Renal Physiology Practice Questions

Q: A patient has a blood glucose level of $200 \mathrm{mg} \%$ and GFR of 90. The transport maximum of the patient is as shown in the picture given below. What is the amount of glucose excreted?

30 mg/min. ***30 mg/min*** - The amount of **glucose filtered** by the kidneys is calculated as GFR × plasma glucose concentration, which is 90 mL/min × 2 mg/mL = **180 mg/min**. - The **transport maximum (Tm)** for glucose is 150 mg/min, meaning the kidneys can reabsorb a maximum of 150 mg/min of glucose per minute. - Since more glucose is filtered (180 mg/min) than can be reabsorbed (150 mg/min), the excess is excreted in urine. - **Glucose excreted** = Filtered - Reabsorbed = 180 mg/min - 150 mg/min = **30 mg/min**. *Incorrect: 50 mg/min* - This would be incorrect as it doesn't match the calculation based on the given values. *Incorrect: 90 mg/min* - This represents the GFR value, not the amount of glucose excreted. *Incorrect: 180 mg/min* - This represents the total amount of glucose filtered, not the amount excreted after reabsorption.

Q: Which of the following will occur on efferent arteriolar constriction?

Increased GFR. ***Increased GFR*** - **Efferent arteriolar constriction** increases resistance to blood flow out of the glomerulus, causing blood to 'back up' and increase the **glomerular hydrostatic pressure (PGC)** - Higher **glomerular hydrostatic pressure** leads to increased net filtration pressure, which directly results in **increased glomerular filtration rate (GFR)** - This is the **primary and most clinically significant effect** of efferent arteriolar constriction - The increased PGC helps maintain GFR even when renal blood flow decreases slightly *Decreased GFR* - This is incorrect because **efferent arteriolar constriction** increases glomerular hydrostatic pressure, thereby **increasing GFR**, not decreasing it - Only severe efferent constriction that critically reduces renal plasma flow would eventually decrease GFR - The immediate and primary effect is always an increase in GFR *Increase flow in vasa recta* - This is incorrect; **efferent arteriolar constriction** actually **decreases** flow to the vasa recta - The constriction reduces blood flow exiting the glomerulus, which means less blood reaches the downstream peritubular capillaries and vasa recta - This decreased perfusion of the vasa recta can enhance urine concentration by reducing washout of the medullary concentration gradient *Decreased flow in vasa recta* - While this is physiologically true (efferent constriction does reduce peritubular and vasa recta blood flow), it is **not the primary or most significant effect** being tested - In the context of efferent arteriolar constriction, the **increased GFR** is the dominant and most clinically relevant consequence - The question asks "which will occur" expecting the primary hemodynamic effect on glomerular function - Decreased vasa recta flow is a secondary consequence, whereas increased GFR is the direct and immediate result

Q: All of the following substances have decreased concentration on the luminal side of the proximal convoluted tubule except:

Chloride. ***Chloride*** - As **water and other solutes** are reabsorbed from the proximal tubule, the concentration of **chloride** actually increases in the remaining luminal fluid due to continued water reabsorption. - This increased luminal **chloride concentration** then drives passive reabsorption of chloride later in the tubule. *Glucose* - **Glucose** is almost completely reabsorbed from the tubular lumen by **secondary active transport** in the early part of the proximal tubule. - Therefore, its concentration in the remaining luminal fluid rapidly decreases. *Amino acids* - Similar to glucose, **amino acids** are extensively reabsorbed by **secondary active transport** mechanisms in the proximal tubule. - Consequently, their luminal concentration significantly decreases. *Bicarbonate* - Most **bicarbonate** is reabsorbed in the proximal tubule through a process involving **carbonic anhydrase**, converting it to CO2 and water, which then diffuse into the cell. - This efficient reabsorption results in a substantial decrease in luminal bicarbonate concentration.

Q: Which region of the nephron reabsorbs the highest percentage of filtered bicarbonate?

Proximal tubule. ***Proximal tubule*** - The **proximal convoluted tubule (PCT)** reabsorbs approximately 80-90% of the **filtered bicarbonate** through a process involving **carbonic anhydrase** and the **Na+/H+ exchanger**. - This vital function ensures that the majority of bicarbonate, a key buffer, is returned to the blood to maintain **acid-base balance**. *Collecting duct* - While the collecting duct does have the ability to reabsorb and secrete bicarbonate, its contribution is minor compared to the PCT, primarily for fine-tuning acid-base balance. - Cells in the collecting duct, particularly **Type A intercalated cells**, are important for secreting acid (H+) in acidosis and therefore reabsorbing bicarbonate, but not the bulk of it. *Thick ascending limb* - The primary role of the **thick ascending limb** is the reabsorption of **sodium**, **potassium**, and **chloride** to create a concentrated interstitium, not significant bicarbonate reabsorption. - It is largely impermeable to water and is relatively impermeable to bicarbonate. *Distal tubule* - The **distal convoluted tubule (DCT)** reabsorbs a small percentage of filtered bicarbonate, but its main role is regulated reabsorption of **sodium** and **calcium**, and secretion of **potassium** and **hydrogen ions**. - Its contribution to bicarbonate reabsorption is much less significant than that of the proximal tubule.

Q: Which mechanism primarily regulates sodium reabsorption in the collecting duct?

Aldosterone. ***Aldosterone*** - **Aldosterone** is the primary hormone that stimulates **sodium reabsorption** and **potassium secretion** in the principal cells of the collecting duct. - It acts by increasing the synthesis and activity of **ENaC channels** on the apical membrane and **Na+/K+-ATPase pumps** on the basolateral membrane. *Glomerulotubular balance* - **Glomerulotubular balance** refers to the mechanism by which the **proximal tubule** reabsorbs a constant fraction of the filtered load, regardless of changes in glomerular filtration rate (GFR). - This mechanism maintains a relatively constant delivery of fluid and solutes to downstream segments but does not primarily regulate sodium in the collecting duct. *Atrial natriuretic peptide* - **Atrial natriuretic peptide (ANP)** primarily **inhibits sodium reabsorption** in the collecting duct, leading to **natriuresis** and **diuresis**, which is the opposite of sodium reabsorption. - ANP is released in response to atrial stretch, indicating increased blood volume. *Antidiuretic hormone* - **Antidiuretic hormone (ADH)** primarily regulates **water reabsorption** in the collecting duct by increasing the insertion of **aquaporin-2 channels** into the apical membrane, making the collecting duct permeable to water. - While ADH can indirectly affect sodium concentration by influencing water movement, it does not directly regulate sodium transport to the same extent as aldosterone.

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.

Question 1

A patient has a blood glucose level of $200 \mathrm{mg} \%$ and GFR of 90. The transport maximum of the patient is as shown in the picture given below. What is the amount of glucose excreted?

Accepted Answer

30 mg/min

Answer

50 mg/min

Answer

90 mg/min

Answer

180 mg/min

Question 2

Which of the following will occur on efferent arteriolar constriction?

Accepted Answer

Increased GFR

Answer

Decreased GFR

Answer

Increase flow in vasa recta

Answer

Decreased flow in vasa recta

Question 3

All of the following substances have decreased concentration on the luminal side of the proximal convoluted tubule except:

Accepted Answer

Chloride

Answer

Glucose

Answer

Amino acids

Answer

Bicarbonate

Question 4

Which region of the nephron reabsorbs the highest percentage of filtered bicarbonate?

Accepted Answer

Proximal tubule

Answer

Collecting duct

Answer

Thick ascending limb

Answer

Distal tubule

Question 5

Which mechanism primarily regulates sodium reabsorption in the collecting duct?

Accepted Answer

Aldosterone

Answer

Glomerulotubular balance

Answer

Atrial natriuretic peptide

Answer

Antidiuretic hormone

Question 6

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 7

Which factor most strongly influences protein filtration at the glomerulus?

Accepted Answer

Molecular size

Answer

Electrical charge

Answer

Shape

Answer

Temperature

Question 8

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

Accepted Answer

ADH

Answer

EPO

Answer

PTH

Answer

Aldosterone

Answer

ANP

Question 9

What is the best stimulus for release of vasopressin?

Accepted Answer

Hyperosmolality of extracellular fluid

Answer

Hypotension

Answer

Hypertension

Answer

Decreased plasma volume

Question 10

NaCl symporter is present in which part of the nephron?

Accepted Answer

DCT

Answer

PCT

Answer

Loop of Henle

Answer

Collecting duct

Renal Physiology — MCQs

Renal Physiology — MCQs

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