Respiratory System Practice Questions

Q: Which of the following conditions causes a leftward shift in the oxygen-hemoglobin dissociation curve?

Hypothermia. **Explanation:** The oxygen-hemoglobin (O2-Hb) dissociation curve represents the relationship between the partial pressure of oxygen (PO2) and the percentage saturation of hemoglobin. A **leftward shift** indicates an **increased affinity** of hemoglobin for oxygen, meaning hemoglobin binds oxygen more tightly and is less willing to release it to the tissues. **1. Why Hypothermia is Correct:** Temperature is inversely proportional to hemoglobin's affinity for oxygen. In **hypothermia** (low body temperature), the metabolic demands of tissues decrease. The curve shifts to the left, increasing O2 affinity. Conversely, hyperthermia (fever) shifts the curve to the right to facilitate oxygen unloading. **2. Analysis of Incorrect Options:** * **Exercise:** During exercise, there is an increase in temperature, CO2 production, and H+ ions (lactic acid). These factors decrease O2 affinity, shifting the curve to the **right** to provide more oxygen to active muscles. * **Acidosis:** An increase in H+ concentration (low pH) decreases O2 affinity. This is known as the **Bohr Effect**, which shifts the curve to the **right**. * **Adult Hemoglobin (HbA):** This is the standard physiological baseline. However, compared to **Fetal Hemoglobin (HbF)**, HbA has a lower affinity for oxygen. Therefore, HbF would cause a leftward shift relative to HbA, but HbA itself is the reference point. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis, **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase. * **Left Shift Factors:** Hypothermia, Alkalosis, Decreased 2,3-BPG, Fetal Hb (HbF), and Carbon Monoxide poisoning (Carboxyhemoglobin). * **P50 Value:** The PO2 at which Hb is 50% saturated. A **left shift decreases the P50**, while a right shift increases it.

Q: Oxygen delivery to tissues in the body is cut in half by a 50% reduction in what?

Haemoglobin. The core concept behind this question is the formula for **Oxygen Delivery ($DO_2$)**, which is the product of Cardiac Output ($CO$) and Arterial Oxygen Content ($CaO_2$): $$DO_2 = CO \times CaO_2$$ The **Arterial Oxygen Content ($CaO_2$)** is determined by: $$CaO_2 = (1.34 \times Hb \times SaO_2) + (0.003 \times PaO_2)$$ ### Why Haemoglobin is Correct In the $CaO_2$ formula, the dissolved oxygen ($0.003 \times PaO_2$) is negligible (only ~0.3 ml/dL). Therefore, $CaO_2$ is almost entirely dependent on the amount of **Haemoglobin (Hb)**. Since $DO_2$ is directly proportional to Hb, a 50% reduction in Hb (as seen in severe anemia) will result in a 50% reduction in oxygen delivery to the tissues, assuming cardiac output remains constant. ### Why Other Options are Incorrect * **B. Oxygen Saturation ($SaO_2$):** While $SaO_2$ affects $CaO_2$, it cannot be reduced by 50% in a living individual (a drop from 100% to 50% saturation is incompatible with life and represents extreme hypoxia, not a simple linear reduction in delivery). * **C. Partial Pressure of Oxygen ($PaO_2$):** $PaO_2$ only accounts for the dissolved oxygen in plasma. Even if $PaO_2$ drops by 50%, the total oxygen content changes minimally because most oxygen is bound to Hb. * **D. Arterial Content of Oxygen ($CaO_2$):** While a 50% reduction in $CaO_2$ *would* halve $DO_2$, the question asks what *component* reduction leads to this. Hb is the primary physiological variable that dictates $CaO_2$. ### High-Yield Clinical Pearls for NEET-PG * **1.34 mL:** The amount of oxygen carried by 1 gram of pure Hb (Hüfner's constant). * **Anemic Hypoxia:** Characterized by low Hb, normal $PaO_2$, and normal $SaO_2$, but decreased $CaO_2$. * **CO Poisoning:** Hb concentration is normal, but the oxygen-carrying capacity is halved because CO occupies binding sites, mimicking a 50% reduction in functional Hb.

Q: Which is a muscle of expiration?

Internal intercostal. **Explanation:** The process of respiration involves two phases: inspiration (active) and expiration (passive during quiet breathing, active during forced breathing). **Why Internal Intercostals are correct:** The **internal intercostal muscles** (specifically the interosseous portion) are primary muscles of **active expiration**. When they contract, they pull the ribs downward and inward (depress the ribs), decreasing the thoracic volume and increasing intra-thoracic pressure, which forces air out of the lungs. Note that the abdominal muscles (rectus abdominis, obliques) also assist in forced expiration. **Analysis of Incorrect Options:** * **External Intercostals:** These are muscles of **inspiration**. They lift the ribs upward and outward (the "bucket-handle" movement), increasing the transverse and anteroposterior diameter of the thorax. * **Diaphragm:** This is the **primary muscle of inspiration**, responsible for about 75% of air movement during quiet breathing. Its contraction increases the vertical diameter of the thoracic cavity. * **Serratus Anterior:** This is an **accessory muscle of inspiration**. It helps in elevating the ribs when the scapula is fixed, typically used during respiratory distress. **High-Yield Clinical Pearls for NEET-PG:** * **Quiet Expiration:** Is a **passive process** resulting from the elastic recoil of the lungs and relaxation of inspiratory muscles. * **Most important muscle of forced expiration:** The **Abdominal muscles** (Rectus abdominis is the most potent). * **Mnemonic for Intercostals:** **"E-I-E-I-O"** — **E**xternal **I**ntercostals are for **I**nspiration; **I**nternal **I**ntercostals are for **E**xpiration (Opposite). * **Pump-handle movement:** Increases AP diameter (mainly upper ribs). * **Bucket-handle movement:** Increases transverse diameter (mainly lower ribs).

Q: Cyanide poisoning causes which type of anoxia?

Histotoxic anoxia. ### Explanation **Correct Answer: A. Histotoxic anoxia** **Mechanism of Action:** Cyanide poisoning is the classic example of **histotoxic anoxia**. In this condition, the oxygen supply to the tissues is normal, and the blood is fully oxygenated. However, the tissues are unable to utilize this oxygen because cyanide binds to the ferric ($Fe^{3+}$) iron of the **Cytochrome oxidase enzyme** ($aa_3$ complex) in the mitochondrial electron transport chain. This inhibits cellular respiration, leading to "internal suffocation" at the cellular level. **Why other options are incorrect:** * **Anoxic anoxia:** Occurs when there is a failure of oxygen to reach the blood (e.g., high altitude, drowning, or strangulation). * **Anemic anoxia:** Occurs when the blood's capacity to carry oxygen is reduced, though the arterial $PO_2$ is normal (e.g., severe anemia or Carbon Monoxide poisoning). * **Stagnant anoxia:** Occurs when blood flow to the tissues is slowed or stopped, despite normal oxygen saturation (e.g., heart failure or shock). **High-Yield Clinical Pearls for NEET-PG:** * **Cherry Red Discoloration:** Because tissues cannot utilize oxygen, the venous blood remains highly oxygenated, leading to a characteristic **cherry-red** appearance of the skin and post-mortem lividity (similar to CO poisoning). * **Odour:** A characteristic **bitter almond smell** is often noted in the breath or during autopsy. * **Antidote:** The standard treatment is the **Cyanide Antidote Kit** (Amyl nitrite, Sodium nitrite, and Sodium thiosulfate) or **Hydroxocobalamin** (Cyanokit), which converts cyanide to non-toxic Vitamin $B_{12}$. * **Ideal Suicidal Poison:** Cyanide is often preferred by chemists and goldsmiths due to its rapid action.

Q: An increase in the P50 of the oxygen-hemoglobin dissociation curve is caused by a decrease in which of the following?

pH. **Explanation:** The **P50** represents the partial pressure of oxygen at which hemoglobin is 50% saturated. An **increase in P50** indicates a **rightward shift** of the oxygen-hemoglobin dissociation curve, signifying a **decreased affinity** of hemoglobin for oxygen (facilitating oxygen unloading to tissues). **1. Why pH is Correct:** A decrease in pH (acidosis) leads to an increase in hydrogen ion concentration. These ions bind to specific amino acid residues in hemoglobin, stabilizing the **T-state (Tense state)**, which has a lower affinity for oxygen. This phenomenon is known as the **Bohr Effect**. Therefore, a decrease in pH increases the P50. **2. Why the other options are incorrect:** * **B. Oxygen concentration:** Changes in $PO_2$ move the point *along* the existing curve rather than shifting the curve itself (and thus do not change the P50). * **C. Temperature:** A **decrease** in temperature shifts the curve to the **left** (decreasing P50). An *increase* in temperature is required to shift the curve to the right. * **D. Carbon dioxide concentration:** A **decrease** in $PCO_2$ shifts the curve to the **left** (decreasing P50). An *increase* in $PCO_2$ (hypercapnia) shifts the curve to the right. **High-Yield Clinical Pearls for NEET-PG:** * **Right Shift (Increased P50/Decreased Affinity):** Remember **"CADET, face Right!"** — **C**O2 increase, **A**cidosis (low pH), **D**PG (2,3-BPG) increase, **E**xercise, and **T**emperature increase. * **Left Shift (Decreased P50/Increased Affinity):** Occurs with Fetal Hemoglobin (HbF), Methemoglobin, Carbon Monoxide poisoning (though CO also decreases the oxygen-carrying capacity), and the opposite of the CADET factors. * **Physiological Significance:** A right shift is a compensatory mechanism during exercise or hypoxia to ensure tissues receive more oxygen.

Q: Physiological dead space in the lung corresponds to which zone?

Zone 1. **Explanation:** The correct answer is **Zone 1**. This question is based on **West’s Zones of the Lung**, which describe the relationship between alveolar pressure ($P_A$), arterial pressure ($P_a$), and venous pressure ($P_v$). **Why Zone 1 is the correct answer:** In Zone 1 (the apex), alveolar pressure is higher than arterial pressure ($P_A > P_a > P_v$). This high alveolar pressure compresses the pulmonary capillaries, leading to **ventilation without perfusion**. By definition, areas that are ventilated but not perfused constitute **Physiological Dead Space** (specifically, alveolar dead space). In a healthy individual breathing normally at sea level, Zone 1 is minimal, but it increases during positive pressure ventilation or severe hemorrhage. **Analysis of Incorrect Options:** * **Zone 2 (Waterfall Zone):** Here, $P_a > P_A > P_v$. Blood flow is determined by the arterial-alveolar pressure gradient. Perfusion occurs but is intermittent, matching ventilation better than in Zone 1. * **Zone 3 (Distension Zone):** Here, $P_a > P_v > P_A$. The capillaries are permanently open due to high hydrostatic pressure, leading to maximum perfusion. This zone represents the "ideal" area for gas exchange and has the lowest V/Q ratio. * **Zone 4:** This is a pathological zone seen in pulmonary edema or at very low lung volumes where interstitial pressure compresses the vessels, reducing flow. **High-Yield Clinical Pearls for NEET-PG:** * **V/Q Ratio:** The V/Q ratio is **highest at the apex** (Zone 1) and **lowest at the base** (Zone 3). * **Tuberculosis:** *M. tuberculosis* prefers the apex (Zone 1) because the high V/Q ratio results in a higher local $PO_2$, favoring the growth of this aerobe. * **Physiological Dead Space Formula:** Calculated using the **Bohr Equation**: $V_D/V_T = (PaCO_2 - PeCO_2) / PaCO_2$.

Question 1

Complex sectioning (transverse) at mid pons level along with vagi results in which of the following?

Accepted Answer

Apneusis

Answer

Asphyxia

Answer

Hyperventilation

Answer

Rapid and shallow breathing

Question 2

Which of the following conditions causes a leftward shift in the oxygen-hemoglobin dissociation curve?

Accepted Answer

Hypothermia

Answer

Exercise

Answer

Acidosis

Answer

Adult hemoglobin

Question 3

Oxygen delivery to tissues in the body is cut in half by a 50% reduction in what?

Accepted Answer

Haemoglobin

Answer

Oxygen saturation

Answer

Partial pressure of oxygen

Answer

Arterial content of oxygen

Question 4

What is the Haldane effect?

Accepted Answer

The binding of O2 to hemoglobin causes the release of CO2 from hemoglobin.

Answer

The binding of CO2 to hemoglobin causes the release of O2 from hemoglobin.

Answer

An important mechanism of O2 transport in the body.

Answer

All of the above

Question 5

Which is a muscle of expiration?

Accepted Answer

Internal intercostal

Answer

External intercostal

Answer

Diaphragm

Answer

Serratus anterior

Question 6

Tracheostomy reduces dead space by which mechanism?

Accepted Answer

By-passing the upper airway

Answer

Increasing the V/Q ratio

Answer

Reducing airflow resistance

Answer

By all of the above mechanisms

Question 7

Which of the following blood gas results is most likely in a patient with hyperventilation caused by anxiety?

Accepted Answer

Decreased PCO2

Answer

Increased PCO2

Answer

Decreased PO2

Answer

Decreased pH

Question 8

Cyanide poisoning causes which type of anoxia?

Accepted Answer

Histotoxic anoxia

Answer

Anoxic anoxia

Answer

Anemic anoxia

Answer

Stagnant anoxia

Question 9

An increase in the P50 of the oxygen-hemoglobin dissociation curve is caused by a decrease in which of the following?

Accepted Answer

pH

Answer

Oxygen concentration

Answer

Temperature

Answer

Carbon dioxide concentration

Question 10

Physiological dead space in the lung corresponds to which zone?

Accepted Answer

Zone 1

Answer

Zone 2

Answer

Zone 3

Answer

Zone 4

Respiratory System — MCQs

Respiratory System — MCQs

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