Respiratory System — Set 1

Correct Answer: C. Larynx

• **Larynx** = The larynx is a cartilaginous organ situated between the pharynx and trachea that houses the vocal cords and produces sound when air passes through. • **Vocal cord structure** — The two vocal cords stretch across the laryngeal cavity; their tension and the rate of airflow together determine the pitch and volume of the voice. • The larynx also serves as a sphincter that prevents food and liquids from entering the lower airway during swallowing. • 💡 Option A (Bronchi) is wrong because bronchi are the paired tubes branching from the trachea into each lung, far removed from sound production; Option B (Pharynx) is wrong because the pharynx is a shared passage for food and air that lies above the larynx; Option D (Trachea) is wrong because the trachea is the rigid windpipe below the larynx and contains no vocal cords.

Correct Answer: A. Alveoli

• **Alveoli** = Alveoli are microscopic, thin-walled air sacs clustered at the terminal ends of bronchioles where oxygen diffuses into the blood and carbon dioxide diffuses out. • **Massive surface area** — The roughly 300 million alveoli in adult lungs create a total surface area of about 70 square metres — comparable to a singles tennis court — maximising gas exchange efficiency. • Each alveolus is surrounded by a dense capillary network; the diffusion distance between alveolar air and blood is only 0.2–0.5 micrometres, allowing near-instant exchange. • 💡 Option B (Pleura) is wrong because pleura are protective membranes enclosing the lungs, not exchange surfaces; Option C (Diaphragm) is wrong because it is the respiratory muscle that drives breathing, not a gas-exchange organ; Option D (Bronchioles) is wrong because bronchioles are conducting airways that deliver air to alveoli but do not themselves exchange gases.

Correct Answer: A. Hemoglobin

• **Hemoglobin** = Hemoglobin is an iron-containing protein packed inside red blood cells; each molecule reversibly binds up to four oxygen molecules via its four heme groups. • **Carries 98% of blood oxygen** — About 97–98% of oxygen in arterial blood is carried bound to hemoglobin as oxyhemoglobin, while only 1–2% is dissolved in plasma. • Iron is the active site in each heme group; iron deficiency reduces hemoglobin production and causes anaemia, directly impairing oxygen delivery to tissues. • 💡 Option B (Bilirubin) is wrong because bilirubin is a yellow breakdown product of old hemoglobin, not an oxygen transporter; Option C (Myoglobin) is wrong because myoglobin stores oxygen in muscle tissue and does not circulate in blood; Option D (Chlorophyll) is wrong because chlorophyll is the light-absorbing pigment in plant cells and is completely absent in human blood.

Correct Answer: B. Diaphragm

• **Diaphragm** = The diaphragm is a dome-shaped skeletal muscle that forms the floor of the thoracic cavity and the roof of the abdominal cavity, serving as the principal muscle of inspiration. • **Mechanics of breathing** — When the diaphragm contracts it flattens downward, increasing thoracic volume and lowering intrapulmonary pressure so that air flows in; relaxation reverses this and drives exhalation. • The diaphragm also assists in coughing, sneezing, vomiting, and even vocal control by varying intra-abdominal pressure. • 💡 Option A (Sternum) is wrong because the sternum is the flat breastbone forming the front of the rib cage — a bony structure, not a muscular partition; Option C (Epiglottis) is wrong because the epiglottis is a small cartilaginous flap in the throat that covers the larynx during swallowing; Option D (Rib cage) is wrong because the rib cage is the bony protective framework around the thorax, not a dividing muscle.

Correct Answer: B. Epiglottis

• **Epiglottis** = The epiglottis is a leaf-shaped flap of elastic cartilage attached to the base of the tongue that folds down over the laryngeal inlet during the swallowing reflex. • **Involuntary valve action** — The moment food or liquid touches the back of the throat, a reflex elevates the larynx and tips the epiglottis over it, sealing the airway and directing the bolus into the oesophagus. • If this reflex fails — as can happen with neurological disease — food enters the trachea, a dangerous condition called aspiration that can lead to aspiration pneumonia. • 💡 Option A (Uvula) is wrong because the uvula is the fleshy lobe hanging at the back of the soft palate that helps seal the nasopharynx during swallowing but does not cover the larynx; Option C (Glottis) is wrong because the glottis is the opening between the vocal cords — it narrows during swallowing but the epiglottis is the primary cover; Option D (Soft palate) is wrong because the soft palate rises to close the nasal passage during swallowing, not to block the windpipe.

Correct Answer: C. Exchange of gases between blood and tissues

• **Internal respiration** = Internal respiration is the diffusion of oxygen from systemic capillaries into body cells and the simultaneous diffusion of carbon dioxide from cells back into the capillary blood. • **Driven by concentration gradients** — Tissues actively respiring have low O2 and high CO2 concentrations; arterial blood arriving has high O2 and low CO2, so gases move passively down their gradients without any energy cost. • Internal respiration occurs continuously at every living cell in the body, unlike external respiration which is limited to the lungs. • 💡 Option A (Formation of ATP in mitochondria) is wrong because that is cellular respiration — the biochemical oxidation of glucose, not the gas-exchange step; Option B (Mechanical movement of air) is wrong because that describes ventilation, the physical act of breathing; Option D (Exchange of gases between lungs and air) is wrong because that defines external respiration at the pulmonary alveoli.

Correct Answer: A. 12-16 times per minute

• **12–16 breaths per minute** = This range is the clinically accepted eupnoea (normal quiet breathing) rate for a resting healthy adult and represents the baseline set by the medulla oblongata. • **Brain-regulated rhythm** — Chemoreceptors in the medulla and carotid bodies continuously sample blood pH and CO2; even a tiny rise in CO2 triggers faster, deeper breaths to restore balance. • A rate below 12 (bradypnoea) may signal drug overdose or CNS depression, while a rate above 20 (tachypnoea) suggests infection, pain, or respiratory distress. • 💡 Option B (20–30 times) is wrong because this range is typical of mild to moderate exercise, not rest; Option C (70–80 times) is wrong because that is the normal resting heart rate, not breathing rate; Option D (5–10 times) is wrong because it falls below the normal range and would indicate dangerously slow breathing in a resting adult.

Correct Answer: C. Pleura

• **Pleura** = The pleura consists of two continuous layers: the visceral pleura, which is tightly fused to the lung surface, and the parietal pleura, which lines the inner chest wall and diaphragm. • **Pleural fluid as lubricant** — The narrow pleural cavity between the two layers contains 5–15 mL of serous fluid that eliminates friction as the lungs slide against the chest wall during every breath. • Abnormal accumulation of fluid (pleural effusion), air (pneumothorax), or blood (haemothorax) in this cavity can collapse a lung and requires urgent drainage. • 💡 Option A (Peritoneum) is wrong because the peritoneum is the serous membrane lining the abdominal cavity and covering abdominal organs; Option B (Pericardium) is wrong because the pericardium is the double-walled sac enclosing the heart; Option D (Meninges) is wrong because meninges are the three protective membranes (dura, arachnoid, pia) surrounding the brain and spinal cord.

Correct Answer: C. Carbon dioxide

• **Carbon dioxide** = CO2 in the blood combines with water to form carbonic acid, lowering blood pH; central chemoreceptors in the medulla detect this pH drop and immediately increase respiratory rate and depth. • **More sensitive than oxygen** — The respiratory centres respond to a rise of even 2–3 mmHg in arterial CO2 (PaCO2) with a measurable increase in ventilation, making CO2 the dominant real-time regulator. • In chronic lung disease patients, CO2 sensitivity can be blunted; in such cases the body switches to using low O2 as a backup stimulus — the "hypoxic drive." • 💡 Option A (Nitrogen) is wrong because nitrogen is physiologically inert at atmospheric pressure and plays no role in respiratory chemoreception; Option B (Carbon monoxide) is wrong because CO is a toxic gas that is not normally present in blood and does not act as a chemoreceptor stimulus; Option D (Oxygen) is wrong because while peripheral chemoreceptors do respond to falling O2, oxygen changes must be far more dramatic before they override the dominant CO2-based control.

Correct Answer: A. Vital Capacity

• **Vital Capacity** = Vital capacity (VC) is the sum of the tidal volume (TV), inspiratory reserve volume (IRV), and expiratory reserve volume (ERV) — representing the maximum movable air. • **Gender and fitness differences** — Average VC is about 4.8 L in adult males and 3.1 L in females, but trained athletes can exceed 6 L due to stronger respiratory muscles and greater thoracic compliance. • Vital capacity declines with age, restrictive lung diseases, and conditions that limit chest wall expansion such as scoliosis or obesity. • 💡 Option B (Tidal Volume) is wrong because tidal volume is only the small amount (~500 mL) of air moved during a single normal resting breath; Option C (Total Lung Capacity) is wrong because TLC includes the residual volume that can never be exhaled, making it larger than vital capacity; Option D (Residual Volume) is wrong because residual volume is the air permanently remaining in the lungs after maximum exhalation, the exact opposite of what is being asked.