Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 841: Which of the following defines vital capacity?

A. Air in lung after normal expiration
B. Maximum air in lung after end of maximal inspiration
C. Maximum air that can be expirated after maximum inspiration (Correct Answer)
D. Maximum air that can be expirated after normal expiration

Explanation: ***Maximum air that can be expirated after maximum inspiration*** - **Vital capacity (VC)** is the maximum volume of air that can be exhaled after a maximal inspiration. - It represents the sum of **tidal volume (TV)**, **inspiratory reserve volume (IRV)**, and **expiratory reserve volume (ERV)**. - VC = TV + IRV + ERV *Air in lung after normal expiration* - This describes the **functional residual capacity (FRC)**, which is the volume of air remaining in the lungs after a normal passive exhalation. - FRC = ERV + RV (expiratory reserve volume + residual volume). *Maximum air that can be expirated after normal expiration* - This refers to the **expiratory reserve volume (ERV)**, the additional amount of air that can be exhaled forcibly after a normal passive exhalation. - It is the extra air expelled beyond tidal volume during forced expiration. *Maximum air in lung after end of maximal inspiration* - This definition corresponds to **total lung capacity (TLC)**, which is the maximum volume of air the lungs can hold after a maximal inspiration. - TLC = VC + RV (vital capacity + residual volume).

Question 842: Gas exchange in tissues takes place at?

A. Artery
B. Capillary (Correct Answer)
C. Vein
D. Venules

Explanation: ***Capillary*** - **Capillaries** are the smallest and most numerous blood vessels, with very thin walls (only one cell thick), which facilitates the efficient exchange of gases, nutrients, and waste products between blood and tissues. - Their extensive network ensures close proximity to nearly every cell in the body, maximizing the surface area and minimizing the diffusion distance for **gas exchange**. *Artery* - Arteries carry **oxygenated blood** away from the heart to the tissues but have thick, muscular walls designed for high pressure and transport, not for direct exchange with tissues. - They branch into smaller arterioles, which then lead to capillaries, making them a conduit rather than an exchange site. *Vein* - Veins carry **deoxygenated blood** back to the heart from the tissues and have relatively thin walls compared to arteries but are still too thick for efficient gas exchange. - They primarily serve as blood return vessels and reservoirs. *Venules* - Venules are small blood vessels that merge from capillaries and eventually combine to form veins; they primarily function in collecting blood from capillary beds. - While slightly more permeable than larger veins, their main role is still collection and transport, not the extensive gas exchange facilitated by capillaries.

Question 843: Transpulmonary pressure is the difference between:

A. Pressure in alveoli and intrapleural pressure (Correct Answer)
B. The pressure in the bronchus and atmospheric pressure
C. The difference between atmospheric pressure and intrapleural pressure
D. The difference between atmospheric pressure and intraalveolar pressure

Explanation: ***Pressure in alveoli and intrapleural pressure*** - Transpulmonary pressure is the **pressure gradient** across the lung wall, which is essential for maintaining alveolar inflation. - It is calculated as the **alveolar pressure minus the intrapleural pressure**. *The pressure in the bronchus and atmospheric pressure* - This difference would represent the pressure driving airflow through the **bronchial tree**, not the pressure across the lung wall itself. - It's a measure relevant to **airway resistance**, not lung distension. *The difference between atmospheric pressure and intrapleural pressure* - This difference is related to the **intrathoracic pressure**, which influences venous return and cardiac function, but not directly the distension of the lungs. - It does not account for the **alveolar pressure**, which is the primary internal pressure expanding the lung. *The difference between atmospheric pressure and intraalveolar pressure* - This difference is the **driving pressure for airflow** into or out of the lungs. - It represents the pressure gradient that causes air to move between the **atmosphere and the alveoli** during inspiration and expiration.

Question 844: True about normal expiration is:

A. In expiration pleural pressure is equal to alveolar pressure
B. Muscles that elevate the chest cage are classified as muscles of expiration
C. At the end of normal expiration, the air in the lung is FRC
D. Chest wall has a tendency to move outward which is balanced by inward recoil of alveoli (Correct Answer)

Explanation: ***Chest wall has a tendency to move outward which is balanced by inward recoil of alveoli*** - At **Functional Residual Capacity (FRC)**, the outward recoil of the **chest wall** balances the inward elastic recoil of the **lungs**, resulting in no net force and a stable lung volume. - This equilibrium point represents the resting volume of the respiratory system when respiratory muscles are relaxed during **normal expiration**. - This statement directly describes the **mechanism** of normal expiration—the passive process driven by balanced recoil forces. *At the end of normal expiration, the air in the lung is FRC* - While **technically true** that FRC is the volume remaining after normal expiration, this option describes the **endpoint volume** rather than the process of normal expiration itself. - The question asks what is true **about normal expiration** (the process), not what is true **at the end** of expiration (the outcome). - The correct answer better addresses the mechanism and forces involved during the expiratory process. *In expiration pleural pressure is equal to alveolar pressure* - **INCORRECT**: Pleural pressure is **always negative** relative to alveolar pressure (typically -5 to -8 cm H₂O at FRC). - During **normal expiration**, pleural pressure becomes *less negative* as lung volume decreases, but **never equals** alveolar pressure. - If pleural pressure equaled alveolar pressure, the lungs would collapse (pneumothorax). *Muscles that elevate the chest cage are classified as muscles of expiration* - **INCORRECT**: Muscles that **elevate the chest cage**, such as the **external intercostals** and **diaphragm**, are primarily involved in **inspiration**. - **Normal expiration** is a *passive process* driven by the elastic recoil of the lungs and chest wall, **not requiring muscle contraction**.

Question 845: What is the estimated PaO2 after giving FiO2 at 0.5 in a normal person?

A. > 200 mmHg (Correct Answer)
B. < 100 mmHg
C. 150–200 mmHg
D. 100–150 mmHg

Explanation: ***> 200 mmHg*** - In a **normal healthy person** breathing FiO2 of 0.5 (50% oxygen), the expected **PaO2** is typically **250-300 mmHg**. - Using the **alveolar gas equation**: PAO2 = FiO2(PB - PH2O) - PaCO2/RQ = 0.5(760 - 47) - 40/0.8 ≈ **306 mmHg** - The normal **A-a gradient** is 5-15 mmHg, so PaO2 = 306 - 10 ≈ **296 mmHg** - **Clinical rule of thumb**: PaO2 ≈ 5 × FiO2% = 5 × 50 = **250 mmHg** (approximation accounting for physiological shunt) - Therefore, the expected range is clearly **> 200 mmHg** in a normal individual *150–200 mmHg* - This range would indicate **mild oxygenation impairment** or increased shunt fraction - While adequate for tissue oxygenation, this is **lower than expected** for a normal person on 50% oxygen - May suggest underlying **mild V/Q mismatch** or early pulmonary dysfunction *100–150 mmHg* - This represents **moderate impairment** in oxygen transfer - Indicates significant **pulmonary pathology** such as pneumonia, ARDS, or substantial shunt - Not consistent with normal lung function on FiO2 0.5 *< 100 mmHg* - This represents **severe hypoxemia** despite supplemental oxygen - Indicates **critical pulmonary dysfunction** with large shunt or severe V/Q mismatch - Requires immediate intervention and is never expected in a healthy individual on 50% oxygen

Question 846: The chloride shift occurs rapidly and is essentially complete within:

A. 1 second (Correct Answer)
B. 2 seconds
C. 5 seconds
D. 60 seconds

Explanation: ***1 second*** - The **chloride shift**, an exchange of bicarbonate and chloride ions across the red blood cell membrane, is a very rapid process. - This rapid kinetics ensures efficient **CO2 transport** from tissues to the lungs. *2 seconds* - While seemingly a short duration, **2 seconds** is generally considered longer than the actual time frame for the completion of the chloride shift. - The physiological need for immediate CO2 buffering necessitates a faster mechanism. *5 seconds* - A duration of **5 seconds** would imply a slower rate of gas exchange than physiologically required for efficient CO2 transport. - Such a delay could lead to transient but significant alterations in **blood pH**. *60 seconds* - **60 seconds** (1 minute) is far too long for a process critical to immediate blood gas regulation. - If the chloride shift took this long, it would severely impair the body's ability to excrete **CO2** and maintain acid-base balance.

Question 847: What is the physiological effect of very high positive end-expiratory pressure in a patient with respiratory distress?

A. Increased blood pressure
B. Decreased body temperature
C. Decreased blood pressure (Correct Answer)
D. Increased body temperature

Explanation: ***Decreased blood pressure*** - Very high **positive end-expiratory pressure (PEEP)** increases intrathoracic pressure, which in turn reduces **venous return** to the heart. - This decreased preload leads to a **reduction in cardiac output**, ultimately causing **hypotension** (decreased blood pressure). - This is a well-recognized hemodynamic complication of excessive PEEP in mechanical ventilation. *Increased blood pressure* - High PEEP typically lowers, rather than increases, blood pressure due to its effects on **venous return** and **cardiac output**. - The elevated intrathoracic pressure acts as a barrier to venous return, reducing preload and thus blood pressure. *Decreased body temperature* - **PEEP** primarily affects cardiovascular and respiratory physiology, not **thermoregulation**. - Body temperature changes are usually related to systemic inflammation, infection, or environmental factors, not directly to PEEP settings. *Increased body temperature* - Similar to decreased body temperature, **PEEP** does not directly regulate body temperature. - An elevated body temperature (fever) would suggest an underlying **infection** or **inflammatory process**, which might be present in a patient with respiratory distress but is not a direct physiological effect of high PEEP.

Question 848: All of the following factors influence the hemoglobin dissociation curve, except.

A. Temperature
B. 2–3 DPG levels
C. Plasma sodium concentration (Correct Answer)
D. CO2 tension

Explanation: ***Plasma sodium concentration*** - While essential for **osmolality** and **electrolyte balance**, plasma sodium concentration does not directly influence the binding affinity of hemoglobin for oxygen. - Changes in sodium concentration primarily affect fluid shifts and neural function, not the **hemoglobin dissociation curve**. *CO2 tension* - An increase in **PCO2** (hypercapnia) leads to a **rightward shift** of the hemoglobin dissociation curve, indicating decreased oxygen affinity. - This effect, known as the **Bohr effect**, facilitates oxygen release in tissues with high metabolic activity. *Temperature* - An increase in **body temperature** causes a **rightward shift** in the hemoglobin dissociation curve, leading to reduced oxygen affinity. - This is beneficial during exercise or fever, as it promotes oxygen unloading to active tissues. *2–3 DPG levels* - **2,3-bisphosphoglycerate (2,3-BPG)** binds to deoxygenated hemoglobin, stabilizing its T-state and reducing its affinity for oxygen, thus shifting the curve to the **right**. - During chronic hypoxia or anemia, 2,3-BPG levels increase to enhance oxygen delivery to tissues.

Question 849: Which test is primarily used to assess gas diffusion capacity in the lungs?

A. DLCO (Correct Answer)
B. Spirometry
C. Both DLCO and Spirometry
D. None of the options

Explanation: ***DLCO*** - **DLCO (Diffusing Capacity of the Lungs for Carbon Monoxide)** specifically measures the transfer of gas from the alveoli to the red blood cells, directly assessing the **gas diffusion capacity** of the lungs. - It is crucial for identifying interstitial lung diseases, emphysema, or other conditions affecting the **alveolar-capillary membrane**. *Spirometry* - **Spirometry** primarily assesses **lung volumes and airflow rates**, such as FEV1 and FVC, to diagnose obstructive or restrictive ventilatory defects. - It does not directly measure the efficiency of **gas exchange** across the alveolar-capillary membrane. *Both DLCO and Spirometry* - While both are important in pulmonary function testing, they measure different aspects of **lung function**. DLCO specifically measures **diffusion capacity**, while spirometry measures **airflow and lung volumes**. - Therefore, they are not primarily used for the *same* assessment. *None of the options* - DLCO is indeed the primary test for assessing **gas diffusion capacity** in the lungs. - This option is incorrect because a correct answer is provided.

Question 850: Which of the following is not a normal stimulus for resting ventilation?

A. Stretch receptors
B. J receptors (Correct Answer)
C. PCO2
D. PO2

Explanation: ***J receptors*** - **J receptors** (juxtacapillary receptors) are located in the alveolar walls and are primarily stimulated by **pulmonary edema**, inflammation, or vascular congestion. - Their stimulation typically causes rapid, shallow breathing, but they are **completely inactive during normal, resting ventilation**. - These receptors only become active under pathological conditions, making them **not a normal stimulus for resting breathing**. *Stretch receptors* - **Pulmonary stretch receptors** in the airways respond to lung distension, mediating the **Hering-Breuer reflex** which helps regulate breathing depth and rate. - These receptors are **active during normal tidal breathing** and contribute to the rhythmic pattern of respiration at rest. *PO2* - **Peripheral chemoreceptors** (carotid and aortic bodies) monitor **arterial PO2** and do have **tonic baseline activity** at normal PO2 levels (~95-100 mmHg). - While their contribution is **minimal at normal oxygen levels**, they are present and functioning, making them technically a (weak) normal stimulus. - They become a **major stimulus only when PO2 drops below 60 mmHg** (hypoxemia). *PCO2* - **PCO2** (specifically, the H+ concentration in the cerebrospinal fluid derived from PCO2) is the **most potent and immediate normal stimulus** for resting ventilation. - **Central chemoreceptors** in the medulla are extremely sensitive to changes in CSF pH, directly regulating breathing to maintain arterial PCO2 within a narrow range (~40 mmHg).

Practice by Chapter

Mechanics of Breathing

Practice Questions

Pulmonary Ventilation

Practice Questions

Pulmonary Circulation

Practice Questions

Gas Exchange in the Lungs

Practice Questions

Oxygen and Carbon Dioxide Transport

Practice Questions

Control of Breathing

Practice Questions

Respiratory Adjustments in Health and Disease

Practice Questions

High Altitude Physiology

Practice Questions

Diving Physiology

Practice Questions

Respiratory Function Tests

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free