Renal Physiology Practice Questions

Q: What is the principal site of sodium absorption in the nephron?

Proximal convoluted tubule. **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the "workhorse" of the nephron. It is the principal site of sodium reabsorption, accounting for approximately **65-70%** of the total filtered sodium load. This process is primarily driven by the basolateral Na+/K+ ATPase pump, which creates an electrochemical gradient that facilitates sodium entry from the lumen via various symporters (like SGLT-2 for glucose) and antiporters (Na+/H+ exchanger). **Analysis of Incorrect Options:** * **Loop of Henle (Thick Ascending Limb):** This segment reabsorbs about **20-25%** of filtered sodium via the **Na+-K+-2Cl- cotransporter (NKCC2)**. It is the site of action for loop diuretics like Furosemide. * **Distal Convoluted Tubule (DCT):** This segment reabsorbs only about **5-8%** of sodium through the **Na+-Cl- symporter**. It is the site of action for Thiazide diuretics. * **Collecting Duct:** This is the site of "fine-tuning" under hormonal control (Aldosterone). It reabsorbs only **2-3%** of sodium via **ENaC (Epithelial Sodium Channels)**. **High-Yield Clinical Pearls for NEET-PG:** 1. **Isosmotic Reabsorption:** In the PCT, water follows sodium passively, meaning the tubular fluid remains **isosmotic** to plasma. 2. **SGLT-2 Inhibitors:** Drugs like Dapagliflozin act on the PCT to inhibit glucose and sodium reabsorption, used in managing Diabetes Mellitus and Heart Failure. 3. **Glomerulotubular Balance:** This mechanism ensures that the percentage of solute reabsorbed in the PCT remains constant (67%) even if the Glomerular Filtration Rate (GFR) changes.

Q: All of the following statements about Renal physiology are true, except:

The kidney receives 5% of the cardiac output. **Explanation** The correct answer is **B** because the statement is factually incorrect. The kidneys are among the most highly perfused organs in the body, receiving approximately **20-25% of the total cardiac output** (roughly 1100–1200 mL/min), not 5%. This high blood flow is not required for the metabolic needs of the renal tissue itself, but rather to ensure a high Glomerular Filtration Rate (GFR) for effective regulation of body fluids and waste excretion. **Analysis of other options:** * **Option A (True):** The ascending limb of the Loop of Henle is impermeable to water but actively reabsorbs solutes (the "diluting segment"). Consequently, the fluid entering the distal convoluted tubule is always **hypoosmotic** (approx. 100 mOsm/L) relative to plasma. * **Option C (True):** GFR is regulated by the hydrostatic pressure in the glomerular capillaries. This is controlled by the tone of the **afferent arteriole** (constriction decreases GFR) and the **efferent arteriole** (constriction increases GFR, up to a point). * **Option D (True):** The glomerulus is a high-pressure capillary network formed by the branching of the **afferent arteriole**. These capillaries then coalesce to form the efferent arteriole. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Fraction:** 20-25% of Cardiac Output. * **Renal Plasma Flow (RPF):** Approximately 600-700 mL/min. * **Filtration Fraction:** GFR/RPF ≈ 20%. * **Oxygen Consumption:** Despite high blood flow, the kidney has a high arteriovenous oxygen difference only in the medulla, making the renal medulla particularly susceptible to ischemic injury (Acute Tubular Necrosis).

Q: Glucose is reabsorbed in the renal tubules and transported via which of the following mechanisms?

Sodium cotransporter. **Explanation:** Glucose reabsorption in the kidney occurs primarily in the **Proximal Convoluted Tubule (PCT)**. This process is a classic example of **Secondary Active Transport**. **Why the correct answer is right:** Glucose is transported across the apical membrane of the tubular cells against its concentration gradient. This is achieved by coupling its movement with the downhill movement of Sodium ($Na^+$) ions. This mechanism is known as **Sodium-Glucose Cotransport (Symport)**. * **SGLT-2** (Sodium-Glucose Linked Transporter 2) reabsorbs ~90% of glucose in the early PCT (S1 segment). * **SGLT-1** reabsorbs the remaining ~10% in the late PCT (S3 segment). The energy for this process is indirectly provided by the $Na^+/K^+$ ATPase pump on the basolateral membrane, which maintains the low intracellular sodium concentration. **Why the incorrect options are wrong:** * **Sodium antiporter:** An antiporter (counter-transporter) moves substances in opposite directions. In the PCT, the $Na^+/H^+$ exchanger is an antiporter, but glucose moves in the *same* direction as sodium. * **Potassium antiporter/symporter:** Potassium transport in the renal tubules is generally independent of glucose reabsorption. Glucose transport is specifically linked to the sodium gradient, not potassium. **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Threshold for Glucose:** Glucose starts appearing in the urine (glycosuria) when blood glucose levels exceed **180 mg/dL**. 2. **Transport Maximum ($T_m$):** The $T_m$ for glucose is approximately **375 mg/min** in men and **300 mg/min** in women. 3. **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A modern class of anti-diabetic drugs that inhibit these transporters to promote glucose excretion in urine. 4. **GLUT-2:** Once inside the cell, glucose exits the basolateral membrane into the blood via **Facilitated Diffusion** through GLUT-2 transporters.

Q: What is the net filtration pressure?

10 mm Hg. ***10 mm Hg*** - Net filtration pressure is calculated using **Starling forces**: NFP = GHP - (BCOP + PCOP) = 60 - (18 + 32) = **10 mmHg**. - This represents the net driving force for **glomerular filtration** after accounting for opposing pressures. *32 mm Hg* - This value represents the **plasma colloid osmotic pressure** (oncotic pressure), which opposes filtration. - It is one of the **opposing forces** in the NFP calculation, not the final net pressure. *60 mm Hg* - This represents the **glomerular hydrostatic pressure** (GHP), the primary driving force for filtration. - It is the **initial pressure** before subtracting the opposing forces, not the net filtration pressure. *20 mm Hg* - This value does not correspond to any specific **Starling force** component in normal glomerular filtration. - It may represent an incorrect calculation or **partial summation** of pressure values.

Q: Which of the following substances does not normally pass through the glomerular filter in the kidney?

Albumin. **Explanation:** The glomerular filtration barrier acts as a highly selective sieve based on two primary factors: **molecular size** and **electrical charge**. It consists of the fenestrated endothelium, the glomerular basement membrane (GBM), and the podocyte slit diaphragms. **Why Albumin is the Correct Answer:** Albumin has a molecular weight of approximately **69,000 Daltons** and a molecular radius of ~3.6 nm. While its size is near the threshold for filtration, it is **negatively charged**. The GBM and the podocyte glycocalyx are coated with negatively charged sialoglycoproteins (like heparan sulfate). This creates **electrostatic repulsion**, preventing albumin from passing into the Bowman’s space. In a healthy kidney, less than 0.1% of plasma albumin is filtered. **Analysis of Incorrect Options:** * **B & C (IgG and Beta globulin):** These are much larger proteins (IgG is ~150,000 Da). While they also do not pass through the filter, **Albumin** is the classic physiological marker used to describe the "charge barrier" in medical exams. However, in the context of this specific question, Albumin is the most clinically significant protein restricted by the healthy glomerular barrier. * **D (Lysozyme):** This is a small molecular weight protein (~14,000 Da). Small proteins and peptides are easily filtered but are typically reabsorbed by the proximal convoluted tubule. **High-Yield Clinical Pearls for NEET-PG:** * **Minimal Change Disease:** The primary pathology is the loss of the negative charge (heparan sulfate) on the GBM, leading to selective proteinuria (mainly albuminuria). * **Neutral vs. Anionic:** For the same molecular radius, a neutral molecule is filtered more easily than an anionic (negative) one. * **Threshold for Filtration:** Molecules with a radius 4 nm are not filtered. Albumin (3.6 nm) sits in the critical zone where its charge determines its exclusion.

Q: Urinary concentrating ability of the kidney is increased by?

ECF volume contraction. **Explanation:** The primary mechanism for concentrating urine is the maintenance of a high medullary osmotic gradient and the action of Antidiuretic Hormone (ADH). **Why Option A is correct:** In **ECF volume contraction** (dehydration or hemorrhage), the body initiates several compensatory mechanisms to conserve water: 1. **ADH Secretion:** Low ECF volume stimulates the posterior pituitary to release ADH, which increases water reabsorption in the collecting ducts. 2. **Decreased Medullary Blood Flow:** Sympathetic activation causes vasoconstriction of the vasa recta. Slower blood flow prevents the "washout" of medullary solutes, thereby enhancing the medullary hyperosmolarity required for water reabsorption. 3. **Increased Urea Recycling:** ADH increases the permeability of the inner medullary collecting duct to urea, further strengthening the osmotic gradient. **Why other options are incorrect:** * **B & D (Increase in Renal Blood Flow/GFR):** An increase in renal blood flow (specifically medullary flow) leads to a **"solute washout"** effect, where the high concentration of NaCl and urea in the medulla is carried away, reducing the osmotic gradient and decreasing concentrating ability. Similarly, a high GFR increases tubular flow rate, leaving less time for water reabsorption. * **C (Reduction of medullary hyperosmolarity):** This is the direct opposite of what is needed. The kidney can only concentrate urine if the medulla is hyperosmotic relative to the tubular fluid. **High-Yield Clinical Pearls for NEET-PG:** * **Countercurrent Multiplier:** Loop of Henle (creates the gradient). * **Countercurrent Exchanger:** Vasa Recta (maintains the gradient). * **Urea:** Responsible for nearly 50% of the medullary hyperosmolarity. * **Obligatory Water Loss:** The minimum urine volume required to excrete metabolic waste (approx. 0.5 L/day).

Q: What is the principal site of urine acidification?

Collecting duct. **Explanation:** The **Collecting Duct (CD)**, specifically the **Medullary Collecting Duct**, is the principal site of urine acidification. While the Proximal Convoluted Tubule (PCT) handles the bulk of bicarbonate reabsorption, the final and most significant drop in urinary pH occurs in the distal segments. 1. **Why the Collecting Duct is correct:** The **Alpha-Intercalated cells** of the collecting duct are responsible for active secretion of hydrogen ions ($H^+$) into the tubular lumen via **$H^+$-ATPase** and **$H^+/K^+$-ATPase** pumps. Because the CD has a low permeability to $H^+$ ions (unlike the "leaky" PCT), it can establish a steep concentration gradient, lowering the urinary pH to its minimum value of approximately **4.5**. 2. **Why other options are incorrect:** * **Proximal Convoluted Tubule (PCT):** This is the site of *bulk* bicarbonate reabsorption (85%). Although it secretes $H^+$, the pH only drops slightly (to ~6.7-7.0) because the secreted $H^+$ is consumed in reabsorbing $HCO_3^-$. * **Loop of Henle:** Primarily functions in establishing the medullary osmotic gradient (countercurrent mechanism) rather than active acidification. * **Distal Convoluted Tubule (DCT):** While some $H^+$ secretion occurs here, the maximum acidification and final pH adjustment are finalized in the collecting ducts. **High-Yield Clinical Pearls for NEET-PG:** * **Type 1 (Distal) Renal Tubular Acidosis (RTA):** Caused by a failure of Alpha-Intercalated cells to secrete $H^+$. Patients cannot acidify urine below pH 5.5. * **Urinary Buffers:** Since free $H^+$ ions would damage the epithelium, they are buffered by **Phosphate** (Titratable acid) and **Ammonia** ($NH_3 \rightarrow NH_4^+$). * **Aldosterone** acts on the collecting duct to increase $H^+$ secretion, which is why hyperaldosteronism leads to metabolic alkalosis.

Question 1

What is the principal site of sodium absorption in the nephron?

Accepted Answer

Proximal convoluted tubule

Answer

Distal convoluted tubule

Answer

Loop of Henle (thick segment)

Answer

Collecting duct

Question 2

A man with diabetes insipidus has a glomerular filtration of 155 L/day. Which range most likely represents the upper limit for urine flow rate in this individual if he were not receiving treatment but had free access to water?

Accepted Answer

15 to 20 L/day

Answer

1 to 3 L/day

Answer

5 to 10 L/day

Answer

30 to 35 L/day

Question 3

What are the major functions of "P" cells present in cortical collecting ducts?

Accepted Answer

All of the above.

Answer

Sodium and water reabsorption from the lumen.

Answer

Potassium secretion into the lumen.

Answer

ADH controlled water reabsorption.

Question 4

All of the following statements about Renal physiology are true, except:

Accepted Answer

The kidney receives 5% of the cardiac output

Answer

Distal tubule always receives hypoosmotic solution

Answer

GFR is controlled by resistance in afferent and efferent arterioles

Answer

The Glomerulus receives capillaries from the afferent arteriole

Question 5

Glucose is reabsorbed in the renal tubules and transported via which of the following mechanisms?

Accepted Answer

Sodium cotransporter

Answer

Sodium antiporter

Answer

Potassium antiporter

Answer

Potassium symporter

Question 6

Urine concentrating ability of the kidney is increased by?

Accepted Answer

Increased medullary hyperosmolarity

Answer

Increase in renal blood flow (RBF)

Answer

Reduction of medullary hyperosmolarity

Answer

Increase in glomerular filtration rate (GFR)

Question 7

What is the net filtration pressure?

Accepted Answer

10 mm Hg

Answer

32 mm Hg

Answer

60 mm Hg

Answer

20 mm Hg

Question 8

Which of the following substances does not normally pass through the glomerular filter in the kidney?

Accepted Answer

Albumin

Answer

Immunoglobulin G (IgG)

Answer

Beta globulin

Answer

Lysozyme

Question 9

Urinary concentrating ability of the kidney is increased by?

Accepted Answer

ECF volume contraction

Answer

Increase in renal blood flow

Answer

Reduction of medullary hyperosmolarity

Answer

Increase in GFR

Question 10

What is the principal site of urine acidification?

Accepted Answer

Collecting duct

Answer

Loop of Henle

Answer

Distal convoluted tubule

Answer

Proximal convoluted tubule

Renal Physiology — MCQs

Renal Physiology — MCQs

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