Renal Physiology Practice Questions

Q: A 24-year-old woman with a large appetite for salt consumes approximately 25g of salt each day. What is the approximate amount of salt (in grams) that is excreted each day by her kidneys?

23. **Explanation** The core concept tested here is **Sodium Balance**. In a healthy individual with normal renal function, the body maintains a steady state where the total output of sodium equals the total intake. 1. **Why Option C is correct:** The patient consumes **25g** of salt daily. Under normal physiological conditions, sodium is excreted through three main routes: the kidneys (urine), the skin (sweat), and the gastrointestinal tract (feces). In a temperate environment with normal activity, approximately **90-95%** of ingested salt is excreted via the kidneys, while the remaining 5-10% is lost through sweat and feces. * Calculation: 90-95% of 25g ≈ **22.5 to 23.75g**. * Therefore, **23g** is the most accurate approximation of renal excretion to maintain homeostasis. 2. **Why other options are incorrect:** * **Option A (4g) & B (12g):** These values are significantly lower than the intake. If a person consumes 25g but only excretes 4g or 12g, they would develop massive positive sodium balance, leading to severe fluid retention, edema, and hypertension. * **Option D (50g):** This represents a negative sodium balance. Excreting more than the intake would lead to volume depletion and hyponatremia, which does not occur in a healthy individual at steady state. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Handling:** 65% of filtered sodium is reabsorbed in the Proximal Convoluted Tubule (PCT). * **Aldosterone:** The primary hormone regulating the "final touch" of sodium excretion in the distal nephron. * **Pressure Natriuresis:** An increase in arterial pressure directly increases sodium excretion to maintain balance. * **Steady State Rule:** Intake = Output. If intake increases, renal excretion must increase proportionally within 24–48 hours to prevent volume overload.

Q: In which segment of the nephron does tubular fluid have the highest osmolarity?

Henle's loop. **Explanation:** The osmolarity of tubular fluid changes significantly as it traverses the nephron due to the **countercurrent multiplier system**. **1. Why Henle’s Loop is Correct:** The highest osmolarity is reached at the **tip of the Loop of Henle** (the hairpin bend). As fluid descends the thin descending limb, it loses water to the hypertonic medullary interstitium but remains impermeable to solutes. This concentrates the tubular fluid, reaching a peak osmolarity of approximately **1200–1400 mOsm/L** in long-looped juxtamedullary nephrons. **2. Why the Other Options are Incorrect:** * **Proximal Tubule:** Fluid here is **isosmotic** to plasma (~300 mOsm/L) because water and solutes are reabsorbed in equal proportions. * **Distal Tubule:** This segment is part of the "diluting segment." Since solutes (NaCl) are reabsorbed without water, the fluid becomes **hypoosmotic** (~100 mOsm/L). * **Collecting Duct:** While the collecting duct passes through the hypertonic medulla, its primary role is regulated water reabsorption via ADH. Even under maximal ADH influence, the fluid osmolarity equilibrates with the medullary tip but does not exceed it. **High-Yield NEET-PG Pearls:** * **Descending Limb:** Permeable to water, impermeable to NaCl (Concentrating segment). * **Ascending Limb:** Impermeable to water, active reabsorption of NaCl (Diluting segment). * **Vasa Recta:** Acts as a **countercurrent exchanger** to maintain the medullary osmotic gradient without washing it out. * **Urea Recycling:** Contributes nearly 40-50% of the hyperosmolarity in the inner medullary interstitium.

Q: What is the most important buffer system present in the distal convoluted tubule?

Ammonia. **Explanation:** The primary role of the distal nephron in acid-base balance is the excretion of fixed acids ($H^+$ ions). While the bicarbonate buffer system is crucial in the extracellular fluid and proximal tubule, it is not the dominant buffer in the distal tubule because most bicarbonate has already been reabsorbed. **Why Ammonia is the Correct Answer:** The **Ammonia ($NH_3/NH_4^+$) buffer system** is the most important and physiologically flexible buffer in the distal tubule. Unlike other buffers, its synthesis can be **upregulated** in response to chronic acidosis. Ammonia is produced in the proximal tubule from Glutamine, but it exerts its major buffering effect in the collecting ducts. $NH_3$ diffuses into the tubular lumen, combines with secreted $H^+$ to form $NH_4^+$ (ammonium), which is "trapped" and excreted. This system accounts for about **60-75%** of net acid excretion. **Analysis of Incorrect Options:** * **Bicarbonate:** While it is the most important ECF buffer, 80-90% is reabsorbed in the proximal tubule. By the time fluid reaches the distal tubule, its concentration is too low to be the primary buffer. * **Phosphate:** Known as the "Titratable Acid" buffer. Although important, its concentration is fixed and cannot be increased during acidosis, making it less significant than the ammonia system. * **Protein:** Proteins are major intracellular and plasma buffers but are normally absent (or present in negligible amounts) in the tubular fluid of a healthy nephron. **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The production of ammonia from **Glutamine** by the enzyme **Glutaminase** is the key adaptive response to metabolic acidosis. * **Diffusion Trapping:** $NH_3$ is lipid-soluble and diffuses easily; $NH_4^+$ is water-soluble and trapped in the lumen (cannot diffuse back). * **Net Acid Excretion (NAE):** Calculated as $[NH_4^+ \text{ excretion} + \text{Titratable Acid}] - [\text{Bicarbonate excretion}]$.

Q: What percentage of glucose is absorbed in the proximal convoluted tubule?

100%. **Explanation:** In a healthy individual, **100% of filtered glucose is reabsorbed in the Proximal Convoluted Tubule (PCT)**. Glucose is a vital nutrient, and the kidneys are designed to ensure none is lost in the urine under normal physiological conditions. **Mechanism:** Reabsorption occurs via **Secondary Active Transport**. * **SGLT-2 (Sodium-Glucose Co-transporter 2):** Located in the early PCT (S1 segment), it reabsorbs about 90% of glucose. * **SGLT-1:** Located in the late PCT (S3 segment), it reabsorbs the remaining 10%. * Once inside the tubular cell, glucose exits into the blood via facilitated diffusion through **GLUT-2** (early PCT) and **GLUT-1** (late PCT). **Analysis of Incorrect Options:** * **B (0%):** This would imply total glycosuria, seen only in severe pathological states or specific drug actions. * **C & D (60% & 50%):** While the PCT reabsorbs roughly 65% of water and electrolytes (like Na+ and K+), it is specialized to reabsorb 100% of glucose and amino acids. **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Threshold for Glucose:** Glucose starts appearing in urine (glycosuria) when blood glucose levels exceed **180 mg/dL**. 2. **Transport Maximum ($T_m$):** The point where all glucose transporters are saturated. For men, $T_mG$ is approx. **375 mg/min**; for women, **300 mg/min**. 3. **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A modern class of anti-diabetic drugs that work by inhibiting glucose reabsorption in the PCT, purposely inducing glycosuria to lower blood sugar. 4. **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, and phosphates in the urine.

Q: A luminal cell membrane Na+ channel is the main pathway for Na+ reabsorption in which of the following cell types?

Collecting duct principal cells. **Explanation:** The correct answer is **Collecting duct principal cells**. The question asks for the cell type where Na+ reabsorption occurs via a **luminal Na+ channel** (rather than a symporter or antiporter). 1. **Why Principal Cells are correct:** In the late distal tubule and cortical collecting duct, Na+ reabsorption is mediated by the **Epithelial Sodium Channel (ENaC)** located on the apical (luminal) membrane. This is a selective channel, not a transporter. This process is electrogenic and is the primary site of action for **Aldosterone**, which increases the number and activity of ENaCs. 2. **Why other options are incorrect:** * **Proximal Tubule Cells:** Na+ reabsorption here occurs primarily via the **Na+-H+ exchanger (NHE3)** and various symporters (e.g., Na+-Glucose, Na+-Amino Acid). * **Thick Ascending Limb (TAL) Cells:** Na+ enters these cells via the **NKCC2 symporter** (Na+-K+-2Cl- cotransporter), which is the target of loop diuretics. * **Distal Convoluted Tubule (DCT) Cells:** Na+ reabsorption is mediated by the **NCC symporter** (Na+-Cl- cotransporter), which is inhibited by thiazide diuretics. **High-Yield Clinical Pearls for NEET-PG:** * **Liddle Syndrome:** A genetic mutation causing "gain of function" in ENaC, leading to hypertension, hypokalemia, and metabolic alkalosis (mimics hyperaldosteronism but with low renin/aldosterone). * **Amiloride & Triamterene:** These are potassium-sparing diuretics that specifically block the **ENaC** in the principal cells. * **ANP (Atrial Natriuretic Peptide):** Inhibits ENaC, promoting natriuresis.

Q: What is the primary function of Antidiuretic Hormone (ADH)?

Promotes water reabsorption. **Explanation:** **Antidiuretic Hormone (ADH)**, also known as Vasopressin, is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and secreted by the posterior pituitary. Its primary role is the regulation of plasma osmolality. **Why Option A is Correct:** ADH acts on the **V2 receptors** located on the basolateral membrane of the **principal cells** in the late distal tubule and collecting ducts. This triggers a cAMP-mediated signaling pathway that leads to the insertion of **Aquaporin-2 (AQP2)** water channels into the apical membrane. This increases the permeability of the collecting duct to water, allowing water to be reabsorbed down the osmotic gradient into the hypertonic renal medullary interstitium. **Why Other Options are Incorrect:** * **Option B:** While ADH results in decreased water excretion, this is a *consequence* of its primary physiological action, which is active reabsorption. * **Option C:** Sodium reabsorption is primarily regulated by **Aldosterone** (acting on ENaC channels). While ADH can have minor effects on sodium transporters (NKCC2), it is not its primary function. * **Option D:** Potassium secretion is a function of Aldosterone and tubular flow rates, not ADH. **High-Yield Clinical Pearls for NEET-PG:** * **V1 Receptors:** Located on vascular smooth muscle; cause vasoconstriction (hence the name Vasopressin). * **Diabetes Insipidus (DI):** Deficiency of ADH (Central DI) or resistance to ADH (Nephrogenic DI) leads to polyuria and dilute urine. * **SIADH:** Excessive ADH secretion leads to water retention, dilutional hyponatremia, and concentrated urine. * **Urea Recycling:** ADH also increases the permeability of the medullary collecting duct to urea (via UT-A1 transporters), which helps maintain the medullary osmotic gradient.

Q: Ethanol increases urine output by which of the following mechanisms?

Suppressing ADH secretion. ### Explanation **Mechanism of Action** The correct answer is **B. Suppressing ADH secretion**. Ethanol (alcohol) acts as a potent diuretic primarily by inhibiting the release of **Antidiuretic Hormone (ADH)**, also known as Vasopressin, from the posterior pituitary gland. Under normal conditions, ADH acts on the V2 receptors of the collecting ducts to insert **aquaporin-2 channels**, allowing water reabsorption. By suppressing ADH, ethanol prevents these channels from being inserted, leading to decreased water permeability in the distal nephron. This results in the excretion of a large volume of dilute urine (**water diuresis**). **Analysis of Incorrect Options** * **A. Inhibiting Na+ reabsorption:** This is the mechanism of action for diuretics like Furosemide or Thiazides (saluretics). Ethanol does not primarily interfere with sodium transport. * **C. Increasing glomerular filtration rate (GFR):** While minor changes in GFR can occur due to systemic effects, it is not the primary or consistent mechanism behind ethanol-induced diuresis. * **D. Suppressing H+ secretion:** This would affect acid-base balance (potentially causing acidosis) but does not directly lead to increased urine volume. **High-Yield Clinical Pearls for NEET-PG** * **Site of Action:** Ethanol acts centrally on the **supraoptic and paraventricular nuclei** of the hypothalamus. * **The "Breaking the Seal" Phenomenon:** The rapid onset of diuresis after alcohol consumption is due to the immediate drop in plasma ADH levels. * **Dehydration:** The diuresis caused by ethanol is often out of proportion to the fluid intake, leading to the cellular dehydration characteristic of a "hangover." * **Other ADH Inhibitors:** Cold exposure and ANP (Atrial Natriuretic Peptide) also inhibit ADH secretion.

Q: What is the most potent stimulus for renin release?

Hypotension. **Explanation:** Renin release is the rate-limiting step of the Renin-Angiotensin-Aldosterone System (RAAS). While multiple factors stimulate the juxtaglomerular (JG) cells, **hypotension** (specifically a decrease in renal perfusion pressure) is considered the most potent and direct stimulus. 1. **Why Hypotension is correct:** The JG cells in the afferent arteriole act as **intrarenal baroreceptors**. A drop in mean arterial pressure directly stretches these cells less, triggering an immediate and robust release of renin to restore systemic blood pressure and maintain glomerular filtration rate (GFR). 2. **Analysis of Incorrect Options:** * **Sympathetic stimulation (Option A):** Activated via $\beta_1$-adrenergic receptors on JG cells during stress or exercise. While significant, it is often a secondary response to systemic hypotension. * **Decreased NaCl in the DCT (Option B):** This is sensed by the **Macula Densa**. When NaCl delivery drops (indicating low GFR), the macula densa signals the JG cells to release renin. This is a vital feedback mechanism but is generally considered less "potent" than direct baroreceptor activation. * **Prostacyclin (Option C):** Prostaglandins ($PGE_2$ and $PGI_2$) act as local paracrine signaling molecules that stimulate renin release, but they usually act as mediators for the macula densa pathway rather than primary stimuli. **Clinical Pearls for NEET-PG:** * **Location:** Renin is synthesized and stored in the **modified smooth muscle cells** (JG cells) of the **afferent arteriole**. * **Inhibitors:** Renin release is inhibited by **Atrial Natriuretic Peptide (ANP)** and high levels of Angiotensin II (negative feedback). * **Mnemonic:** Stimuli for Renin = **"The 3 Bs"** — **B**aroreceptors (Pressure), **B**ath (NaCl content at Macula Densa), and **B**eta-1 receptors (Sympathetic).

Q: Filtration fraction can be best determined by which pair of substances?

PAH and Inulin. **Explanation:** **Filtration Fraction (FF)** is the ratio of the Glomerular Filtration Rate (GFR) to the Renal Plasma Flow (RPF). It represents the fraction of plasma entering the kidneys that is actually filtered across the glomerular capillaries into the Bowman’s space. The formula is: **FF = GFR / RPF** 1. **Why Option B is Correct:** * **Inulin** is the gold standard for measuring **GFR** because it is freely filtered but neither reabsorbed nor secreted by the renal tubules. * **Para-aminohippuric acid (PAH)** is used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and actively secreted, resulting in almost complete clearance from the plasma in a single pass through the kidney. * Therefore, the ratio of Inulin clearance to PAH clearance provides the most accurate determination of the Filtration Fraction. 2. **Why Other Options are Incorrect:** * **Options A & D (Mannitol):** While mannitol is used to measure GFR (similar to inulin), it does not measure RPF. * **Option C (Creatinine):** Creatinine is used to estimate GFR in clinical practice, but it is slightly secreted by tubules, making it less accurate than inulin. Using two GFR markers (Inulin and Creatinine) cannot determine the FF as the RPF component is missing. **High-Yield Facts for NEET-PG:** * **Normal FF:** Approximately **20%** (0.20). * **Clinical Significance:** FF increases in conditions like **Congestive Heart Failure** and **Renal Artery Stenosis** (due to a disproportionate decrease in RPF compared to GFR, maintained by Angiotensin II-mediated efferent vasoconstriction). * **Inulin Clearance = GFR; PAH Clearance = ERPF.** * **Creatinine Clearance** slightly overestimates GFR (due to tubular secretion).

Q: About ADH secretion all are true except?

Level increased by low plasma osmolality.. **Explanation:** The correct answer is **D** because it is a false statement. Antidiuretic Hormone (ADH), also known as Vasopressin, is primarily regulated by plasma osmolality. Secretion is **stimulated by high plasma osmolality** (detected by osmoreceptors in the hypothalamus) and inhibited by low plasma osmolality. When osmolality is low (dilute blood), ADH levels drop to allow the excretion of excess water. **Analysis of other options:** * **Option A:** ADH secretion increases during the **early postoperative period** as part of the body's surgical stress response. Factors like pain, anxiety, and blood loss (hypovolemia) are potent non-osmotic stimuli for ADH release. * **Option B:** ADH acts on the V2 receptors in the **Distal Convoluted Tubule (DCT)** and collecting ducts. It facilitates the insertion of Aquaporin-2 channels, thereby increasing water permeability and reabsorption. * **Option C:** ADH is a classic example of **neurosecretion**. It is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and transported axonally to the posterior pituitary (neurohypophysis) for storage and release. **High-Yield Clinical Pearls for NEET-PG:** * **Most sensitive stimulus:** A change in plasma osmolality as small as 1%. * **Most potent stimulus:** Severe hypovolemia/hypotension (though it requires a >10% change in volume). * **SIADH:** Characterized by inappropriately high ADH despite low plasma osmolality, leading to dilutional hyponatremia. * **Diabetes Insipidus:** Central (lack of ADH) or Nephrogenic (resistance to ADH), resulting in polyuria and high plasma osmolality.

Question 1

A 24-year-old woman with a large appetite for salt consumes approximately 25g of salt each day. What is the approximate amount of salt (in grams) that is excreted each day by her kidneys?

Accepted Answer

23

Answer

4

Answer

12

Answer

50

Question 2

In which segment of the nephron does tubular fluid have the highest osmolarity?

Accepted Answer

Henle's loop

Answer

Distal tubule

Answer

Proximal tubule

Answer

Collecting duct

Question 3

What is the most important buffer system present in the distal convoluted tubule?

Accepted Answer

Ammonia

Answer

Bicarbonate

Answer

Phosphate

Answer

Protein

Question 4

What percentage of glucose is absorbed in the proximal convoluted tubule?

Accepted Answer

100%

Answer

0%

Answer

60%

Answer

50%

Question 5

A luminal cell membrane Na+ channel is the main pathway for Na+ reabsorption in which of the following cell types?

Accepted Answer

Collecting duct principal cells

Answer

Proximal tubule cells

Answer

Thick ascending limb cells

Answer

Distal convoluted tubule cells

Question 6

What is the primary function of Antidiuretic Hormone (ADH)?

Accepted Answer

Promotes water reabsorption

Answer

Inhibits water excretion

Answer

Facilitates sodium absorption

Answer

Stimulates potassium secretion

Question 7

Ethanol increases urine output by which of the following mechanisms?

Accepted Answer

Suppressing ADH secretion

Answer

Inhibiting Na+ reabsorption

Answer

Increasing glomerular filtration rate

Answer

Suppressing H+ secretion

Question 8

What is the most potent stimulus for renin release?

Accepted Answer

Hypotension

Answer

Sympathetic stimulation

Answer

Decreased sodium chloride in the distal convoluted tubule

Answer

Prostacyclin

Question 9

Filtration fraction can be best determined by which pair of substances?

Accepted Answer

PAH and Inulin

Answer

Inulin and mannitol

Answer

Inulin and creatinine

Answer

PAH and mannitol

Question 10

About ADH secretion all are true except?

Accepted Answer

Level increased by low plasma osmolality.

Answer

Secretion increased in early postoperative period.

Answer

Causes increased permeability of DCT.

Answer

Neurosecretion.

Renal Physiology — MCQs

Renal Physiology — MCQs

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