated by increased plasma osmolality?

ADH. ***ADH*** - **Antidiuretic hormone (ADH)**, also known as vasopressin, is primarily released in response to an increase in **plasma osmolality**. - Its main function is to promote water reabsorption in the kidneys, thereby decreasing osmolality and concentrating urine. *EPO* - **Erythropoietin (EPO)** is a hormone primarily produced by the kidneys in response to **hypoxia** (low oxygen levels), not increased plasma osmolality. - It stimulates the production of **red blood cells** in the bone marrow. *PTH* - **Parathyroid hormone (PTH)** regulates **calcium** and phosphate levels in the blood, primarily stimulated by low plasma calcium concentrations. - It does not directly respond to changes in plasma osmolality. *Aldosterone* - **Aldosterone** is a mineralocorticoid hormone involved in regulating **blood pressure** and electrolyte balance, particularly sodium and potassium. - Its secretion is primarily stimulated by the **renin-angiotensin-aldosterone system** in response to low blood volume or pressure, and high potassium levels, not plasma osmolality.

Q: What is the best stimulus for release of vasopressin?

Hyperosmolality of extracellular fluid. ***Hyperosmolality of extracellular fluid*** - **Hyperosmolality** is sensed by **osmoreceptors** in the hypothalamus, which then stimulate the release of vasopressin (ADH). - This response is crucial for **water conservation** to dilute the extracellular fluid and restore normal osmolality. *Hypotension* - While hypotension does stimulate vasopressin release, its effect is less potent than that of hyperosmolality in terms of triggering release. - Baroreceptors sense a decrease in blood pressure, leading to an increase in **ADH** to help maintain blood volume and pressure. *Hypertension* - **Hypertension** would typically inhibit vasopressin release, as the body would attempt to excrete more water to lower blood volume and pressure. - Increased blood pressure signals stretch receptors, leading to a decrease in **ADH** secretion. *Decreased plasma volume* - A decrease in **plasma volume** (hypovolemia) also stimulates ADH release, but this is often accompanied by changes in osmolality. - The primary stimulus for ADH is usually the resulting **increase in plasma osmolality** due to water loss, or significant drops in blood pressure detected by baroreceptors.

Q: NaCl symporter is present in which part of the nephron?

DCT. ***DCT*** - The **NaCl symporter** (also known as the **Na-Cl co-transporter** or NCC) is located in the **luminal membrane** of cells in the **distal convoluted tubule (DCT)**. - This transporter is responsible for reabsorbing approximately 5-10% of filtered sodium and chloride, and it is the target of **thiazide diuretics**. *PCT* - The **proximal convoluted tubule (PCT)** is responsible for bulk reabsorption of Na+ through various mechanisms, including Na+/H+ exchangers and Na+-glucose co-transporters, but not the specific NaCl symporter found in the DCT. - While significant NaCl reabsorption occurs here, it is primarily driven by different transport proteins. *Loop of Henle* - The **thick ascending limb of the loop of Henle** uses the **Na-K-2Cl co-transporter (NKCC2)** for Na+ reabsorption, which is distinct from the NaCl symporter. - This segment is the target for **loop diuretics**. *Collecting duct* - The **collecting duct** reabsorbs Na+ primarily through the **epithelial sodium channel (ENaC)**, which is regulated by aldosterone. - While reabsorption of sodium occurs, the specific NaCl symporter is not present in this segment.

Q: What is the principal stimulus for vasopressin secretion?

Hyperosmolality. ***Hyperosmolality*** - An increase in **plasma osmolality**, even by a small percentage (1-2%), is the **most potent and PRINCIPAL stimulus** for ADH (vasopressin) release. - This is detected by **osmoreceptors** in the hypothalamus (particularly in the organum vasculosum of the lamina terminalis), which are extremely sensitive and respond rapidly. - This response is the primary mechanism for day-to-day regulation of water balance. *Hypovolemia* - A significant decrease in **blood volume** (typically >10%) also stimulates vasopressin release, but it is **much less sensitive** than hyperosmolality and serves as a secondary/backup mechanism. - This response is mediated by **baroreceptors** in the carotid sinuses and aortic arch. - Only activated during substantial volume loss (hemorrhage, severe dehydration). *Hypoosmolality* - **Decreased plasma osmolality** actively **inhibits** vasopressin secretion as the body aims to excrete excess water. - This helps to prevent overhydration and maintain proper fluid balance. *Hypervolemia* - **Increased blood volume** (hypervolemia) **inhibits** vasopressin secretion, as the body needs to excrete excess fluid. - This contributes to diuresis and the lowering of blood pressure.

Q: A healthy 22-year-old female medical student with normal kidneys decreases her sodium intake by 50% for a period of 2 months. Which of the following parameters is expected to increase in response to the reduction in sodium intake?

Renin release. ***Renin release*** - A reduction in **sodium intake** leads to decreased extracellular fluid volume and **reduced renal perfusion pressure**, which stimulates **renin release** from the juxtaglomerular cells. - Renin initiates the **renin-angiotensin-aldosterone system (RAAS)**, leading to **angiotensin II** formation and increased **aldosterone** secretion, aimed at sodium and water retention. *Atrial natriuretic peptide release* - **Atrial natriuretic peptide (ANP)** release is stimulated by **atrial stretch** due to increased blood volume and pressure, which would decrease with reduced sodium intake. - Therefore, ANP release would likely **decrease** or remain unchanged, not increase, in response to chronic sodium restriction. *Extracellular fluid volume* - A decrease in sodium intake directly leads to a **reduction in total body sodium**, which is the primary determinant of **extracellular fluid volume**. - The body attempts to maintain fluid balance, but chronic sodium restriction will ultimately lead to a **decrease** in extracellular fluid volume as the kidneys excrete less water to match the lower sodium intake. *Arterial pressure* - Reduced sodium intake typically leads to a **decrease in extracellular fluid volume** and **cardiac output**, which in turn causes a **reduction in arterial blood pressure**. - The activation of the RAAS aims to mitigate this drop but usually does not fully compensate to increase pressure above baseline in this scenario.

Q: Escape phenomenon in mineralocorticoid excess occurs due to:

ANP (Atrial Natriuretic Peptide). ***ANP (Atrial Natriuretic Peptide)*** - The **escape phenomenon** in mineralocorticoid excess is an adaptive mechanism where initial sodium and water retention is followed by a return to near-normal sodium balance, primarily due to increased release of **ANP**. - **ANP** promotes natriuresis (sodium excretion) and diuresis (water excretion), counteracting the effects of prolonged mineralocorticoid action. *Angiotensin II* - **Angiotensin II** stimulates aldosterone secretion and promotes sodium and water reabsorption, directly opposing the "escape" from mineralocorticoid-induced fluid retention. - Its actions would exacerbate, rather than mitigate, the effects of mineralocorticoid excess. *Renin* - In mineralocorticoid excess, high sodium and fluid volume typically **suppress renin secretion**, which would lead to decreased angiotensin II and aldosterone production. - Suppressed renin is a *consequence* of mineralocorticoid excess, not the mechanism for escape. *Mineralocorticoid-like action of cortisol* - While **cortisol** can exert mineralocorticoid effects, especially at high concentrations or in conditions like apparent mineralocorticoid excess, it would perpetuate rather than resolve the issues of sodium and water retention. - The escape phenomenon describes the body's adaptation *despite* the continued presence of mineralocorticoid activity.

Q: Parathormone increases calcium reabsorption by acting at -

DCT. ***DCT*** - Parathormone (PTH) primarily increases **calcium reabsorption** in the kidneys by acting on the **distal convoluted tubule (DCT)**. - This action involves stimulating the activity of **calcium channels** and pumps in the apical and basolateral membranes of DCT cells. *Loop of Henle* - The loop of Henle is mainly involved in establishing the **osmotic gradient** in the kidney and reabsorbing water and some ions like sodium and chloride, but not significantly calcium under PTH control. - While some calcium reabsorption occurs in the thick ascending limb, it is not the primary site of **PTH-mediated calcium absorption**. *PCT* - The **proximal convoluted tubule (PCT)** is the primary site for reabsorbing filtered substances like glucose, amino acids, and a large portion of sodium and water. - While about 60-70% of filtered calcium is reabsorbed here, this process is largely **PTH-independent**. *Collecting duct* - The collecting duct is mainly involved in fine-tuning water reabsorption (controlled by **ADH**) and potassium and acid-base balance. - It plays a minor role in calcium reabsorption, and its activity is not a primary target for **PTH's calcium-regulating effects**.

Question 1

Which transport mechanism is primarily responsible for calcium reabsorption in the proximal tubule?

Accepted Answer

Paracellular transport

Answer

Facilitated diffusion

Answer

Active transport

Answer

Antiport with sodium

Question 2

Which factor most strongly influences protein filtration at the glomerulus?

Accepted Answer

Molecular size

Answer

Electrical charge

Answer

Shape

Answer

Temperature

Question 3

Which hormone's secretion is primarily stimulated by increased plasma osmolality?

Accepted Answer

ADH

Answer

EPO

Answer

PTH

Answer

Aldosterone

Answer

ANP

Question 4

What is the best stimulus for release of vasopressin?

Accepted Answer

Hyperosmolality of extracellular fluid

Answer

Hypotension

Answer

Hypertension

Answer

Decreased plasma volume

Question 5

NaCl symporter is present in which part of the nephron?

Accepted Answer

DCT

Answer

PCT

Answer

Loop of Henle

Answer

Collecting duct

Question 6

What is the principal stimulus for vasopressin secretion?

Accepted Answer

Hyperosmolality

Answer

Hypovolemia

Answer

Hypoosmolality

Answer

Hypervolemia

Question 7

A healthy 22-year-old female medical student with normal kidneys decreases her sodium intake by 50% for a period of 2 months. Which of the following parameters is expected to increase in response to the reduction in sodium intake?

Accepted Answer

Renin release

Answer

Atrial natriuretic peptide release

Answer

Extracellular fluid volume

Answer

Arterial pressure

Question 8

Escape phenomenon in mineralocorticoid excess occurs due to:

Accepted Answer

ANP (Atrial Natriuretic Peptide)

Answer

Angiotensin II

Answer

Renin

Answer

Mineralocorticoid-like action of cortisol

Question 9

Which of the following rightly describes the mechanism of "Vasopressin Escape" in SIADH?

Accepted Answer

Characterized by sudden increase in urine volume with decrease in urine osmolality independent of circulating vasopressin levels.

Answer

Characterized by sudden decrease in urine volume with increase in urine osmolality independent of circulating vasopressin levels.

Answer

Characterized by sudden decrease in urine volume with increase in urine osmolality dependent on circulating vasopressin levels.

Answer

Characterized by sudden increase in urine volume with decrease in urine osmolality dependent on circulating vasopressin levels.

Question 10

Parathormone increases calcium reabsorption by acting at -

Accepted Answer

DCT

Answer

Loop of Henle

Answer

PCT

Answer

Collecting duct

Renal Physiology — MCQs

Renal Physiology — MCQs

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