Respiratory System Practice Questions

Q: J-receptors, which are responsible for rapid shallow breathing, are located in which of the following structures?

Alveoli-capillary junction. ### Explanation **Correct Answer: C. Alveoli-capillary junction** **Concept Overview:** Juxtacapillary receptors, commonly known as **J-receptors**, are sensory nerve endings located in the interstitial space of the alveolar wall, specifically near the **alveoli-capillary junction**. They are innervated by non-myelinated **vagal C-fibers**. These receptors are primarily stimulated by an increase in interstitial fluid volume (pulmonary edema) or engorgement of pulmonary capillaries. When activated, they trigger the **"J-reflex,"** which results in a characteristic triad: **apnea followed by rapid shallow breathing (tachypnea), bradycardia, and hypotension.** **Analysis of Incorrect Options:** * **A. Thoracic cage and lung:** While the lungs contain various receptors (like irritant receptors in the airway epithelium), J-receptors are specifically localized to the alveolar-capillary interface, not the thoracic cage. * **B. Carotid artery:** This is the site of **peripheral chemoreceptors** (Carotid bodies), which respond to changes in $PaO_2$, $PaCO_2$, and pH, rather than mechanical or fluid changes in the lung parenchyma. * **D. Respiratory muscles:** These contain **muscle spindles** and Golgi tendon organs that sense stretch and tension to coordinate the work of breathing, but they do not mediate the J-reflex. **High-Yield Clinical Pearls for NEET-PG:** * **Stimulus:** The most common clinical trigger for J-receptor activation is **Left Heart Failure** leading to pulmonary congestion/edema. * **Chemical Triggers:** J-receptors are also stimulated by chemicals injected into the pulmonary circulation, such as **capsaicin** or phenyl diguanide. * **Dyspnea:** Activation of these receptors contributes to the sensation of dyspnea (breathlessness) in patients with pulmonary edema or pneumonia. * **Reflex Pathway:** Sensory limb = Vagus nerve (C-fibers) $\rightarrow$ Medullary respiratory centers.

Q: A PO2 of over 8 kPa corresponds to what hemoglobin saturation?

90%. This question tests your understanding of the **Oxygen-Hemoglobin Dissociation Curve (OHDC)**, which describes the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin ($SaO_2$). ### **Explanation of the Correct Answer** The OHDC is sigmoid (S-shaped) due to the cooperative binding of hemoglobin. To solve this, you must convert the units: **1 kPa is approximately 7.5 mmHg.** * $8 \text{ kPa} \times 7.5 \approx \mathbf{60 \text{ mmHg}}$. * On the standard OHDC, a $PO_2$ of **60 mmHg** corresponds to an arterial oxygen saturation ($SaO_2$) of approximately **90%**. * This is a critical physiological "shoulder" point: above 60 mmHg, the curve is relatively flat (ensuring high saturation despite minor drops in $PO_2$), but below 60 mmHg, the curve becomes very steep, leading to rapid desaturation. ### **Analysis of Incorrect Options** * **A (60%):** This corresponds to a $PO_2$ of approximately **30 mmHg**. * **B (70%):** This corresponds to a $PO_2$ of approximately **37-40 mmHg**. * **C (80%):** This corresponds to a $PO_2$ of approximately **45-50 mmHg**. ### **High-Yield NEET-PG Clinical Pearls** 1. **The 40-50-60 Rule:** A quick mnemonic for the OHDC: * $PO_2$ 27 mmHg $\approx$ 50% Saturation ($P_{50}$) * $PO_2$ 40 mmHg $\approx$ 70-75% Saturation (Mixed venous blood) * $PO_2$ 60 mmHg $\approx$ 90% Saturation (Threshold for respiratory failure) 2. **$P_{50}$:** The $PO_2$ at which hemoglobin is 50% saturated. Normal value is **26.7 mmHg**. 3. **Right Shift (Decreased Affinity):** Caused by ↑ $CO_2$, ↑ H+ (Acidosis/Bohr effect), ↑ 2,3-BPG, and ↑ Temperature. 4. **Left Shift (Increased Affinity):** Caused by ↓ $CO_2$, ↓ H+ (Alkalosis), ↓ 2,3-BPG, ↓ Temperature, and **Carbon Monoxide (CO) poisoning**.

Q: All of the following factors influence the oxygen-hemoglobin dissociation curve, except?

Chloride ion concentration. The oxygen-hemoglobin (O2-Hb) dissociation curve represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. Factors that shift this curve influence the affinity of hemoglobin for oxygen. **Why Chloride ion concentration is the correct answer:** While chloride ions are crucial for the **"Chloride Shift" (Hamburger Phenomenon)**—which maintains electrical neutrality during $CO_2$ transport in RBCs—they do not directly influence the O2-Hb dissociation curve. The curve is primarily affected by factors that stabilize either the Tense (T) state or Relaxed (R) state of hemoglobin; chloride concentration is not a primary physiological regulator of this affinity. **Explanation of Incorrect Options:** * **$CO_2$ Tension & pH (Bohr Effect):** Increased $PCO_2$ and decreased pH (increased $H^+$) shift the curve to the **right**, facilitating oxygen unloading in tissues. * **Temperature:** Increased body temperature (e.g., during exercise or fever) shifts the curve to the **right**, decreasing Hb affinity for $O_2$ to meet metabolic demands. * **2,3-DPG (2,3-Bisphosphoglycerate):** This byproduct of glycolysis binds to the beta chains of deoxyhemoglobin, stabilizing the T-state and shifting the curve to the **right**. **High-Yield Clinical Pearls for NEET-PG:** * **Right Shift (CADET, face Right!):** **C**O2, **A**cidosis, **D**PG, **E**xercise, **T**emperature. A right shift means **decreased affinity**, making it easier to unload $O_2$ to tissues. * **Left Shift:** Fetal Hb (HbF), Carbon Monoxide (CO) poisoning, Hypothermia, and Alkalosis. A left shift means **increased affinity**, making Hb "stingy" with $O_2$. * **HbF:** Shifts the curve to the **left** because it has a lower affinity for 2,3-DPG compared to adult Hb (HbA).

Q: Which of the following is the best-known metabolic function of the lung?

Conversion of angiotensin I to angiotensin II. The lungs are not merely organs of gas exchange; they serve a vital **non-respiratory metabolic function** by processing various endogenous substances circulating in the blood. ### **Why Option B is Correct** The conversion of **Angiotensin I to Angiotensin II** is the most clinically significant metabolic function of the lung. This process is mediated by **Angiotensin-Converting Enzyme (ACE)**, which is located on the luminal surface of the pulmonary capillary endothelial cells. Since the entire cardiac output passes through the pulmonary circulation, the lungs provide a massive surface area for this conversion, making it a central component of the Renin-Angiotensin-Aldosterone System (RAAS) for blood pressure regulation. ### **Analysis of Incorrect Options** * **Option A & C:** While the lungs do inactivate **Serotonin** and **Bradykinin** (also via ACE), these are considered secondary metabolic functions. The conversion of Angiotensin I is the "best-known" and most physiologically impactful function tested in exams. * **Option D:** While the lungs contain cytochrome P450 enzymes, the **liver** is the primary site for the metabolism of basic drugs. Pulmonary drug metabolism is negligible compared to hepatic clearance. ### **High-Yield NEET-PG Pearls** * **Substances Inactivated by Lungs:** Bradykinin (up to 80%), Serotonin, Prostaglandins (E and F series), and Noradrenaline (partial). * **Substances NOT affected by Lungs:** Adrenaline, Dopamine, Oxytocin, and Vasopressin (ADH) pass through the pulmonary circulation unchanged. * **Clinical Correlation:** ACE inhibitors (used in hypertension) prevent the conversion of Angiotensin I and the breakdown of Bradykinin; the resulting accumulation of Bradykinin in the lungs is responsible for the common side effect of a **dry cough**.

Q: If at the beginning of inspiration, pulmonary compliance is 0.2 L/cm H2O and the initial trans-pulmonary pressure is 5 cm H2O, what will be the transpulmonary pressure after inhaling 600 mL of air?

8 cm H2O. ### Explanation **1. Understanding the Correct Answer (C: 8 cm H2O)** The core concept here is the relationship between **Pulmonary Compliance ($C$)**, **Volume change ($\Delta V$)**, and **Pressure change ($\Delta P$)**. Compliance is defined as the change in lung volume per unit change in transpulmonary pressure: $$C = \frac{\Delta V}{\Delta P}$$ To find the new transpulmonary pressure, we first calculate the change in pressure ($\Delta P$) required to inhale 600 mL (0.6 L) of air: * **Given:** $C = 0.2 \text{ L/cm H}_2\text{O}$; $\Delta V = 0.6 \text{ L}$ * **Formula:** $\Delta P = \frac{\Delta V}{C}$ * **Calculation:** $\Delta P = \frac{0.6}{0.2} = 3 \text{ cm H}_2\text{O}$ The transpulmonary pressure ($P_{tp}$) must **increase** to expand the lungs. * **Final $P_{tp}$** = Initial $P_{tp}$ + $\Delta P$ * **Final $P_{tp}$** = $5 \text{ cm H}_2\text{O} + 3 \text{ cm H}_2\text{O} = \mathbf{8 \text{ cm H}_2\text{O}}$. **2. Why Other Options are Incorrect** * **Option A (4 cm H2O):** This suggests a decrease in pressure, which occurs during expiration, not inspiration. * **Option B (7 cm H2O):** This would be the result if the tidal volume was only 400 mL ($0.2 \times 2 = 0.4$). * **Option D (9 cm H2O):** This would imply a lower compliance (0.15 L/cm H2O) or a higher inhaled volume (800 mL). **3. Clinical Pearls & High-Yield Facts** * **Transpulmonary Pressure ($P_{tp}$):** It is the difference between Alveolar pressure ($P_{alv}$) and Intrapleural pressure ($P_{ip}$). It is always positive under normal physiological conditions to keep the lungs inflated. * **Compliance Trends:** * **Decreased Compliance:** Seen in Pulmonary Fibrosis, ARDS, and Pulmonary Edema (lungs are "stiff"). * **Increased Compliance:** Seen in Emphysema (loss of elastic recoil) and with aging. * **Specific Compliance:** Compliance divided by Functional Residual Capacity (FRC); used to compare lungs of different sizes.

Q: Which hormone stimulates respiration?

Progesterone. **Explanation:** **Progesterone** is a potent respiratory stimulant. It acts through two primary mechanisms: 1. **Central Stimulation:** It increases the sensitivity of the respiratory center in the medulla to Carbon Dioxide ($CO_2$). 2. **Direct Action:** It acts as a direct stimulant to the respiratory drive, increasing tidal volume and minute ventilation. This physiological effect is most evident during the **luteal phase** of the menstrual cycle and during **pregnancy**, where elevated progesterone levels lead to chronic hyperventilation. This results in a physiological decrease in arterial $PCO_2$ (hypocapnia), creating a favorable $CO_2$ gradient for the fetus to excrete waste. **Analysis of Incorrect Options:** * **A. Estrogen:** While estrogen can enhance the effect of progesterone by increasing progesterone receptor expression, it does not independently stimulate respiration to a significant degree. * **C. Corticosteroids:** These primarily influence metabolic and anti-inflammatory pathways. While they are used to treat respiratory distress (e.g., asthma or fetal lung maturity), they do not act as physiological stimulants of the respiratory center. * **D. Prolactin:** This hormone is primarily involved in lactation and reproductive suppression; it has no documented role in stimulating the respiratory drive. **High-Yield Clinical Pearls for NEET-PG:** * **Pregnancy Physiology:** Pregnant women often experience "physiological dyspnea" due to progesterone-induced hyperventilation, leading to a state of **compensated respiratory alkalosis**. * **Therapeutic Use:** Medroxyprogesterone (a synthetic progestin) has been used clinically to treat **Obstructive Sleep Apnea (OSA)** and **Pickwickian Syndrome** (Obesity Hypoventilation Syndrome) due to its ventilatory stimulatory effects. * **Acid-Base Balance:** In pregnancy, the normal $PaCO_2$ drops to approximately **30–32 mmHg** (Normal: 40 mmHg).

Question 1

What is true about airway resistance?

Accepted Answer

It is increased in asthma.

Answer

It is increased if the lungs are removed and inflated with saline.

Answer

It does not affect the work of breathing.

Answer

It is increased in paraplegic patients.

Question 2

J-receptors, which are responsible for rapid shallow breathing, are located in which of the following structures?

Accepted Answer

Alveoli-capillary junction

Answer

Thoracic cage and lung

Answer

Carotid artery

Answer

Respiratory muscles

Question 3

If the lungs were allowed to recoil without the chest wall during expiration, what would be the lung volume?

Accepted Answer

Minimal volume

Answer

Residual volume

Answer

Functional residual capacity (FRC)

Answer

Zero

Question 4

A PO2 of over 8 kPa corresponds to what hemoglobin saturation?

Accepted Answer

90%

Answer

60%

Answer

70%

Answer

80%

Question 5

Negative intrapleural pressure is due to which of the following?

Accepted Answer

Absorption by lymphatics

Answer

Uniform distribution of surfactant over alveoli

Answer

Negative intraalveolar pressure

Answer

Presence of cartilage in the upper airway

Question 6

All of the following factors influence the oxygen-hemoglobin dissociation curve, except?

Accepted Answer

Chloride ion concentration

Answer

CO2 tension

Answer

Temperature

Answer

2,3-DPG

Question 7

Which of the following is the best-known metabolic function of the lung?

Accepted Answer

Conversion of angiotensin I to angiotensin II

Answer

Inactivation of serotonin

Answer

Inactivation of bradykinin

Answer

Metabolism of basic drugs by the cytochrome P450 system

Question 8

If at the beginning of inspiration, pulmonary compliance is 0.2 L/cm H2O and the initial trans-pulmonary pressure is 5 cm H2O, what will be the transpulmonary pressure after inhaling 600 mL of air?

Accepted Answer

8 cm H2O

Answer

4 cm H2O

Answer

7 cm H2O

Answer

9 cm H2O

Question 9

In a transection at the pontomedullary junction, respiration is maintained with slight irregularity. This is due to which structure?

Accepted Answer

Pre-Botzinger complex

Answer

Pneumotaxic centre

Answer

Apneustic Centre

Answer

Dorsal respiratory group

Question 10

Which hormone stimulates respiration?

Accepted Answer

Progesterone

Answer

Estrogen

Answer

Corticosteroids

Answer

Prolactin

Respiratory System — MCQs

Respiratory System — MCQs

On this page

Practice by Chapter

Want unlimited practice?