Breathing And Exchange of Gases – Class 11

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breathing and exchange of gases

Breathing is an essential process for all living beings as they derive energy from it to perform various activities. The chapter, ‘Breathing and Exchange of Gases’ of  NCERT biology class 11 covers this topic in detail. Through this chapter, students will understand that living cells need a continuous supply of O2 for the process, and the CO2 produced must be released. This process is also referred to as respiration. So let’s get started and go through this interesting chapter.

Overview of Breathing and Exchange of Gases

The body requires a constant exchange of gases in order to survive, and function. Breathing is one of the most prominent and essential exchanges of gases, where oxygen is taken inside, and carbon dioxide produced is taken out. There are different respiratory organs for different organisms.

Breathing And Exchange of Gases

Respiratory Organs

Based on habitats and levels of organization, different groups of living organisms have different types of respiratory organs. As explained in NCERT biology class 11 books, while the lower invertebrates respire through the entire body surface, the aquatic arthropods and molluscs use gills, and the terrestrial animals use the vascularised organs called lungs.      

CBSE Class 11 Biology Syllabus

Human Respiratory System

The human respiratory system consists of the following parts:

  • A pair of nostril
  • Pharynx
  • Larynx
  • Trachea
  • Primary Bronchi 
  • Secondary and Tertiary bronchi
  • Lungs
  • Alveoli
  • Heart 
Breathing And Exchange of Gases - Class 11

Biology Project for Class 11

The Process Of Respiration

As explained in the chapter Breathing and Exchange of Gases, the process of respiration takes place in the following steps:

  • The atmospheric oxygen enters the body through pulmonary respiration, and CO2-rich alveolar is released
  • In the next step, the diffusion of O2 and CO2 occurs across the alveolar membrane
  • The blood transports O2 and CO2 across different parts of the body
  • The diffusion of O2 and CO2 then occurs between blood and tissues
  • The O2 is utilized in cellular respiration and releases energy and CO2

Mechanism of Breathing

The respiration process and the breathing mechanism are two different and important topics of the chapter Breathing and the Exchange of Gases. As the respiration process consists of different steps, the breathing mechanism involves two stages: Inspiration and Expiration. When the atmospheric air is drawn within, it is called inspiration; when alveolar air is released out, it is called expiration.

During inspiration, the pulmonary volume increases due to lower intrapulmonary pressure. On the other hand, the pulmonary volume decreases during expiration due to increased pulmonary pressure. The pulmonary pressure increases and decreases due to the contraction and relaxation of the diaphragm and intercostal muscles.

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Respiratory Volumes and Capacities 

According to the chapter Breathing and Exchange of Gases, the respiration process includes the following terms:

  • Tidal Volume (TV): It is the volume of the air inspired and expired in one breath. 500mL is considered a healthy TV in human adults
  • Inspiratory Reserve Volume (IRV): It is the forcible inspiration or the additional air volume of air. It can be between 2500mL to 3000mL
  • Expiratory Reserve Volume (ERV): It is the forced volume of air expelled after a normal inspiration. It is around 1200mL in addition to 500mL
  • Residual Volume (RV): It is the volume of air remaining in the lungs even after forcible expiration. It is between 1100mL and 1200mL. It cannot be measured by spirometry
  • Inspiratory Capacity (IC): It is TV + IRV and defined as the total air a person can inspire after a normal expiration
  • Expiratory Capacity (EC): It is TV + ERV and defined as the total volume of air a person can exhale after a normal inspiration
  • Functional Residual Capacity (FRC): It is ERV + RV and defined as the volume of air remaining in the lungs after a normal expiration
  • Vital Capacity (VC): The maximum volume of air that can be breathed in after forcible expiration. It is also the maximum volume of air that can be breathed out after forcible inspiration
  • Total Lung Capacity (TLC): It is the sum of vital capacity (VC) and residual capacity (RC)

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Exchange of Gases

As per the class 11 Biology syllabus, the primary site of the exchange of gases is the alveoli. However, it also occurs between blood and tissues. The exchange between O2 and CO2 is the result of the pressure/concentration gradient. The table given below shows the partial pressure (in mmHg) of Oxygen and Carbon dioxide in the diffusion process and in the atmosphere derived from the chapter breathing and exchange of gases. 

Respiratory gasAtmospheric gasAlveoliDeoxygenated BloodOxygenated bloodTissues
Source: NCERT

Transport of Gases

The chapter Breathing and Exchange of Gases explain that O2 and CO2 transportation occurs via the blood. While the blood transports 97% of O2, the remaining 3% is transported through plasma in a dissolved state. Mentioned below are some of the important points related to the transport of gases-

Transportation of Oxygen
As mentioned in the chapter, the O2 molecule binds with haemoglobin present in the RBCs and forms oxyhemoglobin. The 100mL of oxygenated blood delivers around 5mL of O2 to the tissues.

Transportation of Carbon Dioxide
The transportation of CO2 can be explained as carbamino-haemoglobin. The 100mL of deoxygenated blood delivers around 4mL of CO2 to the alveoli.     

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Disorders of Respiratory System

As explained in the breathing and exchange of gases chapter in the NCERT biology book, the disorders associated with respiration are as follows:

  • Asthma This condition causes difficulty in breathing due to inflammation in the bronchi and bronchioles
  • Emphysema – In this condition, the respiratory surface decreases because of damage caused by alveolar walls. The major cause is cigarette smoking 
  • Occupational Respiratory Disorder – The lungs are damaged due to fibrosis. It is common in industries where workers have long exposure to too much dust    

Breathing And Exchange of Gases: NCERT PDF

Breathing And Exchange of Gases: Notes PDF

Breathing And Exchange of Gases: PPT

Courtesy: Navodaya Vidyalaya Samiti

Breathing And Exchange of Gases: Practice MCQs

  1. At rest, most people require alveolar ventilation for about 4 L/minute. Suppose an emphysema victim with a tidal volume of 0.5 L has suffered alveolar damage that has produced a respiratory dead space of 0.3 L, what will be that person’s frequency of breathing at rest?

A. 8 breaths/min   

B. 5 breaths/min    

C. 15 breaths/min   

D. 20 breaths/min   

  1. A zoologist compared the respiratory efficiency and swimming speed of different fish. He found that less efficient fish tended to have:

A. A faster heart rate

B. More hemoglobin

C. A thicker respiratory surface

D. Greater ventilation    

  1. The partial pressure of oxygen in inspired and expired air is ……. and ……. mm of Hg.

A. 150, 40 

B. 100, 46      

C. 158, 90    

D. 100, 95

  1. The chloride shift in red blood cells is the movement of:       

A. Chloride ions out of the cell to balance the release of hydrogen carbonate ions.

B. Chloride ions into the cell to balance its uptake of hydrogen carbonate ions.

C. Cations into the cell to balance its uptake of chloride ions.

D. Chloride ions from the blood into the glomerular filtrate for excretion.

  1. Which of the following statement correctly defines the Bohr effect?

A. Rise in p50 with a decrease in CO2 cone.

B. Rise in p50 with a decrease in pH.

C. Rise in p50 with an increase in O2

D. Fall in p50 with a decrease in pH.      


  1. D
  2. B
  3. A
  4. A
  5. B

Breathing And Exchange of Gases: Important Questions and Answers

Diffusion of gases occurs in the alveolar region only and not in the other parts of the respiratory system. Why?

For effective gas exchange, the respiratory surface must have specific properties such as:

(i) it must be thin, me ist and permeable to respiratory gases 
(ii) having a large surface area, and 
(iii) being highly vascular. 

These features are only seen in the alveolar area. As a result, gas diffusion occurs exclusively in this area.

What are the major transport mechanisms for CO2? Explain.

Approximately 20-25% of CO2 is transported by RBC hemoglobin, 70% is transferred as bicarbonate ion in plasma, and approximately 7% is transported in a dissolved condition through plasma. Hemoglobin transports CO2 as carbamino-hemoglobin. The partial pressure of CO2 is connected to this binding.

Explain the process of inspiration under normal conditions.

The process of taking air from the atmosphere into the lungs is known as inspiration. It is accomplished by establishing a pressure differential between the lungs and the surrounding environment. During inspiration, the diaphragm contracts and the thoracic cavity expands. This is accompanied by the contraction of the external intercostal muscles. The ribs and sternum migrate outward as a result of this. This expands the thoracic chamber along the dorsoventral axis. This eventually leads to an increase in lung volume. As a result, intrapulmonary pressure falls below air pressure. This results in the intake of air from the surrounding environment into the lungs.

How is respiration regulated?

Respiration is controlled by both the neurological and chemical systems.
The brain’s respiratory centre comprises neuron groups situated in the medulla oblongata and pons Varolii. The respiratory centre controls the pace and depth of breathing. Dorsal respiratory neurons are found in the medulla oblongata’s dorsal part. This set of neurons is mostly responsible for inspiration. The ventral group of neurons is situated in the ventrolateral medulla oblongata. These can result in either inspiration or exhaustion. The pneumatic center is positioned on the dorsal side of the pons Varolii. It transmits impulses to all dorsal respiratory group neurons but exclusively to ventral respiratory group inspiratory neurons. Its primary function is to restrict inspiration. Respiration is chemically controlled by a vast number of chemoreceptors situated in the carotid and aortic bodies. Excess carbon dioxide or hydrogen ions primarily activate the brain’s respiratory centre, increasing inspiratory and expiratory impulses to the respiratory muscles. Increased CO2 causes acidosis by lowering the pH. The role of oxygen in the control of respiratory rhythm is minor.

How is respiration regulated?

Increased pCO2 tension in the blood causes a rightward shift in the oxygen dissociation curve of hemoglobin, lowering its affinity for oxygen. This is known as Bohr’s effect. It is essential for the release of oxygen in the tissues.

Breathing And Exchange of Gases: Assignment Questions

  1. Define the oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
  2. Have you heard about hypoxia? Try to gather information about it, and discuss it with your friends.
  3. Distinguish between
    • IRV and ERV
    • Inspiratory capacity and expiratory capacity.
    • Vital capacity and total lung capacity.
  4. What is tidal volume? Find out the tidal volume (approximate value) for a healthy human in an hour.
  5. What will be the p02 and pCO2 in the atmospheric air compared to those in the alveolar air?

(i) pO2 lesser, pCO2 higher

(ii) pO2 higher, pCO2 lesser

(iii) pO2 higher, pCO2 higher

(iv) pO2 lesser, pCO2 lesser

  1. Write the name and important function of the fluid-filled double membranous layer surrounding the lungs.
  2. Why does smoking cigarettes cause emphysema?
  3. Mention the main parts involved in initiating a pressure gradient between the lungs and the atmosphere during normal respiration.
  4. Name the organs of respiration in cockroaches, earthworms, and birds.
  5. Describe the role of the neural system in controlling respiration.
  6. Write a note on the mechanism of breathing.
  7.  What is a Respiratory Quotient?
  8. What are the formulae of Respiratory Quotient (RQ)?
  9. Explain why the diffusion of carbon dioxide by the diffusion membrane per unit difference in partial pressure is much greater compared to oxygen.
  10.  Organize the following in ascending order

a) Tidal volume

b) Residual volume

c) Inspiratory reserve volume

d) Expiratory capacity


1. What are the 4 steps involved in breathing and gas exchange?

The 4 steps involved in breathing and gas exchange are gas transport, ventilation, pulmonary gas exchange, and peripheral gas exchange.

2. What organ is for breathing and gas exchange?

The lung is the main organ for breathing and gas exchange.

3. Where are lungs located?

Lungs are located in te chest cavity of the human body.

Thus, we hope that through these notes, you are all clear about the topic, “breathing and exchange of gases” for CBSE Class 11 NCERT. Our experts at Leverage Edu are here to help you shortlist and get started on your dream career path. Hurry Up! Book an e-meeting.

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