Regional blood flow distribution — MCQs

10 questions

A woman with coronary artery disease is starting to go for a walk. As she begins, her heart rate accelerates from a resting pulse of 60 bpm until it reaches a rate of 120 bpm, at which point she begins to feel a tightening in her chest. She stops walking to rest and the tightening resolves. This has been happening to her consistently for the last 6 months. Which of the following is a true statement?

Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

During exercise, what is the primary mechanism for increased oxygen delivery to active muscles?

A histological examination of the carotid body reveals glomus cells containing dense-core vesicles. These cells function primarily as chemoreceptors for which of the following?

A group of investigators is studying thermoregulatory adaptations of the human body. A subject is seated in a thermally insulated isolation chamber with an internal temperature of 48°C (118°F), a pressure of 1 atmosphere, and a relative humidity of 10%. Which of the following is the primary mechanism of heat loss in this subject?

An investigator is studying the effect of antihypertensive drugs on cardiac output and renal blood flow. For comparison, a healthy volunteer is given a placebo and a continuous infusion of para-aminohippuric acid (PAH) to achieve a plasma concentration of 0.02 mg/ml. His urinary flow rate is 1.5 ml/min and the urinary concentration of PAH is measured to be 8 mg/ml. His hematocrit is 50%. Which of the following values best estimates cardiac output in this volunteer?

Which factor most strongly influences coronary blood flow during exercise?

A 44-year-old man is brought to the emergency department 45 minutes after being involved in a high-speed motor vehicle collision in which he was the restrained driver. On arrival, he has left hip and left leg pain. His pulse is 135/min, respirations are 28/min, and blood pressure is 90/40 mm Hg. Examination shows an open left tibial fracture with active bleeding. The left lower extremity appears shortened, flexed, and internally rotated. Femoral and pedal pulses are decreased bilaterally. Massive transfusion protocol is initiated. An x-ray of the pelvis shows an open pelvis fracture and an open left tibial mid-shaft fracture. A CT scan of the head shows no abnormalities. Laboratory studies show: Hemoglobin 10.2 g/dL Leukocyte count 10,000/mm3 Platelet count <250,000/mm3 Prothrombin time 12 sec Partial thromboplastin time 30 sec Serum Na+ 125 mEq/L K+ 4.5 mEq/L Cl- 98 mEq/L HCO3- 25 mEq/L Urea nitrogen 18 mg/dL Creatinine 1.2 mg/dL The patient is taken emergently to interventional radiology for exploratory angiography and arterial embolization. Which of the following is the most likely explanation for this patient's hyponatremia?

A 66-year-old man is brought to the emergency department 20 minutes after being involved in a high-speed motor vehicle collision in which he was the unrestrained passenger. His wife confirms that he has hypertension, atrial fibrillation, and chronic lower back pain. Current medications include metoprolol, warfarin, hydrochlorothiazide, and oxycodone. On arrival, he is lethargic and confused. His pulse is 112/min, respirations are 10/min, and blood pressure is 172/78 mm Hg. The eyes open spontaneously. The pupils are equal and sluggish. He moves his extremities in response to commands. There is a 3-cm scalp laceration. There are multiple bruises over the right upper extremity. Cardiopulmonary examination shows no abnormalities. The abdomen is soft and nontender. Neurologic examination shows no focal findings. Two large-bore peripheral intravenous catheters are inserted. A 0.9% saline infusion is begun. A focused assessment with sonography in trauma is negative. Plain CT of the brain shows a 5-mm right subdural hematoma with no mass effect. Fresh frozen plasma is administered. Which of the following is most likely to reduce this patient's cerebral blood flow?

Q10

Which of the following cyanotic heart diseases cause increased pulmonary blood flow? 1. Ebstein anomaly 2. Tetralogy of Fallot 3. Transposition of the great arteries (TGA) 4. Total anomalous pulmonary venous communication (TAPVC) Select the correct combination:

Regional blood flow distribution US Medical PG Practice Questions and MCQs

Question 1: A woman with coronary artery disease is starting to go for a walk. As she begins, her heart rate accelerates from a resting pulse of 60 bpm until it reaches a rate of 120 bpm, at which point she begins to feel a tightening in her chest. She stops walking to rest and the tightening resolves. This has been happening to her consistently for the last 6 months. Which of the following is a true statement?

A. This patient's chest pain is indicative of transmural ischemia
B. Perfusion of the myocardium takes place equally throughout the cardiac cycle
C. Increasing the heart rate increases the amount of time spent during each cardiac cycle
D. Increasing the heart rate decreases the relative amount of time spent during diastole (Correct Answer)
E. Perfusion of the myocardium takes place primarily during systole

Explanation: ***Increasing the heart rate decreases the relative amount of time spent during diastole*** - With increasing heart rate, the **duration of the cardiac cycle decreases**, but this reduction is disproportionately greater in **diastole (filling phase)** compared to systole (ejection phase), which becomes critical in patients with coronary artery disease as myocardial perfusion occurs during diastole. - Reduced diastolic time means less time for **coronary artery filling** and **myocardial perfusion**, exacerbating ischemia in the presence of fixed coronary stenosis. *This patient's chest pain is indicative of transmural ischemia* - The patient's symptoms are consistent with **stable angina**, characterized by chest pain with exertion that resolves with rest, suggesting **subendocardial ischemia** rather than transmural. - **Transmural ischemia** typically indicates a more severe, often prolonged, and extensive reduction in blood flow, such as in a **ST-elevation myocardial infarction (STEMI)**. *Perfusion of the myocardium takes place equally throughout the cardiac cycle* - Myocardial perfusion is **not equal throughout the cardiac cycle**; it primarily occurs during **diastole** when the heart muscle is relaxed and coronary arteries are less compressed. - During **systole**, the contracting myocardium compresses the coronary arteries, restricting blood flow, especially to the **subendocardial layers**. *Increasing the heart rate increases the amount of time spent during each cardiac cycle* - **Increasing heart rate** by definition **decreases the total duration of each cardiac cycle** (e.g., if heart rate is 60 bpm, cycle duration is 1 second; if 120 bpm, cycle duration is 0.5 seconds). - While both systole and diastole shorten, the **diastolic phase shortens more significantly**, which is problematic for myocardial perfusion. *Perfusion of the myocardium takes place primarily during systole* - **Myocardial perfusion primarily occurs during diastole**, not systole, because the **intramyocardial pressure is lower** and the coronary arteries are less compressed, allowing for better blood flow. - During **systole**, the high intramyocardial pressure, especially in the left ventricular wall, compresses the coronary vessels, significantly reducing blood flow to the myocardium.

Question 2: Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

A. Inhaling the inspiratory reserve volume (IRV)
B. Exhaling the entire vital capacity (VC)
C. Exhaling the expiratory reserve volume (ERV)
D. Breath holding maneuver at functional residual capacity (FRC)
E. Inhaling the entire vital capacity (VC) (Correct Answer)

Explanation: ***Inhaling the entire vital capacity (VC)*** - As lung volume increases from FRC to TLC (which includes inhaling the entire VC), alveolar vessels are **stretched open**, and extra-alveolar vessels are **pulled open** by the increased radial traction, leading to a decrease in PVR. - This **maximizes the cross-sectional area** of the pulmonary vascular bed, lowering resistance. *Inhaling the inspiratory reserve volume (IRV)* - While inhaling IRV increases lung volume, it's not the maximal inspiration of the entire VC where **PVR is typically at its lowest**. - PVR continues to decrease as lung volume approaches total lung capacity (TLC). *Exhaling the entire vital capacity (VC)* - Exhaling the entire vital capacity leads to very low lung volumes, where PVR significantly **increases**. - At low lung volumes, **alveolar vessels become compressed** and extra-alveolar vessels **narrow**, increasing resistance. *Exhaling the expiratory reserve volume (ERV)* - Exhaling the ERV results in a lung volume below FRC, which causes a **marked increase in PVR**. - This is due to the **compression of alveolar vessels** and decreased radial traction on extra-alveolar vessels. *Breath holding maneuver at functional residual capacity (FRC)* - At FRC, the PVR is at an **intermediate level**, not its lowest. - This is the point where the opposing forces affecting alveolar and extra-alveolar vessels are somewhat balanced, but not optimized for minimal resistance.

Question 3: During exercise, what is the primary mechanism for increased oxygen delivery to active muscles?

A. Decreased blood viscosity
B. Increased cardiac output (Correct Answer)
C. Increased hemoglobin affinity
D. Enhanced oxygen diffusion

Explanation: ***Increased cardiac output*** - During exercise, **cardiac output** increases significantly due to both an elevated **heart rate** and increased **stroke volume**, directly pushing more oxygenated blood to the active muscles. - This augmentation in blood flow is the primary factor ensuring a sufficient supply of oxygen and nutrients to meet the heightened metabolic demands of exercising muscles. *Decreased blood viscosity* - While factors like **hemodilution** can decrease blood viscosity during prolonged exercise, this effect is relatively minor and not the primary mechanism for acute increases in oxygen delivery compared to the dramatic increase in cardiac output. - A decrease in blood viscosity can slightly improve flow efficiency, but it doesn't fundamentally change the amount of blood pumped per minute to the muscles. *Increased hemoglobin affinity* - An *increased* hemoglobin affinity for oxygen would actually make it *harder* for oxygen to unload from hemoglobin to the tissues, which is counterproductive for oxygen delivery during exercise. - In fact, during exercise, local conditions like increased temperature, decreased pH (**Bohr effect**), and increased 2,3-BPG tend to *decrease* hemoglobin's affinity for oxygen, facilitating oxygen release to active muscles. *Enhanced oxygen diffusion* - While exercise does improve the efficiency of oxygen extraction at the tissue level due to a steeper partial pressure gradient and increased capillary recruitment, the *rate* of oxygen diffusion across the capillary membrane isn't the primary modulator of overall oxygen delivery. - The main determinant is the *amount* of oxygenated blood reaching the muscle, which is governed by cardiac output and local blood flow regulation.

Question 4: A histological examination of the carotid body reveals glomus cells containing dense-core vesicles. These cells function primarily as chemoreceptors for which of the following?

A. Partial pressure of oxygen (Correct Answer)
B. Blood pH
C. Temperature
D. Blood glucose levels

Explanation: ***Partial pressure of oxygen*** - Carotid body **glomus cells** are highly specialized **chemoreceptors** that primarily sense changes in the **partial pressure of oxygen (PO2)** in arterial blood. - When PO2 decreases (e.g., hypoxia), these cells are activated and stimulate the respiratory and cardiovascular systems to increase oxygen uptake. *Blood pH* - While carotid body chemoreceptors can sense large changes in blood pH, their primary and most sensitive role is in detecting changes in **PO2**. - Central chemoreceptors in the brainstem are more crucial for routine regulation of respiration in response to changes in **pH and PCO2**. *Temperature* - **Thermoreceptors** located in the skin, hypothalamus, and other internal organs are responsible for sensing body temperature, not the carotid body. - The carotid body's main function is related to blood gas homeostasis, not temperature regulation. *Blood glucose levels* - Blood glucose levels are regulated by specialized cells in the **pancreas** (islets of Langerhans) that secrete hormones like insulin and glucagon. - The carotid body is not directly involved in sensing or regulating glucose homeostasis.

Question 5: A group of investigators is studying thermoregulatory adaptations of the human body. A subject is seated in a thermally insulated isolation chamber with an internal temperature of 48°C (118°F), a pressure of 1 atmosphere, and a relative humidity of 10%. Which of the following is the primary mechanism of heat loss in this subject?

A. Convection
B. Evaporation (Correct Answer)
C. Conduction
D. Piloerection
E. Radiation

Explanation: ***Evaporation*** - In an environment where the ambient temperature (48°C) is **higher than body temperature**, heat gain by convection, conduction, and radiation occurs. Therefore, **evaporation** of sweat is the only significant mechanism for heat loss. - The relatively low humidity (10%) at this high temperature facilitates efficient sweat **evaporation**, which cools the body as it converts liquid sweat into water vapor. *Convection* - **Convection** involves heat transfer through the movement of air or fluid over the body surface. - Since the ambient temperature (48°C) is significantly **above body temperature**, the body would gain heat via convection, not lose it. *Conduction* - **Conduction** is direct heat transfer between objects in contact. - As the ambient temperature (48°C) is much **higher than the skin temperature**, the body would actually **gain heat** through conduction from any surfaces it touched if they were at ambient temperature. *Piloerection* - **Piloerection** (goosebumps) is a mechanism for minimizing heat loss by trapping a layer of warm air close to the skin. - This response is activated in **cold environments** to conserve heat, not in hot environments to dissipate it. *Radiation* - **Radiation** is heat transfer via electromagnetic waves without direct contact. - Since the ambient temperature (48°C) is **higher than body surface temperature**, the body would **gain heat** by radiation, not lose it efficiently, from the surrounding environment.

Question 6: An investigator is studying the effect of antihypertensive drugs on cardiac output and renal blood flow. For comparison, a healthy volunteer is given a placebo and a continuous infusion of para-aminohippuric acid (PAH) to achieve a plasma concentration of 0.02 mg/ml. His urinary flow rate is 1.5 ml/min and the urinary concentration of PAH is measured to be 8 mg/ml. His hematocrit is 50%. Which of the following values best estimates cardiac output in this volunteer?

A. 8 L/min
B. 3 L/min
C. 4 L/min
D. 1.2 L/min
E. 6 L/min (Correct Answer)

Explanation: ***6 L/min*** - This value represents the estimated **cardiac output** based on the calculated renal blood flow. - Step 1: Calculate renal plasma flow (RPF) using PAH clearance: RPF = (Urinary PAH × Urine flow rate) / Plasma PAH = (8 mg/ml × 1.5 ml/min) / 0.02 mg/ml = 600 ml/min = 0.6 L/min - Step 2: Calculate renal blood flow (RBF): Since hematocrit is 50%, RBF = RPF / (1 - Hematocrit) = 0.6 / 0.5 = 1.2 L/min - Step 3: Estimate cardiac output: The kidneys normally receive approximately **20-25% of cardiac output**. Using 20%: Cardiac Output = RBF / 0.20 = 1.2 / 0.20 = **6 L/min** - This is consistent with normal resting cardiac output in a healthy adult. *8 L/min* - This value overestimates cardiac output based on the renal blood flow calculation. - While some individuals may have higher cardiac output during exercise, the calculated RBF of 1.2 L/min suggests a resting cardiac output closer to 6 L/min. *3 L/min* - This value significantly underestimates cardiac output. - If cardiac output were 3 L/min, the kidneys would be receiving 40% of cardiac output (1.2/3), which is physiologically implausible at rest. *4 L/min* - This value underestimates cardiac output based on the renal data. - This would mean kidneys receive 30% of cardiac output (1.2/4), which is higher than the typical 20-25%. *1.2 L/min* - This is the calculated **renal blood flow**, not cardiac output. - While this calculation is correct for RBF, the question specifically asks for cardiac output estimation, which requires accounting for the fact that kidneys receive only about 20-25% of total cardiac output.

Question 7: Which factor most strongly influences coronary blood flow during exercise?

A. Endothelin release
B. Metabolic demand (Correct Answer)
C. Myogenic response
D. Neural regulation
E. Baroreceptor reflex

Explanation: **Metabolic demand** - During exercise, increased **myocardial activity** leads to a higher demand for oxygen and nutrients, prompting a significant increase in coronary blood flow. - Local release of **metabolites** such as adenosine, nitric oxide, and hydrogen ions causes powerful vasodilation of coronary arteries, closely matching blood supply to demand. *Endothelin release* - **Endothelin** is a potent vasoconstrictor and plays a role in regulating vascular tone, but its primary influence is not the immediate or strongest factor dictating increased coronary flow during exercise. - While it can modulate flow, metabolic changes are the dominant driver for the rapid and substantial increases needed during exertion. *Myogenic response* - The **myogenic response** is an intrinsic property of vascular smooth muscle cells to contract when stretched (due to increased pressure) and relax when pressure decreases, helping to maintain relatively constant blood flow. - This mechanism primarily contributes to **autoregulation** and flow stability, but it does not account for the massive increase in flow required by the heart during exercise. *Neural regulation* - **Neural regulation**, primarily sympathetic stimulation, increases heart rate and contractility, which indirectly increases metabolic demand. - However, direct neural effects on coronary arteries can be complex (both vasodilation and vasoconstriction depending on receptor type), and the overriding control during exercise is typically metabolic.

Question 8: A 44-year-old man is brought to the emergency department 45 minutes after being involved in a high-speed motor vehicle collision in which he was the restrained driver. On arrival, he has left hip and left leg pain. His pulse is 135/min, respirations are 28/min, and blood pressure is 90/40 mm Hg. Examination shows an open left tibial fracture with active bleeding. The left lower extremity appears shortened, flexed, and internally rotated. Femoral and pedal pulses are decreased bilaterally. Massive transfusion protocol is initiated. An x-ray of the pelvis shows an open pelvis fracture and an open left tibial mid-shaft fracture. A CT scan of the head shows no abnormalities. Laboratory studies show: Hemoglobin 10.2 g/dL Leukocyte count 10,000/mm3 Platelet count <250,000/mm3 Prothrombin time 12 sec Partial thromboplastin time 30 sec Serum Na+ 125 mEq/L K+ 4.5 mEq/L Cl- 98 mEq/L HCO3- 25 mEq/L Urea nitrogen 18 mg/dL Creatinine 1.2 mg/dL The patient is taken emergently to interventional radiology for exploratory angiography and arterial embolization. Which of the following is the most likely explanation for this patient's hyponatremia?

A. Pathologic aldosterone secretion
B. Physiologic aldosterone secretion
C. Adrenal crisis
D. Pathologic ADH (vasopressin) secretion
E. Physiologic ADH (vasopressin) secretion (Correct Answer)

Explanation: ***Physiologic ADH (vasopressin) secretion*** - The patient has significant **hypovolemia** due to massive bleeding from an open pelvic fracture and an open tibial fracture, leading to **hypotension** (BP 90/40 mmHg) and **tachycardia** (HR 135/min). This severe hypovolemia is a potent non-osmotic stimulus for ADH release. - **Physiologic ADH secretion** in response to hypovolemia acts to conserve water, but in the context of ongoing fluid resuscitation with hypotonic fluids (like normal saline after initial blood loss), it leads to **dilutional hyponatremia** as water is retained disproportionately to sodium. *Pathologic aldosterone secretion* - **Pathologic aldosterone secretion** (e.g., from an adrenal adenoma) causes primary hyperaldosteronism, which typically results in **hypertension**, **hypokalemia**, and **metabolic alkalosis**, none of which are seen in this patient. - While aldosterone does contribute to sodium reabsorption, its primary role in this acute, hypovolemic state is to defend circulating volume, and pathologic excess would not explain the observed hyponatremia. *Physiologic aldosterone secretion* - **Physiologic aldosterone secretion** would be appropriately elevated in response to hypovolemia to promote **sodium and water reabsorption** and **potassium excretion** to maintain circulating volume. - While aldosterone conserves sodium, it does not directly cause hyponatremia; rather, it would tend to increase serum sodium by retaining it, as long as ADH is not excessively retaining free water. *Adrenal crisis* - **Adrenal crisis** (acute adrenal insufficiency) would present with severe hypotension, but it is also characterized by **hyponatremia**, **hyperkalemia**, and often **hypoglycemia** due to cortisol deficiency. - Although hyponatremia is present, the patient's potassium is normal (4.5 mEq/L), making adrenal crisis less likely given the absence of hyperkalemia. *Pathologic ADH (vasopressin) secretion* - **Pathologic ADH secretion** (e.g., Syndrome of Inappropriate Antidiuretic Hormone secretion - SIADH) typically occurs in **euvolemic or mildly hypervolemic** states, often associated with malignancies, CNS disorders, or certain drugs. - In SIADH, patients are typically euvolemic (not hypovolemic as seen here), urine osmolality is inappropriately high, and urine sodium is usually elevated (>20 mEq/L), which contradicts the patient's clinical picture of severe hypovolemia.

Question 9: A 66-year-old man is brought to the emergency department 20 minutes after being involved in a high-speed motor vehicle collision in which he was the unrestrained passenger. His wife confirms that he has hypertension, atrial fibrillation, and chronic lower back pain. Current medications include metoprolol, warfarin, hydrochlorothiazide, and oxycodone. On arrival, he is lethargic and confused. His pulse is 112/min, respirations are 10/min, and blood pressure is 172/78 mm Hg. The eyes open spontaneously. The pupils are equal and sluggish. He moves his extremities in response to commands. There is a 3-cm scalp laceration. There are multiple bruises over the right upper extremity. Cardiopulmonary examination shows no abnormalities. The abdomen is soft and nontender. Neurologic examination shows no focal findings. Two large-bore peripheral intravenous catheters are inserted. A 0.9% saline infusion is begun. A focused assessment with sonography in trauma is negative. Plain CT of the brain shows a 5-mm right subdural hematoma with no mass effect. Fresh frozen plasma is administered. Which of the following is most likely to reduce this patient's cerebral blood flow?

A. Hyperventilation (Correct Answer)
B. Lumbar puncture
C. Decompressive craniectomy
D. Intravenous hypertonic saline
E. Intravenous mannitol

Explanation: ***Hyperventilation*** - **Hyperventilation** reduces arterial partial pressure of carbon dioxide (**PaCO2**), causing **cerebral vasoconstriction** and thereby decreasing cerebral blood flow (CBF). - This effect is used therapeutically to transiently lower **intracranial pressure (ICP)** in cases of acute cerebral edema or herniation by reducing cerebral blood volume. *Lumbar puncture* - A **lumbar puncture** drains cerebrospinal fluid (CSF) from the subarachnoid space, which would reduce ICP. - However, it does not directly impact cerebral blood flow regulations, and in some situations with elevated ICP, it can be hazardous due to the risk of **herniation**. *Decompressive craniectomy* - **Decompressive craniectomy** involves removing a portion of the skull to allow the brain to swell, directly reducing ICP by increasing intracranial volume. - While it lowers ICP, it doesn't directly reduce cerebral blood flow; in fact, by relieving compression, it may help maintain or improve CBF. *Intravenous hypertonic saline* - **Intravenous hypertonic saline** increases serum osmolarity, drawing fluid out of brain cells and into the intravascular space, thereby reducing **cerebral edema** and ICP. - This reduction in edema and ICP can improve rather than reduce cerebral blood flow by reducing extrinsic compression of cerebral vessels. *Intravenous mannitol* - **Intravenous mannitol** is an osmotic diuretic that creates an osmotic gradient, drawing fluid from the brain parenchyma into the intravascular compartment, reducing **cerebral edema** and ICP. - Similar to hypertonic saline, its primary effect is to decrease brain volume and ICP, which tends to improve CBF by reducing vascular compression, not reduce it.

Question 10: Which of the following cyanotic heart diseases cause increased pulmonary blood flow? 1. Ebstein anomaly 2. Tetralogy of Fallot 3. Transposition of the great arteries (TGA) 4. Total anomalous pulmonary venous communication (TAPVC) Select the correct combination:

A. 3,4 (Correct Answer)
B. 1,2
C. 2,4
D. 1,4

Explanation: ***3,4 (TGA and TAPVC)*** - **Transposition of the great arteries (TGA)** involves two parallel circulations with the aorta arising from the right ventricle and pulmonary artery from the left ventricle. Mixing occurs through defects (ASD, VSD, or PDA), leading to **pulmonary overcirculation** as oxygenated blood recirculates through the lungs. - **Total anomalous pulmonary venous connection (TAPVC)** results in all pulmonary veins draining into the systemic venous circulation (typically right atrium). This causes **increased volume load on the right heart** and subsequently increased pulmonary blood flow, with obligatory mixing at the atrial level. *1,2 (Ebstein and ToF)* - Both conditions cause **decreased pulmonary blood flow**. - **Ebstein anomaly** involves apical displacement of the tricuspid valve with "atrialization" of the right ventricle, causing tricuspid regurgitation and right-to-left shunting through an ASD/PFO. - **Tetralogy of Fallot** features right ventricular outflow tract obstruction (pulmonary stenosis) as its defining feature, causing reduced pulmonary blood flow. *2,4* - Incorrect combination: **Tetralogy of Fallot causes decreased pulmonary blood flow** due to RVOT obstruction, not increased. *1,4* - Incorrect combination: **Ebstein anomaly causes decreased pulmonary blood flow**, not increased.

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