Tubular Reabsorption and Secretion Practice Questions
20 free Tubular Reabsorption and Secretion practice questions for the Physiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following transport mechanisms is primarily responsible for the reabsorption of glucose in the early proximal convoluted tubule?
- A Secondary active transport via SGLT2
- B Facilitated diffusion via GLUT1
- C Primary active transport via Na+/K+ ATPase
- D Paracellular solvent drag
Correct answer: Secondary active transport via SGLT2
In the early proximal tubule, SGLT2 uses the sodium gradient established by the basolateral Na+/K+ pump to move glucose into the cell against its concentration gradient. This is a classic example of secondary active transport.
The 'Transport Maximum' (Tm) for a substance like glucose refers to the point where:
- A All available carrier proteins for that substance are saturated
- B The plasma concentration is so low that filtration stops
- C The substance begins to be actively secreted directly into the renal tubule
- D The GFR increases to match the tubular load
Correct answer: All available carrier proteins for that substance are saturated
Tm occurs when the filtered load of a substance exceeds the capacity of the tubular transporters. Once all transporters are occupied, any additional filtered substance cannot be reabsorbed and will be excreted in the urine.
Which part of the nephron is responsible for reabsorbing the largest fraction (approximately 65%) of filtered water and sodium?
- A Proximal convoluted tubule
- B Descending limb of the Loop of Henle
- C Distal convoluted tubule
- D Medullary collecting duct
Correct answer: Proximal convoluted tubule
The proximal tubule is the workhorse of the kidney, performing bulk reabsorption of water, sodium, chloride, and bicarbonate. This process is 'isostatic,' meaning water follows the solutes osmotically in equal proportion.
In the thick ascending limb of the Loop of Henle, the primary apical transporter targeted by 'loop diuretics' like furosemide is the:
- A Na+/H+ exchanger
- B Na+/Cl- symporter
- C Epithelial sodium channels (ENaC)
- D Na+/K+/2Cl- cotransporter (NKCC2)
Correct answer: Na+/K+/2Cl- cotransporter (NKCC2)
NKCC2 is the major transporter in the thick ascending limb that moves sodium, potassium, and chloride into the cell. Inhibition by loop diuretics prevents the establishment of the medullary osmotic gradient, leading to potent diuresis.
Which of the following describes the handling of Urea in the proximal tubule?
- A It is actively secreted into the lumen to ensure rapid excretion
- B It is 100% reabsorbed via primary active transport
- C It is passively reabsorbed down its concentration gradient
- D It is completely impermeable to the proximal tubular epithelium
Correct answer: It is passively reabsorbed down its concentration gradient
As water is reabsorbed in the proximal tubule, the concentration of urea in the lumen increases. This creates a chemical gradient that drives the passive reabsorption of about 50% of the filtered urea.
Which hormone acts on the distal tubule and collecting duct to increase the secretion of Potassium and the reabsorption of Sodium?
- A Antidiuretic hormone (ADH)
- B Atrial natriuretic peptide (ANP)
- C Aldosterone, a steroid hormone
- D Parathyroid hormone (PTH)
Correct answer: Aldosterone, a steroid hormone
Aldosterone stimulates the synthesis and activity of apical ENaC channels and basolateral Na+/K+ pumps in principal cells. This increases sodium reabsorption and creates an electrical gradient that promotes potassium secretion.
The 'Fanconi Syndrome' is a generalized dysfunction of which nephron segment, leading to the loss of glucose, amino acids, and bicarbonate in the urine?
- A Glomerulus
- B Proximal tubule
- C Thick ascending limb
- D Collecting duct
Correct answer: Proximal tubule
Fanconi syndrome involves impaired reabsorptive capacity of the proximal convoluted tubule. Because this is the site for nearly all nutrient reabsorption, patients excrete substances that should normally be absent from urine.
Bicarbonate (HCO3-) reabsorption in the proximal tubule requires the action of which enzyme on the apical brush border?
- A Carbonic anhydrase
- B Na+/K+ ATPase
- C Angiotensin converting enzyme
- D Renin
Correct answer: Carbonic anhydrase
Filtered bicarbonate combines with secreted H+ to form H2CO3, which is then broken down into CO2 and H2O by carbonic anhydrase. The CO2 diffuses into the cell, where the process is reversed to regenerate bicarbonate for reabsorption.
The 'obligatory water reabsorption' occurs in the proximal tubule and descending limb because these areas are always permeable to water due to high levels of:
- A Aquaporin-1
- B Aquaporin-2
- C Vasopressin receptors
- D Sodium-glucose linkers
Correct answer: Aquaporin-1
Aquaporin-1 is constitutively expressed in the proximal tubule and descending limb, allowing water to move freely. In contrast, Aquaporin-2 in the collecting duct is regulated by ADH.
Which substance is actively secreted into the proximal tubule and is used clinically to estimate Renal Plasma Flow (RPF)?
- A Inulin, a plant polysaccharide
- B Creatinine, a muscle metabolite
- C Para-aminohippuric acid (PAH)
- D Albumin, a plasma protein
Correct answer: Para-aminohippuric acid (PAH)
PAH is both filtered and secreted so efficiently that nearly all of it is removed from the plasma in a single pass through the kidney. Consequently, the clearance of PAH approximates the volume of plasma reaching the functional nephrons.
What is the primary effect of Parathyroid Hormone (PTH) on the proximal tubule?
- A Stimulates calcium reabsorption
- B Inhibits phosphate reabsorption
- C Stimulates sodium reabsorption
- D Inhibits bicarbonate secretion
Correct answer: Inhibits phosphate reabsorption
PTH inhibits the Na+/Phosphate cotransporter in the proximal tubule, leading to increased phosphate excretion (phosphaturia). It simultaneously works in the distal tubule to increase calcium reabsorption.
Which of the following describes 'Secondary Active Secretion' of Hydrogen ions in the proximal tubule?
- A H+ is moved out in exchange for Na+ moving into the cell
- B H+ is pumped out by an ATP-driven primary pump directly
- C H+ moves out passively through open ion channels only
- D H+ is cotransported out of the cell together with glucose
Correct answer: H+ is moved out in exchange for Na+ moving into the cell
The Na+/H+ exchanger (NHE3) uses the inward sodium gradient to move H+ out of the cell into the lumen. This process is essential for bicarbonate reabsorption and acid-base balance.
The term 'Renal Threshold' for glucose (approx. 180 mg/dL) describes the plasma concentration at which:
- A SGLT transporters become completely and permanently inhibited
- B Glucose first begins to appear in the urine in trace amounts
- C The kidneys stop filtering glucose at the glomerulus
- D Insulin production reaches its maximum secretion rate
Correct answer: Glucose first begins to appear in the urine in trace amounts
The renal threshold is the plasma level where the filtered load exceeds the reabsorptive capacity of some nephrons. Because of 'splay' (variance between nephrons), glucose appears in the urine before the total Transport Maximum is reached.
In the distal convoluted tubule, the reabsorption of Sodium and Chloride is primarily mediated by which transporter?
- A NKCC2 (Na-K-2Cl) symporter
- B NCC (Na+/Cl- symporter)
- C ENaC sodium channel
- D SGLT1 transporter
Correct answer: NCC (Na+/Cl- symporter)
The NCC symporter is the primary target of thiazide diuretics. This segment is impermeable to water, contributing to the further dilution of the tubular fluid.
Which specialized cells in the collecting duct are responsible for H+ secretion and HCO3- reabsorption during respiratory acidosis?
- A Principal cells
- B Type A intercalated cells
- C Type B intercalated cells
- D Macula densa cells
Correct answer: Type A intercalated cells
Type A intercalated cells use H+-ATPase and H+/K+-ATPase to secrete acid into the lumen. This helps the body compensate for acidosis by eliminating excess protons and regenerating bicarbonate.
How does an increase in Peritubular Capillary Oncotic Pressure affect the rate of fluid reabsorption from the proximal tubule?
- A It decreases reabsorption
- B It increases reabsorption
- C It has no effect on reabsorption
- D It reverses the flow into the tubule
Correct answer: It increases reabsorption
Higher oncotic pressure in the peritubular capillaries acts as a 'pulling' force that favors the movement of fluid from the interstitium into the blood. This facilitates the overall reabsorption process started by the tubular cells.
Which of the following is true regarding the 'Thin Ascending Limb' of the Loop of Henle?
- A It is highly permeable to water along its entire length
- B It actively pumps sodium ions out into the medullary interstitium
- C It is impermeable to water but allows passive diffusion of sodium
- D It reabsorbs glucose from the filtrate via facilitated diffusion carriers
Correct answer: It is impermeable to water but allows passive diffusion of sodium
The thin ascending limb is impermeable to water but allows solutes like NaCl to diffuse out into the interstitium. This contributes to the high osmolarity of the inner medulla without the use of active transport.
In the presence of high Antidiuretic Hormone (ADH), the largest amount of water reabsorption still occurs in the:
- A Proximal tubule
- B Loop of Henle
- C Distal tubule
- D Collecting duct
Correct answer: Proximal tubule
Even though ADH is required for water reabsorption in the collecting duct (facultative), the proximal tubule always reabsorbs the bulk (65%) of water regardless of ADH levels (obligatory).
The secretion of organic anions and cations (such as penicillin or bile salts) occurs primarily in the:
- A Glomerular capsule
- B Distal tubule
- C Collecting duct
- D Proximal tubule
Correct answer: Proximal tubule
The proximal tubule contains various organic anion transporters (OATs) and organic cation transporters (OCTs). These systems allow the kidney to rapidly clear metabolic waste and exogenous drugs from the blood.
Which of the following ions is reabsorbed paracellularly in the thick ascending limb due to the 'lumen-positive' potential created by K+ back-leakage?
- A Sodium ions alone
- B Chloride ions alone
- C Bicarbonate ions alone
- D Calcium and Magnesium
Correct answer: Calcium and Magnesium
The recycling of potassium into the lumen via ROMK channels creates a positive electrical potential. This charge repels divalent cations like Ca2+ and Mg2+, driving them between the cells into the interstitium.