Respiration in Human
🌬️ General / Intro
• Organisms obtain energy from food by cellular respiration (catabolic processes: glycolysis, Krebs cycle, electron transport chain).
• Cellular respiration requires oxygen (O₂) and produces carbon dioxide (CO₂) as waste.
• External respiration = uptake of O₂ from environment and removal of CO₂ at body-system level (breathing + gas exchange in capillaries).
• Organs performing external respiration = respiratory system (main organ: lungs).
• Chapter theme: explain human respiratory system and important respiratory disorders.
🌬️ Properties of Respiratory Surface (requirements for effective gas diffusion)
• Moist and permeable — gases dissolve and pass easily.
• Thin — minimum distance for diffusion.
• Rich blood supply — quick transport of O₂ and CO₂.
• Structural support — prevents collapse; keeps surface open.
• Located internally — prevents loss of moisture to atmosphere.
• Air ventilation — air must move toward and away from surface.
• Branched tubular pathway before surface — air becomes saturated with water vapor before reaching alveoli.
🌬️ Respiratory Tract — Divisions
• Respiratory organs form a continuous respiratory tract (air flows in/out).
• Two major divisions: Upper respiratory tract and Lower respiratory tract.
🌬️ Upper Respiratory Tract
Nasal cavity
• Opens via nostrils; divided by nasal septum.
• Blood vessels warm and humidify incoming air.
• Hairs + mucus trap larger particles (filtering).
• Contains chemoreceptors for smell; contributes to taste.
Pharynx
• Tube connecting nasal & oral cavities to larynx & oesophagus.
• Common passage for air and food — part of both respiratory & digestive systems.
Larynx
• Connects pharynx to trachea; made of muscles + cartilages.
• Contains vocal cords — vibrate with air flow to produce sound; muscles control pitch & volume.
• Epiglottis: cartilaginous flap above glottis; upright during breathing, closes during swallowing to prevent food entering airway.
• If material enters larynx → cough reflex expels it.
🌬️ Lower Respiratory Tract
Trachea (windpipe)
• Wide airway connecting larynx to lungs; about 4–6 inches long, ~1 inch wide.
• Walls contain smooth muscle and C-shaped cartilage rings (keep airway open).
• Lined with mucus + cilia that move trapped particles toward pharynx.
• Trachea branches into two primary bronchi.
Bronchi → Bronchioles → Alveoli
• Primary bronchi: right & left.
• Secondary bronchi: right lung 3, left lung 2.
• Further branching → tertiary bronchi → bronchioles → terminal bronchioles → alveolar ducts → alveoli.
• Bronchioles lack cartilage plates.
• Terminal bronchioles end in alveolar ducts which open into alveolar clusters.
• Each cluster → 20–30 alveoli.
🌬️ Lungs & Alveoli
• Lungs are largest respiratory organs; each lung covered by two membranes: visceral pleura (on lung) and parietal pleura (thoracic wall).
• Pleural cavity between pleurae contains fluid — allows smooth expansion/contraction.
• Right lung: 3 lobes; Left lung: 2 lobes (space for heart).
• Lungs may contain up to ~700 million alveoli → huge total surface area for gas exchange.
• Alveolus: moist epithelial sac, wall thickness ≈ 0.1 micrometre — ideal for diffusion.
• Alveoli surrounded by dense capillary network for gas exchange (O₂ into blood, CO₂ out).
• Some alveolar epithelial cells secrete surfactant — prevents alveoli collapsing/sticking together during expiration. Surfactant is secreted and reabsorbed in healthy lungs.
🌬️ Recap of Blood Flow Related to Lungs
• Pulmonary arteries bring deoxygenated blood to lungs for oxygenation.
• Pulmonary veins carry oxygenated blood from lungs to the heart to be pumped to body.
• Lungs also receive a small oxygenated blood supply (for lung tissue cells) from the heart for their own cellular respiration.
🌬️ Mechanism of Breathing (Ventilation)
• Breathing (ventilation) = movement of air in and out of lungs via pressure changes. Lungs do not actively suck or push air.
• Main muscles: diaphragm (dome-shaped skeletal muscle separating thorax & abdomen) and intercostal muscles (between ribs).
• Inspiration (inhalation): diaphragm contracts & flattens; intercostals contract raising ribs → thoracic cavity volume increases → lung pressure drops below atmospheric → air enters lungs.
• Expiration (exhalation): diaphragm & intercostals relax; diaphragm returns to dome shape & ribs move down → thoracic volume decreases → lung pressure rises above atmospheric → air leaves lungs.
• Elastic tension created during inspiration is released during expiration to help lungs recoil.
🌬️ Control of Breathing
• Breathing initiated and regulated by neurons in the respiratory centre of the medulla oblongata (brain stem).
• These neurons send rhythmic impulses to diaphragm and intercostal muscles (causing inspiration). When impulses stop, muscles relax → expiration.
• Breathing is largely involuntary, maintaining blood O₂ and CO₂ balance.
🌬️ Special Notes
• High altitude: atmospheric pressure is lower → requires greater thoracic expansion to lower lung pressure below atmosphere; body adapts to improve O₂ uptake. (Reason athletes use high-altitude training.)
• Bird respiration: birds have lungs + air sacs and one-way airflow (posterior air sacs → lungs → anterior air sacs → outside). This one-way flow and counter-directional blood flow make gas exchange very efficient — allows breathing at high altitudes.
• Human respiratory tract has an extensive branching network — text mentioned ~2,414 kilometres of airways conducting air (illustrates massive internal airway length).
🧠 MCQs on Human Respiratory System – Class 11 Biology
1. The process of obtaining energy from food molecules is called:
(a) Photosynthesis
(b) Cellular respiration
(c) Fermentation
(d) Metabolism
✅ Answer: (b) Cellular respiration
🟢 Explanation: Cellular respiration includes glycolysis, Krebs cycle, and electron transport chain that release energy by oxidizing food molecules.
2. External respiration involves:
(a) Breakdown of glucose
(b) Exchange of gases between environment and blood
(c) ATP synthesis
(d) Absorption of nutrients
✅ Answer: (b) Exchange of gases between environment and blood
🟢 Explanation: External respiration refers to the uptake of oxygen and removal of carbon dioxide at the body system level (lungs and capillaries).
3. The main organs for external respiration in humans are:
(a) Kidneys
(b) Lungs
(c) Liver
(d) Trachea
✅ Answer: (b) Lungs
🟢 Explanation: Lungs provide the respiratory surface where exchange of oxygen and carbon dioxide occurs.
4. Which property does NOT belong to a good respiratory surface?
(a) Thick and dry
(b) Moist and thin
(c) Rich blood supply
(d) Structural support
✅ Answer: (a) Thick and dry
🟢 Explanation: A good respiratory surface must be thin, moist, and have good blood supply for rapid diffusion of gases.
5. Which part of the respiratory system is responsible for filtering and humidifying air?
(a) Trachea
(b) Nasal cavity
(c) Alveoli
(d) Larynx
✅ Answer: (b) Nasal cavity
🟢 Explanation: The nasal cavity warms, humidifies, and filters the air using mucus and nasal hairs before it enters the lungs.
6. The nasal cavity also contributes to:
(a) Blood circulation
(b) Sense of smell and taste
(c) Digestion
(d) Voice production
✅ Answer: (b) Sense of smell and taste
🟢 Explanation: Chemoreceptors in the nasal cavity help detect odors and contribute to taste perception.
7. Which structure is common for both food and air passage?
(a) Larynx
(b) Trachea
(c) Pharynx
(d) Esophagus
✅ Answer: (c) Pharynx
🟢 Explanation: The pharynx connects nasal and oral cavities to both larynx (for air) and esophagus (for food).
8. The voice box of humans is:
(a) Pharynx
(b) Larynx
(c) Epiglottis
(d) Trachea
✅ Answer: (b) Larynx
🟢 Explanation: The larynx contains vocal cords that vibrate to produce sound when air passes through them.
9. Epiglottis prevents food from entering:
(a) Nasal cavity
(b) Larynx
(c) Pharynx
(d) Esophagus
✅ Answer: (b) Larynx
🟢 Explanation: During swallowing, the epiglottis covers the glottis to prevent food from entering the airway.
10. The trachea is supported by:
(a) Smooth muscles only
(b) C-shaped cartilage rings
(c) Bone rings
(d) Elastic fibers
✅ Answer: (b) C-shaped cartilage rings
🟢 Explanation: Cartilage rings keep the trachea open during pressure changes in breathing.
11. The trachea divides into:
(a) Bronchioles
(b) Alveoli
(c) Primary bronchi
(d) Secondary bronchi
✅ Answer: (c) Primary bronchi
🟢 Explanation: The trachea bifurcates into right and left primary bronchi, each entering a lung.
12. The absence of cartilage plates is a characteristic of:
(a) Trachea
(b) Bronchi
(c) Bronchioles
(d) Larynx
✅ Answer: (c) Bronchioles
🟢 Explanation: Bronchioles are small airways without cartilage; they lead to alveolar ducts.
13. The number of lobes in the right lung is:
(a) One
(b) Two
(c) Three
(d) Four
✅ Answer: (c) Three
🟢 Explanation: Right lung has 3 lobes; left lung has 2 to accommodate the heart.
14. The pleural cavity is filled with:
(a) Mucus
(b) Surfactant
(c) Pleural fluid
(d) Blood
✅ Answer: (c) Pleural fluid
🟢 Explanation: Pleural fluid between visceral and parietal pleura reduces friction during lung movement.
15. The actual site of gas exchange is:
(a) Bronchioles
(b) Alveoli
(c) Larynx
(d) Trachea
✅ Answer: (b) Alveoli
🟢 Explanation: Alveoli provide a thin, moist surface surrounded by capillaries for O₂ and CO₂ diffusion.
16. The function of surfactant in alveoli is to:
(a) Increase oxygen absorption
(b) Keep alveoli moist
(c) Prevent alveolar collapse
(d) Help diffusion of carbon dioxide
✅ Answer: (c) Prevent alveolar collapse
🟢 Explanation: Surfactant reduces surface tension and prevents alveoli from sticking together during exhalation.
17. Pulmonary arteries carry:
(a) Oxygenated blood to lungs
(b) Deoxygenated blood to lungs
(c) Oxygenated blood from lungs
(d) Nutrients to lungs
✅ Answer: (b) Deoxygenated blood to lungs
🟢 Explanation: Pulmonary arteries transport deoxygenated blood from the heart to lungs for oxygenation.
18. Inspiration occurs when:
(a) Diaphragm relaxes
(b) Diaphragm contracts
(c) Thoracic volume decreases
(d) Rib cage moves inward
✅ Answer: (b) Diaphragm contracts
🟢 Explanation: Contraction of diaphragm increases thoracic volume and decreases pressure, drawing air in.
19. Expiration occurs due to:
(a) Diaphragm contraction
(b) Elastic recoil of lungs
(c) Rib cage expansion
(d) Air pressure decrease inside lungs
✅ Answer: (b) Elastic recoil of lungs
🟢 Explanation: Relaxation of respiratory muscles and elastic recoil push air out of the lungs.
20. The breathing rate is controlled by:
(a) Cerebrum
(b) Medulla oblongata
(c) Cerebellum
(d) Spinal cord
✅ Answer: (b) Medulla oblongata
🟢 Explanation: Respiratory center in medulla sends rhythmic signals to diaphragm and intercostal muscles.
21. Which factor makes breathing difficult at high altitudes?
(a) Increased CO₂ level
(b) Low air pressure
(c) Increased humidity
(d) Thick air
✅ Answer: (b) Low air pressure
🟢 Explanation: Lower air pressure means less oxygen availability, making breathing harder.
22. The lungs contain approximately how many alveoli?
(a) 7 million
(b) 70 million
(c) 700 million
(d) 7 billion
✅ Answer: (c) 700 million
🟢 Explanation: The lungs have about 700 million alveoli providing a large surface area for gas exchange.
23. In birds, the flow of air through the lungs is:
(a) Two-way
(b) One-way
(c) Random
(d) Intermittent
✅ Answer: (b) One-way
🟢 Explanation: Birds have air sacs that ensure continuous, one-directional airflow for efficient gas exchange.
24. The diaphragm separates:
(a) Lungs and liver
(b) Thoracic and abdominal cavities
(c) Heart and stomach
(d) Lungs and esophagus
✅ Answer: (b) Thoracic and abdominal cavities
🟢 Explanation: The diaphragm is a dome-shaped muscle that separates thoracic and abdominal cavities.
25. The main reason lungs remain open during breathing is:
(a) Air sacs
(b) Surfactant and pleural pressure
(c) Mucus
(d) Bronchial cartilage
✅ Answer: (b) Surfactant and pleural pressure
🟢 Explanation: Both surfactant and the negative pressure in the pleural cavity prevent lung collapse.
26. Air moves into the lungs when:
(a) Lung pressure > atmospheric pressure
(b) Lung pressure < atmospheric pressure
(c) Rib cage contracts
(d) Diaphragm relaxes
✅ Answer: (b) Lung pressure < atmospheric pressure
🟢 Explanation: Air always moves from higher to lower pressure; during inspiration, lung pressure drops below atmospheric.
27. Which of the following is NOT a part of the lower respiratory tract?
(a) Trachea
(b) Bronchi
(c) Alveoli
(d) Pharynx
✅ Answer: (d) Pharynx
🟢 Explanation: Pharynx belongs to the upper respiratory tract.
28. The pitch of the voice is controlled by:
(a) Lung volume
(b) Laryngeal muscles
(c) Tongue
(d) Pharynx size
✅ Answer: (b) Laryngeal muscles
🟢 Explanation: Laryngeal muscles adjust the tension of vocal cords, altering pitch.
29. The exchange of gases in alveoli occurs through:
(a) Osmosis
(b) Active transport
(c) Diffusion
(d) Filtration
✅ Answer: (c) Diffusion
🟢 Explanation: Oxygen and carbon dioxide move across alveolar and capillary walls by simple diffusion.
30. The inner lining of alveoli is made up of:
(a) Connective tissue
(b) Smooth muscle
(c) Moist epithelial tissue
(d) Elastic cartilage
✅ Answer: (c) Moist epithelial tissue
🟢 Explanation: Alveoli have thin epithelial lining that allows easy diffusion of gases.
🧠 Human Respiratory System – Short Questions with Answers (By Rana Zahid, FFSA)
Q1. What is the main difference between cellular respiration and external respiration?
Cellular respiration occurs inside cells to release energy, while external respiration involves the exchange of gases (O₂ and CO₂) between the organism and its environment.
Q2. Why must the respiratory surface be thin and moist?
Because gases can only diffuse efficiently through a thin, moist membrane, allowing rapid exchange of O₂ and CO₂.
Q3. Why is the respiratory surface richly supplied with blood capillaries?
To quickly transport oxygen to cells and remove carbon dioxide from tissues, maintaining concentration gradients.
Q4. What structural adaptations make alveoli efficient for gas exchange?
Alveoli have a large surface area, thin epithelium, moist lining, rich blood supply, and surfactant to prevent collapse.
Q5. What is the function of the pleural membranes?
They enclose the lungs and secrete pleural fluid, reducing friction during breathing movements.
Q6. Why is the right lung larger than the left lung?
Because the left side of the thoracic cavity houses the heart, reducing available space for the left lung.
Q7. Why are C-shaped cartilage rings present in the trachea?
They keep the airway open and prevent tracheal collapse during pressure changes in breathing.
Q8. Why are the cartilage rings incomplete posteriorly?
To allow flexibility and movement of the esophagus during swallowing.
Q9. What role does mucus play in the respiratory tract?
It traps dust, bacteria, and other particles, preventing them from entering the lungs.
Q10. How do cilia help in cleaning the air passages?
Cilia beat upward to push mucus and trapped particles toward the pharynx, where they are swallowed or expelled.
Q11. What happens if surfactant is absent in alveoli?
Alveoli collapse after exhalation, making reinflation difficult, leading to respiratory distress.
Q12. How does the diaphragm function during inspiration?
It contracts and flattens, increasing thoracic volume and decreasing pressure so air enters the lungs.
Q13. How does the diaphragm function during expiration?
It relaxes and moves upward, reducing thoracic volume and increasing pressure, forcing air out.
Q14. Why is expiration a passive process while inspiration is active?
Inspiration requires muscular contraction, while expiration mainly depends on relaxation and elastic recoil.
Q15. What would happen if the phrenic nerve is damaged?
The diaphragm would be paralyzed, impairing breathing and possibly causing respiratory failure.
Q16. What is the role of the intercostal muscles in breathing?
They lift or lower the ribs to change thoracic volume during inhalation and exhalation.
Q17. Why is alveolar air different in composition from inspired air?
Because of continuous gas exchange and mixing of old and new air within the alveoli.
Q18. Why is carbon dioxide more soluble in blood than oxygen?
CO₂ combines with plasma and hemoglobin more easily, forming bicarbonates for transport.
Q19. What causes the movement of gases between alveoli and capillaries?
It occurs by simple diffusion due to partial pressure differences of O₂ and CO₂.
Q20. What is partial pressure?
It is the pressure exerted by a single gas in a mixture of gases, determining diffusion direction.
Q21. How does high altitude affect respiration?
At high altitudes, air has low oxygen pressure, causing shortness of breath and increased breathing rate.
Q22. How does smoking damage the respiratory system?
It destroys cilia, increases mucus secretion, damages alveoli, and reduces oxygen absorption.
Q23. Why are lungs considered elastic organs?
Because of elastic connective tissue fibers that help them expand and recoil during breathing.
Q24. What prevents food from entering the trachea during swallowing?
The epiglottis closes over the glottis, blocking the airway when swallowing food.
Q25. Why is gas exchange faster in lungs than in tissues?
Because alveolar membranes are thinner and have a much larger surface area than tissue capillaries.
Q26. Why is carbon dioxide exhaled faster than oxygen is absorbed?
Because CO₂ diffuses more readily due to higher solubility and steeper diffusion gradient.
Q27. What is the role of the medulla oblongata in respiration?
It regulates the rhythm of breathing by sending nerve impulses to respiratory muscles.
Q28. How does the body respond to increased CO₂ concentration in blood?
Chemoreceptors detect it, stimulating the medulla to increase breathing rate to expel CO₂.
Q29. Why do we cough or sneeze?
To expel irritants or particles from the respiratory passages through forceful exhalation.
Q30. What is tidal volume?
The amount of air inhaled or exhaled during normal breathing (about 500 mL in adults).
Q31. What is vital capacity?
Maximum amount of air that can be exhaled after maximum inhalation (~4800 mL).
Q32. Why is oxygen transport efficient in the human body?
Because most oxygen binds with hemoglobin, forming oxyhemoglobin for rapid transport.
Q33. What is the physiological significance of residual volume?
It prevents lung collapse by ensuring some air always remains after exhalation.
Q34. How do respiratory centers in pons assist breathing?
They fine-tune rhythm generated by medulla, maintaining smooth inspiration and expiration.
Q35. Why do lungs not completely empty after expiration?
Residual air maintains gas exchange continuity and prevents alveolar collapse.
Q36. How is oxygen transported in blood?
About 97% binds with hemoglobin, and 3% is dissolved in plasma.
Q37. How is carbon dioxide transported in blood?
70% as bicarbonate ions, 23% as carbaminohemoglobin, and 7% dissolved in plasma.
Q38. Why is breathing rate higher in infants than adults?
Infants have higher metabolic rate and smaller lung capacity, requiring more frequent breaths.
Q39. Why do mountaineers face difficulty in breathing?
Due to lower oxygen partial pressure at high altitudes, reducing oxygen diffusion into blood.
Q40. What is the role of hemoglobin in respiration?
It carries oxygen from lungs to tissues and assists in CO₂ transport back to lungs.
Q41. What is hypoxia?
A condition in which body tissues receive insufficient oxygen, leading to fatigue or organ failure.
Q42. What is apnea?
Temporary cessation of breathing, often during sleep (sleep apnea).
Q43. Why does breathing become difficult in asthma?
Because bronchioles constrict and mucus production increases, narrowing air passages.
Q44. How does emphysema affect alveoli?
It damages alveolar walls, reducing surface area for gas exchange and causing breathlessness.
Q45. What happens to breathing rate during exercise?
It increases to meet the higher oxygen demand and remove excess CO₂.
Q46. Why does oxygen diffuse from alveoli into blood and not vice versa?
Because alveolar oxygen partial pressure is higher than that of deoxygenated blood.
Q47. Why is the lung structure compared to an inverted tree?
Because bronchi and bronchioles branch extensively like tree limbs ending in alveolar sacs.
Q48. What are the main protective mechanisms of the respiratory system?
Nasal hairs, mucus, cilia, cough reflex, and immune cells in alveoli.
Q49. What is meant by pulmonary ventilation?
The mechanical process of inhalation and exhalation that moves air into and out of lungs.
Q50. How does oxygen deficiency affect brain function?
It leads to dizziness, loss of consciousness, or brain damage due to insufficient ATP production.
Human Respiratory System – Long Questions with Answers
Q1. Describe the structure and function of the human respiratory system.
The human respiratory system consists of organs responsible for the exchange of gases between the atmosphere and body tissues. It ensures the intake of oxygen and the removal of carbon dioxide.
Structure:
• Nasal cavity – Air enters through nostrils, is filtered by hairs, moistened and warmed by blood vessels.
• Pharynx – A common passage for air and food connecting nasal cavity to larynx.
• Larynx – Contains vocal cords for sound production; guarded by the epiglottis that prevents food entry into the trachea.
• Trachea – A tube supported by C-shaped cartilage rings; lined with cilia and mucus to trap and expel dust.
• Bronchi and bronchioles – The trachea divides into two bronchi, each entering a lung and further branching into bronchioles.
• Alveoli – Microscopic air sacs surrounded by capillaries where actual gas exchange occurs.
• Lungs – Pair of spongy organs enclosed by pleura; right lung has 3 lobes, left has 2 lobes.
Function:
• Inhalation: Air is drawn into lungs as diaphragm contracts and thoracic cavity expands.
• Gas exchange: O₂ diffuses from alveoli into capillaries, and CO₂ diffuses in the opposite direction.
• Exhalation: Diaphragm relaxes, volume decreases, and air rich in CO₂ is expelled.
• The process is regulated by the respiratory centers in the medulla oblongata.
Q2. Explain the mechanism of breathing in humans.
Breathing involves the rhythmic movement of the diaphragm and intercostal muscles, resulting in the inflow and outflow of air.
Inspiration (Inhalation):
• The diaphragm contracts and flattens.
• External intercostal muscles contract, lifting the ribs upward and outward.
• Thoracic cavity volume increases, causing air pressure inside lungs to decrease.
• As a result, air enters the lungs from the atmosphere.
Expiration (Exhalation):
• The diaphragm relaxes and moves upward.
• Intercostal muscles relax, ribs move downward and inward.
• Thoracic cavity volume decreases, pressure inside lungs increases.
• Air is forced out of the lungs.
Control:
• The medulla oblongata and pons regulate breathing rhythm.
• Chemoreceptors detect CO₂ concentration and adjust the breathing rate accordingly.
Q3. Discuss the adaptations of alveoli for efficient gaseous exchange.
Alveoli are specialized structures for maximum gas exchange between air and blood.
Adaptations:
• Large surface area: About 700 million alveoli provide ~70 m² surface area for diffusion.
• Thin walls: Only one cell thick (~0.1 μm), minimizing diffusion distance.
• Rich blood supply: Dense capillary network maintains concentration gradients.
• Moist lining: Dissolves gases, aiding diffusion.
• Elastic fibers: Allow expansion during inspiration and recoil during expiration.
• Surfactant: Prevents collapse by reducing surface tension.
Result:
These adaptations ensure rapid and efficient diffusion of oxygen into blood and carbon dioxide out of it, making the lungs ideal organs for gaseous exchange.
Q4. Describe the role of diaphragm and intercostal muscles in respiration.
The diaphragm and intercostal muscles are the chief muscles involved in the mechanics of breathing.
Role of Diaphragm:
• Dome-shaped sheet separating thoracic and abdominal cavities.
• During inspiration, it contracts and flattens, increasing thoracic volume and drawing air into lungs.
• During expiration, it relaxes and curves upward, reducing thoracic volume and pushing air out.
Role of Intercostal Muscles:
• Located between ribs in two layers: external and internal.
• External intercostal muscles contract during inspiration to lift ribs upward and outward.
• Internal intercostal muscles contract during forced expiration to push ribs downward.
Together, they create rhythmic changes in thoracic pressure essential for breathing.
Q5. Explain how gases are exchanged and transported in the human body.
Gas exchange occurs in lungs (external respiration) and in body tissues (internal respiration).
1. Exchange of gases in lungs:
• In alveoli, oxygen partial pressure is higher than in blood.
• Oxygen diffuses into pulmonary capillaries; CO₂ diffuses out from blood into alveoli.
2. Transport of gases:
Oxygen Transport:
• 97% bound to hemoglobin forming oxyhemoglobin (HbO₂).
• 3% dissolved in plasma.
Carbon Dioxide Transport:
• 70% as bicarbonate ions (HCO₃⁻).
• 23% as carbaminohemoglobin (HbCO₂).
• 7% dissolved in plasma.
3. Exchange in tissues:
• Oxygen leaves blood due to lower tissue O₂ pressure.
• CO₂ produced by metabolism diffuses into blood for exhalation.
Q6. What are the properties of an efficient respiratory surface? Explain how alveoli possess them.
An efficient respiratory surface has specific structural and functional characteristics that allow rapid gas diffusion.
Properties:
• Thin and moist for easy gas diffusion.
• Large surface area.
• Rich blood supply.
• Permeable to gases.
• Ventilated for maintaining concentration gradient.
Alveoli as Ideal Surfaces:
• Walls are one-cell thick and moist.
• Vast number (~700 million) provide large area.
• Surrounded by a dense capillary network.
• Constant ventilation keeps O₂ high and CO₂ low.
Hence, alveoli are the most efficient gas-exchange surfaces in the human body.
Q7. Describe the structure of lungs and their protective coverings.
The lungs are soft, spongy organs located in the thoracic cavity on either side of the heart.
Structure:
• Right lung – 3 lobes; Left lung – 2 lobes.
• Made of bronchi, bronchioles, alveolar ducts, and alveoli.
• Richly supplied with pulmonary blood vessels.
• Elastic tissue allows expansion and recoil.
Protective Coverings:
• Pleura: Double-layered membrane around each lung.
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Visceral pleura: Attached to lung surface.
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Parietal pleura: Lines thoracic wall.
• Between them is pleural fluid, reducing friction and aiding smooth lung movement during breathing.
Human Respiratory System
FFSA Biology | 30 Questions | 20s per Question
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