Renal Physiology Practice Questions

Q: What is the effective renal blood flow in humans?

625 mL/min. ### Explanation **1. Why Option C is Correct:** The effective renal plasma flow (ERPF) is typically measured using **Para-aminohippuric acid (PAH) clearance**, as PAH is both filtered and secreted, resulting in almost complete extraction from the blood. * **The Calculation:** Total Renal Blood Flow (RBF) in a healthy 70 kg adult is approximately **1100–1200 mL/min** (about 20-25% of cardiac output). * Since the average hematocrit is 45%, the Renal Plasma Flow (RPF) is roughly 55% of the RBF. * $1100 \text{ mL/min} \times 0.55 = 605 \text{ mL/min}$. * In standard medical texts (like Guyton and Ganong), the average value for **Effective Renal Plasma Flow** is cited as **625 mL/min**. **2. Why Other Options are Incorrect:** * **Option A (425 mL/min) & B (525 mL/min):** These values are too low for a healthy adult. Such values would indicate significant renal vasoconstriction or a state of shock/hypovolemia. * **Option D (725 mL/min):** This value is higher than the physiological average for plasma flow. While RBF can increase during pregnancy or high protein intake, 625 mL/min remains the standard "textbook" value for exams. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Filtration Fraction (FF):** Calculated as GFR/RPF. Normal FF is approximately **20%** ($125/625 = 0.2$). * **PAH Clearance:** It underestimates true RPF by about 10% because some blood supplies non-secreting portions of the kidney (like the medulla and perirenal fat). Therefore, **True RPF = ERPF / 0.9**. * **Gold Standard:** While PAH measures ERPF, **Inulin clearance** is the gold standard for measuring GFR. * **Renal Oxygen Consumption:** The kidneys have the highest oxygen consumption per gram of tissue after the heart, primarily to fuel the $Na^+/K^+$ ATPase pump for sodium reabsorption.

Q: Urine osmolality in Diabetes insipidus is:

<150 mmol/L. **Explanation:** **Diabetes Insipidus (DI)** is characterized by the inability to concentrate urine due to either a deficiency of Antidiuretic Hormone (ADH/Vasopressin) from the posterior pituitary (**Central DI**) or a lack of renal response to it (**Nephrogenic DI**). 1. **Why Option A is Correct:** In a healthy individual, ADH acts on the V2 receptors of the collecting ducts to reabsorb water via aquaporin-2 channels. In DI, this mechanism fails, leading to the excretion of large volumes of highly dilute urine (polyuria). The hallmark of DI is a **low urine osmolality**, typically ** 295 mOsm/kg). 2. **Why Other Options are Incorrect:** * **<300 mmol/L:** While urine in DI is indeed less than 300 (which is roughly isotonic to plasma), this range is too broad and includes "isosthenuria" (seen in chronic renal failure), rather than the profound dilution characteristic of DI. * **<600 and <900 mmol/L:** These values represent concentrated urine. A normal kidney can concentrate urine up to 1200 mOsm/kg. Values above 600 indicate intact ADH function and concentrated urine, which contradicts a diagnosis of DI. **High-Yield Clinical Pearls for NEET-PG:** * **Water Deprivation Test:** The gold standard for diagnosis. If urine osmolality remains low despite dehydration, DI is confirmed. * **Desmopressin (DDAVP) Challenge:** Used to differentiate Central from Nephrogenic DI. Central DI shows a >50% increase in urine osmolality; Nephrogenic DI shows little to no response. * **Urine Specific Gravity:** In DI, it is typically very low (**≤ 1.005**). * **Plasma Osmolality:** In DI, plasma is hypertonic (>295 mOsm/kg) with hypernatremia, unlike Psychogenic Polydipsia where plasma is dilute.

Q: Which of the following substances are freely filtered by the kidney across glomerular capillaries?

Creatinine, HCO3-, and Glucose. ### Explanation The glomerular filtration barrier consists of the fenestrated endothelium, the glomerular basement membrane (GBM), and the podocyte slit diaphragms. Filtration is determined by two primary factors: **molecular size** and **electrical charge**. **1. Why Option A is Correct:** * **Creatinine (113 Da):** A small metabolic waste product that is freely filtered. * **Bicarbonate (HCO3-) (61 Da):** A small ion that passes easily into the Bowman’s space. * **Glucose (180 Da):** A small organic molecule that is freely filtered (and normally 100% reabsorbed in the proximal tubule). All three substances have a molecular weight well below the threshold of **70,000 Daltons** and are not significantly hindered by the negative charge of the basement membrane. **2. Why Other Options are Incorrect:** * **Options B & D (Albumin):** Albumin has a molecular weight of ~69,000 Da. While it is near the size cutoff, it is **negatively charged**. The GBM contains heparan sulfate (polyanionic), which electrostatically repels albumin, preventing its filtration. * **Option C (Globulin):** Globulins are much larger proteins (90,000 to 150,000+ Da) than albumin. Their large size and charge prevent them from crossing the filtration barrier. **High-Yield Clinical Pearls for NEET-PG:** * **Filtration Fraction:** The ratio of GFR to Renal Plasma Flow (Normal ≈ 0.2 or 20%). * **Neutral vs. Charged:** For the same molecular radius, **cationic** (positive) substances are filtered most easily, followed by neutral substances, while **anionic** (negative) substances are filtered the least. * **Minimal Change Disease:** Characterized by the loss of the negative charge on the GBM (loss of polyanions), leading to selective proteinuria (albuminuria) despite no visible structural change on light microscopy.

Q: Which segment of the nephron has the lowest osmolality under the influence of antidiuretic hormone (ADH)?

Early distal tubule. ***Early distal tubule*** - This segment is known as the **cortical diluting segment** because it actively reabsorbs solutes (Na+ and Cl-) via the **Na+-Cl- cotransporter** while being impermeable to water. - This mandatory solute removal ensures the tubular fluid is maximally diluted (hypoosmolar, often around 100 mOsm/L), a process that is independent of **ADH** levels. *Descending limb of the loop of Henle* - This segment is highly permeable to **water** but largely impermeable to solutes, causing water to flow out into the surrounding hypertonic medulla. - Consequently, the osmolality of the tubular fluid **increases** significantly as it moves down toward the loop hairpin turn, making it highly concentrated, not dilute. *Collecting duct* - The influence of **ADH** is to insert **aquaporin 2** channels into the apical membrane, making this segment highly permeable to water. - This allows massive water reabsorption out of the tubule, concentrating the urine and leading to a **high osmolality** within the tubule lumen, especially near the end. *Late distal tubule* - While the fluid here is dilute, some water reabsorption is possible in the presence of **ADH** due to ADH-sensitive aquaporins, similar to the collecting duct. - This water movement slightly increases the osmolality compared to the maximally dilute fluid produced earlier in the **early distal tubule**.

Q: What renal function is represented by the formula UV/P?

Clearance. ***Clearance*** - The formula **(U × V) / P** represents the **renal clearance** (C) of a substance, where U is the urine concentration, V is the urine flow rate, and P is the plasma concentration of that substance - Clearance measures the virtual volume of **plasma** from which a substance is completely removed by the kidney per unit of time (typically expressed in ml/min) - This is the fundamental definition and calculation of renal clearance *Filtration* - Filtration is the bulk flow of fluid and solutes from the glomerular capillaries into Bowman's capsule, quantified by the Glomerular Filtration Rate (GFR) - The net filtration rate of a substance is calculated as GFR multiplied by the plasma concentration (P), which is distinct from the clearance formula UV/P *Tubular Secretion* - Tubular secretion is the active transfer of substances from the peritubular capillaries into the tubular lumen - While UV/P is used to calculate clearance, if a substance's clearance exceeds the GFR, it indicates that net tubular secretion has occurred *Tubular Reabsorption* - Tubular reabsorption is the process where filtered solutes move from the tubular lumen back into the peritubular capillaries - If the clearance (UV/P) of a substance is significantly less than the GFR, it signifies that net reabsorption of the substance has taken place

Q: What is the typical Glomerular Filtration Rate (GFR) in a healthy adult?

125 ml/min. **Correct: 125 ml/min** - This is the accepted average value for **Glomerular Filtration Rate (GFR)** in a standard 70 kg healthy young adult - Represents the total volume of fluid filtered from plasma into Bowman's capsule per minute - Totals approximately **180 liters per day** *Incorrect: 150 ml/min* - Higher than the established average GFR (125 ml/min) - Indicative of **renal hyperfiltration**, often an early sign preceding kidney damage - Not the normal rate for a healthy adult *Incorrect: 100 ml/min* - Within the lower limits of normal for some demographics or mild age-related decline - Not the typical average GFR used in standard physiology - GFR below 90 ml/min suggests potential **renal impairment** (Stage 2 CKD) *Incorrect: 200 ml/min* - Significantly higher than physiological GFR - Suggests **hyperfiltration**, a pathological state associated with conditions like early **diabetes mellitus** - Indicates increased pressure or permeability in glomerular capillaries

Q: Which of the following statements is true about GFR? GFR-Glomerular filtration rate RPF-Renal Plasma Flow

An increase in RPF increases GFR. ***An increase in RPF increases GFR***- **GFR** is directly proportional to the amount of **plasma delivered** to the glomerular capillaries for filtration, meaning higher **Renal Plasma Flow (RPF)** generally increases GFR.- Increased RPF contributes to a higher **glomerular hydrostatic pressure** and delivers more solute/fluid load to the capillary surface area for filtration.*An increase in sympathetic function increases GFR*- Strong sympathetic activation causes generalized **vasoconstriction** of the renal blood vessels (via alpha-1 receptors), significantly reducing **RPF** and thus GFR.- This response is critical during conditions like hemorrhage or shock to conserve fluid volume by prioritizing systemic circulation over **renal filtration**.*Afferent arteriolar constriction increases GFR*- Constriction of the **afferent arteriole** restricts blood flow into the glomerulus, immediately dropping the **glomerular capillary hydrostatic pressure (PGC)**.- Since PGC is the chief driving force for filtration, afferent constriction invariably leads to a *decrease* in GFR; this mechanism is essential for **GFR autoregulation**.*A decrease in RPF increases GFR*- A decrease in **Renal Plasma Flow (RPF)** means less plasma is physically available to be filtered across the glomerular membrane per unit time.- A primary reduction in RPF, assuming stable filtration dynamics, generally results in a proportional *decrease* in the absolute **GFR**.

Q: Which among the following is true regarding creatinine clearance?

It is more than inulin clearance. ***It is more than inulin clearance*** - Creatinine clearance **overestimates GFR** by approximately 10-20% compared to inulin clearance - This is because creatinine undergoes both **glomerular filtration AND tubular secretion** - The additional secretion increases the amount of creatinine excreted, making the calculated clearance higher - **Inulin clearance** remains the gold standard as inulin is only filtered (not secreted or reabsorbed) *Equal to inulin clearance* - Incorrect: Creatinine undergoes tubular secretion in addition to filtration, so clearances are not equal *It is less than inulin clearance* - Incorrect: This would only be true if creatinine were reabsorbed, but it is actually secreted, making its clearance higher *Equal to renal plasma flow* - Incorrect: Renal plasma flow is measured by PAH (para-aminohippuric acid) clearance (~650 mL/min), which is much higher than creatinine clearance (~120-130 mL/min) or GFR (~120 mL/min)

Question 1

What is the effective renal blood flow in humans?

Accepted Answer

625 mL/min

Answer

425 mL/min

Answer

525 mL/min

Answer

725 mL/min

Question 2

Urine osmolality in Diabetes insipidus is:

Accepted Answer

<150 mmol/L

Answer

<300 mmol/L

Answer

<600 mmol/L

Answer

<900 mmol/L

Question 3

Which of the following conditions leads to an increase in the secretion of antidiuretic hormone (ADH)?

Accepted Answer

Hypovolemia

Answer

Hypervolemia

Answer

Hypertension

Answer

Decreased osmolarity

Question 4

Which of the following substances are freely filtered by the kidney across glomerular capillaries?

Accepted Answer

Creatinine, HCO3-, and Glucose

Answer

Albumin, Creatinine, and HCO3-

Answer

Globulin, HCO3-, and Glucose

Answer

Albumin, HCO3-, and Glucose

Question 5

Which segment of the nephron has the lowest osmolality under the influence of antidiuretic hormone (ADH)?

Accepted Answer

Early distal tubule

Answer

Descending limb of the loop of Henle

Answer

Collecting duct

Answer

Late distal tubule

Question 6

What renal function is represented by the formula UV/P?

Accepted Answer

Clearance

Answer

Tubular Secretion

Answer

Tubular Reabsorption

Answer

Filtration

Question 7

What is the typical Glomerular Filtration Rate (GFR) in a healthy adult?

Accepted Answer

125 ml/min

Answer

150 ml/min

Answer

100 ml/min

Answer

200 ml/min

Question 8

Which of the following statements is true about GFR? GFR-Glomerular filtration rate RPF-Renal Plasma Flow

Accepted Answer

An increase in RPF increases GFR

Answer

An increase in sympathetic function increases GFR

Answer

Afferent arteriolar constriction increases GFR

Answer

A decrease in RPF increases GFR

Question 9

Which among the following is true regarding creatinine clearance?

Accepted Answer

It is more than inulin clearance

Answer

Equal to inulin clearance

Answer

It is less than inulin clearance

Answer

Equal to renal plasma flow

Question 10

Which of the following is not a mechanism of action of ADH?

Accepted Answer

Increases absorption of urea in descending limb of loop of Henle

Answer

Increases absorption of NaCl in thin ascending limb

Answer

Increases water permeability in collecting ducts

Answer

Increases absorption of urea in medullary collecting duct

Renal Physiology — MCQs

Renal Physiology — MCQs

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