Respiratory System Practice Questions

Q: During normal inspiration, by how much does the diaphragm descend?

1-2 cm. The diaphragm is the primary muscle of inspiration, responsible for approximately 75% of the change in intrathoracic volume during quiet breathing. ### **Explanation of the Correct Answer** During **normal (quiet) inspiration**, the diaphragm contracts and moves caudally (downward) by approximately **1 to 2 cm**. This descent increases the vertical dimension of the thoracic cavity. Because the lungs are compliant, this volume increase creates a negative intrapulmonary pressure (approx. -1 mmHg), allowing roughly 500 mL of tidal air to enter the lungs. ### **Analysis of Incorrect Options** * **Options B, C, and D (3-9 cm):** These values represent excessive movement for quiet breathing. While the diaphragm can descend significantly more during **forced inspiration** (deep breathing or exercise), it typically reaches a maximum excursion of **7 to 10 cm** only under extreme respiratory effort. Therefore, any value above 2 cm does not characterize "normal" or "quiet" inspiration. ### **High-Yield Clinical Pearls for NEET-PG** * **Nerve Supply:** The diaphragm is supplied by the **Phrenic Nerve (C3, C4, C5)**. "C3, 4, 5 keep the diaphragm alive." * **Piston Movement:** The movement of the diaphragm is often compared to a piston. For every 1 cm of descent, the intrathoracic volume increases by approximately 200–300 mL. * **Paradoxical Respiration:** In cases of phrenic nerve palsy, the paralyzed side of the diaphragm moves *upward* (cranially) during inspiration due to the negative pressure generated by the healthy side. * **Quiet vs. Forced:** In quiet breathing, expiration is entirely **passive** (elastic recoil). The diaphragm only performs active work during inspiration.

Q: What is the expected mixed venous oxygen tension, in mm Hg, in a normal adult after breathing 100% oxygen for 10 minutes?

45. **Explanation:** The correct answer is **45 mm Hg**. **1. Underlying Concept:** Mixed venous oxygen tension ($PvO_2$) is primarily determined by the balance between oxygen delivery and tissue oxygen consumption ($VO_2$). In a normal adult, arterial blood is already nearly 100% saturated with oxygen while breathing room air ($PaO_2 \approx 100$ mm Hg). When breathing 100% oxygen, the $PaO_2$ rises significantly (up to 600+ mm Hg), but this adds very little extra oxygen content to the blood because hemoglobin is already saturated; the increase is almost entirely due to a small amount of additional dissolved oxygen. As blood passes through systemic capillaries, tissues extract a fixed amount of oxygen to meet metabolic demands. Because the total oxygen content increase is marginal and tissue extraction remains constant, the $PvO_2$ rises only slightly from its baseline of 40 mm Hg to approximately **45 mm Hg**. **2. Analysis of Incorrect Options:** * **Option A (150 mm Hg):** This is the approximate $PiO_2$ (inspired oxygen tension) at sea level on room air, not venous tension. * **Option B (740 mm Hg):** This value approaches the total atmospheric pressure at sea level. It is impossible for venous blood to reach this tension. * **Option D (573 mm Hg):** This represents a typical alveolar oxygen tension ($PAO_2$) while breathing 100% oxygen, but it does not reflect venous levels after tissue extraction. **3. High-Yield Clinical Pearls for NEET-PG:** * **Normal $PvO_2$:** 40 mm Hg (Saturation $\approx$ 75%). * **The "Venous Oxygen Paradox":** Even with massive increases in $FiO_2$, $PvO_2$ remains low because the sigmoid shape of the oxyhemoglobin dissociation curve ensures that most "extra" oxygen is consumed or remains bound, preventing a massive rise in dissolved partial pressure in the veins. * **Mixed Venous Blood:** Best sampled from the **Pulmonary Artery** using a Swan-Ganz catheter.

Q: Transection of the brain stem at the mid-pontine level with bilateral vagotomy causes what?

Apneusis. To understand the control of respiration, one must visualize the brainstem respiratory centers and their inhibitory/excitatory inputs. ### **Mechanism of Apneusis** The **Apneustic Center** (located in the lower pons) promotes inhalation by stimulating the Dorsal Respiratory Group (DRG). Under normal physiological conditions, this center is inhibited by two main "off-switches": 1. **Pneumotaxic Center:** Located in the upper pons (nucleus parabrachialis). 2. **Vagus Nerve:** Carries inhibitory signals from pulmonary stretch receptors (Hering-Breuer reflex). When a transection occurs at the **mid-pontine level**, the Pneumotaxic center is separated from the lower respiratory centers. If the **Vagus nerves** are also cut, both inhibitory inputs are removed. This results in unchecked stimulation of the DRG, leading to **Apneusis**—characterized by prolonged, gasping inspiratory efforts with short, inefficient expirations. ### **Analysis of Incorrect Options** * **A & B:** Spontaneous respiration continues because the Medullary Rhythmicity Centers (DRG/VRG) are still intact and below the level of transection. However, the breathing pattern becomes pathological, not regular. * **C:** Irregular/Ataxic respiration (Biot’s breathing) typically occurs with lesions involving the **medulla** itself, not the mid-pons. ### **NEET-PG High-Yield Pearls** * **Upper Pontine Transection + Vagus Intact:** Breathing remains near normal because the Vagus compensates for the loss of the Pneumotaxic center. * **Medullary Transection:** Leads to immediate cessation of spontaneous respiration (separates the spinal cord from the rhythm generators). * **Pneumotaxic Center Function:** Its primary role is to limit inspiration, thereby increasing the respiratory rate. * **Location Summary:** Pneumotaxic (Upper Pons), Apneustic (Lower Pons), Rhythmicity Centers (Medulla).

Q: Which of the following statements is true concerning the contraction of the diaphragm?

It tends to flatten the diaphragm.. ### Explanation The diaphragm is the primary muscle of inspiration, responsible for approximately 75% of the change in intrathoracic volume during quiet breathing. **Why Option B is Correct:** The diaphragm is a dome-shaped muscular partition. When the muscle fibers contract, they pull the central tendon downward toward the abdominal cavity. This action **flattens the dome**, increasing the vertical dimension of the thoracic cavity. This expansion creates a negative intrapulmonary pressure, allowing air to flow into the lungs. **Why the Other Options are Incorrect:** * **Option A:** The diaphragm is innervated by the **phrenic nerves**, which arise from the cervical plexus, specifically spinal segments **C3, C4, and C5** ("C3, 4, 5 keep the diaphragm alive"). They do not emerge from the lower thorax. * **Option C:** During contraction, the diaphragm pushes down on the abdominal viscera, which in turn creates an outward force on the lower ribs (the "bucket-handle" and "pump-handle" mechanisms). This **increases**, rather than reduces, the lateral and transverse diameters of the lower rib cage. * **Option D:** As the diaphragm descends, it increases intra-abdominal pressure. This causes the **anterior abdominal wall to move outward** (protrude) during inspiration, not inward. **High-Yield NEET-PG Pearls:** * **Shape:** At rest, the right dome is slightly higher than the left due to the presence of the liver. * **Openings:** Remember the levels of major diaphragmatic openings: **Vena Cava (T8), Esophagus (T10), and Aorta (T12)** (Mnemonic: **I** **E**at **A**pples—IVC, Esophagus, Aorta). * **Paradoxical Respiration:** If the diaphragm is paralyzed (phrenic nerve injury), the abdominal wall moves *inward* during inspiration due to the negative intrathoracic pressure pulling the flaccid muscle upward.

Q: Pulmonary endothelium is NOT concerned with which of the following?

Factor X. **Explanation:** The pulmonary endothelium is not merely a passive barrier for gas exchange; it is a metabolically active organ responsible for the synthesis, activation, and inactivation of various substances. **Why Factor X is the correct answer:** **Factor X** is a clotting factor synthesized primarily in the **liver**. It is not produced, stored, or significantly metabolized by the pulmonary endothelium. While the lungs play a role in the coagulation cascade (e.g., through the production of tissue factor or thrombomodulin), Factor X itself is a systemic plasma protein of hepatic origin. **Analysis of Incorrect Options:** * **A. Lipoprotein Lipase (LPL):** The pulmonary capillaries contain high levels of LPL. This enzyme is responsible for the hydrolysis of circulating triglycerides into free fatty acids and glycerol, playing a key role in lipid metabolism. * **B. Plasminogen Activator:** Pulmonary endothelial cells synthesize and release **Tissue Plasminogen Activator (tPA)**. This is a vital fibrinolytic function that helps dissolve microthrombi, ensuring the pulmonary circulation remains patent. * **C. Thrombin:** The pulmonary endothelium interacts with thrombin through receptors like **thrombomodulin**. This interaction converts thrombin from a procoagulant to an anticoagulant (by activating Protein C), effectively "clearing" or modulating thrombin levels in the lungs. **High-Yield Facts for NEET-PG:** * **ACE Activity:** The pulmonary endothelium is the primary site for **Angiotensin-Converting Enzyme (ACE)**, which converts Angiotensin I to II and inactivates Bradykinin. * **Vasoactive Substances:** It inactivates **Serotonin, Norepinephrine, and Bradykinin**, but does *not* affect Epinephrine, Dopamine, or Histamine. * **Prostaglandins:** It inactivates Prostaglandins E and F, but synthesizes **Prostacyclin (PGI2)**, a potent vasodilator and inhibitor of platelet aggregation.

Q: An increase in the concentration of 2,3 DPG may be seen in all of the following, except?

Hypoxanthine. **Explanation:** The concentration of **2,3-Bisphosphoglycerate (2,3-DPG)** in red blood cells is a critical regulator of hemoglobin's affinity for oxygen. An increase in 2,3-DPG shifts the Oxygen-Dissociation Curve (ODC) to the **right**, facilitating the unloading of oxygen to tissues. **Why Hypoxanthine is the Correct Answer:** Hypoxanthine is a purine derivative and a breakdown product of adenosine monophosphate (AMP) metabolism. It does not play a role in the glycolytic pathway (Luebering-Rapoport shunt) where 2,3-DPG is synthesized. Therefore, it does not increase 2,3-DPG levels. **Analysis of Incorrect Options:** * **Anemia:** In anemia, the reduced hemoglobin concentration leads to tissue hypoxia. The body compensates by increasing 2,3-DPG production to enhance oxygen delivery to tissues. * **Hypoxia:** Chronic hypoxia (e.g., high altitude or chronic lung disease) stimulates 2,3-DPG production. This is a key adaptive mechanism to maintain oxygenation despite lower arterial oxygen tension. * **Inosine:** In blood banking, inosine is added to stored blood. It can be metabolized into ribose-5-phosphate and eventually into glycolytic intermediates (like glyceraldehyde-3-phosphate), which **increases** the synthesis of 2,3-DPG, thereby restoring the oxygen-carrying efficiency of stored blood. **High-Yield Clinical Pearls for NEET-PG:** * **Luebering-Rapoport Shunt:** The specific pathway in RBCs that produces 2,3-DPG. * **Right Shift Factors:** "CADET, face Right!" (**C**O2, **A**cidosis, **D**PG, **E**xercise, **T**emperature). * **Stored Blood:** 2,3-DPG levels **decrease** in stored blood over time, causing a left shift (increased O2 affinity). This is why inosine is used as a preservative. * **Fetal Hemoglobin (HbF):** Has a lower affinity for 2,3-DPG compared to HbA, resulting in a **left shift**, allowing the fetus to "pull" oxygen from maternal blood.

Q: Which of the following changes occur secondary to hypercarbia?

Hypertension. **Explanation:** Hypercarbia (or hypercapnia) refers to an abnormally high concentration of carbon dioxide ($CO_2$) in the blood. The physiological response to hypercarbia is primarily mediated by the **activation of the sympathetic nervous system**. **Why Hypertension is Correct:** Excess $CO_2$ acts as a potent stimulus for the central and peripheral chemoreceptors. This triggers the vasomotor center in the medulla to increase sympathetic outflow. The resulting release of catecholamines leads to peripheral vasoconstriction and increased cardiac output, manifesting clinically as **hypertension** and tachycardia. While $CO_2$ has a direct local vasodilatory effect on blood vessels, the systemic sympathetic response typically overrides this, leading to an overall rise in blood pressure. **Analysis of Incorrect Options:** * **Miosis:** Hypercarbia typically causes **mydriasis** (pupillary dilation) due to sympathetic overactivity. Miosis (pinpoint pupils) is more characteristic of opioid overdose or pontine hemorrhage. * **Cool extremities:** Hypercarbia causes peripheral vasodilation (direct effect) and increased skin blood flow, leading to **warm, flushed extremities** and a bounding pulse. * **Bradycardia:** The sympathetic surge usually results in **tachycardia**. Bradycardia is generally a late, pre-terminal sign of severe respiratory failure or CO2 narcosis. **High-Yield Clinical Pearls for NEET-PG:** * **CO2 Narcosis:** Extremely high levels of $PaCO_2$ (>70–80 mmHg) can lead to CNS depression, confusion, and coma. * **Cerebral Blood Flow:** $CO_2$ is a potent cerebral vasodilator. Hypercarbia increases intracranial pressure (ICP), which is why therapeutic hyperventilation (to lower $CO_2$) is used to acutely reduce ICP. * **Flapping Tremors (Asterixis):** A classic clinical sign of severe hypercapnia, often seen in COPD patients.

Q: Carbon dioxide (CO2) is primarily transported in the arterial blood as which of the following?

Bicarbonate. **Explanation:** Carbon dioxide (CO2) is a metabolic waste product that must be transported from the tissues to the lungs. In arterial blood, it exists in three distinct forms, but the distribution is unequal. **1. Why Bicarbonate (D) is Correct:** The majority of CO2 (**approximately 70%**) is transported as **Bicarbonate (HCO3⁻)**. This process occurs primarily within Red Blood Cells (RBCs), where the enzyme **Carbonic Anhydrase** catalyzes the reaction: $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$. The bicarbonate then leaves the RBC in exchange for Chloride ions (the **Chloride Shift** or Hamburger phenomenon), allowing for efficient transport in the plasma. **2. Why the other options are incorrect:** * **A. Dissolved CO2:** Only about **7%** of CO2 is transported physically dissolved in plasma. While small, this portion is crucial because it determines the partial pressure of CO2 ($PaCO_2$). * **B. Carbonic Acid:** This is a transient intermediate molecule. It is highly unstable and rapidly dissociates into $H^+$ and $HCO_3^-$; therefore, it is never a primary transport form. * **C. Carbamino-hemoglobin:** About **23%** of CO2 binds directly to the globin portion (amino groups) of the hemoglobin molecule. This binding is influenced by the **Haldane Effect** (deoxygenated blood has a higher affinity for CO2). **High-Yield NEET-PG Pearls:** * **Chloride Shift (Hamburger Phenomenon):** In systemic tissues, $Cl^-$ enters the RBC as $HCO_3^-$ leaves. In the lungs, this process reverses. * **Haldane Effect:** Oxygenation of Hb in the lungs promotes the dissociation of $CO_2$ from Hb. This is the most important factor for $CO_2$ uptake/release. * **Enzyme Fact:** Carbonic Anhydrase is one of the fastest known enzymes and is absent in plasma (it is found inside RBCs).

Q: Which of the following is a cause of shock?

Stagnant hypoxia. **Explanation:** The core pathophysiology of **Shock** is defined as a state of generalized hypoperfusion where the delivery of oxygen and nutrients is insufficient to meet metabolic demands. **1. Why Stagnant Hypoxia is Correct:** Stagnant hypoxia (also known as ischemic hypoxia) occurs when the arterial oxygen content is normal, but the **blood flow to tissues is reduced**. In shock—whether cardiogenic (pump failure), hypovolemic (low volume), or obstructive—the cardiac output falls significantly. This leads to a "stagnant" flow of blood, preventing adequate oxygen delivery to the peripheral tissues despite normal hemoglobin levels and oxygen saturation. **2. Analysis of Incorrect Options:** * **Anemic Hypoxia:** Occurs when the oxygen-carrying capacity of the blood is reduced (e.g., anemia, CO poisoning). While it reduces oxygen delivery, it is not the primary mechanism defining shock. * **Hypoxic Hypoxia:** Characterized by low arterial $PO_2$ (e.g., high altitude, lung disease). The circulation is usually intact, but the blood itself is poorly oxygenated. * **Histotoxic Hypoxia:** Occurs when tissues cannot utilize oxygen despite adequate delivery (e.g., Cyanide poisoning). The blood flow and oxygen content are normal, but the cellular respiratory chain is inhibited. **Clinical Pearls for NEET-PG:** * **V/Q Mismatch:** The most common cause of Hypoxic Hypoxia. * **Cyanosis:** Stagnant hypoxia often presents with **peripheral cyanosis** (increased oxygen extraction at tissues), whereas hypoxic hypoxia presents with **central cyanosis**. * **Shock Hallmark:** The transition from aerobic to **anaerobic metabolism**, leading to lactic acidosis, is the metabolic hallmark of stagnant hypoxia in shock.

Question 1

During normal inspiration, by how much does the diaphragm descend?

Accepted Answer

1-2 cm

Answer

3-5 cm

Answer

5-7 cm

Answer

7-9 cm

Question 2

What is the expected mixed venous oxygen tension, in mm Hg, in a normal adult after breathing 100% oxygen for 10 minutes?

Accepted Answer

45

Answer

150

Answer

740

Answer

573

Question 3

Paradoxical respiration is seen in which of the following conditions?

Accepted Answer

Diaphragmatic palsy

Answer

Multiple fractured ribs

Answer

Bulbar polio

Answer

Severe asthma

Question 4

Transection of the brain stem at the mid-pontine level with bilateral vagotomy causes what?

Accepted Answer

Apneusis

Answer

Cessation of spontaneous respiration

Answer

Continuation of regular breathing

Answer

Irregular but rhythmic respiration

Question 5

Which of the following statements is true concerning the contraction of the diaphragm?

Accepted Answer

It tends to flatten the diaphragm.

Answer

The nerves responsible for its innervation emerge from the spinal cord at the level of the lower thorax.

Answer

It reduces the lateral distance between the lower rib margins.

Answer

It causes the anterior abdominal wall to move inward.

Question 6

Pulmonary endothelium is NOT concerned with which of the following?

Accepted Answer

Factor X

Answer

Lipoprotein lipase

Answer

Plasminogen activator

Answer

Thrombin

Question 7

An increase in the concentration of 2,3 DPG may be seen in all of the following, except?

Accepted Answer

Hypoxanthine

Answer

Anemia

Answer

Hypoxia

Answer

Inosine

Question 8

Which of the following changes occur secondary to hypercarbia?

Accepted Answer

Hypertension

Answer

Miosis

Answer

Cool extremities

Answer

Bradycardia

Question 9

Carbon dioxide (CO2) is primarily transported in the arterial blood as which of the following?

Accepted Answer

Bicarbonate

Answer

Dissolved CO2

Answer

Carbonic Acid

Answer

Carbamino-hemoglobin

Question 10

Which of the following is a cause of shock?

Accepted Answer

Stagnant hypoxia

Answer

Anemic hypoxia

Answer

Hypoxic hypoxia

Answer

Histotoxic hypoxia

Respiratory System — MCQs

Respiratory System — MCQs

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