Renal Physiology Practice Questions

Q: Which substance is secreted by the proximal convoluted tubule (PCT)?

Hydrogen ions. ### Explanation The **Proximal Convoluted Tubule (PCT)** is the most metabolically active segment of the nephron, responsible for both bulk reabsorption and selective secretion. **Why Hydrogen ions (H⁺) is correct:** The PCT plays a vital role in acid-base balance. It secretes **Hydrogen ions** into the tubular lumen, primarily via the **Sodium-Hydrogen Exchanger 3 (NHE3)**. This secretion is coupled with the reabsorption of filtered bicarbonate ($HCO_3^-$). For every $H^+$ ion secreted, one $HCO_3^-$ molecule is returned to the blood, making the PCT the primary site for maintaining the body's alkali reserve. **Why the other options are incorrect:** * **Sodium (A):** Sodium is **reabsorbed** (not secreted) in the PCT. Approximately 65% of filtered sodium is reabsorbed here through various symporters and antiporters. * **Glucose (B) & Amino acids (D):** These are essential nutrients that are **100% reabsorbed** in the early PCT under normal physiological conditions. Glucose is reabsorbed via **SGLT-2** (and some SGLT-1), while amino acids use specific sodium-coupled transporters. Their presence in urine (glycosuria/aminoaciduria) indicates a pathological state or exceeding the renal threshold. **High-Yield NEET-PG Pearls:** * **Secreted substances in PCT:** $H^+$, Ammonium ($NH_4^+$), Creatinine, and organic acids/bases (e.g., Penicillin, PAH, Urate). * **Carbonic Anhydrase:** The enzyme essential for $H^+$ secretion and $HCO_3^-$ reabsorption in the PCT. **Acetazolamide** inhibits this enzyme, leading to proximal renal tubular acidosis. * **Fanconi Syndrome:** A clinical condition resulting from generalized dysfunction of the PCT, leading to the wasting of glucose, amino acids, phosphate, and bicarbonate in the urine.

Q: Which part of the nephron is the dilution segment?

Ascending limb of the Loop of Henle. ### Explanation The correct answer is **A. Ascending limb of the Loop of Henle**. **Why it is the Diluting Segment:** The Thick Ascending Limb (TAL) of the Loop of Henle is termed the "diluting segment" because it is **impermeable to water** but actively reabsorbs solutes (Na⁺, K⁺, and Cl⁻) via the **NKCC2 transporter**. As electrolytes are pumped out into the medullary interstitium while water remains trapped in the tubule, the tubular fluid becomes increasingly dilute (hypotonic) relative to plasma. By the time the fluid reaches the distal convoluted tubule, its osmolarity drops to approximately 100 mOsm/L. **Analysis of Incorrect Options:** * **B. Descending limb of the Loop of Henle:** This is the "concentrating segment." It is highly permeable to water but impermeable to solutes. Water leaves the tubule via osmosis, making the tubular fluid hypertonic. * **C. Collecting tubule:** While dilution can occur here in the absence of ADH, it is primarily responsible for the final concentration of urine under the influence of ADH (Vasopressin). * **D. Proximal tubule:** Reabsorption here is **isostatic**. Both water and solutes are reabsorbed in equal proportions, so the tubular fluid remains isotonic (300 mOsm/L) to plasma. **NEET-PG High-Yield Pearls:** * **NKCC2 Transporter:** This is the target of **Loop Diuretics** (e.g., Furosemide). By inhibiting this transporter, these drugs abolish the corticomedullary gradient, preventing both dilution and concentration of urine. * **Bartter Syndrome:** A genetic defect in the NKCC2 transporter or associated channels in the TAL, mimicking chronic loop diuretic use. * **Countercurrent Multiplier:** The TAL provides the "single effect" that drives the countercurrent multiplier system, essential for the kidney's ability to concentrate urine.

Q: Destruction of the zona glomerulosa will deplete which hormone?

Aldosterone. **Explanation:** The adrenal gland is divided into an outer cortex and an inner medulla. The adrenal cortex consists of three distinct histological layers, often remembered by the mnemonic **"GFR"** (from superficial to deep), which correspond to the hormones they produce: **"Salt, Sugar, Sex."** 1. **Zona Glomerulosa (Outer):** Produces Mineralocorticoids, primarily **Aldosterone**. It is regulated by Angiotensin II and extracellular potassium levels. Destruction of this layer directly leads to a depletion of aldosterone, resulting in hyperkalemia and hyponatremia. 2. **Zona Fasciculata (Middle):** Produces Glucocorticoids, primarily **Cortisol**. It is the largest layer and is regulated by ACTH. 3. **Zona Reticularis (Inner):** Produces Androgens, such as Dehydroepiandrosterone (DHEA) and **Testosterone** precursors. **Analysis of Incorrect Options:** * **B. Cortisol:** Produced by the Zona Fasciculata, not the Glomerulosa. * **C. Testosterone:** Primarily produced by the testes in males; adrenal contributions come from the Zona Reticularis. * **D. Catecholamines:** These (Epinephrine and Norepinephrine) are produced by the **Adrenal Medulla** (chromaffin cells), which is embryologically derived from the neural crest. **NEET-PG High-Yield Pearls:** * **Conn’s Syndrome:** Primary hyperaldosteronism usually caused by an adenoma in the zona glomerulosa. * **Addison’s Disease:** Primary adrenal insufficiency involving destruction of all three cortical layers, leading to deficiencies in aldosterone, cortisol, and androgens. * **Regulation:** Unlike the Fasciculata and Reticularis, the Zona Glomerulosa is relatively independent of the anterior pituitary (ACTH) and is primarily controlled by the **Renin-Angiotensin-Aldosterone System (RAAS)**.

Q: Anuria is defined as urine output less than:

4 ml/hr. **Explanation:** In clinical physiology, urine output is a critical indicator of renal perfusion and function. The definitions of reduced urine output are categorized based on the volume produced over a 24-hour period or per hour. **1. Why Option A is Correct:** **Anuria** is clinically defined as a urine output of **less than 100 ml in 24 hours**. To find the hourly rate, we divide 100 ml by 24 hours, which equals approximately **4.16 ml/hr**. Therefore, 4 ml/hr is the standard threshold used to define anuric states in a clinical setting. Anuria usually indicates a serious condition such as complete urinary tract obstruction, bilateral renal artery occlusion, or severe acute tubular necrosis. **2. Why Other Options are Incorrect:** * **Options B, C, and D (8, 12, and 16 ml/hr):** These values fall within the range of **Oliguria**. Oliguria is defined as urine output between **100 ml and 400 ml per day**. * 400 ml / 24 hours ≈ **17 ml/hr**. * Any value between 4 ml/hr and 17 ml/hr signifies oliguria rather than anuria. **3. High-Yield Clinical Pearls for NEET-PG:** * **Oliguria:** 3 Liters/day (commonly seen in Diabetes Mellitus and Diabetes Insipidus). * **Fixed Specific Gravity (1.010):** Known as **Isosthenuria**, this indicates chronic renal failure where the kidney loses its ability to concentrate or dilute urine. * **Urine Output Monitoring:** It is the most sensitive non-invasive indicator of cardiac output and tissue perfusion in critically ill patients.

Q: What is the approximate length of the distal convoluted tubule?

5 mm. **Explanation:** The **Distal Convoluted Tubule (DCT)** is a critical segment of the nephron responsible for the fine-tuning of electrolytes and acid-base balance. Anatomically, it begins at the macula densa (at the end of the thick ascending limb) and extends to the collecting duct. **1. Why Option A (5 mm) is Correct:** In standard physiological texts (such as Ganong and Guyton), the DCT is described as being approximately **5 mm** in length. It is significantly shorter than the Proximal Convoluted Tubule (PCT), which measures about 15 mm. Despite its shorter length, the DCT plays a vital role in the reabsorption of sodium and calcium (regulated by PTH) and is the primary site of action for thiazide diuretics. **2. Analysis of Incorrect Options:** * **Option B (2 mm):** This is too short for the DCT. This length is more characteristic of specific sub-segments or transition zones within the renal medulla. * **Option C (12 mm):** This value is closer to the length of the **Proximal Convoluted Tubule (PCT)**, which is the longest and most convoluted part of the nephron (approx. 12–15 mm). * **Option D (8 mm):** While closer than other options, 8 mm overestimates the average length of the DCT in a standard human nephron. **High-Yield Clinical Pearls for NEET-PG:** * **Histology:** Unlike the PCT, the DCT **lacks a brush border** (microvilli), making its lumen appear clearer and more distinct under a microscope. * **Macula Densa:** The very beginning of the DCT contains specialized cells called the macula densa, which sense distal sodium chloride delivery and regulate the **Tubuloglomerular Feedback (TGF)**. * **Hormonal Control:** The late DCT and collecting duct are the sites where **Aldosterone** acts to increase sodium reabsorption and potassium secretion. * **Diuretic Site:** Thiazides inhibit the **Na+-Cl- symporter** specifically in the early DCT.

Q: What is the most important extracellular buffer?

Bicarbonate. **Explanation:** The **Bicarbonate buffer system ($HCO_3^- / CO_2$)** is the most important extracellular fluid (ECF) buffer. Its primary importance stems not from its concentration or pKa, but from its status as an **"open system."** The lungs can rapidly regulate the concentration of $CO_2$ (the acid component), while the kidneys regulate the concentration of $HCO_3^-$ (the base component). This dual physiological control allows the body to maintain a stable pH of 7.4 despite constant metabolic acid production. **Analysis of Options:** * **B. Plasma Proteins:** These are important intracellular and intravascular buffers (e.g., Albumin), but they are less significant than bicarbonate in the overall ECF because their concentration is lower and they cannot be regulated as rapidly as $CO_2$. * **C. Phosphate:** While the phosphate buffer has a pKa (6.8) closer to physiological pH than bicarbonate, its concentration in the ECF is very low. It is, however, a **major intracellular buffer** and the most important **tubular buffer** in the kidneys (titratable acidity). * **D. Nitrate:** This is not a physiological buffer system in the human body. **NEET-PG High-Yield Pearls:** * **Most important ECF buffer:** Bicarbonate. * **Most important ICF buffer:** Proteins and Phosphates. * **Most important buffer in RBCs:** Hemoglobin (due to the imidazole group of Histidine). * **Henderson-Hasselbalch Equation:** $pH = pKa + \log ([Base]/[Acid])$. For the bicarbonate system, the ratio of $HCO_3^-$ to $CO_2$ is normally **20:1**.

Question 1

A 50-year-old female goes into shock with a blood pressure of 50 mmHg. Which of the following concerning her Glomerular Filtration Rate (GFR) is most likely?

Accepted Answer

GFR will virtually cease.

Answer

Myogenic autoregulation will keep GFR constant.

Answer

Angiotensin II release will maintain normal GFR.

Answer

The macula densa will cause constriction of the afferent arteriole to maintain GFR.

Question 2

Which substance is secreted by the proximal convoluted tubule (PCT)?

Accepted Answer

Hydrogen ions

Answer

Sodium

Answer

Glucose

Answer

Amino acids

Question 3

Which part of the nephron is the dilution segment?

Accepted Answer

Ascending limb of the Loop of Henle

Answer

Descending limb of the Loop of Henle

Answer

Collecting tubule

Answer

Proximal tubule

Question 4

In the kidney, which of the following statements is true?

Accepted Answer

The osmolarity of the medullary interstitium is greater than that of plasma.

Answer

The osmolarity of tubular fluid in the PCT gradually increases.

Answer

The osmolarity of fluid in the DCT is more than in Bowman's capsule.

Answer

Fluid coming out of the descending limb of the loop of Henle is hypotonic.

Question 5

Destruction of the zona glomerulosa will deplete which hormone?

Accepted Answer

Aldosterone

Answer

Cortisol

Answer

Testosterone

Answer

Catecholamines

Question 6

Anuria is defined as urine output less than:

Accepted Answer

4 ml/hr

Answer

8 ml/hr

Answer

12 ml/hr

Answer

16 ml/hr

Question 7

What is the approximate length of the distal convoluted tubule?

Accepted Answer

5 mm

Answer

2 mm

Answer

12 mm

Answer

8 mm

Question 8

Sympathectomy of the kidney results in which of the following responses?

Accepted Answer

Increased urine output

Answer

No change to the glomerular filtration rate, nor to the urine output

Answer

Decreased urine output

Answer

Reduction of glomerular filtration rate and decreased urinary output

Question 9

What is the most important extracellular buffer?

Accepted Answer

Bicarbonate

Answer

Plasma protein

Answer

Phosphate

Answer

Nitrate

Question 10

Which of the following are actions of angiotensin II?

Accepted Answer

Systemic vasodilation

Answer

Systemic vasoconstriction

Answer

Renal vasodilatation

Answer

Reabsorption of Na+ in the proximal renal tubule

Renal Physiology — MCQs

Renal Physiology — MCQs

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