Renal Physiology Practice Questions

Q: What percentage of total nephrons are juxtamedullary nephrons?

15%. **Explanation:** In the human kidney, nephrons are classified into two distinct types based on their location and structural characteristics: **Cortical nephrons** and **Juxtamedullary nephrons**. 1. **Why 15% is correct:** Approximately **15%** (range 12–15%) of all nephrons are juxtamedullary nephrons. Their glomeruli are located in the inner third of the renal cortex, near the medulla. They possess long Loops of Henle that descend deep into the renal medulla, often reaching the tips of the renal papillae. Their primary physiological role is the **concentration of urine** via the countercurrent multiplier system. 2. **Why other options are incorrect:** * **85% (Inverse of Option A):** This represents the majority of nephrons, which are **Cortical nephrons**. These have short loops and are primarily responsible for waste excretion and nutrient reabsorption. * **50%, 70%, and 90%:** These values do not correspond to any standard anatomical distribution of nephron types in a healthy human kidney. **High-Yield Facts for NEET-PG:** * **Vasa Recta:** Juxtamedullary nephrons are uniquely associated with the *vasa recta* (specialized peritubular capillaries), which are essential for maintaining the medullary osmotic gradient. * **Concentrating Capacity:** Species living in arid environments (like the kangaroo rat) have a much higher percentage of juxtamedullary nephrons to conserve water. * **Renin Content:** Juxtamedullary nephrons generally contain higher amounts of renin compared to cortical nephrons. * **Blood Flow:** Although juxtamedullary nephrons are vital for concentration, the majority of renal blood flow (approx. 90%) is directed to the renal cortex.

Q: What is the normal number of RBCs excreted per day in urine?

1 million. **Explanation:** In a healthy individual, the glomerular filtration barrier is highly selective, preventing the passage of most cellular elements. However, a minute number of Red Blood Cells (RBCs) can pass through the glomerular capillaries or enter the tubular lumen via diapedesis. **1. Why Option A is Correct:** Under physiological conditions, the normal excretion of RBCs in the urine is approximately **0.5 to 1 million cells per 24 hours**. This is considered the upper limit of normal for "microscopic hematuria" when measured via a timed 24-hour urine collection or an Addis count. In a standard microscopic examination of urinary sediment (High Power Field - HPF), this correlates to roughly **0–2 RBCs/HPF**. **2. Why Options B, C, and D are Incorrect:** * **2 million to 4 million:** These values exceed the physiological threshold. Excretion consistently above 1 million cells/day (or >3 RBCs/HPF) is clinically defined as **hematuria**, which may indicate glomerular disease (e.g., glomerulonephritis), malignancy, or urolithiasis. **Clinical Pearls for NEET-PG:** * **Addis Count:** This is the traditional method used to quantify the formed elements (RBCs, WBCs, casts) in a 12 or 24-hour urine sample. * **Dysmorphic RBCs:** The presence of "acanthocytes" (mickey-mouse shaped RBCs) in urine suggests a **glomerular origin** of bleeding, as cells are distorted while passing through the basement membrane. * **Isomorphic RBCs:** Uniformly shaped RBCs usually indicate **post-glomerular** bleeding (e.g., bladder or ureter). * **Hematuria Definition:** Clinically, significant microscopic hematuria is defined as **≥3 RBCs/HPF** in two out of three properly collected samples.

Q: Maximum absorption of NaCl in the proximal convoluted tubule occurs due to the effect of:

Angiotensin II. **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for the reabsorption of water and solutes (approximately 65-70% of the filtered load). **Angiotensin II** is the most potent stimulator of NaCl and water reabsorption in the PCT. **Why Angiotensin II is correct:** Angiotensin II acts on the **NHE3 (Sodium-Hydrogen Exchanger)** on the apical membrane and the **Na+/K+ ATPase** on the basolateral membrane of the PCT cells. By stimulating these transporters, it directly increases sodium reabsorption. Additionally, by constricting the efferent arteriole, it increases the filtration fraction, leading to increased peritubular capillary oncotic pressure, which further facilitates fluid reabsorption from the PCT. **Why other options are incorrect:** * **ADH (Vasopressin):** Primarily acts on the **Collecting Ducts** (V2 receptors) to increase water permeability via Aquaporin-2 channels. It has minimal effect on NaCl reabsorption in the PCT. * **Aldosterone:** Acts on the **Principal cells of the Late Distal Tubule and Collecting Duct**. It promotes Na+ reabsorption and K+ secretion by upregulating ENaC channels and Na+/K+ ATPase. * **Atrial Natriuretic Peptide (ANP):** This is a "natriuretic" hormone; it **inhibits** Na+ reabsorption in the medullary collecting duct and inhibits the secretion of Renin and Aldosterone to decrease blood pressure. **High-Yield Clinical Pearls for NEET-PG:** * **PCT** is the site for 100% reabsorption of Glucose and Amino acids. * **Carbonic Anhydrase inhibitors (Acetazolamide)** act specifically on the PCT. * **Angiotensin II** is unique because it maintains GFR (via efferent constriction) while simultaneously increasing Na+ reabsorption to expand ECF volume.

Q: Which cells are present in the collecting duct?

Principal and intercalated cells. The collecting duct is the final segment of the nephron responsible for fine-tuning water and electrolyte balance. It is composed of two distinct cell types: 1. **Principal Cells:** These are the most abundant cells. They are responsible for **sodium (Na+) reabsorption** and **potassium (K+) secretion**. This process is regulated by **Aldosterone**. They also contain aquaporins (AQP2) for water reabsorption under the influence of **ADH (Vasopressin)**. 2. **Intercalated Cells:** These are primarily involved in acid-base balance. **Type A** cells secrete H+ (acid) and reabsorb HCO3- (base), while **Type B** cells secrete HCO3- and reabsorb H+. **Analysis of Incorrect Options:** * **Parietal and Oxyntic cells:** These are found in the **stomach (gastric glands)**. Parietal (oxyntic) cells secrete Hydrochloric acid (HCl) and Intrinsic Factor. * **Leydig cells:** These are interstitial cells located in the **testes** that produce testosterone in response to Luteinizing Hormone (LH). * **Podocytes:** These are specialized epithelial cells located in **Bowman’s capsule** (visceral layer) that form the filtration slits of the glomerular basement membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Liddle’s Syndrome:** A gain-of-function mutation in the ENaC channels of **Principal cells**, leading to hypertension and hypokalemia. * **Distal Renal Tubular Acidosis (Type 1 RTA):** Caused by a defect in the H+ secretion by **Alpha-intercalated cells** in the collecting duct. * **Potassium Sparing Diuretics:** Spironolactone (Aldosterone antagonist) and Amiloride (ENaC blocker) act specifically on the **Principal cells**.

Q: If the lumen of the renal artery is reduced by half, by what factor will the blood flow be reduced?

It will reduce by 1/16. ### Explanation **1. Why Option C is Correct: The Poiseuille-Hagen Equation** The relationship between the radius of a blood vessel and the flow of blood is governed by **Poiseuille’s Law**. According to this law, the flow ($Q$) is directly proportional to the fourth power of the radius ($r^4$): $$Q \propto r^4$$ If the lumen (radius) of the renal artery is reduced by half ($1/2$), the new flow rate is calculated as: $$(1/2)^4 = 1/2 \times 1/2 \times 1/2 \times 1/2 = \mathbf{1/16}$$ Therefore, even a small decrease in the radius leads to a massive reduction in blood flow because of this exponential relationship. **2. Why Other Options are Incorrect** * **Option A (1/2):** This assumes a linear relationship between radius and flow, which is incorrect. * **Option B (1/4):** This assumes flow is proportional to the cross-sectional area ($\pi r^2$). While area decreases by 1/4, the flow decreases further due to increased resistance. * **Option D (Autoregulation):** While the kidney has autoregulatory mechanisms (Myogenic and Tubuloglomerular feedback) to maintain GFR between 80–180 mmHg, these cannot compensate for a structural 50% reduction in the main renal artery diameter. Such a significant mechanical obstruction leads to **Renovascular Hypertension**. **3. High-Yield Clinical Pearls for NEET-PG** * **Resistance ($R$):** Resistance is inversely proportional to the fourth power of the radius ($R \propto 1/r^4$). If the radius is halved, resistance increases **16-fold**. * **Arterioles:** These are the "resistance vessels" of the systemic circulation because they have the greatest ability to change their radius. * **Goldblatt Kidney:** A classic experimental model where constricting one renal artery (simulating the question's scenario) leads to systemic hypertension via the Renin-Angiotensin-Aldosterone System (RAAS).

Q: Glomerular filtration rate (GFR) is opposed by which of the following pressures?

Osmotic pressure in the glomerular capillaries. **Explanation:** The Glomerular Filtration Rate (GFR) is determined by the **Starling forces** acting across the glomerular capillary membrane. The net filtration pressure (NFP) is calculated as: **NFP = (Pgc + πbs) – (Pbs + πgc)** 1. **Why Option B is Correct:** **Osmotic (Oncotic) pressure in the glomerular capillaries (πgc)** is exerted by plasma proteins (mainly albumin) that cannot pass through the filtration barrier. This pressure acts to "hold" or pull fluid back into the capillary, thereby **opposing** filtration. As blood moves from the afferent to the efferent arteriole, the concentration of proteins increases, further increasing this opposing force. 2. **Why Other Options are Incorrect:** * **Option A & C:** Peritubular capillaries are involved in tubular reabsorption and secretion, not the initial process of glomerular filtration. Their pressures do not directly influence GFR. * **Option D:** **Hydrostatic pressure in the glomerular capillaries (Pgc)** is the primary **driving force** that favors filtration, pushing fluid out of the blood into Bowman’s space. **High-Yield Clinical Pearls for NEET-PG:** * **Net Filtration Pressure:** In a healthy individual, it is approximately **10 mmHg**. * **Bowman’s Space Hydrostatic Pressure (Pbs):** This also opposes GFR. Conditions like kidney stones (ureteral obstruction) increase Pbs, leading to a decrease in GFR. * **Hypoproteinemia:** In conditions like Nephrotic Syndrome, a decrease in πgc (due to low plasma albumin) actually leads to an *increase* in GFR, though it often results in systemic edema. * **Filtration Fraction (FF):** GFR / Renal Plasma Flow (Normal ≈ 20%).

Q: Erythropoietin secretion decreases in which of the following conditions?

Severe renal disease. ### Explanation **Correct Option: B. Severe renal disease** **Underlying Medical Concept:** Erythropoietin (EPO) is a glycoprotein hormone essential for erythropoiesis. In adults, approximately **85–90% of EPO is produced by the peritubular interstitial cells (fibroblasts)** in the renal cortex, while the remaining 10–15% is produced by the liver. The primary stimulus for EPO secretion is **hypoxia** (detected via Hypoxia-Inducible Factor, HIF-1α). In **severe renal disease** (such as Chronic Kidney Disease), the functional parenchyma and peritubular cells are destroyed or replaced by fibrosis. Consequently, the kidneys lose their ability to synthesize EPO, leading to the classic "normocytic normochromic anemia of chronic renal failure." **Analysis of Incorrect Options:** * **A. High altitude:** Decreased atmospheric pressure leads to a lower partial pressure of oxygen ($PO_2$). This systemic hypoxia stimulates the kidneys to **increase** EPO production to enhance oxygen-carrying capacity. * **C. Congestive heart failure:** Reduced cardiac output leads to decreased tissue perfusion and stagnant hypoxia. This triggers a compensatory **increase** in EPO secretion. * **D. Lung disease:** Conditions like COPD or interstitial lung disease impair gas exchange, causing arterial hypoxemia. This is a potent stimulus that **increases** EPO, often resulting in secondary polycythemia. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Action:** EPO acts on the **CFU-E** (Colony Forming Unit-Erythroid) receptors in the bone marrow. * **Recombinant EPO:** Used clinically to treat anemia in CKD patients; however, it can cause **hypertension** as a side effect. * **Non-Renal EPO:** In the fetus, the **liver** is the primary source of EPO. * **Tumor Association:** Certain tumors (e.g., Renal Cell Carcinoma, Hemangioblastoma, Hepatocellular Carcinoma) can cause **ectopic EPO production**, leading to paraneoplastic polycythemia.

Question 1

What percentage of total nephrons are juxtamedullary nephrons?

Accepted Answer

15%

Answer

50%

Answer

70%

Answer

90%

Question 2

What is the normal number of RBCs excreted per day in urine?

Accepted Answer

1 million

Answer

2 million

Answer

3 million

Answer

4 million

Question 3

Maximum absorption of NaCl in the proximal convoluted tubule occurs due to the effect of:

Accepted Answer

Angiotensin II

Answer

ADH

Answer

Aldosterone

Answer

Atrial natriuretic peptide

Question 4

Which of the following renal alterations would most likely preserve the Glomerular Filtration Rate (GFR)?

Accepted Answer

Decreased intrarenal nitric oxide

Answer

Defect in the elimination of hydrogen ions in the distal tubule

Answer

Luminal obstruction by renal tubular casts

Answer

Increased interstitial pressure

Question 5

Which cells are present in the collecting duct?

Accepted Answer

Principal and intercalated cells

Answer

Parietal and oxyntic cells

Answer

Leydig cells

Answer

Podocytes

Question 6

If the lumen of the renal artery is reduced by half, by what factor will the blood flow be reduced?

Accepted Answer

It will reduce by 1/16

Answer

It will reduce by 1/2

Answer

It will reduce by 1/4

Answer

It will return to normal after autoregulation

Question 7

Glomerular filtration rate (GFR) is opposed by which of the following pressures?

Accepted Answer

Osmotic pressure in the glomerular capillaries

Answer

Osmotic pressure in the peritubular capillaries

Answer

Hydrostatic pressure in the peritubular capillaries

Answer

Hydrostatic pressure in the glomerular capillaries

Question 8

A substance is present in a concentration of 2 mg% in the afferent arteriole and zero mg% in the efferent arteriole. What is true about this substance?

Accepted Answer

It is freely filtered at the glomerulus.

Answer

It is secreted in the cortical nephron.

Answer

It is absorbed in the proximal convoluted tubule.

Answer

It is impermeable in the loop of Henle.

Question 9

Erythropoietin secretion decreases in which of the following conditions?

Accepted Answer

Severe renal disease

Answer

High altitude

Answer

Congestive heart failure

Answer

Lung disease

Question 10

Net filtration pressure of the glomerulus increases when which of the following occurs?

Accepted Answer

Increase in hydrostatic pressure of the glomerular capillary

Answer

Increase in oncotic pressure of the glomerular capillary

Answer

Increase in hydrostatic pressure of Bowman's space

Answer

All of the above

Renal Physiology — MCQs

Renal Physiology — MCQs

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