CBSE Class 11 Biology Chapter 17 Notes: Breathing and Exchange of Gases

Chapter Introduction

Breathing and Exchange of Gases is Chapter 17 of the NCERT Class 11 Biology textbook, falling under the Human Physiology unit (Unit V). This chapter bridges the circulatory system (Chapter 16) with body fluids and circulation, explaining how oxygen reaches every cell and how carbon dioxide is removed.

Respiration at the organismal level involves two fundamental steps:

1. Breathing (ventilation) — the mechanical process of moving air into and out of the lungs. This is a purely physical process involving muscle action and pressure changes. 2. Gas exchange — the diffusion of oxygen and carbon dioxide across membranes, driven by partial pressure gradients. This occurs at two sites: between alveolar air and blood (external respiration), and between blood and tissues (internal respiration).

Note that cellular respiration — the biochemical oxidation of glucose to release energy (ATP) — is a separate, intracellular process covered in Chapter 14. This chapter focuses on the physical transport and exchange steps that supply O2 to cells and remove CO2.

The chapter covers the diversity of respiratory organs across animal groups, the detailed anatomy of the human respiratory system, the mechanism of breathing, gas exchange at the alveoli, transport of O2 and CO2 in blood (including the oxygen-haemoglobin dissociation curve and Bohr effect), regulation of respiration by neural and chemical mechanisms, lung volumes and capacities, and common respiratory disorders.

Key Definitions

1. Respiration — The biological process involving the uptake of oxygen (O2) and release of carbon dioxide (CO2) at the organismal level. At the cellular level, it includes the oxidation of nutrients to produce ATP.

2. Breathing (ventilation) — The mechanical process of inspiration (drawing air into the lungs) and expiration (pushing air out of the lungs). It is a physical process, not a biochemical one.

3. Diffusion — The movement of gases from a region of higher partial pressure to a region of lower partial pressure across a permeable membrane. Gas exchange in the lungs and tissues occurs entirely by diffusion.

4. Partial pressure — The individual pressure exerted by a gas in a mixture of gases. For respiratory physiology, the key partial pressures are pO2 (partial pressure of oxygen) and pCO2 (partial pressure of carbon dioxide), measured in mm Hg.

5. Haemoglobin (Hb) — An iron-containing respiratory pigment found in red blood cells (RBCs). Each Hb molecule has four haem groups, each capable of binding one O2 molecule, giving a maximum capacity of four O2 per Hb.

6. Oxyhaemoglobin (HbO2) — The compound formed when oxygen binds reversibly to haemoglobin. Formation occurs in the lungs (high pO2); dissociation occurs in the tissues (low pO2).

7. Carbaminohaemoglobin — The compound formed when CO2 binds to the amino group of haemoglobin. It accounts for about 23% of CO2 transport in blood.

8. Bohr effect — The phenomenon where increased pCO2, decreased pH (increased H+ concentration), or increased temperature reduces the oxygen affinity of haemoglobin, shifting the oxygen-haemoglobin dissociation curve to the right and promoting O2 release in tissues.

9. Tidal volume (TV) — The volume of air inspired or expired during normal breathing. In a healthy adult, it is approximately 500 mL.

10. Vital capacity (VC) — The maximum volume of air that can be expelled from the lungs after a maximum inspiration. It equals TV + IRV + ERV (approximately 3400–4800 mL).

11. Residual volume (RV) — The volume of air that remains in the lungs even after maximum expiration (approximately 1100–1200 mL). It prevents the lungs from collapsing and maintains some gas exchange between breaths.

Detailed Concepts

Respiratory Organs in Animals

Different animal groups have evolved different respiratory structures suited to their habitats and metabolic demands:

Key point: In insects, the tracheal system is so efficient that haemoglobin is absent in their blood. In vertebrates, the circulatory system works alongside lungs (or gills) to transport gases.

Human Respiratory System

The human respiratory system is a branching network of tubes that conduct air from the nostrils to the alveoli, where gas exchange occurs. Each component has a specific role:

Nostrils and Nasal Cavity

• Air enters through the nostrils into the nasal cavity.
• The nasal cavity is lined by a mucous membrane and fine hairs that filter dust and particles.
• The rich blood supply warms the incoming air, and mucus moistens it.

Pharynx

• The nasal cavity opens into the pharynx, which is a common passage for air and food.
• It is divided into nasopharynx (air passage), oropharynx (common), and laryngopharynx (leads to larynx and oesophagus).

Larynx (Voice Box)

• Located at the entrance of the trachea.
• Contains the vocal cords, which vibrate to produce sound when air passes through them.
• The glottis is the opening of the larynx. During swallowing, the epiglottis (a flap of cartilage) covers the glottis to prevent food from entering the trachea.

Trachea (Windpipe)

• A straight tube about 11 cm long and 2.5 cm in diameter.
• Supported by C-shaped cartilaginous rings that prevent the trachea from collapsing when air pressure inside drops during inspiration.
• The open part of the C faces the oesophagus posteriorly, allowing the oesophagus to expand during swallowing.

Bronchi and Bronchioles

• At its lower end, the trachea divides into the right and left primary bronchi — one entering each lung.
• Within the lungs, each primary bronchus branches into secondary (lobar) bronchi, then tertiary (segmental) bronchi, and continues branching into smaller bronchioles.
• Terminal bronchioles give rise to respiratory bronchioles, which open into alveolar ducts and then alveoli.
• Cartilage is present in bronchi but gradually decreases and is absent in bronchioles (which have smooth muscle instead).

Alveoli

• The functional units of gas exchange. Each lung contains approximately 300 million alveoli.
• Alveolar walls are extremely thin (about 0.5 micrometres), lined by a single layer of squamous epithelium.
• Surrounded by a dense network of pulmonary capillaries.
• The total alveolar surface area for gas exchange is approximately 70–100 square metres — roughly the area of a tennis court.
• Surfactant (a lipoprotein secreted by Type II alveolar cells) reduces surface tension and prevents alveoli from collapsing.

Pleural Membranes

• Each lung is enclosed in a double-layered membrane called the pleura.
• The visceral pleura adheres to the lung surface; the parietal pleura lines the thoracic wall.
• The pleural cavity between the two layers contains pleural fluid, which lubricates the membranes and reduces friction during breathing movements.
• The negative pressure in the pleural cavity keeps the lungs inflated.

Diaphragm

• A dome-shaped muscular partition separating the thoracic cavity (above) from the abdominal cavity (below).
• It is the primary muscle of breathing. Its contraction and relaxation drive the pressure changes that cause inspiration and expiration.

Mechanism of Breathing

Breathing involves two phases: inspiration (inhaling) and expiration (exhaling). The mechanism is based on pressure differences between the lungs and the atmosphere.

Inspiration (Active Process)

Inspiration at rest is an active process because it requires muscle contraction:

1. The diaphragm contracts and moves downward (flattens from its dome shape), increasing the vertical dimension of the thoracic cavity. 2. The external intercostal muscles contract, pulling the ribs upward and outward, increasing the anteroposterior and lateral dimensions of the thoracic cavity. 3. The overall thoracic cavity volume increases. 4. As lung volume increases (lungs follow the thoracic wall due to the pleural fluid coupling), the intrapulmonary (intra-alveolar) pressure decreases below atmospheric pressure (by about 1–2 mm Hg). 5. Air rushes into the lungs down the pressure gradient until intrapulmonary pressure equals atmospheric pressure.

Expiration (Passive Process at Rest)

Normal quiet expiration is a passive process — it does not require muscle contraction:

1. The diaphragm relaxes and returns to its dome shape, moving upward. 2. The external intercostal muscles relax, and the ribs move downward and inward (due to gravity and elastic recoil). 3. The thoracic cavity volume decreases. 4. The intrapulmonary pressure increases above atmospheric pressure (by about 1–2 mm Hg). 5. Air moves out of the lungs down the pressure gradient.

Forced expiration (during exercise or voluntary exhalation) is active. It involves:

• Contraction of internal intercostal muscles (pull ribs downward and inward more forcefully).
• Contraction of abdominal muscles (push the diaphragm upward by increasing abdominal pressure).

Gas Exchange at the Alveoli

Gas exchange between the alveolar air and pulmonary blood occurs by simple diffusion across the respiratory membrane (alveolar-capillary membrane). The driving force is the partial pressure gradient of each gas.

Partial Pressure Values (in mm Hg):

Diffusion of O2 into blood:

• Alveolar pO2 = 104 mm Hg; venous blood pO2 = 40 mm Hg
• O2 diffuses from the alveolus into the pulmonary capillary blood (gradient = 64 mm Hg).

Diffusion of CO2 into alveoli:

• Venous blood pCO2 = 45 mm Hg; alveolar pCO2 = 40 mm Hg
• CO2 diffuses from the blood into the alveolus (gradient = 5 mm Hg). Although the gradient is smaller, CO2 diffuses about 20 times faster than O2 because it is much more soluble in water.

The Diffusion Membrane (Respiratory Membrane): The total thickness of the barrier is only about 0.5 micrometres, consisting of three layers: 1. Alveolar epithelium — thin squamous epithelium 2. Basement membrane — shared by alveolar and capillary epithelia 3. Capillary endothelium — single layer of flat endothelial cells

This extremely thin barrier allows rapid diffusion of gases by keeping the diffusion path minimal.

Transport of Oxygen

Oxygen is transported in the blood by two methods:

1. As oxyhaemoglobin (HbO2) — 98.5%: Oxygen binds reversibly to haemoglobin in the RBCs.

• In the lungs (high pO2): Hb + 4O2 → Hb(O2)4 (oxyhaemoglobin forms)
• In the tissues (low pO2): HbO2 → Hb + O2 (oxygen dissociates and diffuses to tissues)

2. Dissolved in plasma — 1.5%: A small fraction of O2 is carried in physical solution in the blood plasma.

Haemoglobin structure and oxygen binding:

• Each Hb molecule has 4 haem groups (each with an iron atom in the Fe2+ state).
• Each haem group can bind one O2 molecule.
• Therefore, one Hb molecule can carry a maximum of 4 O2 molecules.
• The binding of the first O2 molecule increases the affinity for the second (cooperative binding), and so on — this is called cooperative binding and explains the sigmoid shape of the dissociation curve.

Oxygen-Haemoglobin Dissociation Curve

The oxygen-haemoglobin dissociation curve is a graph plotting pO2 (x-axis, in mm Hg) against percentage saturation of haemoglobin (y-axis, in %).

The curve is sigmoid (S-shaped), not linear, because of cooperative binding of O2 to haemoglobin.

Key points on the curve:

• At pO2 = 100 mm Hg (alveolar air): haemoglobin is approximately 97% saturated — nearly fully loaded with O2.
• At pO2 = 40 mm Hg (tissues): haemoglobin is approximately 70% saturated — about 27% of O2 has been released to the tissues.
• The steep portion of the curve (between pO2 40–60 mm Hg) is physiologically crucial: a small drop in pO2 in this range causes a large amount of O2 to be released. This allows tissues with higher metabolic demand to extract more O2 efficiently.

Bohr Effect

The Bohr effect describes how the affinity of haemoglobin for oxygen is influenced by pCO2, pH, and temperature:

Physiological significance of the Bohr effect:

• In active tissues (e.g., exercising muscle): high pCO2, low pH, and high temperature all cause a right shift, promoting O2 unloading exactly where it is needed most.
• In the lungs: low pCO2, high pH, and normal temperature cause a left shift, promoting O2 loading onto haemoglobin.

Transport of CO2

Carbon dioxide is transported in blood by three methods:

Bicarbonate Pathway (Most Important — 70%)

The bicarbonate pathway is the dominant method of CO2 transport:

In the tissues (CO2 loading): 1. CO2 produced by tissue metabolism diffuses into RBCs. 2. Carbonic anhydrase (enzyme in RBCs) catalyses the reaction:

3. HCO3- accumulates in RBCs and diffuses out into the plasma down its concentration gradient. 4. To maintain electrical neutrality, Cl- ions from the plasma move into the RBCs — this exchange is called the chloride shift (Hamburger phenomenon). 5. The H+ ions released bind to haemoglobin, forming reduced haemoglobin (HHb), which has lower O2 affinity (contributing to the Bohr effect).

• CO2 + H2O → H2CO3 (carbonic acid)
• H2CO3 → H+ + HCO3- (bicarbonate)

In the lungs (CO2 unloading): 1. The reactions are reversed. 2. HCO3- re-enters the RBC from plasma (Cl- moves back out — reverse chloride shift). 3. HCO3- + H+ → H2CO3 (carbonic acid) 4. Carbonic anhydrase catalyses: H2CO3 → CO2 + H2O 5. CO2 diffuses into the alveoli and is exhaled.

Carbaminohaemoglobin (23%)

• CO2 binds directly to the globin part of haemoglobin at the amino (NH2) groups.
• This binding is reversible: CO2 is released in the lungs where pCO2 is lower.
• CO2 and O2 bind at different sites on Hb. CO2 binds to the globin (protein) part; O2 binds to the haem (iron) part. Therefore, they do not directly compete for the same binding site.

Regulation of Respiration

Respiration is regulated by both neural and chemical mechanisms to match breathing rate and depth to the body's metabolic needs.

Neural Regulation

• Respiratory rhythm centre — located in the medulla oblongata. It generates the basic rhythm of breathing by alternating signals to inspiratory and expiratory muscles.
• Pneumotaxic centre — located in the pons (upper part of the brainstem). It regulates the rate of breathing by modulating the duration of inspiration. When the pneumotaxic centre sends strong signals, it shortens inspiration and increases the breathing rate.
• Aortic arch and carotid artery contain peripheral chemoreceptors that detect changes in blood pO2, pCO2, and H+ and send signals to the rhythm centre.

Chemical Regulation

Chemoreceptors respond to changes in the chemical composition of blood and cerebrospinal fluid:

• pCO2 (most important chemical stimulus): A rise in arterial pCO2 (even by a small amount, e.g., from 40 to 45 mm Hg) strongly stimulates the respiratory centres to increase the rate and depth of breathing. This is the primary driver of respiratory regulation under normal conditions. The chemosensitive areas near the rhythm centre in the medulla are most sensitive to CO2 (via H+ formed when CO2 dissolves in CSF).

• H+ concentration (pH): Increased H+ in the blood (decreased pH) stimulates breathing. Peripheral chemoreceptors in the aortic arch and carotid artery are sensitive to changes in blood pH.

• pO2: Under normal conditions, pO2 has very little effect on respiratory regulation because haemoglobin saturation remains high (above 90%) until pO2 drops below 60 mm Hg. Only when pO2 falls below approximately 60 mm Hg do the peripheral chemoreceptors (aortic and carotid bodies) significantly stimulate increased breathing. This is a safety backup mechanism — pO2 becomes important only in severe hypoxia.

Summary of regulatory hierarchy: pCO2 > H+ > pO2

Lung Volumes and Capacities

Spirometry is the technique used to measure lung volumes and capacities. A spirometer records the volume of air inspired and expired.

Lung Volumes (Four Primary Volumes)

Lung Capacities (Combinations of Two or More Volumes)

Important notes:

• RV cannot be expelled — it prevents the lungs from collapsing and maintains some gas exchange between breaths.
• VC does not include RV — it is the maximum exhalable air. VC = TV + IRV + ERV only.
• TLC = VC + RV — this is the total air the lungs can hold at maximum inflation.
• Vital capacity is higher in athletes and lower in individuals with restrictive or obstructive lung diseases.

Disorders of the Respiratory System

Additional note on COPD: Chronic Obstructive Pulmonary Disease (COPD) is a term that encompasses both chronic bronchitis and emphysema. It is a progressive disease most commonly caused by smoking, characterised by persistent airflow limitation that is not fully reversible.

Important Diagrams

The following diagrams are essential for CBSE exams. Practise drawing them with correct labelling:

1. Human Respiratory System — Label: nostrils, nasal cavity, pharynx, larynx, trachea, right and left bronchi, bronchioles, alveoli, lungs, diaphragm, ribs, intercostal muscles.

2. Mechanism of Breathing — Draw two figures side by side: (a) Inspiration — show diaphragm flattened/contracted, ribs upward and outward, lungs expanded, arrows showing air entering; (b) Expiration — show diaphragm domed/relaxed, ribs downward and inward, lungs compressed, arrows showing air leaving.

3. Alveolar Gas Exchange — Draw an alveolus surrounded by a capillary network. Label: alveolar pO2 = 104 mm Hg, alveolar pCO2 = 40 mm Hg, venous blood pO2 = 40 mm Hg, venous blood pCO2 = 45 mm Hg. Show arrows for O2 moving into blood and CO2 moving into alveolus.

4. Oxygen-Haemoglobin Dissociation Curve — Draw a sigmoid (S-shaped) curve with pO2 on the x-axis (0–100 mm Hg) and % saturation on the y-axis (0–100%). Mark key points: 97% at pO2 100, 70% at pO2 40. Show the Bohr effect as a right shift and a left shift curve, clearly labelled.

5. CO2 Transport as Bicarbonate (Chloride Shift) — Draw an RBC in the tissues showing CO2 entering, carbonic anhydrase converting CO2 + H2O to H2CO3 to H+ + HCO3-, HCO3- exiting the RBC, and Cl- entering (chloride shift). Show the reverse in the lungs.

6. Lung Volumes and Capacities Diagram (Spirometry Tracing) — Draw a spirogram showing TV, IRV, ERV, and RV. Mark IC, EC, FRC, VC, and TLC as brackets above the tracing.

Common Mistakes

1. Confusing breathing with respiration — Breathing is the mechanical process of moving air in and out of the lungs. Respiration is a broader term that includes gas exchange at the lungs and tissues AND cellular respiration (oxidation of glucose to produce ATP inside cells). Do not use them interchangeably.

2. Saying expiration is always active — Normal quiet expiration at rest is a passive process. The diaphragm simply relaxes, and elastic recoil of the lungs drives air out. Only forced expiration (e.g., during exercise, blowing out candles) is active, involving internal intercostal muscles and abdominal muscles.

3. Forgetting the chloride shift when explaining bicarbonate transport — When HCO3- leaves the RBC, Cl- must enter to maintain electrical neutrality. This chloride shift is an integral part of the bicarbonate transport mechanism and is frequently asked in exams.

4. Drawing the oxygen-Hb dissociation curve as a straight line — The curve is sigmoid (S-shaped), not linear. The S-shape results from cooperative binding of O2 to haemoglobin. Drawing it as a straight line will cost marks.

5. Confusing IRV with ERV — IRV (Inspiratory Reserve Volume) is the extra air you can inhale after a normal inspiration. ERV (Expiratory Reserve Volume) is the extra air you can exhale after a normal expiration. "I" for inhale, "E" for exhale.

6. Saying pO2 is the main driver of respiratory regulation — pCO2 is the primary chemical stimulus for regulating breathing under normal conditions. pO2 becomes significant only when it drops below approximately 60 mm Hg (severe hypoxia). Stating that pO2 is the main driver is incorrect.

7. Writing that CO2 is mainly transported as carbaminohaemoglobin — The dominant method of CO2 transport is as bicarbonate (HCO3-), which accounts for 70%. Carbaminohaemoglobin accounts for only 23%. The most common exam mistake is inverting these percentages.

Exam Tips

1. Gas exchange questions — Always mention specific partial pressure values: alveolar pO2 = 104 mm Hg, venous blood pO2 = 40 mm Hg, alveolar pCO2 = 40 mm Hg, venous blood pCO2 = 45 mm Hg. These values earn full marks.

2. Draw the oxygen-Hb dissociation curve — This is a guaranteed 3–5 mark question in the board exam. Draw a clear sigmoid curve, label both axes, mark the 97% and 70% saturation points, and show the Bohr effect (right shift for tissues, left shift for lungs) with arrows and labels.

3. CO2 transport — Write all three methods with their exact percentages: 7% dissolved in plasma, 23% carbaminohaemoglobin, 70% bicarbonate. Mention carbonic anhydrase and the chloride shift for full credit.

4. Breathing mechanism — Clearly distinguish inspiration (active — diaphragm contracts, thoracic volume increases, intrapulmonary pressure decreases, air enters) from expiration (passive at rest — diaphragm relaxes, thoracic volume decreases, intrapulmonary pressure increases, air leaves). Use a table format for clarity.

5. Lung capacities — Remember that VC = TV + IRV + ERV (does NOT include RV). TLC = VC + RV (includes everything). A common trick question asks whether RV is part of VC — it is NOT.

6. Disorder questions — Always mention both the cause AND the structural/functional change. For example: Emphysema — caused by cigarette smoking; structural change is damage to alveolar walls reducing surface area for gas exchange.

7. C-shaped cartilaginous rings — If asked about the trachea, mention that the C-shape allows the oesophagus to expand during swallowing. This is a frequently tested detail.

8. Surfactant — Mention its role in reducing surface tension and preventing alveolar collapse, especially in premature infants (leading to respiratory distress syndrome if deficient).

Quick Revision

• Respiratory organs across animals: skin (earthworm), gills (fish), tracheal system (insects), lungs (mammals)
• Human respiratory pathway: nostrils → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
• Inspiration: diaphragm contracts (flattens), external intercostals contract (ribs up and out), thoracic volume increases, pressure decreases, air enters — active process
• Expiration at rest: diaphragm relaxes (domes), external intercostals relax (ribs down and in), thoracic volume decreases, pressure increases, air leaves — passive process
• Gas exchange: simple diffusion across alveolar-capillary membrane (0.5 micrometres thick), driven by partial pressure gradients (O2: 104 → 40 mm Hg; CO2: 45 → 40 mm Hg)
• O2 transport: 98.5% as oxyhaemoglobin (HbO2), 1.5% dissolved in plasma; sigmoid dissociation curve; Bohr effect (high pCO2, low pH, high temperature → right shift → O2 released)
• CO2 transport: 70% bicarbonate (carbonic anhydrase, chloride shift), 23% carbaminohaemoglobin, 7% dissolved
• Regulation: medulla (neural rhythm centre), pons (pneumotaxic centre); pCO2 is the most important chemical stimulus; pO2 matters only below approximately 60 mm Hg
• Lung volumes: TV = 500 mL, IRV = 2500–3000 mL, ERV = 1000–1100 mL, RV = 1100–1200 mL; VC = TV + IRV + ERV; TLC = VC + RV
• Disorders: asthma (airway inflammation and narrowing), emphysema (alveolar wall damage, smoking), occupational (silicosis, asbestosis, pneumoconiosis), bronchitis (bronchial inflammation)