🫁 Chapter 9 · MDCAT Biology
📅 Updated Jan 2026
⏱️ 30 min read
🎯 High-yield
Human Respiratory System — Complete MDCAT Notes & MCQs
Full MDCAT Biology Chapter 9 on the human respiratory system: respiratory tract anatomy (nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli), surfactant, gas exchange, oxygen and carbon dioxide transport (Bohr effect, Haldane effect, chloride shift), lung volumes, respiratory center, and the effects of smoking (chronic bronchitis, emphysema, lung cancer). Includes 200 solved MCQs with explanations and year-wise past papers.
❓People Also Ask — Human Respiratory System
Questions students most frequently search about this chapter. Answers are direct and exam-focused.
QWhat is the difference between the conducting zone and respiratory zone?
Conducting zone (nose → terminal bronchioles) warms, humidifies, and filters air but does NOT perform gas exchange. Respiratory zone (respiratory bronchioles → alveoli) is where gas exchange occurs.
QWhy does the trachea have C-shaped cartilage?
The C-shaped cartilage rings keep the trachea open (prevent collapse) while the posterior gap allows the esophagus to expand during swallowing.
QWhy do bronchioles lack cartilage?
Bronchioles lack cartilage so that their smooth muscle walls can regulate airflow (bronchodilation/bronchoconstriction) and control resistance.
QWhat is the pleural cavity?
The pleural cavity is a potential space between the visceral and parietal pleura, filled with pleural fluid that lubricates and reduces friction during breathing.
QWhat is the difference between external and internal respiration?
External respiration: gas exchange between alveoli and blood in the lungs. Internal respiration: gas exchange between blood and tissues.
QWhy is CO₂ more important than O₂ in stimulating breathing?
Central chemoreceptors in the medulla respond primarily to increased H⁺ in the CSF (derived from CO₂). O₂ acts only as a backup via peripheral chemoreceptors when pO₂ drops below 60 mmHg.
QWhat is the oxygen-hemoglobin dissociation curve?
A sigmoid curve showing hemoglobin's oxygen saturation vs partial pressure of oxygen. Its shape reflects cooperative binding — binding one O₂ increases affinity for the next three.
QWhat is the Bohr effect?
Increased CO₂, H⁺ (low pH), and temperature shift the oxygen-hemoglobin dissociation curve to the right, reducing hemoglobin's affinity for O₂ and enhancing oxygen unloading in active tissues.
QWhat is the Haldane effect?
Deoxygenated hemoglobin has a higher affinity for CO₂ than oxygenated hemoglobin. This facilitates CO₂ loading in tissues and unloading in lungs.
QWhy does carbon monoxide poisoning cause tissue hypoxia?
CO binds to hemoglobin with ~200× greater affinity than O₂, reducing oxygen-carrying capacity. It also shifts the curve left, preventing remaining hemes from releasing O₂ to tissues.
QWhat are the three main lung volumes?
Tidal volume (~500 mL, normal breath), Inspiratory reserve volume (~3000 mL, extra forced inspiration), Expiratory reserve volume (~1100 mL, extra forced expiration). Residual volume (~1200 mL) remains after maximum expiration.
QWhat is the difference between obstructive and restrictive lung disease?
Obstructive (asthma, COPD): decreased airflow, low FEV₁/FVC ratio. Restrictive (fibrosis): decreased lung compliance, reduced lung volumes but preserved FEV₁/FVC ratio.
QWhat is the function of the nasal conchae (turbinates)?
Nasal conchae increase surface area inside the nasal cavity, causing air turbulence that warms, humidifies, and filters inspired air.
QWhat is the respiratory quotient (RQ)?
RQ = CO₂ produced / O₂ consumed. Carbohydrates = 1.0, fats = 0.7, proteins = 0.8. Used to determine metabolic fuel being used.
QWhat is respiratory distress syndrome (RDS)?
RDS is a serious condition in premature infants caused by surfactant deficiency, leading to alveolar collapse (atelectasis) and difficulty breathing.
QWhat happens to the diaphragm during inspiration?
The diaphragm contracts and flattens (moves downward), increasing thoracic volume and decreasing intrapulmonary pressure, drawing air into the lungs.
QWhat is the Hering-Breuer reflex?
A protective reflex triggered by stretch receptors in the lungs when over-inflated, inhibiting further inspiration to prevent over-expansion.
QWhat is cyanosis?
Cyanosis is bluish discoloration of the skin and mucous membranes caused by an increased amount of reduced (deoxygenated) hemoglobin (>5 g/dL) in the blood.
QWhat is the dead space?
Anatomical dead space (~150 mL) is the volume of air in the conducting airways that does not participate in gas exchange. Physiological dead space = anatomical + alveolar dead space (non-functional alveoli).
QWhat is the difference between myoglobin and hemoglobin?
Myoglobin has one heme, no cooperativity (hyperbolic curve), and higher O₂ affinity — stores O₂ in muscle. Hemoglobin has four hemes, cooperative binding (sigmoid curve), lower affinity — transports O₂ in blood.
📋Syllabus & Learning Outcomes
According to PMDC/PMC MDCAT syllabus, Human Respiratory System covers:
- 13.1 Discuss the functions of main part of respiratory system.
- 13.2 Discuss the process of gas exchange in human lungs.
- 13.3 Discuss the effect of smoking on respiratory system.
📝Full Notes — Human Respiratory System
🫁 Human Respiratory System – Schematic Notes
🧬 13.1 Main Parts & Their Functions
- Nose / Nasal cavity: Filters, warms, moistens air; detects smell (olfactory epithelium); mucous traps particles, cilia sweep them to pharynx.
- Pharynx (throat): Common passage for air and food; leads to larynx.
- Larynx (voice box): Contains vocal cords; epiglottis covers it during swallowing to prevent aspiration.
- Trachea (windpipe): Tube with C‑shaped cartilage rings (keep it open); lined with ciliated mucous membrane – mucus traps particles, cilia beat upward to pharynx ("mucociliary escalator").
- Bronchi (left and right): Trachea divides; similar structure, cartilage rings; enter lungs at hilum.
- Bronchioles: Smaller branches, no cartilage, smooth muscle walls; regulate airflow (bronchodilation / bronchoconstriction).
- Alveoli (air sacs): Functional units; walls one cell thick, surrounded by pulmonary capillaries; site of gas exchange; coated with surfactant (reduces surface tension, prevents collapse).
🔑 Key: The conducting zone (nose → terminal bronchioles) warms/humidifies air; respiratory zone (respiratory bronchioles → alveoli) performs gas exchange.
💨 13.2 Gas Exchange in Human Lungs
- Occurs by simple diffusion across the respiratory membrane (alveolar wall + capillary wall).
- Driven by partial pressure gradients:
- O₂: alveolar air (PO₂ ≈ 104 mm Hg) → blood (PO₂ ≈ 40 mm Hg).
- CO₂: blood (PCO₂ ≈ 45 mm Hg) → alveolar air (PCO₂ ≈ 40 mm Hg).
- O₂ binds haemoglobin (oxyhaemoglobin); CO₂ transported as bicarbonate (70%), dissolved (7%), or carbaminohaemoglobin (23%).
- Efficient because: huge surface area (≈70 m²), short diffusion distance, rich blood supply.
- Breathing movements: inspiration (diaphragm contracts, ribs rise) and expiration (passive at rest).
🚬 13.3 Effects of Smoking on Respiratory System
- Destroys cilia → impaired mucociliary clearance → chronic cough, infections.
- Chronic bronchitis: inflammation and excess mucus → narrowed airways, persistent cough.
- Emphysema: alveolar walls break down → reduced surface area → breathlessness; loss of elastic recoil.
- Lung cancer: carcinogens in tar cause mutations in bronchial epithelium → uncontrolled growth.
- Carbon monoxide in smoke binds haemoglobin → reduces oxygen‑carrying capacity.
- Increases risk of respiratory infections, asthma exacerbations.
🔑 High‑Yield MCQs & Traps
- Trachea has C‑shaped cartilage; bronchioles have smooth muscle but no cartilage.
- Alveoli are the site of gas exchange; surfactant prevents collapse.
- Gas exchange is by simple diffusion; O₂ moves from alveoli to blood, CO₂ vice versa.
- Epiglottis prevents food entering larynx.
- Smoking paralyses cilia → mucus accumulation.
- Emphysema: loss of alveolar walls → decreased surface area.
- Chronic bronchitis: excess mucus and narrowed airways.
- CO binds haemoglobin more strongly than O₂ → reduces O₂ supply.
- ⚠️ Traps: "Gas exchange occurs in bronchi" → False (alveoli). "O₂ diffuses from blood to alveoli" → False (opposite). "Smoking only causes lung cancer" → False (also bronchitis, emphysema, COPD).
💡 MDCAT focus: Structure‑function relationship of respiratory tract, partial pressure gradients, and three major smoking‑related diseases.
❔Frequently Asked Questions
What is the human respiratory system?
The human respiratory system is the network of organs (nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli) responsible for taking in oxygen and expelling carbon dioxide through breathing and gas exchange.
What are the main parts of the respiratory system?
Nose/nasal cavity, pharynx, larynx, trachea, primary bronchi, secondary bronchi, bronchioles, and alveoli. The conducting zone warms and filters air; the respiratory zone (alveoli) performs gas exchange.
What is the function of the epiglottis?
The epiglottis is a cartilaginous flap that covers the glottis (opening to the larynx) during swallowing, preventing food and liquids from entering the trachea.
What is the function of surfactant?
Surfactant (secreted by Type II alveolar cells) reduces surface tension in the alveoli, preventing their collapse at end-expiration (atelectasis).
Where does gas exchange occur in the lungs?
Gas exchange occurs in the alveoli (air sacs), across the respiratory membrane (alveolar wall + capillary wall), driven by partial pressure gradients of O₂ and CO₂.
How is oxygen transported in blood?
About 98.5% of oxygen is transported bound to hemoglobin as oxyhemoglobin. A small amount (~1.5%) dissolves in plasma.
How is carbon dioxide transported in blood?
About 70% as bicarbonate ions (HCO₃⁻), 23% as carbaminohemoglobin, and 7% dissolved in plasma.
What is the Bohr effect?
The Bohr effect describes how increased CO₂ and H⁺ (decreased pH) reduce hemoglobin's affinity for oxygen, shifting the oxygen-hemoglobin dissociation curve to the right, which enhances oxygen unloading in tissues.
What is the Haldane effect?
The Haldane effect states that deoxygenated hemoglobin has a higher affinity for CO₂ than oxygenated hemoglobin, facilitating CO₂ loading in tissues and unloading in lungs.
What is the chloride shift?
The chloride shift (Hamburger phenomenon) is the movement of Cl⁻ into red blood cells in exchange for HCO₃⁻ leaving, maintaining electrical neutrality during CO₂ transport.
What are the effects of smoking on the respiratory system?
Smoking paralyses cilia, causes chronic bronchitis, destroys alveolar walls (emphysema), causes lung cancer, and carbon monoxide reduces oxygen-carrying capacity. Nicotine is addictive and causes vasoconstriction.
What is emphysema?
Emphysema is a chronic obstructive pulmonary disease in which alveolar walls are destroyed, reducing surface area for gas exchange and causing loss of elastic recoil, leading to breathlessness.
What is chronic bronchitis?
Chronic bronchitis is inflammation of the bronchi with excessive mucus secretion, causing a persistent productive cough (≥3 months in 2 consecutive years).
What is the difference between external and internal respiration?
External respiration is gas exchange between alveoli and blood in the lungs. Internal respiration is gas exchange between blood and tissues.
What is tidal volume?
Tidal volume (TV) is the volume of air inhaled or exhaled during a normal quiet breath (~500 mL in adults).
What is vital capacity?
Vital capacity (VC) = Tidal volume + Inspiratory reserve volume + Expiratory reserve volume. It's the maximum volume of air a person can exhale after a maximum inhalation (~4.8 L).
What is residual volume?
Residual volume (RV) is the air remaining in the lungs after maximum exhalation (~1.2 L). It prevents lung collapse.
Where is the respiratory center located?
The respiratory center is in the medulla oblongata (DRG, VRG) and pons (pneumotaxic, apneustic centers). It controls the rate and depth of breathing.
How many MCQs come from Human Respiratory System in MDCAT?
Human Respiratory System typically contributes 4–6 MCQs out of 81 Biology MCQs in MDCAT.
Which topics from Human Respiratory System are most frequently asked in MDCAT?
Alveoli as gas exchange sites, surfactant function, hemoglobin oxygen transport, CO₂ transport forms, Bohr effect, tidal volume, vital capacity, respiratory center in medulla, and smoking effects (bronchitis, emphysema, lung cancer).