Breathing And Exchange Of Gases — Complete NCERT Guide with Diagrams

The Air We Don’t Think About

Most students memorise the steps of breathing without ever understanding why the chest moves the way it does. That’s the gap we’re fixing here.

Breathing isn’t just “air in, air out.” It’s a precisely coordinated mechanical and chemical process involving pressure gradients, partial pressures, haemoglobin chemistry, and neural regulation — all happening 12-20 times per minute without conscious effort. NEET tests this topic hard: expect 3-4 questions per paper, often from gas exchange mechanisms and transport of O₂/CO₂.

The chapter splits into four logical blocks: (1) how air moves in and out, (2) how gases cross membranes, (3) how blood carries gases, and (4) how the brain controls breathing rate. Once you see that structure, the entire chapter becomes predictable.

Key Terms & Definitions

Tidal Volume (TV): Volume of air inspired or expired in a single normal breath. Value: ~500 mL (0.5 L).

Inspiratory Reserve Volume (IRV): Additional air you can forcibly inspire after a normal inspiration. Value: ~2500-3000 mL.

Expiratory Reserve Volume (ERV): Additional air you can forcibly expire after a normal expiration. Value: ~1000-1100 mL.

Residual Volume (RV): Air remaining in lungs after maximum forced expiration. Value: ~1100-1200 mL. This is why lungs don’t fully collapse — RV cannot be expelled voluntarily.

Vital Capacity (VC): Maximum air that can be moved in one breath = IRV + TV + ERV. Value: ~3500-4500 mL.

Total Lung Capacity (TLC): VC + RV ≈ 5800-6000 mL.

Functional Residual Capacity (FRC): ERV + RV — air remaining after normal (not forced) expiration.

Inspiratory Capacity (IC): TV + IRV — total air that can be inspired from resting position.

Partial Pressure: In a gas mixture, the pressure exerted by one gas independently. Denoted $pO_2$, $pCO_2$. Key for understanding gas exchange direction.

Diffusion Capacity: Rate of gas transfer across the respiratory membrane per unit pressure difference.

The Mechanics of Breathing

Inspiration (Active Process)

Inspiration requires muscular work — it’s never passive. Here’s the sequence:

1. Diaphragm contracts → flattens downward → thoracic volume increases vertically
2. External intercostal muscles contract → ribs move up and outward → thoracic volume increases laterally and anteroposteriorly
3. Increased thoracic volume → decreased intrapulmonary pressure (below atmospheric: ~758 mmHg vs atmospheric 760 mmHg)
4. Pressure gradient drives air into lungs

For forceful inspiration, the accessory muscles (sternocleidomastoid, scalene) also assist.

Expiration (Passive in Normal Breathing)

Normal expiration requires no muscular effort:

1. Diaphragm and external intercostals relax
2. Thoracic volume decreases
3. Intrapulmonary pressure rises above atmospheric (~763 mmHg)
4. Air flows out

Forceful expiration involves internal intercostals and abdominal muscles contracting actively.

Why the Pleural Fluid Matters

The space between visceral and parietal pleura contains fluid at slightly negative pressure (~756 mmHg). This creates surface tension that keeps the lung pressed against the thoracic wall. When this pressure equilibrates with atmosphere (pneumothorax — lung puncture), the lung collapses.

Exchange of Gases

The Respiratory Membrane

Gas exchange happens at the alveolar-capillary interface, a 4-layer structure:

• Alveolar epithelium (squamous, type I pneumocytes)
• Basement membrane of epithelium
• Basement membrane of endothelium
• Capillary endothelium

Total thickness: ~0.1 μm. Enormous surface area: ~70 m² (size of a tennis court). This geometry ensures rapid diffusion.

Fick’s Law Applied

Rate of diffusion ∝ (Surface area × Pressure difference × Solubility) ÷ (Thickness × √Molecular weight)

This tells us three things about $CO_2$ vs $O_2$:

• $CO_2$ is ~20-25× more soluble in plasma than $O_2$
• Despite $CO_2$ having a smaller pressure gradient, it diffuses ~20× faster
• NEET often asks to compare diffusion rates of these two gases

Partial Pressures: The Direction of Diffusion

Gas always moves from higher to lower partial pressure. At alveoli: $O_2$ moves into blood (104 → 40 mmHg gradient), $CO_2$ moves out (45 → 40 mmHg gradient). At tissues: the reverse.

Transport of Gases in Blood

Oxygen Transport

97% of $O_2$ is carried by haemoglobin as oxyhaemoglobin ($HbO_2$). Only 3% dissolves in plasma.

Each haemoglobin molecule has 4 haem groups, each binding one $O_2$. The binding is cooperative: binding of the first $O_2$ increases affinity for subsequent molecules. This produces the characteristic S-shaped (sigmoid) oxygen-dissociation curve.

Factors shifting the curve:

Right shift (reduced $O_2$ affinity — more $O_2$ released to tissues):

• ↑ $pCO_2$
• ↑ Temperature
• ↓ pH (acidosis)
• ↑ 2,3-BPG

Left shift (increased $O_2$ affinity — less $O_2$ released):

• ↓ $pCO_2$
• ↓ Temperature
• ↑ pH
• Fetal Hb (HbF)

Bohr Effect: The phenomenon where increased $CO_2$ / decreased pH reduces Hb’s affinity for $O_2$, facilitating release at active tissues.

Carbon Dioxide Transport

$CO_2$ travels in three forms:

1. As bicarbonate ions ($HCO_3^-$): ~70% — the dominant form
2. Carbaminohaemoglobin ($HbCO_2$): ~20-25% — binds to globin (not haem)
3. Dissolved in plasma: ~7%

The chloride shift (Hamburger effect): When $CO_2$ enters RBCs and forms $HCO_3^-$, this diffuses out into plasma. To maintain electrical neutrality, $Cl^-$ moves into RBCs. This happens at tissues and reverses at alveoli.

Haldane Effect: Deoxygenated Hb carries more $CO_2$ as carbaminohaemoglobin than oxygenated Hb. So at tissues where $O_2$ is released, $CO_2$ uptake by Hb increases.

Regulation of Breathing

The respiratory rhythm centre is in the medulla oblongata. It has two groups:

• Dorsal Respiratory Group (DRG): Sets basic inspiratory rhythm, active during normal breathing
• Ventral Respiratory Group (VRG): Active during forceful breathing; drives both inspiration and expiration

The pneumotaxic centre (upper pons) modulates DRG activity, limiting inspiration duration and helping set breathing rate.

The apneustic centre (lower pons) tends to prolong inspiration — pneumotaxic centre antagonises it.

Chemical Regulation

$CO_2$ is the primary driver of breathing rate — not $O_2$ (a common misconception).

Central chemoreceptors (medulla): Respond to $H^+$ ions in cerebrospinal fluid (CSF). Since $CO_2$ crosses the blood-brain barrier and forms $H^+$, rising $pCO_2$ → increased breathing rate.

Peripheral chemoreceptors (carotid and aortic bodies): Respond to ↓ $pO_2$, ↑ $pCO_2$, ↑ $H^+$ in blood.

Hering-Breuer Reflex: Stretch receptors in lungs send signals via vagus nerve to stop inspiration when lungs are sufficiently inflated. Prevents over-inflation.

Respiratory Disorders

Solved Examples

Example 1 — Easy (CBSE Level)

Q: What is the total lung capacity if vital capacity is 4200 mL and residual volume is 1200 mL?

Solution: TLC = VC + RV = 4200 + 1200 = 5400 mL

Example 2 — Medium (NEET Level)

Q: During strenuous exercise in a person, $pCO_2$ in tissues rises to 60 mmHg and temperature increases. How does this affect $O_2$ delivery to muscles?

Solution:

• Increased $pCO_2$ → Bohr effect → right shift in ODC → Hb releases $O_2$ more readily
• Increased temperature → further right shift → more $O_2$ released
• Net effect: significantly enhanced $O_2$ delivery to active muscles

Both factors act synergistically — this is a physiologically elegant design. During exercise, exactly when muscles need more $O_2$, the conditions for releasing it improve automatically.

Example 3 — Hard (NEET Level)

Q: A patient has damaged pneumotaxic centre but intact apneustic centre. Predict the breathing pattern and explain the mechanism.

Solution: The pneumotaxic centre’s role is to send inhibitory signals to terminate inspiration, preventing prolonged inhalation. Without this inhibition:

• Apneustic centre activity dominates
• Inspiration becomes prolonged and gasping
• The pattern is called apneusis — deep, gasping breaths with very brief expiration
• Breathing rate decreases but tidal volume dramatically increases

This distinguishes the pneumotaxic centre’s role (switch off inspiration) from the DRG’s role (initiate inspiration).

Exam-Specific Tips

For CBSE Board (Class 11):

• Diagrams of diaphragm position during inspiration vs expiration carry marks — practice labelling
• The table of lung volumes with values is frequently asked as a fill-in question
• “Describe the mechanism of breathing” — always mention the pressure changes explicitly with mmHg values

For NEET:

• Memorise the exact partial pressure values in the table above — numerical MCQs appear
• The chloride shift and Haldane effect are favourite “application” question topics
• Know the difference: Bohr effect (CO₂ affects Hb-O₂ affinity) vs Haldane effect (O₂ affects Hb-CO₂ carrying capacity)

Common Mistakes to Avoid

Practice Questions

Q1. Which of the following cannot be estimated by spirometry? (a) Tidal volume (b) Vital capacity (c) Total lung capacity (d) IRV

Q2. The oxygen dissociation curve shifts to the right when: (a) pH increases (b) Temperature decreases (c) $pCO_2$ increases (d) $pO_2$ increases

Q3. What percentage of $CO_2$ is transported as carbaminohaemoglobin?

Q4. During inspiration, the intrapulmonary pressure: (a) Equals atmospheric (b) Rises above atmospheric (c) Falls below atmospheric (d) Becomes zero

Q5. The dorsal respiratory group (DRG) is located in: (a) Pons (b) Cerebellum (c) Medulla oblongata (d) Hypothalamus

Q6. If a person’s tidal volume is 500 mL, IRV is 3000 mL, and ERV is 1000 mL, what is the vital capacity?

Q7. The Hering-Breuer reflex prevents: (a) Hypoxia (b) Hyperventilation (c) Over-inflation of lungs (d) Carbon dioxide accumulation

Q8. Chloride shift occurs because: (a) $Cl^-$ is needed for haemoglobin function
(b) $HCO_3^-$ moving out of RBCs must be electrically balanced
(c) Atmospheric $Cl^-$ enters blood during breathing
(d) $Na^+$ binds $CO_2$ in plasma

FAQs

Why does breathing rate increase during exercise, not just depth?

Both rate and depth increase during exercise. The primary trigger is rising $pCO_2$ from increased cellular metabolism. Central chemoreceptors detect the increased $H^+$ in CSF (from dissolved $CO_2$) and the respiratory centre fires more frequently. Muscle proprioceptors also send signals at exercise onset, causing immediate rate increase even before $CO_2$ builds up.

What happens to breathing at high altitude?

$pO_2$ in inspired air falls. Initially, peripheral chemoreceptors detect low $pO_2$ and increase breathing rate (hyperventilation). This also blows off $CO_2$, causing respiratory alkalosis, which actually slows breathing temporarily — a conflict. Over days, kidneys excrete $HCO_3^-$ to compensate, and the body acclimatises with increased RBC production (via EPO).

Why is residual volume necessary?

Without RV, alveoli would completely collapse after every expiration (atelectasis). Reopening collapsed alveoli requires enormous pressure — the work of breathing would be prohibitive. RV maintains a basal inflation that keeps surfactant spread across alveolar surfaces.

What is the difference between breathing and respiration?

In biology, these are distinct: breathing (ventilation) is the mechanical process of air movement. Cellular respiration is the biochemical process of ATP production inside mitochondria. CBSE and NEET use “external respiration” for gas exchange at alveoli and “internal respiration” for gas exchange at tissues.

Why does CO₂ diffuse faster than O₂ despite a smaller pressure gradient?

Diffusion rate ∝ solubility ÷ √molecular weight. $CO_2$ is ~24× more soluble in water/plasma than $O_2$, more than compensating for its slightly larger molecular weight. The net diffusion coefficient of $CO_2$ is approximately 20× that of $O_2$.

Why does haemoglobin have a sigmoid (S-shaped) dissociation curve rather than a simple hyperbola?

Cooperative binding. Haemoglobin has 4 subunits. When the first $O_2$ binds, it induces a conformational change in adjacent subunits, increasing their affinity. This makes binding accelerate non-linearly. The sigmoid shape is physiologically critical: it enables near-complete loading at alveolar $pO_2$ (~104 mmHg) and meaningful unloading at tissue $pO_2$ (~40 mmHg).

What is the functional significance of 2,3-BPG?

2,3-bisphosphoglycerate binds to the β-chains of deoxygenated haemoglobin, stabilising the deoxy form and reducing $O_2$ affinity (right shift). RBCs produce more 2,3-BPG during chronic hypoxia (altitude, anaemia), helping deliver more $O_2$ to tissues from the same blood $pO_2$.