Renal Physiology Practice Questions

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.

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 for the reabsorption of the majority of the glomerular filtrate. Approximately **65-70% of filtered sodium (Na+)** is reabsorbed here, primarily through the Na+/H+ exchanger (NHE3) and co-transport with glucose, amino acids, and phosphate. This reabsorption is iso-osmotic, meaning water follows sodium passively to maintain osmotic balance. **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 (e.g., Furosemide). * **Distal Convoluted Tubule (DCT):** This segment reabsorbs only about **5-8%** of sodium via the **Na+-Cl- symporter**. It is the site of action for Thiazide diuretics. * **Collecting Duct:** This is the final site for "fine-tuning" sodium balance, accounting for only **2-3%** of reabsorption. Sodium entry here occurs through **ENaC (Epithelial Sodium Channels)**, which are regulated by Aldosterone. **High-Yield Clinical Pearls for NEET-PG:** * **Obligatory Reabsorption:** Reabsorption in the PCT is "obligatory" and independent of hormonal control, unlike the distal segments. * **Glucose Reabsorption:** 100% of filtered glucose is reabsorbed in the PCT (via SGLT2/SGLT1) unless the renal threshold (180 mg/dL) is exceeded. * **Carbonic Anhydrase:** The PCT is the site of action for Acetazolamide, which inhibits sodium bicarbonate reabsorption. * **Smallest fraction of Na+ reabsorption:** Occurs in the medullary collecting ducts.

Question 1

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 2

What is the net filtration pressure?

Accepted Answer

10 mm Hg

Answer

32 mm Hg

Answer

60 mm Hg

Answer

20 mm Hg

Question 3

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 4

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 5

What is the principal site of urine acidification?

Accepted Answer

Collecting duct

Answer

Loop of Henle

Answer

Distal convoluted tubule

Answer

Proximal convoluted tubule

Question 6

Why is the actual tubular maximum for the kidney in practice less than the calculated value?

Accepted Answer

Different nephrons have different transport maximums.

Answer

It depends on the glomerular filtration rate.

Answer

It depends on the renal blood flow.

Answer

It depends on the blood pressure.

Question 7

In renal disease, why does albumin first appear in the urine?

Accepted Answer

It has a molecular weight slightly greater than molecules normally filtered.

Answer

Its high concentration in plasma.

Answer

A high albumin to globulin ratio.

Answer

Tubular epithelial cells are sensitive to albumin.

Question 8

What would tend to decrease GFR by more than 10% in a normal kidney?

Accepted Answer

50% decrease in efferent arteriolar resistance

Answer

Decrease in mean arterial pressure from 100 to 85 mm Hg

Answer

50% decrease in afferent arteriolar resistance

Answer

50% increase in the glomerular capillary filtration coefficient

Question 9

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 portion)

Answer

Collecting duct

Question 10

Fractional excretion of sodium (FENa) is calculated as:

Accepted Answer

(Urine sodium / Serum sodium) / (Urine potassium / Serum potassium) x 100

Answer

(Urine sodium x Serum potassium) / (Serum sodium x Urine potassium) x 100

Answer

(Urine potassium x Serum sodium) / (Serum potassium x Urine sodium) x 100

Answer

(Urine sodium x Urine potassium) / (Serum sodium x Serum potassium) x 100

Renal Physiology — MCQs

Renal Physiology — MCQs

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