Physiology
10 questionsWhich of the following parameters indicates the elimination of CO2 from the lungs?
In zero gravity, the V/Q ratio is?
What is the normal O2 extraction ratio of tissues?
Aortic valve closure occurs in which part of cardiac cycle?
How many phases are there in the action potential of cardiac muscles?
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:
NEET-PG 2015 - Physiology NEET-PG Practice Questions and MCQs
Question 531: Which of the following parameters indicates the elimination of CO2 from the lungs?
- A. pH
- B. PaCO2 (Correct Answer)
- C. PaO2
- D. HCO3 level
Explanation: ***PaCO2*** - **Partial pressure of carbon dioxide in arterial blood (PaCO2)** directly reflects the efficiency of **alveolar ventilation**, which is the process of eliminating CO2 from the lungs. - When CO2 elimination is adequate, PaCO2 remains within the normal range (35-45 mmHg); higher or lower values indicate ventilatory impairment or hyperventilation, respectively. *PaO2* - **PaO2** measures the partial pressure of **oxygen in arterial blood** and indicates oxygenation, not the efficiency of carbon dioxide elimination. - While CO2 elimination and oxygenation are interdependent, **PaO2** primarily reflects how well oxygen is being transported from the lungs to the blood. *pH* - **pH** indicates the **acidity or alkalinity of the blood**, which is influenced by both respiratory (CO2) and metabolic (bicarbonate) components. - Although CO2 elimination affects pH through the carbonic acid-bicarbonate buffer system, pH itself is an overall measure of acid-base balance, not a direct indicator of CO2 elimination. *HCO3 level* - **Bicarbonate (HCO3-)** is a **metabolic component** of the acid-base balance, primarily regulated by the kidneys. - While it helps buffer CO2-induced acid changes, HCO3 level alone does not directly reflect the efficiency of CO2 elimination from the lungs.
Question 532: In zero gravity, the V/Q ratio is?
- A. 0.8
- B. 1 (Correct Answer)
- C. 2
- D. 3
Explanation: ***Correct: 1*** - In **zero gravity**, the normal physiological effects of gravity on both ventilation and perfusion are eliminated, leading to a more uniform distribution. - Without gravity, blood flow and gas distribution become more even throughout the lungs, resulting in a V/Q ratio that approaches **unity (1)** across all lung regions. - This represents the ideal physiological state where ventilation perfectly matches perfusion. *Incorrect: 0.8* - A V/Q ratio of **0.8** represents the **average normal V/Q ratio** in an upright individual on Earth, where gravity creates disparities in ventilation and perfusion. - This value is an average, with regional variations (apex ~3.3, base ~0.6) in the lungs; it does not reflect the uniform conditions of zero gravity. *Incorrect: 2* - A V/Q ratio of **2** would indicate a significant **ventilation-perfusion mismatch** where ventilation greatly exceeds perfusion. - This scenario suggests substantial **dead space ventilation**, which is not the expected outcome in a zero-gravity environment where distribution is balanced. *Incorrect: 3* - A V/Q ratio of **3** represents an even more extreme case of **ventilation exceeding perfusion**, indicating severe physiologic dead space. - Such a high V/Q ratio would signify a major functional impairment, which is contrary to the more ideal and uniform distribution expected in zero gravity.
Question 533: What is the normal O2 extraction ratio of tissues?
- A. 5 percent
- B. 15 percent
- C. 25 percent (Correct Answer)
- D. 40 percent
Explanation: ***25 percent*** - The normal **O2 extraction ratio** (or **oxygen utilization coefficient**) is approximately 25%, meaning tissues extract about one-fourth of the oxygen delivered by arterial blood. - This ratio is crucial for understanding **tissue oxygenation** and can increase significantly during times of high metabolic demand, such as exercise. *5 percent* - An O2 extraction ratio of 5% is **too low** for normal physiological function, indicating that tissues are receiving much more oxygen than they are utilizing. - Such a low ratio would be seen only in situations of **excessive oxygen delivery** or **severely reduced metabolic demand**. *15 percent* - While 15% represents some oxygen extraction, it is **below the normal physiological range** for resting tissues. - An extraction ratio of 15% would mean the tissues are not extracting sufficient oxygen to meet their typical metabolic needs efficiently. *40 percent* - An O2 extraction ratio of 40% is **higher than the normal resting value** and suggests increased oxygen demand by the tissues. - This level of extraction is typically seen during **strenuous exercise** or in conditions of **reduced oxygen delivery** where tissues compensate by extracting more oxygen from available blood.
Question 534: Aortic valve closure occurs in which part of cardiac cycle?
- A. Beginning of isovolumetric contraction
- B. During rapid ventricular filling
- C. Beginning of ventricular ejection
- D. Beginning of isovolumetric relaxation (Correct Answer)
Explanation: ***Beginning of isovolumetric relaxation*** - Aortic valve closure marks the end of **ventricular systole** and the start of **isovolumetric relaxation**, as blood ceases to be ejected and the ventricle begins to relax while remaining closed. - This event corresponds to the **second heart sound (S2)** and signifies the beginning of a period where ventricular volume remains constant, but pressure drops. *Beginning of isovolumetric contraction* - This phase begins with the closure of the **mitral and tricuspid valves** (first heart sound, S1), as ventricular pressure rises but volume remains constant before ejection. - The aortic valve is still closed at this point, as ventricular pressure is not yet high enough to open it. *Beginning of ventricular ejection* - This phase begins when the **aortic valve opens** as ventricular pressure exceeds aortic pressure, allowing blood to be ejected from the left ventricle. - Aortic valve closure occurs *after* ejection, not at its beginning. *During rapid ventricular filling* - Rapid ventricular filling occurs when the **mitral valve opens** (following isovolumetric relaxation), allowing blood to flow from the atria into the ventricles. - During this phase, the aortic valve is closed, but its closure happened earlier, at the beginning of isovolumetric relaxation.
Question 535: How many phases are there in the action potential of cardiac muscles?
- A. 2 phases
- B. 3 phases
- C. 4 phases
- D. 5 phases (Correct Answer)
Explanation: ***5 phases*** - The cardiac myocyte action potential is classically described in **five phases** (phases 0, 1, 2, 3, and 4), which encompass depolarization, repolarization, and the resting state. - Each phase is characterized by specific ion channel activities leading to distinct electrical changes essential for proper cardiac function. *2 phases* - Action potentials in nerve cells typically follow a simpler two-phase model: **depolarization** and **repolarization**. - This model does not account for the additional plateau and resting phases characteristic of cardiac muscle cells. *3 phases* - Some simplified models might describe three phases (depolarization, repolarization, and a resting phase), but this still **omits specific nuances** of cardiac repolarization and the sustained plateau phase. - This simplification leaves out the early repolarization and the critical plateau phase (phase 2), which is vital for the prolonged contraction of the heart. *4 phases* - While some sources might refer to four phases, they typically combine certain repolarization steps or omit the distinct early repolarization phase. - This description would likely miss the **early, rapid repolarization phase (phase 1)**, understating the complex ion movements.
Question 536: 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 537: 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 538: 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 539: 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 540: 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.