Breathing and Exchange of Gases Class 11 Notes

These notes summarise NCERT Class 11 Biology Chapter 14, Breathing and Exchange of Gases, as a revision-ready study page for the current syllabus; they follow the 2026-27 session.

The chapter’s core problem is simple: every cell needs oxygen to break down glucose, amino acids and fatty acids for energy, and the carbon dioxide released is harmful, so O₂ must keep entering and CO₂ must keep leaving the body (NCERT, p. 183).

Below you get the respiratory organs of different animal groups, the human air passage, the mechanism of breathing, the lung volumes and capacities, gas exchange and transport, regulation of respiration, and the disorders — with tables, memory devices, worked calculations and a map of the textbook’s own exercises.

This page is part of the Class 11 Biology notes collection; for the other chapters in the class, see the complete Class 11 notes list.

Breathing and Respiration: What This Chapter Covers

Respiration is not a single event. The textbook lists five steps (NCERT, p. 185):

1. Pulmonary ventilation or breathing — atmospheric air is drawn in and CO₂-rich alveolar air is released out.
2. Diffusion of gases (\( O_2 \) and \( CO_2 \)) across the alveolar membrane.
3. Transport of gases by the blood.
4. Diffusion of O₂ and CO₂ between blood and tissues.
5. Utilisation of O₂ by the cells for catabolic reactions, with CO₂ released — cellular respiration, covered in Chapter 12 of the same book.

This chapter deals with steps (i) to (iv); step (v), the chemistry inside cells, belongs to the chapter on cellular respiration. Removing CO₂ is one half of the body’s waste disposal — the other metabolic wastes are handled by systems described in the notes on Excretory Products and Their Elimination.

Respiratory Organs Across Animals

Which respiratory surface an animal uses depends mainly on its habitat and level of organisation (NCERT, p. 183). The body-plan levels behind these surfaces are covered in the Animal Kingdom notes.

One compact takeaway: simple aquatic animals exchange over the whole body surface, insects pipe air directly to the tissues through tracheal tubes, and vertebrates use gills in water or lungs on land — with frogs adding their moist skin.

Human Respiratory System: Pathway and Parts

Air follows one fixed passage from the outside to the exchange surfaces (NCERT, p. 184):

1. External nostrils → nasal chamber through the nasal passage; the chamber opens into the pharynx, a portion of which is the common passage for food and air.
2. Pharynx → larynx, a cartilaginous sound box; during swallowing the glottis is covered by the thin elastic epiglottis so food cannot enter the larynx.
3. Larynx → trachea, a straight tube that divides at the level of the 5th thoracic vertebra into right and left primary bronchi.
4. Primary bronchi → secondary and tertiary bronchi → bronchioles → terminal bronchioles.
5. Each terminal bronchiole gives rise to thin, irregular-walled, vascularised bags called alveoli — the exchange site.

Keep the order with a mnemonic: ‘Never Play Lazy Tennis, Breathe Big Air’ = Nostrils, Pharynx, Larynx, Trachea, Bronchi, Bronchioles, Alveoli.

As Figure 14.1 shows, the branching network of bronchi, bronchioles and alveoli makes up the lungs, with the sectional view of the left lung exposing the branching airways.

The whole system is supported: the trachea, bronchi and initial bronchioles carry incomplete cartilaginous rings, and each lung is covered by a double-layered pleura whose pleural fluid reduces friction on the lung surface (NCERT, p. 184).

Functionally the system splits into two parts (NCERT, p. 185):

The lungs sit in an air-tight thoracic chamber — vertebral column dorsally, sternum ventrally, ribs laterally, and a dome-shaped diaphragm on the lower side (NCERT, p. 185). Because the chamber is airtight, any change in its volume is reflected in the lung cavity; we cannot alter pulmonary volume directly, so this arrangement is essential for breathing.

Mechanism of Breathing: Inspiration and Expiration

Air moves into and out of the lungs only when a pressure gradient exists between the lungs and the atmosphere (NCERT, p. 185). Inspiration occurs when the intra-pulmonary pressure is less than atmospheric pressure — a negative pressure in the lungs. Expiration occurs when the intra-pulmonary pressure is higher than atmospheric pressure.

The diaphragm and the external and internal intercostal muscles create this gradient. Contraction of the diaphragm enlarges the thoracic chamber in the antero-posterior axis; contraction of the external intercostals lifts the ribs and sternum and enlarges it in the dorso-ventral axis. The bigger thoracic volume pulls the pulmonary volume up, pressure falls, and air rushes in — inspiration.

Relaxation of the diaphragm and intercostals reverses everything, pressure rises slightly above atmospheric, and air is expelled — expiration (NCERT, p. 185–186).

Figure 14.2 shows the two stages side by side: panel (a) is inspiration, when the thoracic and pulmonary volumes enlarge and intra-pulmonary pressure falls below atmospheric; panel (b) is expiration, after the diaphragm and intercostals relax and the pressure rises slightly above atmospheric.

We can strengthen both acts using additional muscles in the abdomen, which is how forced inspiration and expiration work. On average a healthy human breathes 12–16 times per minute, and the volume of air involved can be estimated with a spirometer, which is used in clinical assessment of pulmonary function (NCERT, p. 186).

Respiratory Volumes and Capacities: The Numbers and Formulas

Four volumes describe single breaths; five capacities are derived by adding volumes and are used in clinical diagnosis (NCERT, p. 186–187).

The Four Respiratory Volumes

The residual volume is the air you cannot breathe out no matter how hard you try — it stays behind even after a maximum forced expiration (NCERT, p. 187).

The Five Pulmonary Capacities and Their Formulas

Remember the order of the capacities with the mnemonic ‘I F V T — I Feel Very Tired’: Inspiratory Capacity, Functional Residual Capacity, Vital Capacity, Total Lung Capacity (NCERT, p. 187).

Exchange of Gases: Partial Pressure Gradients

Exchange happens at two sites — the alveoli (primary site) and the body tissues. Both gases cross by simple diffusion, driven mainly by the pressure or concentration gradient; the gas’s solubility and the thickness of the diffusion membrane also affect the rate (NCERT, p. 187).

Partial pressure is the pressure contributed by an individual gas in a mixture, written \( pO_2 \) for oxygen and \( pCO_2 \) for carbon dioxide. NCERT Table 14.1 gives the partial pressures in mm Hg at five locations:

Reading the numbers (NCERT, p. 187–188): \( O_2 \) runs 159 → 104 → 40 → 95 → 40, so it is always higher on the entry side — the alveolus holds 104 against the deoxygenated blood’s 40, and oxygenated blood holds 95 against the tissues’ 40. \( O_2 \) therefore diffuses from alveoli to blood and from blood to tissues. \( CO_2 \)

walks the gradient backwards: deoxygenated blood carries 45 against the alveolus’s 40, and the tissues sit at 45 against the arriving oxygenated blood’s 40, so \( CO_2 \) diffuses from tissues to blood and from blood to alveoli.

As Figure 14.3 shows, the same pattern repeats at both ends of the journey: at the alveolus, \( O_2 \) enters the blood while \( CO_2 \) leaves it; at the tissues the directions reverse.

Two factors modify the rate. First, solubility: \( CO_2 \) is 20–25 times more soluble than \( O_2 \), so far more \( CO_2 \) diffuses per unit difference in partial pressure. Second, the thickness of the diffusion membrane. The diffusion membrane has just three layers (NCERT, p. 188):

• the thin squamous epithelium of the alveolus,
• the endothelium of the alveolar capillaries, and
• the basement substance between them — a thin basement membrane supporting the epithelium plus the basement membrane surrounding the capillary endothelial cells.

Figure 14.4 zooms into one alveolus with its pulmonary capillary — the membrane the gases must cross. Its total thickness is much less than a millimetre. So every factor in the body — gradient direction, solubility, thinness — favours \( O_2 \) diffusion from alveoli to tissues and \( CO_2 \) diffusion from tissues to alveoli (NCERT, p. 188).

Transport of Gases: How Blood Carries Oxygen and Carbon Dioxide

Blood is the medium of transport for both gases. The headline splits (NCERT, p. 189):

• \( O_2 \): about 97% carried by red blood cells; the remaining 3% dissolved in plasma.
• \( CO_2 \): about 70% carried as bicarbonate, 20–25% as carbamino-haemoglobin in RBCs, and about 7% dissolved in plasma.

Transport of Oxygen

Haemoglobin is a red, iron-containing pigment in the RBCs. Oxygen binds to it in a reversible manner to form oxyhaemoglobin, and each haemoglobin molecule can carry a maximum of four \( O_2 \) molecules (NCERT, p. 189). Binding is governed mainly by \( pO_2 \), but \( pCO_2 \), hydrogen-ion concentration and temperature interfere with it. This gives the loading and unloading sites their logic:

• Alveoli — high \( pO_2 \), low \( pCO_2 \), lesser \( H^+ \), lower temperature: all factors favour formation of oxyhaemoglobin (loading).
• Tissues — low \( pO_2 \), high \( pCO_2 \), high \( H^+ \), higher temperature: all factors favour dissociation of oxygen from oxyhaemoglobin (unloading).

So \( O_2 \) gets bound at the lung surface and dissociates at the tissues; every 100 mL of oxygenated blood delivers about 5 mL of \( O_2 \) to the tissues under normal physiological conditions (NCERT, p. 189).

Transport of Carbon Dioxide

About 70% of \( CO_2 \) travels as bicarbonate (\( HCO_3^- \)). RBCs contain a very high concentration of the enzyme carbonic anhydrase, with minute amounts in plasma too; this enzyme facilitates the reaction in both directions (NCERT, p. 189–190):

\[ CO_2 + H_2O \underset{\text{carbonic anhydrase}}{\rightleftharpoons} H_2CO_3 \underset{\text{carbonic anhydrase}}{\rightleftharpoons} HCO_3^- + H^+ \]

At the tissue site, where catabolism keeps \( pCO_2 \) high, \( CO_2 \) diffuses into the blood and the reaction runs forward, trapping \( CO_2 \) as \( HCO_3^- \) and \( H^+ \). At the alveoli, where \( pCO_2 \) is low, the reaction reverses, releasing \( CO_2 \) and \( H_2O \) — and the \( CO_2 \) is breathed out.

The remaining 20–25% binds to haemoglobin as carbamino-haemoglobin: binding is favoured by high \( pCO_2 \) and low \( pO_2 \) (tissues) and released where \( pCO_2 \) is low and \( pO_2 \) is high (alveoli). Every 100 mL of deoxygenated blood delivers about 4 mL of \( CO_2 \) to the alveoli (NCERT, p. 190).

The two gases use different carriers; this side-by-side is the one to revise from:

Oxygen Dissociation Curve: Reading the S-Shape

The oxygen dissociation curve plots the percentage saturation of haemoglobin with \( O_2 \) against \( pO_2 \), and its shape is sigmoid, or S-shaped (NCERT, p. 189).

Reading Figure 14.5 from left to right: at very low \( pO_2 \) saturation rises slowly; through the middle range it climbs steeply, so a small rise in \( pO_2 \) loads a lot of oxygen; near the top it flattens again as haemoglobin approaches full saturation.

The curve is highly useful for studying how factors like \( pCO_2 \) and \( H^+ \) concentration affect the binding of \( O_2 \) with haemoglobin (NCERT, p. 189).

Its practical message matches the mechanism section: at the alveoli the conditions are right for oxyhaemoglobin to form, and at the tissues they are right for it to dissociate — oxygen loads at the lung surface and unloads at the tissues. A memory hook: the tissues are hot, acidic and \( CO_2 \)-rich — exactly where haemoglobin lets go of \( O_2 \).

If a question asks you to suggest a reason for the sigmoid pattern, as NCERT Exercise Q11 does, a sound line of reasoning is this: each haemoglobin carries up to four \( O_2 \) molecules, and the binding sites work cooperatively — after the first \( O_2 \) binds, the remaining sites accept further \( O_2 \) more readily.

That cooperation produces the steep middle section and the flattened ends of the S.

Regulation of Respiration: Control Centres in the Brain

Human beings can maintain and moderate the respiratory rhythm to suit the demands of the body tissues; this is done by the neural system (NCERT, p. 190). The chain of control:

The counterintuitive fact to memorise: the role of oxygen in the regulation of respiratory rhythm is quite insignificant (NCERT, p. 190). What the brain senses is \( CO_2 \) and \( H^+ \), not low \( O_2 \).

Respiratory Disorders: What Goes Wrong

Three disorders are covered; each links a cause to an effect on the respiratory passage or surface (NCERT, p. 190–191):

Notice how emphysema connects back to the exchange section: a smaller respiratory surface means less area for diffusion, so the same gradient moves less gas.

Worked Examples: Breathing Volume Calculations

The values below are illustrative — the NCERT exercises use their own numbers — but the method is exactly what you apply to them.

Example 1 — Minute ventilation (air moved per minute and per hour).

This is the same skill NCERT Exercise Q14 asks for (tidal volume over an hour), just with different data.

Example 2 — Computing capacities from given volumes.

Key Terms and Definitions at a Glance

The chapter’s key vocabulary in one table; each meaning is written so you can use it directly in an answer:

Common Mistakes Students Make

Six classic traps from this chapter; each row gives the correction, the reason, and a quick self-check:

Misconception autopsy: low oxygen is not the breathing trigger

This is the chapter’s most counterintuitive statement, so it deserves a full autopsy. The confusion is natural: sprinting leaves you breathless and gasping, so it feels as if low \( O_2 \) forces you to breathe harder.

But look at the regulation pathway (NCERT, p. 190): the chemosensitive area adjacent to the rhythm centre is sensitive to \( CO_2 \) and hydrogen ions, and a rise in these — not a fall in \( O_2 \) — activates the adjustment. The receptors at the aortic arch and carotid artery behave the same way.

The textbook states the conclusion plainly: the role of oxygen in the regulation of respiratory rhythm is quite insignificant. In any answer on regulation, name \( CO_2 \) and \( H^+ \) as the sensed signals and keep \( O_2 \) out of the mechanism.

Exam Notes: How to Write the NCERT Exercise Answers

The 14 NCERT exercises (NCERT, p. 192) fall into a few answer types. For each type, here is what a complete answer must contain — apply the recipe to the question number given:

Two general tips. For any definition question, the complete answer names the term first and then the quantity or function that defines it — never just an example. For any process question, keep the pressure rule visible: air moves only when a pressure gradient exists between the lungs and the atmosphere.

You can verify the exercise list and wording in the official NCERT Class 11 Biology textbook PDF (Chapter 14) at ncert.nic.in. Once you have finished revising, you can browse all CBSE notes on this site for the other chapters.

Revision Summary: The Chapter in One Page

Last-night revision needs just two things from this chapter: the numbers and the process.

Numbers to memorise

The five steps as a process map

1. Ventilation — lungs: pressure gradient moves air in and out.
2. Diffusion across the alveolar membrane — alveoli: driven by partial-pressure gradients.
3. Transport by blood — haemoglobin and bicarbonate carry the gases.
4. Diffusion between blood and tissues — tissues: \( O_2 \) unloads, \( CO_2 \) loads.
5. Cellular utilisation — cells: catabolic reactions use \( O_2 \) and release \( CO_2 \) (covered in Chapter 12).

One-sentence takeaway: \( O_2 \) flows down its partial-pressure gradient from the alveoli to the tissues, while \( CO_2 \) flows the opposite way, from the tissues to the alveoli.

FAQs

What is the difference between breathing and respiration in Class 11 Biology?

Breathing is step (i) of respiration — pulmonary ventilation, the mechanical drawing in of air and release of alveolar air. Respiration is the whole chain (steps i–v): ventilation, diffusion across the alveolar membrane, transport by blood, diffusion into tissues, and the cellular utilisation of \( O_2 \) that releases \( CO_2 \).

Why is the oxygen dissociation curve sigmoid in shape?

A suggested reason is cooperative binding: each haemoglobin molecule carries up to four \( O_2 \) molecules, and once the first \( O_2 \) binds, the remaining sites bind further \( O_2 \) more readily. That produces the steep middle of the S-curve, with the curve flattening as saturation approaches its maximum.

Why is the role of oxygen insignificant in regulating respiration?

The respiratory rhythm centre in the medulla and the adjacent chemosensitive area respond mainly to \( CO_2 \) and hydrogen ions. When these rise, the centre adjusts breathing to eliminate them; the chapter concludes that the role of oxygen in regulating respiratory rhythm is quite insignificant.

What is the volume of air remaining in the lungs after a normal breathing?

After a normal expiration, the air remaining is the functional residual capacity (FRC = \( ERV + RV \)). Do not confuse it with the residual volume — RV (1100–1200 mL) is what remains after a forcible expiration.

How is most of the carbon dioxide transported in blood?

About 70% is carried as bicarbonate (\( HCO_3^- \)), formed through the carbonic anhydrase reaction; 20–25% binds to haemoglobin as carbamino-haemoglobin, and about 7% is dissolved in plasma.

Why does a person going up a hill breathe faster?

At altitude, atmospheric \( pO_2 \) is lower, so the partial-pressure gradient driving \( O_2 \) into the blood is reduced. The respiratory process responds with faster, deeper breathing. Note the chapter’s nuance: although the threat is to the oxygen supply, the regulatory centres sense \( CO_2 \) and \( H^+ \), not \( O_2 \).

Reference: NCERT Class 11 Biology textbook, chapter 14 Breathing and Exchange of Gases.

Official source: download the NCERT textbook free from ncert.nic.in.