Aortic valve closure corresponds to the beginning of which phase of the cardiac cycle?
Cerebral blood flow is most directly increased by?
What does the ST Segment of an ECG correspond to?
Cerebral blood flow is regulated by all, EXCEPT:
In an ECG the cardiac event corresponding to the ST segment is:
The ST Segment of an ECG corresponds to which phase of the action potential?
Which of the following structures contains baroreceptors that detect changes in blood pressure?
Which of the following conditions can lead to a decrease in afterload?
What is the typical oxygen saturation level of venous blood?
Which of the following factors is most commonly targeted therapeutically for blood pressure control?
NEET-PG 2015 - Physiology NEET-PG Practice Questions and MCQs
Question 61: Aortic valve closure corresponds to the beginning of which phase of the cardiac cycle?
- A. Systole
- B. Parasystole
- C. Isovolumetric contraction
- D. Isovolumetric relaxation (Correct Answer)
Explanation: ***Isovolumetric relaxation*** - **Aortic valve closure** marks the end of **ventricular ejection** and the beginning of **isovolumetric relaxation** as both the aortic and mitral valves are closed, and ventricular pressure drops without a change in volume. - This phase is vital for the heart to relax and prepare for filling, corresponding to the **second heart sound (S2)**. *Systole* - **Systole** refers to the **contraction phase** of the heart, encompassing both isovolumetric contraction and ventricular ejection. - Aortic valve closure signifies the end of the **ejection phase** of systole, not its beginning. *Parasystole* - **Parasystole** is an **arrhythmia** where an ectopic pacemaker competes with the normal sinus rhythm, leading to independent atrial or ventricular contractions. - It is a **pathological condition** and not a normal phase of the cardiac cycle. *Isovolumetric contraction* - **Isovolumetric contraction** occurs after the **mitral valve closes** and before the aortic valve opens, causing pressure to build in the ventricle. - This phase precedes **ventricular ejection** and is initiated by mitral valve closure, not aortic valve closure.
Question 62: Cerebral blood flow is most directly increased by?
- A. Increase in PO2
- B. Increase in PCO2 (Correct Answer)
- C. Decrease metabolic rate
- D. Increase in metabolic rate
Explanation: ***Increase in PCO2*** - An increase in **arterial PCO2** (partial pressure of carbon dioxide) causes **cerebral vasodilation**, leading to a direct increase in cerebral blood flow. - This is a potent regulatory mechanism to ensure adequate **carbon dioxide removal** and **oxygen supply** to the brain. *Increase in PO2* - An increase in **arterial PO2** (partial pressure of oxygen) causes **mild cerebral vasoconstriction**, which would tend to decrease cerebral blood flow, not increase it. - Cerebral blood flow is generally **less sensitive** to changes in PO2 within the normal range compared to PCO2. *Decrease metabolic rate* - A decrease in the brain's **metabolic rate** would typically lead to a **decrease in local demand** for oxygen and nutrients, resulting in **decreased cerebral blood flow**. - Cerebral blood flow is intrinsically linked to the metabolic needs of brain tissue. *Increase in metabolic rate* - An increase in the brain's **metabolic rate** would lead to an **increase in demand** for oxygen and glucose, which in turn causes **vasodilation** and an increase in cerebral blood flow. - However, this is an indirect effect, whereas an increase in PCO2 directly causes vasodilation.
Question 63: What does the ST Segment of an ECG correspond to?
- A. Ventricular depolarization
- B. Plateau phase between ventricular depolarization and repolarization (Correct Answer)
- C. Atrial depolarization
- D. AV Conduction
Explanation: ***Plateau phase between ventricular depolarization and repolarization*** - The **ST segment** represents the electrically neutral period between ventricular depolarization and repolarization, corresponding to the **plateau phase (phase 2)** of the ventricular action potential. - During this phase, the entire ventricular myocardium is depolarized, and there is minimal electrical activity, typically causing the ST segment to be **isoelectric**. *Ventricular depolarization* - This electrical event is represented by the **QRS complex** on the ECG, not the ST segment. - The QRS complex signifies the rapid spread of electrical impulses through the ventricles, leading to their contraction. *Atrial depolarization* - **Atrial depolarization** is represented by the **P wave** on the ECG. - This wave indicates the electrical activation of the atria, which precedes atrial contraction. *AV Conduction* - **AV conduction** time is primarily represented by the **PR interval** on the ECG. - The PR interval measures the time from the beginning of atrial depolarization to the beginning of ventricular depolarization, encompassing the delay at the AV node.
Question 64: Cerebral blood flow is regulated by all, EXCEPT:
- A. Intracranial pressure
- B. Cerebral metabolic rate
- C. Potassium ions (Correct Answer)
- D. Arterial PCO2
Explanation: ***Potassium ions*** - While potassium ions play a crucial role in neuronal excitability and membrane potential, they are **not a primary direct regulator** of cerebral blood flow (CBF) in the same way as other factors listed. - Changes in extracellular potassium can affect vascular smooth muscle, but their direct impact on CBF auto-regulation is less pronounced compared to metabolic or pressure-related factors. *Intracranial pressure* - **Increased intracranial pressure (ICP)** can significantly decrease cerebral blood flow due to the **Monro-Kellie doctrine**, which states that an increase in ICP reduces the cerebral perfusion pressure (CPP). - A sustained and significant elevation in ICP can lead to **cerebral ischemia** as it opposes the arterial pressure driving blood into the brain. *Arterial PCO2* - **Arterial PCO2** is a potent regulator of cerebral blood flow, with **hypercapnia (high PCO2)** causing **vasodilation** and increased CBF. - Conversely, **hypocapnia (low PCO2)** leads to **vasoconstriction** and decreased CBF, which is a key mechanism in the management of cerebral edema. *Cerebral metabolic rate* - **Cerebral metabolic rate (CMR)** is directly coupled to cerebral blood flow, meaning that regions of the brain with higher metabolic activity receive increased blood flow. - This **neurovascular coupling** ensures adequate supply of oxygen and nutrients to meet the brain's metabolic demands.
Question 65: In an ECG the cardiac event corresponding to the ST segment is:
- A. Atrial depolarisation
- B. Ventricular depolarisation
- C. Atrial repolarisation
- D. Ventricular repolarisation (Correct Answer)
Explanation: ***Ventricular repolarisation*** - The **ST segment** represents the **early phase of ventricular repolarization**, corresponding to the **plateau phase (Phase 2)** of the ventricular action potential. - During this phase, the ventricles are completely depolarized and calcium influx balances potassium efflux, creating an isoelectric (flat) segment on the ECG. - The ST segment extends from the **end of the QRS complex (J point)** to the **beginning of the T wave**, after which rapid repolarization occurs. - Together, the **ST segment and T wave** represent the complete process of ventricular repolarization. *Atrial depolarisation* - **Atrial depolarization** is represented by the **P wave** on the ECG, not the ST segment. - This occurs first in the cardiac cycle, triggering atrial contraction and filling of the ventricles. *Ventricular depolarisation* - **Ventricular depolarization** is represented by the **QRS complex**, which immediately **precedes** the ST segment. - This event triggers ventricular contraction (systole) and occurs before the plateau phase. *Atrial repolarisation* - **Atrial repolarization** occurs during the QRS complex and is **obscured** by the much larger electrical signal from ventricular depolarization. - It is not visible as a separate deflection on the standard ECG.
Question 66: The ST Segment of an ECG corresponds to which phase of the action potential?
- A. Rapid repolarization
- B. Final repolarization
- C. Plateau phase (Correct Answer)
- D. Rapid depolarization
Explanation: ***Plateau phase*** - The **ST segment** of the ECG represents the period when the ventricles are completely depolarized and corresponds to the **plateau phase (phase 2)** of the ventricular myocardial action potential. - During this phase, there is a balance between **calcium influx** and **potassium efflux**, maintaining the depolarized state and contributing to the sustained contraction of the ventricles. *Rapid depolarization* - This phase, represented by the **QRS complex** on the ECG, signifies the rapid influx of sodium ions into the ventricular cells. - It corresponds to **phase 0** of the action potential, where there is a sharp upstroke. *Rapid repolarization* - This corresponds to **phase 3** of the ventricular action potential, where potassium ions rapidly exit the cell, leading to repolarization. - On the ECG, this phase is represented by the **T wave**. *Final repolarization* - This is **not a standard electrophysiological term** in cardiac action potential nomenclature. - The complete repolarization process is represented by the **T wave** (phase 3), which returns the ventricle to its resting potential (phase 4). - The term may cause confusion as it doesn't correspond to a specific phase or ECG component.
Question 67: Which of the following structures contains baroreceptors that detect changes in blood pressure?
- A. Carotid body
- B. Carotid sinus (Correct Answer)
- C. Aortic body
- D. None of the options
Explanation: ***Carotid sinus*** - The **carotid sinus** is a dilation at the bifurcation of the common carotid artery, containing **baroreceptors** sensitive to changes in blood pressure [1]. - These baroreceptors are **mechanoreceptors** that respond to the stretching of the vessel wall due to increased arterial pressure, sending signals to the brainstem to regulate blood pressure. *Carotid body* - The **carotid body** is a chemoreceptor that primarily detects changes in **blood oxygen, carbon dioxide, and pH** levels, not blood pressure [2]. - It plays a crucial role in regulating **respiration** in response to hypoxemia. *Aortic body* - The **aortic body** is a **chemoreceptor** located near the aortic arch that primarily monitors **blood oxygen, carbon dioxide, and pH levels**. - Note: While the aortic body itself is a chemoreceptor, the **aortic arch** (a different structure) does contain baroreceptors [1]. However, this option specifically refers to the aortic body, which is not a baroreceptor. - The aortic body contributes to the regulation of **respiration** in response to hypoxemia, not directly blood pressure. *None of the options* - This option is incorrect because the **carotid sinus** is a well-known site for baroreceptors involved in blood pressure regulation.
Question 68: Which of the following conditions can lead to a decrease in afterload?
- A. Severe anemia (Correct Answer)
- B. Hypothyroidism
- C. Increased physical activity
- D. None of the options
Explanation: ***Severe anemia*** - In **severe anemia**, the **blood viscosity** is reduced, and the body compensates by decreasing systemic vascular resistance to maintain tissue perfusion, thereby lowering **afterload**. - The reduced **oxygen-carrying capacity** triggers vasodilation to maximize blood flow to tissues, contributing to decreased afterload. - This represents a **chronic compensatory mechanism** that results in sustained reduction of afterload. *Hypothyroidism* - **Hypothyroidism** typically leads to an **increase in systemic vascular resistance** and thus can increase afterload. - It often results in **bradycardia** and reduced cardiac output, which can further elevate afterload to maintain pressure. *Increased physical activity* - During **physical activity**, there is **vasodilation in exercising muscles**, which acutely decreases systemic vascular resistance. - However, this is accompanied by **increased cardiac output** and **elevated blood pressure** due to sympathetic stimulation, and the afterload reduction is **transient** rather than sustained. - In the context of this question asking about conditions that lead to decreased afterload, **severe anemia** is the better answer as it represents a chronic pathological state with sustained afterload reduction, whereas exercise represents a temporary physiological response. *None of the options* - This option is incorrect because **severe anemia** is a recognized cause of decreased afterload.
Question 69: What is the typical oxygen saturation level of venous blood?
- A. 30%
- B. 50%
- C. 70% (Correct Answer)
- D. 90%
Explanation: ***70%*** - Venous blood has a lower oxygen saturation compared to arterial blood because tissues have extracted a significant amount of oxygen for **cellular respiration**. - A typical mixed venous oxygen saturation (SvO2) is around **70-75%**, indicating the amount of oxygen remaining after tissues have taken what they need. *30%* - This level of oxygen saturation is **too low** for typical venous blood and would indicate severe tissue hypoperfusion or extreme oxygen extraction. - Such low levels are usually not compatible with normal physiological function for prolonged periods. *50%* - While lower than normal, a 50% venous oxygen saturation is still indicative of **increased oxygen extraction** by tissues, often seen in conditions of increased metabolic demand or decreased oxygen delivery. - It's not the typical resting value for healthy individuals. *90%* - An oxygen saturation of 90% is more characteristic of **arterial blood** (normal arterial saturation is 95-100%). - Venous blood, having already delivered oxygen to tissues, would normally have a lower saturation.
Question 70: Which of the following factors is most commonly targeted therapeutically for blood pressure control?
- A. Heart rate
- B. Peripheral resistance (Correct Answer)
- C. Cardiac output
- D. Stroke volume
Explanation: ***Peripheral resistance*** - **Peripheral resistance** is primarily determined by the **arteriolar tone**, which can be effectively modulated by various antihypertensive medications. - Medications like **ACE inhibitors**, **ARBs**, **calcium channel blockers**, and **diuretics** all influence peripheral resistance to lower blood pressure. *Heart rate* - While heart rate contributes to **cardiac output** and thus blood pressure, it is not the most common primary target for hypertension management. - **Beta-blockers** reduce heart rate, but they are often used for specific indications beyond essential hypertension, such as angina or post-MI. *Cardiac output* - **Cardiac output** is a product of **heart rate** and **stroke volume**, and while it directly impacts blood pressure, directly targeting cardiac output as a whole is less common than modulating its individual components or peripheral resistance. - Many antihypertensive drugs reduce cardiac output as a secondary effect of reducing blood volume or heart rate, but directly reducing cardiac output is not the primary mechanism for the most common medications. *Stroke volume* - **Stroke volume** is influenced by **preload**, **afterload**, and **contractility**, and while it impacts cardiac output, it is generally less accessible for direct pharmacological manipulation in hypertension management compared to peripheral resistance. - **Diuretics** can indirectly reduce stroke volume by decreasing preload, but this is often considered a mechanism related to volume status rather than a direct myocardial effect.