Respiratory System — Set 5

Correct Answer: D. 0.04%

• **0.04% CO₂ in inhaled air** = Atmospheric air contains only about 0.04% (400 ppm) carbon dioxide — a trace amount that the body barely registers as inhaled CO₂. • **100-fold rise on exhalation** — After gas exchange in the alveoli, exhaled air contains ~4% CO₂ as metabolically produced CO₂ from every cell is expelled. • This contrast — 0.04% in vs. 4% out — clearly shows the scale of metabolic CO₂ production in the body. • 💡 Option A (21%) is wrong because 21% is the concentration of oxygen in inhaled air, not CO₂; Option B (4%) is wrong because 4% is the CO₂ percentage in exhaled air after cellular respiration, not the air we breathe in; Option C (78%) is wrong because 78% is the share of nitrogen in the atmosphere.

Correct Answer: A. Pons

• **Pons** = The pneumotaxic center is located in the upper (rostral) pons of the brainstem; it sends inhibitory signals to the inspiratory neurons in the medulla to switch off inhalation and allow exhalation to begin. • **Rhythm fine-tuning** — By adjusting the duration of inspiration, the pneumotaxic center controls breathing rate; stronger signals from the center create faster, shallower breaths. • It works alongside the apneustic center (lower pons) and the dorsal/ventral respiratory groups in the medulla to produce the seamless breathing rhythm. • 💡 Option B (Medulla) is wrong because the medulla houses the dorsal and ventral respiratory groups (the primary rhythm generators), not the pneumotaxic center; Option C (Cerebrum) is wrong because the cerebrum handles conscious thought and voluntary motor actions — it can voluntarily override breathing but is not the pneumotaxic center; Option D (Midbrain) is wrong because the midbrain processes visual and auditory reflexes and relays signals but does not contain respiratory control centers.

Correct Answer: D. Left lung

• **Left lung** = The heart is positioned slightly left of the body midline; to accommodate it, the left lung is smaller and has only two lobes (superior and inferior), compared to the right lung's three lobes. • **Cardiac notch** — The left lung has a distinctive indentation called the cardiac notch on its medial surface where the left ventricle of the heart rests against it. • Despite being smaller, the left lung contains the lingula — a tongue-shaped projection that mirrors the middle lobe of the right lung. • 💡 Option A (Right lung) is wrong because the right lung is actually larger with three lobes (superior, middle, inferior) and greater volume; Option B (Both are equal) is wrong because the asymmetry is consistent and anatomically universal in humans, not a coincidence; Option C (It depends on the individual) is wrong because the cardiac notch and two-lobe structure of the left lung is a fixed anatomical feature present in all humans.

Correct Answer: B. Prevent alveolar collapse

• **Prevent alveolar collapse** = Surfactant (surface-active agent) is a phospholipid-protein mixture secreted by type II pneumocytes; it reduces the surface tension at the air-water interface inside alveoli, keeping them inflated and preventing them from collapsing (atelectasis) at the end of exhalation. • **Critical in premature infants** — Surfactant production begins only around 28–36 weeks of gestation; premature babies born before this develop Respiratory Distress Syndrome (RDS) because their alveoli lack surfactant and collapse with every breath. • Without surfactant, the energy needed to re-inflate collapsed alveoli would be enormous — breathing would become impossible. • 💡 Option A (Fight infections) is wrong because immune defence in the alveoli is handled by alveolar macrophages and immunoglobulins in the mucus, not surfactant; Option C (Increase surface tension) is wrong because surfactant does the exact opposite — it lowers surface tension; Option D (Warm the air) is wrong because warming and humidifying inhaled air is the job of the nasal mucosa and turbinates, not surfactant.

Correct Answer: A. Carbon dioxide

• **Carbon dioxide** = CO₂ is approximately 20–25 times more soluble in blood plasma than oxygen, meaning it dissolves readily without requiring a carrier protein for a significant fraction of its transport. • **Multiple transport forms** — CO₂ travels in blood as dissolved CO₂ (~7%), bicarbonate ions HCO₃⁻ (~70%, via carbonic anhydrase reaction), and carbaminohaemoglobin (~23%), leveraging its high solubility for efficient clearance. • This high solubility is the reason CO₂ can be removed from tissues rapidly despite being produced in large amounts by cellular respiration. • 💡 Option B (Helium) is wrong because helium, being a noble gas, has very low solubility in plasma and is essentially physiologically inert; Option C (Oxygen) is wrong because O₂ has much lower plasma solubility — only about 1.5% of oxygen is transported dissolved, the rest requires hemoglobin; Option D (Nitrogen) is wrong because nitrogen is poorly soluble in plasma under normal conditions, though it can dissolve dangerously in compressed-air diving (causing decompression sickness).

Correct Answer: D. Cyanosis

• **Cyanosis** = Cyanosis is the bluish or purplish discoloration of the skin, lips, and mucous membranes caused by excessive deoxygenated hemoglobin (deoxyhemoglobin) in the superficial blood vessels. • **Threshold and visibility** — Cyanosis becomes clinically visible when deoxygenated hemoglobin exceeds approximately 5 g/dL in capillary blood, corresponding to blood oxygen saturation falling below ~85–90%. • Central cyanosis (lips, tongue) indicates systemic hypoxia; peripheral cyanosis (fingers, toes) can also result from poor local circulation. • 💡 Option A (Hypertension) is wrong because high blood pressure causes no characteristic skin color change — it is a pressure problem, not an oxygenation problem; Option B (Anemia) is wrong because anemia causes pallor (pale skin) due to reduced total hemoglobin, not the blue tint from deoxygenated hemoglobin; Option C (Jaundice) is wrong because jaundice produces yellow-orange discoloration of skin and eyes from accumulated bilirubin, unrelated to oxygen saturation.

Correct Answer: A. Nasopharynx

• **Nasopharynx** = The nasopharynx is the uppermost section of the pharynx, positioned directly posterior to the nasal cavity and above the soft palate; it is exclusively an air passage. • **Immune structures** — The nasopharynx contains the pharyngeal tonsil (adenoids) and the openings of the Eustachian (auditory) tubes that connect to the middle ear, helping equalize pressure. • Unlike the oropharynx and hypopharynx, the nasopharynx never carries food — it is dedicated entirely to airflow. • 💡 Option B (Oropharynx) is wrong because the oropharynx lies behind the oral cavity (mouth), below the soft palate — it carries both air and food; Option C (Hypopharynx) is wrong because the hypopharynx (laryngopharynx) is the lowest segment of the pharynx, sitting behind the larynx before dividing into the esophagus and trachea; Option D (Laryngopharynx) is wrong because laryngopharynx is simply another name for the hypopharynx — both refer to the lowest pharynx region, not the part behind the nose.

Correct Answer: A. 70-100 square meters

• **70–100 square metres** = An adult human has approximately 300–500 million alveoli in both lungs; their combined internal surface area is roughly 70–100 m², comparable to the size of a singles tennis court. • **Thin diffusion barrier** — This vast area is separated from pulmonary capillary blood by only the alveolar-capillary membrane (~0.5 µm thick), enabling rapid gas exchange to meet the body's oxygen demand of ~250 mL/min at rest. • The enormous surface-to-volume ratio is possible because each alveolus is a tiny sphere (~200 µm diameter), maximising surface area within the limited thoracic space. • 💡 Option B (20 square metres) is wrong because 20 m² is roughly one-quarter of the actual area — far too small to supply enough O₂ for sustained activity; Option C (500 square metres) is wrong because 500 m² is five times the true value and physically impossible within the human thoracic cage; Option D (5 square metres) is wrong because 5 m² is twenty times too small and would cause fatal hypoxia even at rest.

Correct Answer: C. Larynx

• **Larynx** = The larynx is the cartilaginous structure that connects the pharynx (throat) above to the trachea below; it acts as the gateway between the upper and lower respiratory tracts. • **Dual function** — Besides conducting air, the larynx houses the vocal cords (true vocal folds) which vibrate to produce sound, and the epiglottis which flips down during swallowing to prevent food from entering the trachea. • The larynx is made of nine cartilages, the largest being the thyroid cartilage (Adam's apple), and is located at vertebral levels C3–C6. • 💡 Option A (Bronchi) is wrong because the bronchi are downstream of the trachea — they carry air into each individual lung and do not connect the pharynx to the trachea; Option B (Esophagus) is wrong because the esophagus runs parallel to the trachea but connects the pharynx to the stomach for food passage, not air; Option D (Alveoli) is wrong because alveoli are terminal dead-end sacs deep in the lung tissue where gas exchange occurs — they are not connecting structures.

Correct Answer: B. Breakdown of glucose with oxygen

• **Breakdown of glucose with oxygen** = Cellular respiration is the aerobic metabolic process in which glucose (C₆H₁₂O₆) is oxidized in the presence of oxygen to produce ATP energy, carbon dioxide, and water — equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~36–38 ATP. • **Site: mitochondria** — The process occurs in three stages — glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane) — yielding far more ATP than anaerobic fermentation's 2 ATP. • This is distinct from external (pulmonary) respiration, which refers to the mechanical act of breathing and gas exchange in the lungs. • 💡 Option A (Physical breathing) is wrong because physical breathing is external respiration — the mechanical movement of air in and out of the lungs, not energy production inside cells; Option C (Digestion of proteins) is wrong because protein digestion is a function of the gastrointestinal system involving proteases, completely separate from energy metabolism; Option D (Movement of blood) is wrong because blood circulation is the function of the cardiovascular system — the heart and blood vessels have no role in intracellular glucose oxidation.