Renal Physiology Practice Questions

Q: In the following Pressure-Volume curve of the bladder, what is the physiological basis for the segment labeled 1b?

Law of Laplace. ***Law of Laplace*** - Segment 1b on a bladder pressure-volume curve represents the **filling phase** of the bladder (cystometrogram), where the bladder wall exhibits **high compliance**. - This allows the bladder to accommodate increasing **volumes of urine** with minimal change in **intravesical pressure**, a property directly related to the **Law of Laplace** (P = 2T/r), which explains how the bladder wall tension adjusts to maintain low pressure even as the radius increases. *Sympathetic control* - Sympathetic stimulation occurs primarily during the **storage (filling) phase** via the **hypogastric nerve**, which promotes relaxation of the **detrusor muscle** (beta-3 receptors) and contraction of the **internal urethral sphincter** (alpha-1 receptors). - While the sympathetic system facilitates storage, segment 1b specifically describes the **passive mechanical property** of the bladder wall rather than the neural input itself. *Parasympathetic control* - Parasympathetic activation is the primary driver of the **voiding (micturition) phase**, mediated by the **pelvic splanchnic nerves** releasing acetylcholine onto **M3 receptors**. - This control leads to active **detrusor contraction** and a sharp rise in **intravesical pressure**, which corresponds to a different phase of the curve than the compliant filling phase. *Urethral valve* - The **urethra** and its associated sphincters act as the outlet mechanism, but they do not dictate the passive pressure-volume relationship of the **bladder wall** itself during filling. - The function of the urethral sphincter is to maintain **continence** to prevent leakage, rather than determining the **compliance** of the bladder reservoir.

Q: Which structure in the nephron functions as the counter-current multiplier to help concentrate urine?

Thick ascending loop of Henle. ***Thick ascending loop of Henle*** - This segment actively reabsorbs **sodium, potassium, and chloride** via the **NKCC2 cotransporter** while being **impermeable to water**, which creates the high medullary interstitial osmolarity required for urine concentration. - The active transport of solutes in this segment provides the driving force for the **counter-current multiplier system**, allowing the kidney to produce urine that is hypertonic to plasma. *Thin ascending loop of Henle* - This segment is **passive** in its function, allowing for the diffusion of **sodium and chloride** along their concentration gradients due to the high osmolarity of the surrounding interstitium. - It does not actively transport solutes and therefore lacks the metabolic engine to

Q: Damage to the glomerular capillary wall causes which of the following to appear in urine?

Proteins. ***Proteins*** - The **glomerular capillary wall** serves as a size-selective barrier, normally preventing the filtration of large molecules like **plasma proteins (albumin)**. - Damage to the glomerular basement membrane or podocytes (as seen in **glomerulonephritis** or **nephrotic syndrome**) increases permeability, leading to the leakage of proteins into the urine, a condition known as **proteinuria**. *Uric acid* - Uric acid is typically elevated due to **metabolic disorders** like gout or high cell turnover states, rather than structural glomerular damage. - While uric acid crystals can form in acidic urine, their presence is not a diagnostic marker of **glomerular capillary wall injury**. *Glucose* - Glucose appears in the urine (**glycosuria**) when blood glucose levels exceed the **renal threshold** for reabsorption in the **proximal convoluted tubule**. - This is primarily a marker of **hyperglycemia** or tubular dysfunction, not glomerular filtration barrier integrity. *Ketones* - The presence of ketones in urine (**ketonuria**) indicates increased **fat metabolism**, often seen in **diabetic ketoacidosis** or prolonged fasting. - Ketones are small molecules that are freely filtered by the glomerulus and their presence is not related to damage of the capillary wall.

Q: Which physiological abnormality increases renin secretion?

Reduced renal perfusion pressure. ***Reduced renal perfusion pressure*** - Detected by **intrarenal baroreceptors** in the **afferent arteriole**, this drop in pressure directly stimulates the juxtaglomerular cells to release renin. - This is a critical compensatory mechanism to activate the **Renin-Angiotensin-Aldosterone System (RAAS)** to restore blood pressure and organ perfusion. *Increased extracellular fluid volume* - Expansion of volume suppresses renin release through both **atrial natriuretic peptide (ANP)** release and increased renal perfusion. - The body senses this as adequate pressure, leading to a down-regulation of the RAAS pathway to promote sodium excretion. *Elevated renal perfusion pressure* - High pressures stretch the **afferent arterioles**, which provides negative feedback to inhibit renin secretion. - This works in opposition to the release mechanism, ensuring systemic arterial pressure does not become excessively high under normal physiological conditions. *Elevated sodium load to macula densa* - Increased sodium-chloride delivery is sensed by the **macula densa** via the **NKCC2 co-transporter**, causing an influx of chloride that triggers the release of adenosine, which inhibits renin release. - This process is part of **tubuloglomerular feedback**, intended to regulate the glomerular filtration rate by decreasing renin when sodium delivery is high.

Q: What is the clearance of a substance if its concentration in plasma is 10 mg%, concentration in urine is 100 mg%, and urine flow is 2 ml/min?

20 ml/min. ***20 ml/min*** - Renal clearance ($C$) is calculated using the formula **$C = (U \times V) / P$**, where $U$ is urine concentration, $V$ is urine flow rate, and $P$ is plasma concentration. - By substituting the given values: **$(100\, mg\% \times 2\, ml/min) / 10\, mg\% = 20\, ml/min$**, which represents the volume of plasma cleared of the substance per minute. *0.02 ml/min* - This value is significantly lower than the calculated result and likely stems from a **calculation error** involving incorrect decimal placement. - It does not follow the standard **clearance equation** logic based on the provided concentrations and flow rate. *0.2 ml/min* - This incorrect value could result from a **calculation error** such as dividing $P$ by $U$ instead of $U$ by $P$. - It fails to account for the fact that the substance is **concentrated 10-fold** in the urine compared to the plasma. *2 ml/min* - This result would be obtained if the **U/P ratio** was 1, meaning the urine and plasma concentrations were identical. - Since the urine concentration is **10 times higher** than the plasma concentration, the clearance must be higher than the urine flow rate itself.

Q: Rapid diffusion of water across cell membranes depends on the presence of water channels, called aquaporins. Which aquaporin is present in the proximal convoluted tubule?

Aquaporin 1. **Explanation:** The correct answer is **Aquaporin 1 (AQP1)**. In the kidney, approximately 65% of filtered water is reabsorbed in the **Proximal Convoluted Tubule (PCT)**. This rapid, constitutive movement of water occurs via AQP1 channels located on both the apical and basolateral membranes, as well as the descending limb of the Loop of Henle. Unlike other segments, AQP1 in the PCT is **not regulated by ADH** (Vasopressin), ensuring constant water reabsorption coupled with solute transport. **Analysis of Incorrect Options:** * **Aquaporin 2 (AQP2):** Found exclusively in the **principal cells** of the collecting ducts. It is the only aquaporin regulated by **ADH**. ADH binding to V2 receptors causes the insertion of AQP2 into the apical membrane. * **Aquaporin 5 (AQP5):** Primarily located in **secretory glands** (salivary, lacrimal, and sweat glands) and alveolar type I cells in the lungs. It is not a major renal aquaporin. * **Aquaporin 9 (AQP9):** Functions as an "aquaglyceroporin" (transporting water and glycerol) and is mainly expressed in the **liver**, leukocytes, and brain. **High-Yield NEET-PG Pearls:** * **AQP1:** PCT and Thin Descending Limb (responsible for "obligatory" water reabsorption). * **AQP2:** Apical membrane of Collecting Duct (target of ADH; deficiency causes **Nephrogenic Diabetes Insipidus**). * **AQP3 & AQP4:** Basolateral membrane of Collecting Duct (provide the exit pathway for water into the interstitium). * **Mnemonic:** "AQP**1** is **First** (PCT), AQP**2** is **Two**-wards the end (Collecting Duct)."

Question 1

In the following Pressure-Volume curve of the bladder, what is the physiological basis for the segment labeled 1b?

Accepted Answer

Law of Laplace

Answer

Sympathetic control

Answer

Parasympathetic control

Answer

Urethral valve

Question 2

Which structure in the nephron functions as the counter-current multiplier to help concentrate
urine?

Accepted Answer

Thick ascending loop of Henle

Answer

Thin ascending loop of Henle

Answer

Collecting duct

Answer

Vasa recta

Question 3

In a graph of glomerular filterability vs. effective molecular radius, if X and Y are two molecules,
which statement is correct?

1. Y has more filterability than X
2. Y has more filterability than Z
3. X has less filterability than Y
4. X represents positively charged molecules

Accepted Answer

3 and 4

Answer

1 and 2

Answer

2 and 3

Answer

1 and 4

Question 4

Damage to the glomerular capillary wall causes which of the following to appear in urine?

Accepted Answer

Proteins

Answer

Uric acid

Answer

Glucose

Answer

Ketones

Question 5

Which physiological abnormality increases renin secretion?

Accepted Answer

Reduced renal perfusion pressure

Answer

Increased extracellular fluid volume

Answer

Elevated renal perfusion pressure

Answer

Elevated sodium load to macula densa

Question 6

A patient with chronic heart failure shows elevated ADH levels despite low serum osmolality (270 mOsm/kg) and hyponatremia (125 mEq/L). Critically evaluating this paradox, which represents the most physiologically sound explanation?

Accepted Answer

Non-osmotic stimuli (baroreceptors sensing decreased effective volume) override osmotic inhibition

Answer

Osmoreceptor dysfunction causing inappropriate ADH secretion

Answer

Syndrome of inappropriate ADH secretion (SIADH) from cardiac medications

Answer

Primary polydipsia causing dilutional hyponatremia with secondary ADH elevation

Question 7

What is the clearance of a substance if its concentration in plasma is 10 mg%, concentration in urine is 100 mg%, and urine flow is 2 ml/min?

Accepted Answer

20 ml/min

Answer

0.02 ml/min

Answer

0.2 ml/min

Answer

2 ml/min

Question 8

Rapid diffusion of water across cell membranes depends on the presence of water channels, called aquaporins. Which aquaporin is present in the proximal convoluted tubule?

Accepted Answer

Aquaporin 1

Answer

Aquaporin 2

Answer

Aquaporin 5

Answer

Aquaporin 9

Question 9

Which protein in the glomerular basement membrane (GBM) is responsible for charge-dependent filtration?

Accepted Answer

Proteoglycan

Answer

Albumin

Answer

Collagen type IV

Answer

Fibronectin

Question 10

Which is the most sensitive index for renal tubular function?

Accepted Answer

Specific gravity of urine

Answer

Blood urea

Answer

Glomerular Filtration Rate (GFR)

Answer

Creatinine clearance

Renal Physiology — MCQs

Renal Physiology — MCQs

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