Respiratory System Practice Questions

Q: True regarding vascularity of the lung is:

Distended pulmonary veins in the lower lobe. **Explanation:** The vascularity of the lung is governed by gravity and unique local regulatory mechanisms. **Why Option D is Correct:** In an upright position, **gravity** significantly influences pulmonary blood flow. Hydrostatic pressure is highest at the base (lower lobes) of the lung. This increased pressure leads to the recruitment and distension of pulmonary vessels (West Zone 3), resulting in **distended pulmonary veins in the lower lobes** compared to the apex. **Analysis of Incorrect Options:** * **Option A:** Unlike systemic vessels which vasodilate in response to hypoxia, pulmonary arterioles undergo **Hypoxic Pulmonary Vasoconstriction (HPV)**. This shunts blood away from poorly ventilated alveoli to well-ventilated ones to optimize V/Q matching. * **Option B:** Pulmonary vascular resistance (PVR) is significantly lower than systemic vascular resistance (SVR). PVR is approximately **1/10th to 1/12th** of SVR, not half. This allows the right ventricle to pump the same cardiac output at much lower pressures. * **Option C:** Due to gravity, both ventilation and perfusion increase from the apex to the base. However, **perfusion increases more steeply** than ventilation. Therefore, the base of the lung is better perfused than the apex. **High-Yield Clinical Pearls for NEET-PG:** * **West Zones:** Zone 1 (Apex: $P_A > P_a > P_v$), Zone 2 (Middle: $P_a > P_A > P_v$), Zone 3 (Base: $P_a > P_v > P_A$). * **Cephalization:** On a chest X-ray, if pulmonary veins in the upper lobes become distended (Antler sign), it indicates pulmonary venous hypertension (e.g., Mitral Stenosis or Left Heart Failure). * **V/Q Ratio:** Highest at the apex (~3.3) and lowest at the base (~0.6).

Q: How is the volume of inspired air that actually ventilates the alveoli calculated?

Single breath N2 method. The volume of inspired air that actually reaches the alveoli is determined by subtracting the **Anatomical Dead Space** from the Tidal Volume. The **Single Breath Nitrogen (N₂) Method** (also known as Fowler’s Method) is the gold standard for measuring this anatomical dead space. ### Why Option A is Correct: In Fowler’s method, a subject takes a single breath of 100% oxygen. As they exhale, a nitrogen analyzer measures the N₂ concentration. * The initial part of the breath contains 0% N₂ (pure oxygen from the dead space). * As exhalation continues, N₂ levels rise as alveolar air (which contains N₂) mixes in. * By plotting N₂ concentration against expired volume, the **Anatomical Dead Space** is calculated. Subtracting this from the Tidal Volume gives the volume that effectively ventilates the alveoli. ### Why Other Options are Incorrect: * **B. Dalton’s Law:** States that the total pressure of a gas mixture is the sum of the partial pressures of individual gases. It is used to calculate $PO_2$ at different altitudes, not lung volumes. * **C. Bohr Equation:** This measures **Physiological Dead Space** using $CO_2$ concentrations. While related, "Anatomical Dead Space" (measured by Fowler's) specifically refers to the volume of the conducting airways. * **D. Boyle’s Law:** States that pressure is inversely proportional to volume ($P \propto 1/V$). This is the principle behind **Body Plethysmography** used to measure Functional Residual Capacity (FRC). ### High-Yield Clinical Pearls for NEET-PG: * **Anatomical Dead Space:** Volume of conducting zones (approx. 150 ml or 2 ml/kg). Measured by **Fowler’s Method**. * **Physiological Dead Space:** Anatomical dead space + Alveolar dead space (wasted ventilation). Measured by **Bohr’s Equation**. * In healthy individuals, Anatomical $\approx$ Physiological dead space. In lung diseases (like PE or COPD), Physiological dead space increases significantly.

Q: What is the most important stimulus controlling the level of resting ventilation?

pH of CSF on central chemoreceptors. **Explanation:** The primary drive for resting ventilation is the concentration of **hydrogen ions (H+) in the brain extracellular fluid**, which is directly reflected by the **pH of the Cerebrospinal Fluid (CSF)**. **Why Option D is Correct:** Central chemoreceptors, located on the ventral surface of the medulla, are exquisitely sensitive to changes in pH. While CO2 can easily cross the blood-brain barrier (BBB), H+ and HCO3- cannot. Once CO2 enters the CSF, it reacts with water to form carbonic acid, which dissociates into H+ and HCO3-. The resulting increase in H+ (drop in pH) stimulates the central chemoreceptors, accounting for approximately **70-80% of the respiratory drive** under normal resting conditions. **Why Other Options are Incorrect:** * **Option A:** PO2 only becomes a major stimulus for ventilation when it drops significantly (Hypoxic drive, <60 mmHg). In healthy individuals at rest, oxygen levels play a minimal role. * **Option B & C:** Peripheral chemoreceptors (Carotid and Aortic bodies) do respond to PCO2 and pH. However, they are responsible for only about **20-30%** of the ventilatory response to CO2. They act faster than central receptors but are not the *most important* for resting ventilation. **High-Yield NEET-PG Pearls:** * **Main Stimulus:** The central chemoreceptor responds to **H+**, but the stimulus that triggers it from the blood is **CO2** (because only CO2 crosses the BBB). * **Location:** Central chemoreceptors are in the **medulla**; Peripheral chemoreceptors are in the **Carotid bodies** (CN IX) and **Aortic bodies** (CN X). * **CO2 Narcosis:** In chronic CO2 retainers (like COPD patients), the central receptors become desensitized, and respiration becomes driven by PO2 (Hypoxic drive). Giving high-flow oxygen to these patients can paradoxically cause respiratory arrest.

Q: Oxygen delivery to tissues is decreased by:

Increased hemoglobin level. **Explanation:** The delivery of oxygen to tissues depends on the **Oxygen Delivery Index ($DO_2$)**, which is calculated by the formula: **$DO_2 = \text{Cardiac Output (CO)} \times \text{Arterial Oxygen Content } (CaO_2)$** 1. **Why Option A is Correct:** While hemoglobin (Hb) carries oxygen, an excessive increase in hemoglobin (as seen in Polycythemia) significantly increases **blood viscosity**. According to Poiseuille’s Law, increased viscosity leads to increased peripheral resistance and a subsequent **decrease in Cardiac Output**. When the rise in viscosity outweighs the oxygen-carrying capacity, the net oxygen delivery to tissues decreases. 2. **Why Options B and C are Incorrect:** * **Increased $PaCO_2$ (Hypercapnia):** An increase in $CO_2$ causes a **rightward shift** of the Oxygen-Dissociation Curve (Bohr Effect). This decreases the affinity of Hb for oxygen, actually **facilitating** the unloading and delivery of oxygen to the tissues. * **Increased $HCO_3$:** This typically reflects a metabolic alkalosis or a compensatory response to respiratory acidosis. While alkalosis shifts the curve to the left (increasing affinity), it does not inherently decrease the total delivery index in the same systemic manner as increased viscosity. **High-Yield Clinical Pearls for NEET-PG:** * **The Bohr Effect:** Shift to the **Right** (facilitates delivery) is caused by: **↑** $CO_2$, **↑** H+ (Acidosis), **↑** 2,3-DPG, and **↑** Temperature (**CADET**, face Right!). * **Optimal Hematocrit:** For maximal oxygen delivery, a hematocrit of approximately 40-45% is ideal; beyond this, viscosity becomes the limiting factor. * **Formula for $CaO_2$:** $(1.34 \times Hb \times SaO_2) + (0.003 \times PaO_2)$. Note that dissolved oxygen ($PaO_2$) contributes minimally compared to Hb-bound oxygen.

Q: Lung compliance is increased in which of the following conditions?

Emphysema. **Explanation:** **Compliance** is defined as the change in lung volume per unit change in transpulmonary pressure ($C = \Delta V / \Delta P$). It represents the "stretchability" or ease with which the lungs expand. **Why Emphysema is Correct:** In **Emphysema**, there is permanent destruction of the alveolar walls and loss of **elastic recoil** due to the breakdown of elastin fibers (often by elastase). Since elasticity and compliance are inversely related, a loss of elastic "snap-back" makes the lung highly distensible. Therefore, the lung expands very easily at low pressures, resulting in **increased compliance**. **Why Other Options are Incorrect:** * **A. Intra-alveolar fluid:** Fluid (as seen in pulmonary edema) increases surface tension and occupies air space, making the lungs stiffer and **decreasing** compliance. * **B. Acute Respiratory Distress Syndrome (ARDS):** ARDS involves a massive inflammatory response and loss of surfactant. Increased surface tension and "wet lungs" lead to a significant **decrease** in compliance (often called "stiff lungs"). * **C. Idiopathic Pulmonary Fibrosis:** This is a restrictive lung disease where healthy lung tissue is replaced by thick, scarred collagen fibers. This increases the work required to expand the lungs, thereby **decreasing** compliance. **High-Yield Clinical Pearls for NEET-PG:** * **Static Compliance** is primarily affected by the elastic properties of the lung and surface tension. * **Surfactant** increases compliance by reducing alveolar surface tension. * **Aging:** Compliance increases with age due to the natural loss of elastic fibers. * **Pressure-Volume Loop:** In Emphysema, the curve shifts **upward and to the left** (higher volume for lower pressure). In Fibrosis, it shifts **downward and to the right**.

Q: Curve A signifies which of the following?

Emphysema. ***Emphysema*** - Curve A shows **increased lung compliance** with an upward-left shift, characteristic of emphysema due to **destruction of alveolar walls** and reduced **elastic recoil**. - The **loss of elastic tissue** allows lungs to expand more easily at lower pressures, resulting in higher lung volumes at any given pressure. *Pulmonary fibrosis* - Shows **decreased compliance** with a downward-right shift on pressure-volume curves due to **increased collagen deposition** and lung stiffness. - Requires **higher pressures** to achieve the same lung volumes, opposite to what Curve A demonstrates. *Atelectasis* - Characterized by **collapsed alveoli** leading to **reduced lung volumes** and decreased compliance. - Pressure-volume curve would shift **downward-right**, showing smaller volumes at given pressures, not the pattern seen in Curve A. *ARDS* - Results in **severely decreased compliance** due to **inflammatory fluid accumulation** and **surfactant dysfunction**. - Shows markedly **reduced lung volumes** and rightward shift, requiring much higher pressures for ventilation.

Question 1

True regarding vascularity of the lung is:

Accepted Answer

Distended pulmonary veins in the lower lobe

Answer

Hypoxia causes vasodilation

Answer

Pulmonary resistance is half of the systemic vascular resistance

Answer

Perfusion is more in the apical lobe than in the base

Question 2

How is the volume of inspired air that actually ventilates the alveoli calculated?

Accepted Answer

Single breath N2 method

Answer

Dalton's law

Answer

Bohr equation

Answer

Boyle's law

Question 3

What is the most important stimulus controlling the level of resting ventilation?

Accepted Answer

pH of CSF on central chemoreceptors

Answer

PO2 on peripheral chemoreceptors

Answer

PCO2 on peripheral chemoreceptors

Answer

pH on peripheral chemoreceptors

Question 4

Oxygen delivery to tissues is decreased by:

Accepted Answer

Increased hemoglobin level

Answer

Increased HCO3

Answer

Increased PaCO2

Answer

All of the above

Question 5

Lung compliance is increased in which of the following conditions?

Accepted Answer

Emphysema

Answer

The presence of intra-alveolar fluid

Answer

Acute respiratory distress syndrome

Answer

Idiopathic pulmonary fibrosis

Question 6

Cyanosis does not occur in severe anaemia because:

Accepted Answer

The critical concentration of haemoglobin required to produce cyanosis is reduced

Answer

Hypoxia stimulates erythropoietin production

Answer

Oxygen carrying capacity of available haemoglobin is increased

Answer

The oxygen-haemoglobin dissociation curve shifts to the right

Question 7

What is the formula for transthoracic compliance?

Accepted Answer

C = V / (Alv p - atmospheric pressure)

Answer

C = Vt x RR / PEEP

Answer

C = Vt / (Pplt - PEEP)

Answer

C = Vt / (Pk - Pplt)

Question 8

Curve A signifies which of the following?

Accepted Answer

Emphysema

Answer

Pulmonary fibrosis

Answer

Atelectasis

Answer

ARDS

Question 9

What type of blood flow is typically seen in the apex of the lung?

Accepted Answer

Intermittent flow, alveolar pressure less than pulmonary capillary pressure during systole

Answer

No flow, alveolar pressure greater than pulmonary capillary pressure

Answer

Intermittent flow, alveolar pressure less than pulmonary capillary pressure during diastole

Answer

Continuous flow, alveolar pressure less than pulmonary capillary pressure

Question 10

Which of the following will not cause a low lung diffusing capacity (DL)?

Accepted Answer

Increased diffusion distance

Answer

Decreased capillary blood volume

Answer

Decreased surface area

Answer

Decreased cardiac output

Respiratory System — MCQs

Respiratory System — MCQs

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