Respiratory System Practice Questions

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.

Q: What causes effort during normal respiration?

Lung elasticity. **Explanation:** In normal, quiet breathing (eupnea), inspiration is an active process, while expiration is passive. The "effort" or work of breathing is primarily required to overcome the **elastic recoil** of the lungs and the chest wall. **1. Why Lung Elasticity is correct:** During inspiration, the respiratory muscles (mainly the diaphragm) must perform work to stretch the elastic fibers of the lung parenchyma and overcome surface tension in the alveoli. This stored elastic energy is then released during expiration, allowing the lungs to recoil to their original volume without further muscular effort. Therefore, the primary resistance to expansion in a healthy individual is the lung's inherent elasticity. **2. Analysis of Incorrect Options:** * **B. Respiratory air passages:** While airway resistance exists, it accounts for only a small fraction of the work in normal respiration. It becomes a major factor only in obstructive pathologies like asthma or COPD. * **C. Alveolar air spaces:** The spaces themselves do not cause effort; rather, it is the *surface tension* at the air-liquid interface within the alveoli that contributes to elastic work. * **D. Creating negative pleural pressure:** This is the *mechanism* by which inspiration occurs, not the cause of the effort. The effort is expended *to* create this negative pressure against the resistance of lung elasticity. **NEET-PG High-Yield Pearls:** * **Compliance:** Defined as $\Delta V / \Delta P$. High elasticity means low compliance (stiff lungs), which significantly increases the work of breathing (e.g., Pulmonary Fibrosis). * **Surfactant:** Reduces the work of breathing by lowering alveolar surface tension, thereby increasing lung compliance. * **Active Expiration:** Becomes necessary during exercise or in diseases like emphysema, where elastic recoil is lost.

Question 1

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 2

Oxygen delivery to tissues is decreased by:

Accepted Answer

Increased hemoglobin level

Answer

Increased HCO3

Answer

Increased PaCO2

Answer

All of the above

Question 3

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 4

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 5

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 6

Curve A signifies which of the following?

Accepted Answer

Emphysema

Answer

Pulmonary fibrosis

Answer

Atelectasis

Answer

ARDS

Question 7

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 8

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

Question 9

Activation of which receptor causes pulmonary vasoconstriction?

Accepted Answer

ETA epithelial receptor

Answer

Alpha 2 adrenergic

Answer

H2 histamine

Answer

M3 cholinergic

Question 10

What causes effort during normal respiration?

Accepted Answer

Lung elasticity

Answer

Respiratory air passages

Answer

Alveolar air spaces

Answer

Creating negative pleural pressure

Respiratory System — MCQs

Respiratory System — MCQs

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