Renal Physiology Practice Questions

Q: If the renal plasma flow (RPF) is 600 mL/min and the hematocrit (hct) is 45%, what is the renal blood flow?

1090 mL/min. ### Explanation **1. Understanding the Correct Answer (Option B: 1090 mL/min)** Renal Blood Flow (RBF) represents the total volume of blood delivered to the kidneys per unit time. Blood consists of two main components: **Plasma** and **Formed Elements (primarily RBCs)**. The Hematocrit (Hct) represents the fraction of blood occupied by cells. Therefore, the remaining fraction $(1 - \text{Hct})$ represents the Plasma. The mathematical relationship is: $$\text{RBF} = \frac{\text{Renal Plasma Flow (RPF)}}{1 - \text{Hematocrit}}$$ **Calculation:** * RPF = 600 mL/min * Hct = 45% (0.45) * Plasma fraction = $1 - 0.45 = 0.55$ * $\text{RBF} = \frac{600}{0.55} \approx \mathbf{1090.9 \text{ mL/min}}$ **2. Analysis of Incorrect Options** * **Option A (1000 mL/min):** This is a common distractor for students who incorrectly assume a standard RBF without calculating based on the provided Hct. * **Option C (1200 mL/min):** This value is often cited as the "average" RBF in a 70kg male, but it does not fit the specific parameters given in this mathematical problem. * **Option D (1250 mL/min):** This would be the result if the Hct were 52% or if the student confused the calculation with GFR/Filtration Fraction constants. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Normal RBF:** Approximately 1100–1200 mL/min (roughly 20-25% of Cardiac Output). * **Normal RPF:** Approximately 600–650 mL/min. * **Filtration Fraction (FF):** $\text{GFR} / \text{RPF}$ (Normal $\approx 0.20$ or 20%). * **Gold Standard for RPF:** Para-aminohippuric acid (PAH) clearance, as it is both filtered and secreted. * **Autoregulation:** RBF remains constant between a Mean Arterial Pressure (MAP) of **80–180 mmHg** via myogenic and tubuloglomerular feedback mechanisms.

Q: Autosomal variant of nephrogenic diabetes insipidus is due to defect in which of the following?

Aquaporin 2. **Explanation:** **Nephrogenic Diabetes Insipidus (NDI)** is characterized by the kidney's inability to concentrate urine despite adequate levels of Antidiuretic Hormone (ADH/Vasopressin). * **Why Aquaporin 2 is correct:** ADH normally binds to V2 receptors on the basolateral membrane of the collecting duct, triggering a cAMP pathway that inserts **Aquaporin-2 (AQP2)** channels into the apical membrane to facilitate water reabsorption. * The most common form of hereditary NDI is **X-linked recessive** (90%), caused by a mutation in the **V2 receptor**. * The **Autosomal (recessive or dominant) variant** of NDI is caused by mutations in the **Aquaporin-2 gene**, leading to defective water channels. **Analysis of Incorrect Options:** * **Options B & C (Angiotensin I & II receptors):** These are involved in blood pressure regulation and aldosterone secretion via the Renin-Angiotensin-Aldosterone System (RAAS). They do not directly mediate the water-permeability effects of ADH in the collecting duct. * **Option D (VIP receptors):** Vasoactive Intestinal Peptide (VIP) is involved in smooth muscle relaxation and intestinal secretion, but it plays no role in the renal concentrating mechanism associated with Diabetes Insipidus. **High-Yield Clinical Pearls for NEET-PG:** * **Lithium:** The most common cause of *acquired* nephrogenic DI (it inhibits cAMP formation). * **V1 Receptors:** Located on vascular smooth muscle (cause vasoconstriction). * **V2 Receptors:** Located in the principal cells of the collecting duct (mediate water reabsorption). * **Diagnosis:** NDI is confirmed when there is no increase in urine osmolality following the administration of exogenous ADH (Desmopressin) during a water deprivation test.

Q: Ion ligand receptors are:

Glycine receptor. **Explanation:** The question asks to identify an **Ionotropic receptor** (Ion-ligand gated channel). These are membrane proteins that act as both a receptor and an ion channel, allowing rapid flux of ions across the cell membrane upon ligand binding. **1. Why Glycine receptor is correct:** The **Glycine receptor** is a classic example of an ionotropic receptor. It is a ligand-gated **chloride (Cl⁻) channel**. When glycine binds, the channel opens, leading to Cl⁻ influx, which causes hyperpolarization of the postsynaptic membrane, resulting in fast inhibitory neurotransmission (primarily in the spinal cord and brainstem). Other examples include Nicotinic ACh, GABA-A, and NMDA receptors. **2. Why other options are incorrect:** * **Beta-adrenergic receptors:** These belong to the **G-Protein Coupled Receptor (GPCR)** family (specifically Gs). They work via the adenylyl cyclase-cAMP second messenger system, not by direct ion gating. * **V1 receptor:** These are Vasopressin receptors (V1a and V1b) that are **GPCRs** coupled to the Gq pathway, activating Phospholipase C and increasing intracellular calcium. * **Insulin receptor:** This is an **Enzyme-linked receptor** (specifically a Receptor Tyrosine Kinase). Binding triggers autophosphorylation of tyrosine residues, initiating a phosphorylation cascade. **High-Yield Clinical Pearls for NEET-PG:** * **Strychnine:** A potent convulsant that acts as a competitive antagonist at Glycine receptors, leading to unchecked muscular contractions. * **Fastest Receptors:** Ionotropic receptors (milliseconds) > GPCRs (seconds) > Enzyme-linked (minutes/hours) > Nuclear receptors (hours/days). * **V2 Receptors:** Unlike V1, V2 receptors (found in renal collecting ducts) are GPCRs coupled to **Gs**, increasing cAMP to insert Aquaporin-2 channels.

Q: Which cells are responsible for acid secretion in the kidney?

I cells. **Explanation:** The correct answer is **I cells (Intercalated cells)**. These cells are located in the late distal tubule and the collecting duct and play a vital role in acid-base balance. 1. **Why I cells are correct:** There are two types of Intercalated cells: * **Type A (Alpha) cells:** Responsible for **acid secretion**. They utilize H⁺-ATPase and H⁺/K⁺-ATPase pumps on their apical membrane to secrete hydrogen ions into the tubular lumen while reabsorbing bicarbonate (HCO₃⁻) via the basolateral Cl⁻/HCO₃⁻ exchanger (Anion Exchanger 1). * **Type B (Beta) cells:** Responsible for **bicarbonate secretion** (active during alkalosis). 2. **Why other options are incorrect:** * **P cells (Principal cells):** These are the most abundant cells in the collecting duct. Their primary function is **sodium reabsorption** (via ENaC channels) and **potassium secretion**, regulated by Aldosterone. They also reabsorb water via Aquaporin-2 under the influence of ADH. * **Mesangial cells:** These are specialized smooth-muscle-like cells located in the glomerulus. They provide structural support, regulate the glomerular filtration rate (GFR) through contraction, and have phagocytic properties. * **Pericytes:** In the kidney, pericytes are found around the vasa recta. They regulate medullary blood flow and are the primary source of **Erythropoietin (EPO)** production. **High-Yield Clinical Pearls for NEET-PG:** * **Type 1 Renal Tubular Acidosis (Distal RTA):** Caused by a failure of Type A Intercalated cells to secrete H⁺, leading to a high urinary pH (>5.5). * **Aldosterone's dual action:** It acts on P cells to reabsorb Na⁺ and on Type A I-cells to stimulate H⁺ secretion. This is why hyperaldosteronism leads to metabolic alkalosis. * **Memory Aid:** **A**lpha cells secrete **A**cid; **B**eta cells secrete **B**ase (Bicarbonate).

Q: Alpha-adrenergic receptors are located in which part of the urinary bladder?

Base of bladder. The innervation of the urinary bladder is a high-yield topic in renal physiology, governed by the principle of "reciprocal innervation" between the detrusor muscle and the internal urethral sphincter. ### **Explanation of the Correct Answer** The **base of the bladder** (including the bladder neck and the internal urethral sphincter) is rich in **Alpha-1 ($\alpha_1$) adrenergic receptors**. * **Mechanism:** Stimulation of these receptors by the sympathetic nervous system (via the Hypogastric nerve, T11-L2) causes **contraction** of the internal sphincter. * **Function:** This action promotes urinary storage by increasing outlet resistance and preventing the voiding of urine during the filling phase. ### **Analysis of Incorrect Options** * **A. Dome of bladder:** The dome and body of the bladder consist primarily of the detrusor muscle. This area is dominated by **Beta-3 ($\beta_3$) adrenergic receptors**, which cause muscle relaxation to allow for bladder filling, and **Muscarinic (M3) receptors**, which cause contraction during micturition. * **C & D:** These are incorrect because the distribution of adrenergic receptors is anatomically distinct to ensure coordinated storage and voiding. ### **High-Yield Clinical Pearls for NEET-PG** 1. **Pharmacology Link:** Alpha-blockers (e.g., **Tamsulosin**) are used in Benign Prostatic Hyperplasia (BPH) to relax the bladder neck (base) and improve urine flow. 2. **Sympathetic vs. Parasympathetic:** * **Sympathetic (Hypogastric N.):** "Fills" the bladder (Relaxes detrusor via $\beta_3$, contracts sphincter via $\alpha_1$). * **Parasympathetic (Pelvic N.):** "Empties" the bladder (Contracts detrusor via $M_3$). 3. **Somatic Control:** The external urethral sphincter is under voluntary control via the **Pudendal nerve** (S2-S4) acting on Nicotinic receptors.

Q: A patient has a GFR of 100 ml/min, her urine flow rate is 2.0 ml/min, and her plasma glucose concentration is 200 mg/100 ml. If the kidney transport maximum for glucose is 250 mg/min, what would be her approximate rate of glucose excretion?

0 mg/min. To solve this problem, we must compare the **Filtered Load** of glucose with the **Transport Maximum ($T_m$)**. ### 1. Calculation of Filtered Load The filtered load is the amount of glucose that enters the Bowman’s capsule per minute. * **Formula:** Filtered Load = GFR × Plasma Concentration ($P_x$) * **Given:** GFR = 100 ml/min; $P_x$ = 200 mg/100 ml (which is 2 mg/ml). * **Calculation:** $100 \text{ ml/min} \times 2 \text{ mg/ml} = \mathbf{200 \text{ mg/min}}$. ### 2. Comparison with Transport Maximum ($T_m$) The $T_m$ is the maximum rate at which the proximal tubule can reabsorb glucose via SGLT transporters. * **Given:** $T_m = 250 \text{ mg/min}$. * **Logic:** Since the Filtered Load (200 mg/min) is **less than** the $T_m$ (250 mg/min), the kidneys have the capacity to reabsorb 100% of the filtered glucose. * **Excretion:** Excretion = Filtered Load – Reabsorption. Here, $200 - 200 = \mathbf{0 \text{ mg/min}}$. --- ### Why Incorrect Options are Wrong: * **B, C, and D:** These values assume that the $T_m$ has been exceeded. If the filtered load were, for example, 350 mg/min, the excretion would be $350 - 250 = 100 \text{ mg/min}$. In this case, however, the reabsorptive capacity is sufficient to ensure no glucose is lost in the urine. --- ### High-Yield Clinical Pearls for NEET-PG: * **Renal Threshold for Glucose:** This is the plasma concentration at which glucose first appears in the urine (glycosuria). It is typically **180 mg/dL**. * **Splay Phenomenon:** The renal threshold is lower than the $T_m$ would predict due to the "splay" phenomenon, caused by the heterogeneity of nephrons and the low affinity of SGLT transporters near saturation. * **Transporters:** Glucose is reabsorbed in the PCT via **SGLT-2** (early PCT, high capacity) and **SGLT-1** (late PCT, high affinity). SGLT-2 inhibitors (e.g., Dapagliflozin) are now key drugs in managing Diabetes and Heart Failure.

Question 1

Which of the following conditions results in increased secretion of Renin?

Accepted Answer

Decreased amount of Na+ in the proximal convoluted tubule (PCT)

Answer

Renal ischemia

Answer

Decreased amount of Na+ in the distal convoluted tubule (DCT)

Answer

Narrowing of afferent arterioles

Question 2

What is the minimum urine osmolality that the human kidney can achieve?

Accepted Answer

50 mOsm/L

Answer

100 mOsm/L

Answer

80 mOsm/L

Answer

20 mOsm/L

Question 3

If the renal plasma flow (RPF) is 600 mL/min and the hematocrit (hct) is 45%, what is the renal blood flow?

Accepted Answer

1090 mL/min

Answer

1000 mL/min

Answer

1200 mL/min

Answer

1250 mL/min

Question 4

Autosomal variant of nephrogenic diabetes insipidus is due to defect in which of the following?

Accepted Answer

Aquaporin 2

Answer

Angiotensin I receptor

Answer

Angiotensin II receptor

Answer

VIP receptors

Question 5

Ion ligand receptors are:

Accepted Answer

Glycine receptor

Answer

beta adrenergic receptors

Answer

V1 receptor

Answer

Insulin receptor

Question 6

A freely filterable substance that is neither reabsorbed nor secreted has a renal artery concentration of 12 mg/mL and a renal vein concentration of 9 mg/mL. Which of the following is the filtration fraction?

Accepted Answer

0.25

Answer

0.05

Answer

0.15

Answer

0.35

Question 7

Which mechanism does NOT contribute to the countercurrent multiplier in the nephron?

Accepted Answer

Sodium reabsorption out of the thin descending limb of the loop of Henle

Answer

Active transport of sodium and chloride out of the thick ascending limb of the loop of Henle

Answer

Water reabsorption in the thin descending limb of the loop of Henle

Answer

Urea concentration in the collecting duct

Question 8

Which cells are responsible for acid secretion in the kidney?

Accepted Answer

I cells

Answer

P cells

Answer

Mesangial cells

Answer

Pericytes

Question 9

Alpha-adrenergic receptors are located in which part of the urinary bladder?

Accepted Answer

Base of bladder

Answer

Dome of bladder

Answer

Both of above

Answer

None of above

Question 10

A patient has a GFR of 100 ml/min, her urine flow rate is 2.0 ml/min, and her plasma glucose concentration is 200 mg/100 ml. If the kidney transport maximum for glucose is 250 mg/min, what would be her approximate rate of glucose excretion?

Accepted Answer

0 mg/min

Answer

50 mg/min

Answer

100 mg/min

Answer

150 mg/min

Renal Physiology — MCQs

Renal Physiology — MCQs

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