Which of the following is the MOST important factor determining whether a substance can be filtered at the glomerulus?
Tubuloglomerular feedback is for regulation of?
Which of the following is the primary factor involved in mesangial cell contraction?
Mechanism of secretion of ammonia in distal tubule is?
A substance has a clearance similar to inulin clearance. How is this substance primarily excreted in urine?
What is the normal range of urinary pH?
Increased aldosterone and ADH secretion following major trauma results in all the following except?
Which carrier pump is responsible for transporting solutes in the thick ascending limb of the loop of Henle?
What is the primary function of the myenteric plexus?
Secretion of bile out of hepatocytes occurs via?
NEET-PG 2015 - Physiology NEET-PG Practice Questions and MCQs
Question 81: Which of the following is the MOST important factor determining whether a substance can be filtered at the glomerulus?
- A. Lipid solubility of the substance
- B. Molecular weight of the substance (Correct Answer)
- C. Binding capacity to albumin
- D. None of the options
Explanation: ***Molecular weight of the substance*** - The **glomerular filtration barrier** acts as a size-selective filter, generally permeable to substances with a molecular weight less than 5,000-10,000 Daltons - Larger molecules are typically restricted from filtration due to the **size exclusion** property of the glomerular basement membrane and podocyte slit diaphragms - This is the **primary determinant** of whether a substance can be filtered at all, making it the most important factor among the given options *Lipid solubility of the substance* - **Lipid solubility** is more relevant for reabsorption and secretion in the renal tubules, particularly for passive diffusion across tubular cell membranes - It has minimal direct influence on the initial filtration process at the glomerulus, which is primarily a **pressure-driven, size- and charge-selective ultrafiltration** process - The glomerular capillary wall is not a lipid membrane barrier for the filtration process *Binding capacity to albumin* - Substances bound to **large plasma proteins** like albumin (molecular weight ~67,000 Daltons) cannot pass through the glomerular filtration barrier - While important for determining the *free, filterable fraction* of a substance in plasma, the binding itself is secondary to the fundamental molecular weight/size restriction - Only the **free (unbound) fraction** of a substance is available for filtration, and whether it filters depends primarily on its molecular weight *None of the options* - This option is incorrect because **molecular weight** is indeed the most critical factor among the given options for determining whether a substance can be filtered at the glomerulus
Question 82: Tubuloglomerular feedback is for regulation of?
- A. BP
- B. Blood volume
- C. Na+ reabsorption
- D. Renal blood flow (Correct Answer)
Explanation: ***Renal blood flow*** - **Tubuloglomerular feedback (TGF)** is a key mechanism that helps to tightly regulate **renal blood flow** and **glomerular filtration rate (GFR)** by sensing tubular fluid composition. - This mechanism involves the **macula densa** cells in the distal tubule sensing changes in **sodium chloride (NaCl) delivery**, which then signals the afferent arteriole to adjust its caliber. *BP* - While renal blood flow ultimately influences **blood pressure (BP)**, tubuloglomerular feedback is primarily focused on **local autoregulation** within the kidney, not systemic BP control. - Systemic BP is regulated by much broader mechanisms involving the **renin-angiotensin-aldosterone system** and **autonomic nervous system**. *Blood volume* - **Blood volume** is primarily regulated by hormones like **ADH (vasopressin)** and **aldosterone**, which influence water and sodium reabsorption in the collecting ducts and other parts of the nephron. - Although renal function impacts blood volume, tubuloglomerular feedback's direct role is not in regulating the overall volume of blood. *Na+ reabsorption* - While the macula densa senses **Na+ delivery** to the distal tubule and this influences GFR, the primary role of tubuloglomerular feedback is not to modulate the total amount of **Na+ reabsorbed** throughout the nephron. - Na+ reabsorption is regulated by multiple factors and occurs extensively in the **proximal tubule**, **loop of Henle**, and **distal tubule**, often under hormonal control.
Question 83: Which of the following is the primary factor involved in mesangial cell contraction?
- A. ANP
- B. Endothelin-1
- C. Angiotensin II (Correct Answer)
- D. Platelet-activating factor (PAF)
Explanation: ***Angiotensin II*** - **Angiotensin II** is a potent vasoconstrictor that directly stimulates **mesangial cell contraction**. - Contraction of mesangial cells reduces the **glomerular surface area** available for filtration, thereby decreasing the **glomerular filtration rate (GFR)**. *Endothelin-1* - **Endothelin-1** is a potent vasoconstrictor produced by endothelial cells, which can also induce mesangial cell contraction. - However, its role in **mesangial cell contraction** is generally considered secondary to **angiotensin II** in physiological regulation. *ANP* - **Atrial natriuretic peptide (ANP)** is a hormone that causes **vasodilation** and relaxation of mesangial cells. - Its primary effect is to **increase GFR** and sodium excretion, opposing the effects of vasoconstrictors. *Platelet-activating factor (PAF)* - PAF is a **phospholipid mediator** involved in inflammation and allergic reactions. - While it can affect renal hemodynamics, its role in directly and primarily causing **mesangial cell contraction** is less significant compared to angiotensin II.
Question 84: Mechanism of secretion of ammonia in distal tubule is?
- A. Primary active transport
- B. Symport
- C. Antiport
- D. Passive diffusion (Correct Answer)
Explanation: ***Passive diffusion*** - Ammonia (NH3) is a **lipid-soluble molecule** that can readily cross cell membranes, including those of the distal tubule and collecting duct, down its **concentration gradient**. - This process is crucial for regulating **acid-base balance**, as NH3 traps H+ ions to form NH4+, which is then excreted. *Primary active transport* - This mechanism involves the direct use of **ATP hydrolysis** to move ions against their concentration gradient, which is not the primary way ammonia is secreted in the distal tubule. - While NH4+ can be secreted via active transport in some segments (e.g., substituting for K+ on the Na-K-2Cl cotransporter in the thick ascending limb), free ammonia diffusion is distinct. *Symport* - **Symport** involves the co-transport of two or more different molecules or ions in the same direction across a cell membrane, powered by an electrochemical gradient. - This mechanism is not typically involved in the secretion of uncharged, lipid-soluble ammonia. *Antiport* - **Antiport** is a type of coupled transport where two different ions or molecules move in opposite directions across a membrane. - While antiport systems are essential for various renal functions (e.g., Na+/H+ exchanger), they are not the primary mechanism for the secretion of free ammonia in the distal tubule.
Question 85: A substance has a clearance similar to inulin clearance. How is this substance primarily excreted in urine?
- A. Tubular Secretion
- B. Glomerular filtration (Correct Answer)
- C. Vascular leakage
- D. Both tubular secretion and glomerular filtration
Explanation: ***Glomerular filtration*** - **Inulin** is a gold standard for measuring **glomerular filtration rate** (GFR) because it is freely filtered by the glomeruli and is neither reabsorbed nor secreted by the renal tubules. - Therefore, a substance with clearance similar to inulin is primarily excreted via **glomerular filtration**. *Tubular Secretion* - If a substance were primarily excreted by tubular secretion, its clearance would be **higher than the GFR**, as secretion adds more of the substance to the urine than filtration alone. - This mechanism is characteristic of substances like **para-aminohippurate (PAH)**, which is used to measure renal plasma flow. *Vascular leakage* - **Vascular leakage** is not a normal mechanism of substance excretion in the urine. - It refers to the abnormal passage of fluid and macromolecules from blood vessels into tissues, often seen in conditions like inflammation or sepsis, and does not directly contribute to renal clearance. *Both tubular secretion and glomerular filtration* - If a substance were excreted by both **tubular secretion and glomerular filtration**, its clearance would also be **higher than the GFR**, similar to substances that undergo significant tubular secretion. - The fact that its clearance is *similar* to inulin specifically points to filtration as the predominant and almost exclusive mechanism.
Question 86: What is the normal range of urinary pH?
- A. 4.5 - 5.0
- B. 6.0 - 6.5 (Correct Answer)
- C. 5.0 - 5.5
- D. 7.0 - 7.5
Explanation: ***6.0 - 6.5*** - This represents the **average normal urinary pH range** in healthy individuals. - While the kidney can produce urine with pH ranging from 4.5 to 8.0 (the full physiological range), the **typical urinary pH** in most healthy people is around 6.0, making this the most representative normal range. - This slightly acidic pH reflects normal renal handling of dietary acids and metabolic processes. *4.5 - 5.0* - This represents the **lower acidic end** of the physiological range. - While kidneys can produce urine this acidic in response to acid loads, this is **not the average normal range**. - Persistently low pH may indicate **metabolic acidosis**, high protein diet, or conditions like diabetic ketoacidosis. *5.0 - 5.5* - This range is **more acidic than average** but still within physiological limits. - This may be seen with high protein intake or mild acid loading, but it's not the most representative of typical normal urinary pH. *7.0 - 7.5* - This represents a **more alkaline urine**, which is at the upper end of the physiological range. - While healthy kidneys can produce alkaline urine (especially with alkaline diets), persistently elevated pH may indicate **urinary tract infections** with urea-splitting bacteria (Proteus species), renal tubular acidosis, or alkaline diet.
Question 87: Increased aldosterone and ADH secretion following major trauma results in all the following except?
- A. Increased osmolarity of urine
- B. Increased water excretion (Correct Answer)
- C. Increased K+ excretion in urine
- D. Decreased Na+ excretion in urine
Explanation: ***Increased water excretion*** - **ADH (antidiuretic hormone)** increases water reabsorption in the collecting ducts, leading to a *decrease* in water excretion, not an increase. - Increased aldosterone and ADH would promote fluid retention to maintain blood volume following trauma, thus reducing water loss via urine. *Decreased Na+ excretion in urine* - **Aldosterone** acts on the renal tubules to increase **sodium reabsorption** and potassium excretion. - This response is crucial in **conserving sodium** and thereby maintaining extracellular fluid volume after trauma. *Increased K+ excretion in urine* - **Aldosterone** directly stimulates **potassium secretion** into the urine in the principal cells of the collecting ducts. - This is a normal physiological consequence of increased aldosterone levels. *Increased osmolarity of urine* - **ADH** increases the permeability of the collecting ducts to water, leading to **more water reabsorption** back into the bloodstream. - This removal of water from the urine concentrates the solutes, resulting in a **more concentrated (higher osmolarity)** urine.
Question 88: Which carrier pump is responsible for transporting solutes in the thick ascending limb of the loop of Henle?
- A. NaCl cotransporter
- B. Na+-H+ exchanger
- C. Na+-K+ exchanger
- D. Sodium-potassium-chloride cotransporter (Correct Answer)
Explanation: * **Sodium-potassium-chloride cotransporter.** * This transporter, specifically the **Na+-K+-2Cl- cotransporter (NKCC2)**, is highly expressed in the apical membrane of the thick ascending limb. * It actively reabsorbs **sodium, potassium, and chloride ions** from the filtrate, contributing significantly to the medullary interstitial osmotic gradient. * *NaCl- cotransporter* * The **NaCl cotransporter (NCC)** is primarily found in the **distal convoluted tubule**, not the thick ascending limb. * It reabsorbs sodium and chloride in a 1:1 ratio and is the target of thiazide diuretics. * *Na+-H+ exchanger* * The **Na+-H+ exchanger (NHE3)** is predominantly located in the **proximal tubule** where it plays a crucial role in bicarbonate reabsorption and acid-base balance. * While some NHE activity exists in other nephron segments, it is not the primary carrier in the thick ascending limb. * *Na+-K+ exchanger* * The **Na+-K+ exchanger** or **Na+/K+-ATPase pump** is located on the basolateral membrane of most renal tubular cells, including the thick ascending limb. * Its main function is to maintain the electrochemical gradient by pumping **sodium out of the cell** and potassium into the cell, which indirectly drives other transporters but is not the apical cotransporter responsible for initial solute reabsorption in the thick ascending limb.
Question 89: What is the primary function of the myenteric plexus?
- A. Regulating GI secretion
- B. Regulating local blood flow
- C. Regulating motility (Correct Answer)
- D. Regulating absorption
Explanation: ***Regulating motility*** - The myenteric plexus, also known as **Auerbach's plexus**, is primarily responsible for coordinating the **rhythmic contractions** and **relaxation of the gastrointestinal (GI) smooth muscle**. - Its strategic location between the **longitudinal and circular muscle layers** allows it to directly influence the strength and frequency of peristalsis, thus regulating the movement of food through the digestive tract. *Regulating GI secretion* - While it has some indirect influence, the **submucosal plexus** (Meissner's plexus) is the primary neural network regulating **secretory functions** of the GI tract. - The myenteric plexus's main role is more directly related to muscle contraction and relaxation rather than glandular secretion. *Regulating local blood flow* - Local blood flow in the GI tract is primarily regulated by the **sympathetic and parasympathetic nervous systems**, along with local metabolic factors and hormones. - The myenteric plexus has a minimal direct role in the control of **GI blood vessel smooth muscle**. *Regulating absorption* - Absorption is primarily a function of the **intestinal epithelial cells** and is regulated by various transport mechanisms, hormones, and local factors. - While the enteric nervous system influences mucosal function indirectly, the myenteric plexus's primary role is **motor control** rather than directly regulating nutrient absorption processes.
Question 90: Secretion of bile out of hepatocytes occurs via?
- A. Passive diffusion
- B. Facilitated diffusion
- C. Osmosis
- D. Active transport (Correct Answer)
Explanation: ***Active transport*** - Bile secretion by hepatocytes is an **energy-dependent process** against concentration gradients, requiring specific transporter proteins. - This active mechanism ensures efficient bile flow and prevents reflux, crucial for processes like **fat digestion and absorption**. *Passive diffusion* - **Passive diffusion** involves substances moving down their concentration gradient without energy expenditure. - Bile components, like bile salts, are highly concentrated within hepatocytes and need to be moved against this gradient. *Facilitated diffusion* - This type of diffusion utilizes **transmembrane proteins** to move substances down their concentration gradient, but still does not directly consume ATP. - Bile components are often transported against their concentration gradient, which is not facilitated diffusion. *Osmosis* - **Osmosis** specifically refers to the movement of water across a semipermeable membrane from a region of higher water concentration to a region of lower water concentration. - While water is a component of bile, the primary mechanism of moving bile solutes out of the hepatocyte is not osmosis.