Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 581: What is the functional residual capacity?

A. The amount of air remaining in the lungs after a normal exhalation.
B. The maximum amount of air that can be forcibly exhaled from the lungs after maximum inhalation.
C. The amount of air that can be forcibly exhaled from the lungs after normal exhalation. (Correct Answer)
D. The total amount of air in the lungs after maximum inhalation.

Explanation: **Explanation** Functional Residual Capacity (FRC) is the volume of air remaining in the lungs at the end of a passive, normal expiration. It represents the equilibrium point of the respiratory system where the inward elastic recoil of the lungs exactly balances the outward chest wall recoil. **Formula:** $FRC = ERV + RV$ (Expiratory Reserve Volume + Residual Volume). **Analysis of Options:** * **Option C (Correct):** This describes the volume remaining after a normal breath. It consists of the air that *could* be exhaled (ERV) and the air that *cannot* be exhaled (RV). * **Option A:** While similar in wording, Option C is the more precise physiological definition used in standard textbooks (like Guyton/Ganong) to describe the equilibrium state. * **Option B:** This defines **Vital Capacity (VC)**. * **Option D:** This defines **Total Lung Capacity (TLC)**. **High-Yield NEET-PG Pearls:** 1. **Measurement:** FRC cannot be measured by simple spirometry because it contains the Residual Volume (RV). It is measured via **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography**. 2. **Clinical Significance:** FRC acts as a buffer for gas exchange, preventing large fluctuations in arterial $PO_2$ during the respiratory cycle. 3. **Pathology:** * **Decreased FRC:** Seen in restrictive lung diseases, obesity, and in the **supine position** (due to cephalad movement of the diaphragm). * **Increased FRC:** Seen in obstructive diseases like **Emphysema** (due to hyperinflation and loss of elastic recoil). 4. **Anesthesia:** Induction of general anesthesia typically reduces FRC by 15-20%, often leading to atelectasis.

Question 582: What is characteristic of the base of the lung compared to the apex of the lung?

A. Higher pulmonary arterial oxygen pressure
B. Higher pulmonary arterial carbon dioxide pressure
C. Higher ventilation/perfusion ratio (Correct Answer)
D. Same ventilation/perfusion ratio

Explanation: **Explanation:** In the upright position, both ventilation (V) and perfusion (Q) increase from the apex to the base of the lung due to the effects of gravity. However, **perfusion increases much more steeply than ventilation** as we move downwards. 1. **Why Option C is Correct:** The Ventilation/Perfusion (V/Q) ratio is determined by the relative rates of these two parameters. At the **apex**, ventilation is higher than perfusion, resulting in a high V/Q ratio (approx. 3.3). At the **base**, perfusion is significantly higher than ventilation, leading to a **lower V/Q ratio (approx. 0.6)**. Therefore, the apex has a higher V/Q ratio compared to the base. 2. **Why Other Options are Incorrect:** * **Option A & B:** Because the apex has a high V/Q ratio (over-ventilated relative to blood flow), the alveolar $P_{O2}$ is higher and $P_{CO2}$ is lower at the apex. Conversely, the base has a lower $P_{O2}$ and higher $P_{CO2}$ in the capillary blood leaving the alveoli. * **Option D:** V/Q ratios are never uniform across the lung in a vertical position due to the uneven distribution of blood flow (West Zones). **High-Yield Clinical Pearls for NEET-PG:** * **Zone of West:** The lung is divided into three zones. Zone 1 (Apex) has the highest V/Q ratio; Zone 3 (Base) has the lowest. * **Tuberculosis:** *M. tuberculosis* prefers the **apex** of the lung because the high V/Q ratio there provides a high-oxygen environment favorable for its growth. * **Compliance:** Alveoli at the **base** are more compliant and expand more during inspiration compared to the already "stretched" alveoli at the apex.

Question 583: Increased ventilation at the start of exercise is primarily due to which of the following?

A. Stretch receptors
B. Proprioceptors (Correct Answer)
C. Pain receptors
D. Arterial PCO2

Explanation: **Explanation:** The control of ventilation during exercise occurs in three distinct phases. The **immediate increase** in ventilation at the very onset of exercise (Phase I) occurs before any metabolic changes can be detected in the blood. **Why Proprioceptors are correct:** This rapid initial rise is mediated by **neural mechanisms** rather than chemical ones. As muscles begin to contract and joints move, **proprioceptors** (mechanoreceptors in joints and muscles) send excitatory impulses to the respiratory center in the medulla. Simultaneously, the "central command" from the motor cortex stimulates both the muscles and the respiratory centers. This anticipatory response ensures that oxygen delivery and CO₂ removal increase the moment physical activity begins. **Analysis of Incorrect Options:** * **A. Stretch Receptors:** These are involved in the Hering-Breuer reflex, which prevents over-inflation of the lungs by inhibiting inspiration; they do not initiate the exercise response. * **C. Pain Receptors:** While pain can increase respiratory rate (hyperpnea), it is not the physiological trigger for the coordinated ventilatory increase seen at the start of exercise. * **D. Arterial PCO₂:** This is the most potent stimulus for respiration at **rest**. However, during exercise, arterial PCO₂ actually tends to remain constant or even slightly decrease due to alveolar hyperventilation. It is responsible for the gradual adjustment (Phase II), not the immediate start. **High-Yield NEET-PG Pearls:** * **Phase I (Start):** Neural (Proprioceptors + Central Command). * **Phase II (During):** Humoral/Chemical factors (though PaO₂ and PaCO₂ remain remarkably stable in healthy individuals). * **Phase III (Recovery):** Gradual decline as neural stimulus ceases and "oxygen debt" is repaid. * **Key Fact:** The primary driver for increased ventilation during exercise is **not** hypoxia or hypercapnia, but neural feed-forward mechanisms.

Question 584: Ventilation-perfusion ratio is least at which part of the lung?

A. Apex
B. Middle lobe
C. Base (Correct Answer)
D. None

Explanation: **Explanation:** The Ventilation-Perfusion ratio (V/Q) is determined by the relationship between alveolar ventilation (V) and pulmonary blood flow (Q). In a standing individual, both ventilation and perfusion increase from the apex to the base due to the effects of gravity. However, **perfusion (Q) increases much more steeply than ventilation (V)** as we move down the lung. 1. **Why the Base is Correct:** At the base, the increase in blood flow is disproportionately higher than the increase in ventilation. Because the denominator (Q) is significantly larger, the **V/Q ratio is lowest at the base** (approximately **0.6**). 2. **Why the Apex is Incorrect:** At the apex, both V and Q are lower than at the base, but blood flow is particularly low due to gravity. Since the denominator (Q) is very small, the **V/Q ratio is highest at the apex** (approximately **3.0**). 3. **Why the Middle Lobe is Incorrect:** The middle lobe represents a transitional zone where the V/Q ratio is intermediate (closer to the ideal 0.8–1.0) compared to the extremes of the apex and base. **High-Yield NEET-PG Pearls:** * **V/Q Ratio Values:** Apex ≈ 3.0 (High); Base ≈ 0.6 (Low). * **Gas Exchange:** Because the V/Q is highest at the apex, $P_AO_2$ is highest and $P_ACO_2$ is lowest there. * **Clinical Correlation:** *Mycobacterium tuberculosis* prefers the apex because the high V/Q ratio results in a high oxygen tension ($P_AO_2$), which favors the growth of this aerobe. * **Zone 3 of West:** Located at the base, where both arterial and venous pressures exceed alveolar pressure ($Pa > Pv > PA$), leading to maximum perfusion.

Question 585: Surfactant contains?

A. DPCC (Correct Answer)
B. Nitrous oxide
C. Angiotensin
D. VIP

Explanation: **Explanation:** **1. Why DPCC is Correct:** Pulmonary surfactant is a complex mixture of lipids and proteins secreted by **Type II alveolar epithelial cells**. Its primary component (approx. 90%) is phospholipids, the most abundant and functionally significant being **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Lecithin**. * **Mechanism:** DPPC is an amphipathic molecule that reduces **surface tension** at the air-liquid interface of the alveoli. By lowering surface tension, it prevents alveolar collapse (atelectasis) during expiration and increases lung compliance, thereby reducing the work of breathing. **2. Why Incorrect Options are Wrong:** * **B. Nitrous oxide:** This is an inorganic gas used primarily as an anesthetic and analgesic agent; it is not a structural component of the lung or surfactant. * **C. Angiotensin:** While the lungs are the primary site for the conversion of Angiotensin I to Angiotensin II (via ACE located in the pulmonary capillary endothelium), Angiotensin itself is a hormone involved in blood pressure regulation, not a component of surfactant. * **D. VIP (Vasoactive Intestinal Peptide):** This is a neuropeptide that acts as a bronchodilator and vasodilator in the lungs, but it is not a constituent of the surfactant complex. **3. High-Yield Clinical Pearls for NEET-PG:** * **L/S Ratio:** A Lecithin/Sphingomyelin ratio **> 2.0** in amniotic fluid indicates fetal lung maturity. * **Surfactant Proteins:** Contains four proteins: **SP-A and SP-D** (innate immunity/opsonization) and **SP-B and SP-C** (surface activity). **SP-B** is the most critical for surfactant function. * **Clinical Correlation:** Deficiency of surfactant in premature infants leads to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease. * **Stimulus for Secretion:** Alveolar expansion (deep breathing/stretching) is the most important physiological stimulus for surfactant release.

Question 586: Why do males have deeper voices compared to females?

A. Longer vocal cords (Correct Answer)
B. Increased vibration due to testosterone
C. Infrathyroid position of the larynx
D. Thickened arytenoids

Explanation: **Explanation:** The pitch of the human voice is primarily determined by the **length, tension, and mass** of the vocal cords. During puberty, the surge of testosterone in males leads to significant anatomical changes in the larynx. **1. Why "Longer vocal cords" is correct:** In males, the vocal cords grow significantly longer (approx. 17–25 mm) compared to females (approx. 12–17 mm). According to the principles of acoustics, longer and thicker vocal cords vibrate at a **lower frequency**. This lower fundamental frequency results in a deeper, more resonant voice. **2. Analysis of Incorrect Options:** * **B. Increased vibration due to testosterone:** This is physiologically incorrect. Testosterone causes the cords to become thicker and longer, which actually leads to a **decrease** in the frequency of vibration (slower vibration), resulting in a lower pitch. * **C. Infrathyroid position of the larynx:** The larynx is located anterior to the pharynx; "infrathyroid" is not a standard anatomical term describing laryngeal position in relation to gender-based pitch. * **D. Thickened arytenoids:** While the laryngeal cartilages enlarge, the pitch is specifically determined by the vocal folds (cords) themselves, not the thickness of the arytenoid cartilages. **Clinical Pearls for NEET-PG:** * **The "Adam’s Apple":** The male larynx grows larger and tilts anteriorly, creating the laryngeal prominence (thyroid cartilage). * **Hormonal Influence:** Voice deepening is a **secondary sexual characteristic**. If prepubertal castration occurs (historical "Castrati"), the larynx does not enlarge, and the high-pitched prepubescent voice is maintained. * **Pitch vs. Intensity:** Pitch is determined by frequency (vocal cord length/tension), while intensity (loudness) is determined by the amplitude of vibration and subglottic air pressure.

Question 587: Which of the following is required for pulmonary maturation during gestation?

A. Cortisol
B. Surfactant (Correct Answer)
C. Amniotic fluid
D. Acetylcholine

Explanation: **Explanation:** The correct answer is **Surfactant**. Pulmonary maturation is defined by the lung's ability to maintain alveolar stability and facilitate gas exchange after birth. The most critical factor for this process is the production of pulmonary surfactant by **Type II pneumocytes**. **Why Surfactant is Correct:** Surfactant is a lipoprotein complex (primarily Dipalmitoylphosphatidylcholine - DPPC) that reduces **surface tension** at the air-liquid interface of the alveoli. According to the Law of Laplace ($P = 2T/r$), smaller alveoli have a higher collapsing pressure. Surfactant prevents this collapse (atelectasis) during expiration, increases lung compliance, and reduces the work of breathing. Its production begins around 24–28 weeks of gestation and reaches maturity by 35 weeks. **Analysis of Incorrect Options:** * **A. Cortisol:** While cortisol is the most important *hormonal stimulus* that triggers surfactant synthesis, it is not the substance that provides maturation itself. It is a catalyst, not the functional end-product. * **C. Amniotic Fluid:** Though the fetus "breathes" amniotic fluid to help expand the lungs structurally, it does not contribute to biochemical maturation or surface tension reduction. * **D. Acetylcholine:** This is a neurotransmitter involved in parasympathetic signaling and has no direct role in the structural or biochemical maturation of the fetal lung. **High-Yield NEET-PG Pearls:** * **L/S Ratio:** A Lecithin/Sphingomyelin ratio **>2.0** in amniotic fluid indicates fetal lung maturity. * **Glucocorticoids:** Betamethasone or Dexamethasone is administered to mothers in preterm labor (before 34 weeks) to accelerate surfactant production. * **NRDS:** Deficiency of surfactant leads to Neonatal Respiratory Distress Syndrome (Hyaline Membrane Disease), characterized by widespread atelectasis and ground-glass appearance on X-ray.

Question 588: Toxic effects of high oxygen tension include all of the following except?

A. Pulmonary edema
B. Decreased cerebral blood flow (Correct Answer)
C. Retinal damage
D. CNS excitation and convulsion

Explanation: This question tests your understanding of **Oxygen Toxicity** (the Paul Bert and Lorrain Smith effects). ### **Why "Decreased Cerebral Blood Flow" is the Correct Answer** While high oxygen tension ($PaO_2$) does cause cerebral vasoconstriction leading to a slight decrease in cerebral blood flow (CBF), this is a **physiological compensatory mechanism** to protect the brain from oxidative stress. It is **not considered a "toxic effect"** in the clinical sense; rather, it is a protective response. In the context of this question, the other three options represent direct pathological damage (toxicity) caused by free radicals (ROS). ### **Explanation of Incorrect Options (Toxic Effects)** * **A. Pulmonary Edema (Lorrain Smith Effect):** Prolonged exposure to high $FiO_2$ (>0.6) leads to the formation of Reactive Oxygen Species (ROS), which damage the alveolar-capillary membrane. This causes increased permeability, leading to pulmonary edema, atelectasis, and "Oxygen Lung." * **C. Retinal Damage:** In neonates, high oxygen causes **Retinopathy of Prematurity (ROP)**. High $PaO_2$ causes initial vasoconstriction followed by abnormal neovascularization and retinal detachment. * **D. CNS Excitation and Convulsion (Paul Bert Effect):** At very high partial pressures (usually >2 atm, as in hyperbaric oxygen therapy), oxygen inhibits enzymes like glutamate decarboxylase, leading to decreased GABA (an inhibitory neurotransmitter). This results in neuronal hyperexcitability and seizures. ### **High-Yield Clinical Pearls for NEET-PG** * **Lorrain Smith Effect:** Pulmonary toxicity due to chronic exposure to 1 atm of $O_2$. * **Paul Bert Effect:** CNS toxicity (seizures) due to acute exposure to hyperbaric $O_2$ (>2 atm). * **Mechanism:** All toxic effects are mediated by **Reactive Oxygen Species (ROS)** like superoxide ($O_2^-$) and hydrogen peroxide ($H_2O_2$), which cause lipid peroxidation of cell membranes. * **CO2 Retention:** In COPD patients, high $O_2$ can cause hypoventilation by removing the "hypoxic drive," leading to $CO_2$ narcosis.

Question 589: Holding one's breath after hyperventilating for some time is dangerous because:

A. It can lead to CO2 necrosis.
B. Lack of CO2 stimulation can lead to dangerous levels of anoxia.
C. Decreased CO2 shifts the oxygen dissociation curve to the left.
D. Alkalosis can lead to tetany. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** Hyperventilation causes excessive "washout" of Carbon Dioxide ($CO_2$), leading to **respiratory alkalosis** (increased blood pH). In an alkaline state, hydrogen ions ($H^+$) dissociate from plasma proteins like albumin. This frees up binding sites on albumin, which then bind to ionized calcium ($Ca^{2+}$). This results in **hypocalcemia** (specifically a drop in the physiologically active ionized fraction). Low extracellular calcium lowers the threshold for action potentials in peripheral nerves, causing neuronal hyperexcitability, which manifests as **tetany** (carpopedal spasm, Chvostek’s sign). **2. Why Other Options are Incorrect:** * **Option A:** $CO_2$ necrosis (narcosis) occurs due to $CO_2$ *retention* (hypercapnia), typically seen in end-stage COPD, not after hyperventilation which causes hypocapnia. * **Option B:** While hyperventilation *does* delay the "breaking point" of breath-holding (because the urge to breathe is driven primarily by $CO_2$ levels), the question asks for the immediate danger associated with the physiological state induced by hyperventilation itself. While hypoxia can occur, the classic acute complication of hyperventilation-induced alkalosis is tetany. * **Option C:** Decreased $CO_2$ (and increased pH) does shift the Oxygen-Hemoglobin Dissociation Curve to the **left** (Bohr Effect). While true, this increases hemoglobin's affinity for $O_2$ and is a physiological shift, not the primary "dangerous" clinical complication compared to tetany. **3. High-Yield Clinical Pearls for NEET-PG:** * **Trousseau’s Sign:** Induction of carpal spasm by inflating a BP cuff above systolic pressure for 3 minutes (highly specific for latent tetany). * **Chvostek’s Sign:** Tapping the facial nerve leads to twitching of facial muscles. * **Management:** Breathing into a paper bag helps the patient re-breathe $CO_2$, reversing the alkalosis and restoring ionized calcium levels. * **The "Breaking Point":** The point at which breathing can no longer be voluntarily inhibited is usually when $PaCO_2$ reaches ~50 mmHg.

Question 590: If the concentration of inhaled O2 is doubled in a normal person, what will be the resultant effect?

A. O2 dissolved in plasma increases, but O2 bound to Hb remains relatively constant (Correct Answer)
B. O2 dissolved in plasma increases, but O2 bound to Hb decreases
C. O2 dissolved in plasma remains constant, but O2 bound to Hb increases
D. O2 dissolved in plasma remains constant, but O2 bound to Hb remains constant

Explanation: ### Explanation **1. Why Option A is Correct:** In a normal person breathing room air (21% O2), Hemoglobin (Hb) is already approximately **97-98% saturated**. According to the **Oxyhemoglobin Dissociation Curve**, once the partial pressure of arterial oxygen ($PaO_2$) exceeds 100 mmHg, the curve reaches a plateau. Doubling the inhaled $O_2$ (to 42%) significantly increases the $PaO_2$, but since Hb is already nearly fully saturated, it can carry very little additional oxygen. However, according to **Henry’s Law**, the amount of gas dissolved in a liquid is directly proportional to its partial pressure. Therefore, doubling the inhaled $O_2$ concentration increases the $PaO_2$, which directly increases the amount of **oxygen dissolved in the plasma** (0.003 ml/dL per mmHg of $PaO_2$). **2. Why Other Options are Incorrect:** * **Option B:** O2 bound to Hb cannot decrease when the inspired oxygen concentration increases; it follows the saturation curve and remains stable or increases marginally. * **Option C & D:** These are incorrect because the dissolved O2 is a physical process governed by pressure. If $FiO_2$ (Fraction of inspired oxygen) increases, the dissolved fraction *must* increase. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Oxygen Carrying Capacity:** 1 gram of Hb carries **1.34 ml** of $O_2$. * **Dissolved O2:** In normal conditions, dissolved $O_2$ is only about **0.3 ml/dL**, which is insufficient to meet tissue demands (requiring cardiac output). * **Hyperbaric Oxygen Therapy:** This is the clinical application of Henry's Law. By increasing pressure to 3 atmospheres, enough $O_2$ can be dissolved in plasma to support life even in the absence of Hb. * **Plateau Phase:** The plateau of the OHDC (above 60 mmHg) ensures that $SaO_2$ remains high even if $PaO_2$ drops moderately, providing a safety buffer.

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