What is the normal cerebral blood flow in milliliters per minute for a healthy adult?
What is the primary action observed in the withdrawal reflex?
Which of the following substances has the same concentration in cerebrospinal fluid (CSF) and plasma?
Cell bodies of orexigenic neurons are present in?
Cushing reflex is associated with all except?
Which of the following does not have sympathetic noradrenergic fibers?
All should be features of a substance to measure GFR, except?
What is the normal range of renal blood flow in humans?
Which of the following is the primary mechanism that drives sodium reabsorption in the proximal tubule?
Which of the following is the most accurate measure of Glomerular Filtration Rate (GFR)?
NEET-PG 2013 - Physiology NEET-PG Practice Questions and MCQs
Question 61: What is the normal cerebral blood flow in milliliters per minute for a healthy adult?
- A. 55 ml/min
- B. 150 ml/min
- C. 750 ml/min (Correct Answer)
- D. 1000 ml/min
Explanation: ***750 ml/min*** - The brain receives approximately **15% of the cardiac output**, which for an average adult with a cardiac output of 5 L/min (5000 ml/min) translates to about **750 ml/min**. - This flow rate is essential to meet the high metabolic demands of the brain, which consumes about **20% of the body's total oxygen**. - For reference, this corresponds to approximately **50-55 ml/100g/min** when normalized to brain tissue weight. *55 ml/min* - This value represents the **cerebral blood flow per 100 grams of brain tissue** (50-55 ml/100g/min), not the **total cerebral blood flow**. - As a total flow value, 55 ml/min would be severely **inadequate** for the entire brain (~1400g) and would lead to immediate **ischemia** and neurological dysfunction. *150 ml/min* - While higher than 55 ml/min, this rate is still **grossly insufficient** to maintain the metabolic needs of the entire adult brain. - Such a low total flow would result in widespread **cerebral hypoperfusion** and severe neurological deficits. *1000 ml/min* - Although the brain has significant blood flow, 1000 ml/min is generally **higher than the normal average** for a healthy adult at rest. - The normal range is typically **750-800 ml/min**; sustained flow at 1000 ml/min might be seen in hyperemia or certain physiological states but is not the typical baseline.
Question 62: What is the primary action observed in the withdrawal reflex?
- A. Extension
- B. Flexion (Correct Answer)
- C. Flexion followed by extension
- D. Not applicable
Explanation: ***Flexion*** - The **withdrawal reflex** is a protective reflex that causes the affected limb to **flex** and withdraw from a painful stimulus. - This **flexion** is mediated by the contraction of flexor muscles and relaxation of extensor muscles, moving the limb away from danger. *Extension* - **Extension** is the opposite of flexion and would move the limb closer to or maintain its position relative to the painful stimulus. - This action is typically observed in the **crossed extensor reflex**, where the contralateral limb extends to support the body, not in the direct withdrawal of the stimulated limb. *Flexion followed by extension* - While **flexion** is the primary action, it is not typically followed immediately by extension within the same limb in a simple withdrawal reflex. - If a coordinated movement were to occur, such as shifting weight, the **crossed extensor reflex** would involve extension in the opposite limb. *Not applicable* - The withdrawal reflex involves a clear and defined muscle action which is **flexion**, making "not applicable" incorrect. - This reflex is a fundamental component of the nervous system's response to noxious stimuli.
Question 63: Which of the following substances has the same concentration in cerebrospinal fluid (CSF) and plasma?
- A. Glucose
- B. Ca
- C. HCO3
- D. Cl (Correct Answer)
Explanation: ***Cl*** - **Chloride ions (Cl-)** have the **closest concentration** between CSF and plasma among the listed options, with a CSF-to-plasma ratio of approximately 1.1-1.15. - CSF chloride is **slightly higher** than plasma chloride (CSF: ~120-130 mEq/L; Plasma: ~100-110 mEq/L) because chloride ions freely cross the **blood-brain barrier** and help maintain **electroneutrality** in CSF due to the low protein content. - The elevated chloride compensates for the absence of negatively charged proteins in CSF, making it the **best answer** among the given options. *Glucose* - **Glucose** concentration in CSF is approximately **60-70%** of plasma glucose concentration (CSF: 50-80 mg/dL; Plasma: 70-110 mg/dL). - Transport across the **blood-brain barrier** occurs via **GLUT1 transporters**, which are tightly regulated to meet brain metabolic demands. *Ca* - **Calcium (Ca2+)** concentration in CSF is **significantly lower** than in plasma (CSF: ~2.1-2.5 mg/dL; Plasma: ~8.5-10.5 mg/dL). - Only the **ionized, unbound fraction** can cross the blood-brain barrier, as protein-bound calcium cannot pass through. *HCO3* - **Bicarbonate (HCO3-)** concentration in CSF is typically **slightly lower** than in plasma (CSF: ~20-25 mEq/L; Plasma: ~22-28 mEq/L). - Active regulation maintains **CSF pH** and CO2 buffering capacity independent of plasma bicarbonate levels.
Question 64: Cell bodies of orexigenic neurons are present in?
- A. Dorsal raphe
- B. Locus coeruleus
- C. Lateral hypothalamic area (Correct Answer)
- D. Hippocampus
Explanation: ***Lateral hypothalamic area*** - The **lateral hypothalamic area** (LHA) contains neurons that produce **orexin (hypocretin)**, a neuropeptide critical for promoting appetite and wakefulness. - Stimulation of the LHA leads to increased food seeking and consumption, earning it the moniker "**feeding center**." *Dorsal raphe* - The **dorsal raphe nucleus** is a key source of **serotonin** in the brain, involved in mood, sleep-wake cycles, and appetite regulation (often promoting satiety). - It does not primarily house orexigenic neurons that directly stimulate appetite. *Locus coerulus* - The **locus coeruleus** is the primary source of **norepinephrine** in the brain, playing a significant role in arousal, attention, and stress response. - While it modulates appetitive behaviors indirectly, its neurons are not the primary orexigenic cell bodies. *Hippocampus* - The **hippocampus** is crucial for **learning, memory formation**, and spatial navigation. - It is not directly involved in the primary neural circuits that control hunger and satiety through orexigenic neuropeptides.
Question 65: Cushing reflex is associated with all except?
- A. Irregular respiration
- B. Hypotension (Correct Answer)
- C. Increased intracranial pressure
- D. Bradycardia
Explanation: ***Hypotension*** - The **Cushing reflex** is a compensatory response to increased intracranial pressure (ICP) aiming to maintain cerebral perfusion, which typically involves **hypertension**, not hypotension. - While prolonged or severe ICP can lead to decompensation and eventual hypotension, it is not a direct component of the reflex itself. *Increased intracranial pressure* - The **Cushing reflex** is triggered by an elevation in **intracranial pressure (ICP)**, as the body attempts to maintain blood flow to the brain. - This increased ICP reduces cerebral perfusion pressure, prompting a systemic response to raise mean arterial pressure. *Bradycardia* - **Bradycardia** is a classic component of the **Cushing reflex**, occurring as a compensatory response to the reflex hypertension. - The increased arterial blood pressure stimulates carotid and aortic baroreceptors, leading to a vagal response that slows the heart rate. *Irregular respiration* - **Irregular respiration** is another key component of the **Cushing reflex**, often manifesting as **Cheyne-Stokes breathing** or **ataxic breathing**. - This respiratory dysregulation is due to direct compression and dysfunction of the brainstem, specifically the medullary respiratory centers, caused by increased ICP.
Question 66: Which of the following does not have sympathetic noradrenergic fibers?
- A. Heart
- B. Eye
- C. Sweat gland (Correct Answer)
- D. Blood vessels
Explanation: ***Sweat gland*** - While sweat glands are innervated by the **sympathetic nervous system**, their postganglionic fibers are **cholinergic**, releasing **acetylcholine** rather than noradrenaline. - This is an important exception where sympathetic stimulation leads to acetylcholine release, causing sweating. *Blood vessels* - Most blood vessels, particularly resistance vessels such as **arterioles**, receive substantial **sympathetic noradrenergic innervation** that causes vasoconstriction. - This sympathetic tone is crucial for regulating **blood pressure** and distributing blood flow. *Heart* - The heart is richly innervated by **sympathetic noradrenergic fibers** that increase **heart rate**, **contractility**, and **conduction velocity** via beta-1 adrenergic receptors. - This makes noradrenaline a key neurotransmitter in the sympathetic regulation of cardiac function. *Eye* - The eye receives sympathetic noradrenergic innervation primarily to the **dilator pupillae muscle**, causing **mydriasis** (pupil dilation) upon activation. - These fibers also contribute to the sympathetic control of the **tarsal muscle** (Müller's muscle) in the eyelid.
Question 67: All should be features of a substance to measure GFR, except?
- A. Freely reabsorbed (Correct Answer)
- B. Not secreted by kidney
- C. Freely filtered across the glomerulus membrane
- D. None of the options
Explanation: ***Freely reabsorbed*** - A substance used to measure GFR should **not be reabsorbed** by the kidney tubules. If it were reabsorbed, the amount excreted in the urine would be less than the amount filtered, leading to an **underestimation of GFR**. - The ideal GFR marker is **neither reabsorbed nor secreted**, ensuring that its excretion rate directly reflects the filtration rate. *Freely filtered across the glomerulus membrane* - For a substance to accurately measure GFR, it must be **freely filtered** from the blood into the Bowman's capsule, without any restriction due to its size or charge. - This ensures that its concentration in the glomerular filtrate is the same as in the plasma, allowing for a direct calculation of the filtration rate. *Not secreted by kidney* - An ideal GFR marker should **not be secreted** by the renal tubules, as active secretion would add to the amount excreted in the urine, leading to an **overestimation of GFR**. - This property, along with not being reabsorbed, ensures that the amount of the substance appearing in the urine solely reflects the amount filtered. *None of the options* - This option is incorrect because there is a definitive feature listed among the choices that is *not* a characteristic of an ideal GFR marker. The ability to be "freely reabsorbed" is a disqualifying trait.
Question 68: What is the normal range of renal blood flow in humans?
- A. 1 to 1.2 L/min (Correct Answer)
- B. 1.5 to 2 L/min
- C. 2 to 2.5 L/min
- D. 2.5 to 3 L/min
Explanation: ***1 to 1.2 L/min*** - The **kidneys** receive a substantial portion of the **cardiac output**, typically around 20-25%, to perform their filtration and regulatory functions. - This translates to an absolute renal blood flow of approximately **1000 to 1200 mL/min**, or **1 to 1.2 liters per minute**. - This represents the normal physiological range for healthy adults at rest. *1.5 to 2 L/min* - This range is **higher than the normal physiological** renal blood flow. - While renal blood flow can be influenced by various factors, sustained flow in this range would typically be considered **above the average baseline** for healthy individuals. *2 to 2.5 L/min* - This range significantly **exceeds the typical** renal blood flow observed in healthy humans. - Such high flow rates would be **unusual** and are not representative of normal renal perfusion. *2.5 to 3 L/min* - This range represents an **extremely high** renal blood flow, far beyond what is considered normal. - Sustained perfusion at this level would be **pathological** or indicative of an experimental setting rather than a physiological state.
Question 69: Which of the following is the primary mechanism that drives sodium reabsorption in the proximal tubule?
- A. Sodium reabsorption through cotransport with amino acids at the luminal membrane.
- B. Sodium reabsorption through cotransport with glucose at the luminal membrane.
- C. Sodium reabsorption through countertransport with hydrogen ions at the luminal membrane.
- D. Active sodium transport via the Na+-K+-ATPase pump at the basolateral membrane. (Correct Answer)
Explanation: ***Active sodium transport via the Na+-K+-ATPase pump at the basolateral membrane.*** - This pump **actively transports sodium out of the cell** into the interstitial fluid, creating a low intracellular sodium concentration. - The **Na+-K+-ATPase** is the primary driver of sodium reabsorption throughout the nephron, creating the electrochemical gradient for other sodium transporters. *Sodium reabsorption through cotransport with amino acids at the luminal membrane.* - While **sodium-amino acid cotransport** does occur in the proximal tubule, it accounts for only a fraction of total sodium reabsorption. - The primary driving force for this cotransport is the **low intracellular sodium concentration** maintained by the Na+-K+-ATPase. *Sodium reabsorption through cotransport with glucose at the luminal membrane.* - **Sodium-glucose cotransporters (SGLTs)** are crucial for glucose reabsorption in the proximal tubule, moving glucose into the cell along with sodium. - However, glucose cotransport represents a specific mechanism for glucose handling, not the overarching mechanism for sodium reabsorption. *Sodium reabsorption through countertransport with hydrogen ions at the luminal membrane.* - The **Na+-H+ exchanger (NHE3)** is significant for exchanging sodium for hydrogen ions at the luminal membrane in the proximal tubule. - This mechanism is important for **acid-base balance** and some sodium reabsorption, but it is secondary to the Na+-K+-ATPase in driving the overall sodium gradient.
Question 70: Which of the following is the most accurate measure of Glomerular Filtration Rate (GFR)?
- A. Cystatin C
- B. Serum creatinine
- C. Creatinine Clearance
- D. Iothalamate Clearance (Correct Answer)
Explanation: ***Iothalamate Clearance*** - **Iothalamate clearance** is considered the **gold standard** for directly measuring GFR in clinical practice because it is a substance that is freely filtered by the glomerulus and is neither reabsorbed nor secreted by the renal tubules. - This method provides the most accurate and precise assessment of kidney function by quantifying the actual GFR, often used in research settings or for precise diagnosis. - **Note:** Inulin clearance is the traditional reference standard, but iothalamate is more practical and widely used clinically as it can be measured using radioactive or non-radioactive methods. *Serum creatinine* - **Serum creatinine** is a commonly used biomarker but is an **imperfect measure** of GFR because it can be influenced by factors like muscle mass, diet, and certain medications. - Its levels can remain within the normal range even when GFR has significantly decreased, especially in the early stages of kidney disease. *Cystatin C* - **Cystatin C** is a protein produced by most nucleated cells and is also freely filtered by the glomerulus, with less influence from muscle mass and diet compared to creatinine. - While considered a better marker than serum creatinine, it is still an **estimated measure** and is more expensive and less widely available than creatinine, and can be affected by inflammation or thyroid dysfunction. *Creatinine Clearance* - **Creatinine clearance** (often estimated using urine and serum creatinine levels over a timed collection) attempts to approximate GFR but can be **inaccurate** due to incomplete urine collection and tubular secretion of creatinine. - The **creatinine secretion** by the renal tubules leads to an overestimation of the true GFR, making it less accurate than direct measurement methods.