Breathing and Exchange of Gases - Complete NEET Biology Notes

Breathing and Exchange of Gases is a high-weightage chapter contributing 3-5 questions in NEET annually. This guide covers the complete respiratory physiology for NEET 2026.

Overview of Respiration

Types of Respiration:

Steps in Respiration:

1. Breathing (ventilation)
2. Diffusion of gases across alveolar membrane
3. Transport of gases in blood
4. Diffusion of gases between blood and tissues
5. Cellular respiration

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Respiratory System

Respiratory Organs in Different Animals

Human Respiratory System

Components:

1. Conducting portion: Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles (up to terminal bronchioles)
2. Respiratory portion: Respiratory bronchioles → Alveolar ducts → Alveoli

Upper Respiratory Tract

Nose:

• External nares (nostrils)
• Nasal cavity lined with mucous membrane
• Functions: Filtering, warming, humidifying air

Pharynx:

• Common passage for air and food
• Three parts: Nasopharynx, Oropharynx, Laryngopharynx

Larynx (Voice Box):

• Contains vocal cords
• Cartilages: Thyroid (Adam's apple), Cricoid, Arytenoid, Epiglottis
• Glottis: Opening between vocal cords

Lower Respiratory Tract

Trachea (Windpipe):

• ~10-12 cm long
• C-shaped cartilaginous rings (open posteriorly)
• Lined with ciliated epithelium and goblet cells

Bronchi:

• Trachea divides into right and left primary bronchi
• Right bronchus: Wider, shorter, more vertical (foreign objects lodge here)
• Secondary and tertiary bronchi within lungs

Bronchioles:

• Lack cartilage
• Smooth muscle for diameter regulation
• Terminal bronchioles: End of conducting zone

Alveoli:

• ~300 million per lung
• Site of gas exchange
• Covered by pulmonary capillaries
• Surfactant reduces surface tension

Lungs

Pleura:

• Double membrane covering lungs
• Parietal pleura: Lines thoracic cavity
• Visceral pleura: Covers lungs
• Pleural cavity: Contains pleural fluid (reduces friction)

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Mechanism of Breathing

Breathing Muscles

Inspiration (Active Process)

1. Diaphragm contracts and flattens
2. External intercostals contract, ribs move up and out
3. Thoracic cavity volume increases
4. Intrapulmonary pressure decreases (below atmospheric)
5. Air rushes into lungs

Expiration (Passive Process - at rest)

1. Diaphragm and intercostals relax
2. Thoracic cavity volume decreases
3. Intrapulmonary pressure increases (above atmospheric)
4. Air pushed out of lungs

Forced Expiration: Internal intercostals and abdominal muscles contract

Pressure Changes

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Lung Volumes and Capacities

Lung Volumes

Lung Capacities

NEET Important:

• Vital Capacity: Maximum air that can be exhaled after maximum inhalation
• Residual Volume: Prevents lung collapse, allows continuous gas exchange

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Exchange of Gases

Composition of Atmospheric Air

Partial Pressures (at sea level)

Gas Exchange at Alveoli

Factors affecting diffusion:

1. Pressure gradient (most important)
2. Solubility of gas
3. Thickness of membrane
4. Surface area

Respiratory membrane (0.2 μm thick):

1. Alveolar epithelium (Type I pneumocytes)
2. Epithelial basement membrane
3. Interstitial space
4. Capillary basement membrane
5. Capillary endothelium

Note: CO₂ diffuses 20× faster than O₂ despite lower gradient (higher solubility)

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Transport of Gases

Oxygen Transport

1. Dissolved in Plasma (3%)

• pO₂ determines dissolved oxygen
• ~0.3 mL O₂/100 mL blood

2. Bound to Hemoglobin (97%)

• Hemoglobin: 4 heme groups + 4 polypeptide chains
• Each Hb binds 4 O₂ molecules
• Hb + O₂ ⇌ HbO₂ (Oxyhemoglobin)

Oxygen Carrying Capacity:

• 1 g Hb binds 1.34 mL O₂
• Normal Hb: 15 g/100 mL blood
• Max O₂: ~20 mL/100 mL blood

Oxygen-Hemoglobin Dissociation Curve

Shape: Sigmoidal (S-shaped)

Explanation: Cooperative binding - binding of first O₂ increases affinity for subsequent O₂

Important Values:

Factors Shifting the Curve

NEET Tip: Right shift = releases O₂ more easily (beneficial in active tissues)

Carbon Dioxide Transport

Bicarbonate Formation (Chloride Shift)

In Tissues:

1. CO₂ diffuses into RBC
2. CO₂ + H₂O → H₂CO₃ (carbonic anhydrase)
3. H₂CO₃ → H⁺ + HCO₃⁻
4. HCO₃⁻ moves out, Cl⁻ moves in (chloride shift)
5. H⁺ buffered by hemoglobin

In Lungs: Reverse process occurs

Haldane Effect

• Binding of O₂ to Hb reduces its affinity for CO₂
• In lungs: High O₂ → CO₂ released
• In tissues: Low O₂ → CO₂ binds easily

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Regulation of Respiration

Respiratory Center

Location: Medulla oblongata and pons

Chemical Regulation

NEET Important: CO₂ is the primary chemical regulator of breathing, not O₂.

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Respiratory Disorders

COPD (Chronic Obstructive Pulmonary Disease): Includes emphysema and chronic bronchitis

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Previous Year NEET Questions

Q1 (NEET 2023): The oxygen-hemoglobin dissociation curve is:

• (a) Linear
• (b) Sigmoidal ✓
• (c) Hyperbolic
• (d) Exponential

Q2 (NEET 2022): Bohr effect refers to:

• (a) Decreased O₂ binding at high CO₂ ✓
• (b) Increased O₂ binding at high CO₂
• (c) Decreased CO₂ binding at high O₂
• (d) Increased CO₂ binding at high O₂

Q3 (NEET 2021): Residual volume is:

• (a) Air remaining after normal expiration
• (b) Air remaining after forced expiration ✓
• (c) Maximum air that can be exhaled
• (d) Air in tidal breathing

Q4 (NEET 2020): Major transport form of CO₂ in blood is:

• (a) Dissolved CO₂
• (b) Carbaminohemoglobin
• (c) Bicarbonate ✓
• (d) Carbonic acid

Q5 (NEET 2019): Pneumotaxic center is located in:

• (a) Medulla
• (b) Pons ✓
• (c) Cerebellum
• (d) Hypothalamus

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Quick Revision Points

1. Conducting portion: Nose to terminal bronchioles
2. Respiratory portion: Respiratory bronchioles to alveoli
3. Number of alveoli: ~300 million per lung
4. Vital capacity: TV + IRV + ERV (~4600 mL)
5. Residual volume: ~1200 mL (never exhaled)
6. O₂ transport: 97% as oxyhemoglobin
7. CO₂ transport: 70% as bicarbonate
8. Respiratory center: Medulla oblongata
9. Primary regulator: CO₂ (not O₂)
10. Bohr effect: ↑CO₂ → ↓O₂ affinity (right shift)
11. Haldane effect: ↑O₂ → ↓CO₂ affinity
12. Chloride shift: HCO₃⁻ out, Cl⁻ in

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FAQs

Q: Why is the oxygen-hemoglobin curve sigmoidal? A: Due to cooperative binding. When one O₂ binds, hemoglobin changes shape making it easier for subsequent O₂ molecules to bind. This creates the characteristic S-shape.

Q: Why is CO₂ the primary regulator of breathing, not O₂? A: Central chemoreceptors in the medulla are highly sensitive to CO₂/H⁺ changes. O₂ levels must drop significantly before peripheral chemoreceptors respond. Normal breathing maintains CO₂ levels, which indirectly maintains O₂.

Q: What is the significance of residual volume? A: Residual volume prevents lung collapse (atelectasis) and ensures continuous gas exchange even between breaths. Without it, alveoli would collapse during expiration.

Q: Why do we breathe faster during exercise? A: Exercise increases CO₂ production and H⁺ levels. These stimulate chemoreceptors, which signal the respiratory center to increase breathing rate and depth.

Q: How does smoking affect the respiratory system? A: Smoking destroys cilia (impairs mucus clearance), causes chronic inflammation, destroys alveolar walls (emphysema), and increases risk of lung cancer. It also produces carboxyhemoglobin, reducing oxygen-carrying capacity.