Respiratory System — Set 4

Correct Answer: A. 16%

• **16% oxygen in exhaled air** = We inhale 21% oxygen but exhale about 16% because the body absorbs roughly 5% during gas exchange in the alveoli. • **Gas exchange efficiency** — Only a fraction of available oxygen is extracted per breath, leaving the rest exhaled as a biological safety buffer. • Exhaled air also contains ~4% CO₂ (up from 0.04% inhaled), showing active cellular respiration. • 💡 Option B (21%) is wrong because that is the concentration in inhaled air, not exhaled; Option C (4%) is wrong because 4% is the CO₂ percentage in exhaled air, not O₂; Option D (0%) is wrong because oxygen is never fully extracted — exhaled air still contains usable oxygen.

Correct Answer: D. Difficulty breathing at high altitudes

• **Difficulty breathing at high altitudes** = Mountain sickness (Acute Mountain Sickness, AMS) occurs because atmospheric pressure drops with altitude, reducing the partial pressure of oxygen and making each breath deliver less O₂. • **Hypoxia symptoms** — The body responds with headache, fatigue, nausea, and shortness of breath as tissues struggle to receive adequate oxygen. • The condition typically appears above 2,400 m and worsens if ascent is rapid without acclimatization. • 💡 Option A (Nausea due to high pressure) is wrong because high pressure (diving) causes different conditions like barotrauma, not AMS; Option B (Allergy to mountain air) is wrong because mountain air composition is identical to sea-level air — only pressure is lower; Option C (Fever caused by cold weather) is wrong because cold exposure causes hypothermia, not the breathing difficulty that defines AMS.

Correct Answer: C. pH and CO2 concentration

• **pH and CO₂ concentration** = The Bohr effect states that a rise in blood CO₂ and a fall in pH cause hemoglobin to release oxygen more readily, shifting the oxygen dissociation curve to the right. • **Physiological relevance** — In actively metabolising tissues CO₂ is high and pH is low, so hemoglobin offloads O₂ exactly where it is most needed. • This rightward shift means hemoglobin's affinity for oxygen decreases under acidic, high-CO₂ conditions — a perfect tissue-demand feedback. • 💡 Option A (Light) is wrong because light has no effect on hemoglobin-oxygen binding; Option B (Blood volume) is wrong because total blood volume does not change oxygen affinity of hemoglobin; Option D (Temperature) is wrong because while raised temperature does shift the curve slightly, it is not the defining variable of the Bohr effect — pH and CO₂ are.

Correct Answer: C. Esophagus

• **Esophagus** = The esophagus is a muscular tube of the digestive system that carries food and liquid from the throat to the stomach — it carries no air. • **Different system, shared opening** — Both the esophagus and trachea open from the pharynx, but the epiglottis acts as a flap valve to ensure food goes into the esophagus, not the airway. • Confusion arises because both tubes run parallel through the neck, but their functions are entirely separate. • 💡 Option A (Bronchi) is wrong because bronchi are the two main branches of the trachea carrying air into each lung; Option B (Trachea) is wrong because the trachea is the central windpipe conducting air from the larynx; Option D (Alveoli) is wrong because alveoli are the tiny air sacs where O₂–CO₂ gas exchange actually occurs.

Correct Answer: C. 6000 mL

• **6000 mL (6 litres)** = Total Lung Capacity (TLC) is the maximum volume of air the lungs can hold after a maximal inhalation; in adult males it averages ~6,000 mL. • **TLC components** — TLC = Tidal Volume (500 mL) + Inspiratory Reserve (3,000 mL) + Expiratory Reserve (1,100 mL) + Residual Volume (1,200 mL), all adding to ~5,800–6,000 mL. • Adult females average ~4,200 mL due to smaller thoracic cage; athletes may exceed 7,000 mL. • 💡 Option A (3000 mL) is wrong because 3,000 mL is roughly half the actual TLC and corresponds more to Vital Capacity alone in smaller individuals; Option B (500 mL) is wrong because 500 mL is merely the tidal volume — the amount moved in a single normal breath; Option D (1500 mL) is wrong because 1,500 mL is less than the Inspiratory Reserve Volume alone.

Correct Answer: A. Nitrogen

• **Nitrogen — 78%** = Nitrogen makes up approximately 78% of atmospheric air, making it by far the most abundant gas we breathe with every breath. • **Biologically inert** — The human body cannot use nitrogen for cellular respiration; it is inhaled and exhaled unchanged, serving no metabolic role under normal conditions. • The full atmospheric composition is N₂ (~78%) > O₂ (~21%) > Ar (~0.93%) > CO₂ (~0.04%). • 💡 Option B (Oxygen) is wrong because oxygen is the second-most abundant at only ~21%, essential for respiration but far less than nitrogen; Option C (Argon) is wrong because argon is a noble gas at just 0.93% — barely detectable; Option D (Carbon dioxide) is wrong because CO₂ in inhaled air is only 0.04%, the smallest of all common atmospheric gases.

Correct Answer: C. Diaphragm

• **Diaphragm** = The diaphragm is the large dome-shaped muscle at the base of the thoracic cavity; its contraction flattens the dome downward, increasing thoracic volume and decreasing intrapulmonary pressure, drawing air in. • **Assisted expansion** — External intercostal muscles contract simultaneously, swinging the ribs upward and outward to further expand chest dimensions during deeper breaths. • Expiration at rest is passive — the diaphragm simply relaxes and the elastic recoil of lungs pushes air out without muscular effort. • 💡 Option A (Stomach muscles) is wrong because abdominal muscles assist in forced expiration, not inspiration; Option B (Heart) is wrong because the heart is a circulatory pump with no role in generating the pressure gradient for breathing; Option D (Biceps) is wrong because biceps are arm flexors and have no anatomical connection to the thoracic cavity.

Correct Answer: D. Difficulty in exhaling air

• **Difficulty exhaling** = Emphysema destroys the elastic fibres in alveolar walls, causing them to lose their natural recoil; without that recoil, the airways collapse during exhalation, trapping air in the lungs. • **Chronic air trapping** — Over time the lungs become hyperinflated, the chest takes on a barrel-shaped appearance, and even resting exhalation requires deliberate muscular effort. • Emphysema is a type of COPD, most commonly caused by long-term smoking or exposure to airborne pollutants. • 💡 Option A (Sudden fever) is wrong because emphysema is a structural lung disease, not an infection — fever suggests pneumonia or bronchitis, not emphysema; Option B (Nosebleeds) is wrong because nosebleeds relate to nasal mucosal damage or hypertension, not alveolar destruction; Option C (Chest tightness and wheezing) is wrong because while wheezing can occur in COPD, the hallmark feature of emphysema specifically is the inability to fully exhale, not wheezing.

Correct Answer: C. Four

• **Four oxygen molecules** = Each hemoglobin molecule is made of four polypeptide chains (2α + 2β), each bearing one iron-containing heme group, and each heme binds exactly one O₂ molecule — giving a total of four. • **Cooperative binding (Bohr effect basis)** — Binding the first O₂ triggers a conformational change that makes the remaining three heme groups bind oxygen more easily, maximising loading efficiency in the lungs. • In 100 mL of blood, ~14–16 g of hemoglobin carries about 20 mL of O₂ when fully saturated. • 💡 Option A (Eight) is wrong because eight would require eight heme groups, but hemoglobin has only four subunits and therefore four heme groups; Option B (Two) is wrong because two is only half the actual binding capacity; Option D (One) is wrong because one would make hemoglobin an extremely inefficient carrier — each RBC contains ~270 million hemoglobin molecules precisely because each carries four, not one.

Correct Answer: A. Trachea

• **Trachea** = The trachea is the main conducting airway extending from the larynx (voice box) down to the carina where it bifurcates into the left and right main bronchi, colloquially called the windpipe. • **Structural reinforcement** — The trachea is kept permanently open by 16–20 incomplete C-shaped hyaline cartilage rings; the open posterior part faces the esophagus, allowing food boluses to pass. • It is approximately 10–12 cm long and 2.5 cm in diameter in an adult. • 💡 Option B (Bronchus) is wrong because bronchi are the secondary airways that branch off from the trachea into each lung — they are downstream of the windpipe, not the windpipe itself; Option C (Larynx) is wrong because the larynx (voice box) sits above the trachea and is responsible for sound production and protecting the airway during swallowing; Option D (Esophagus) is wrong because the esophagus is the food tube running alongside the trachea — it is part of the digestive, not respiratory, system.