CBSE Class 11 Biology Breathing And Exchange Of Gases Notes

Download CBSE Class 11 Biology Breathing And Exchange Of Gases Notes in PDF format. All Revision notes for Class 11 Biology have been designed as per the latest syllabus and updated chapters given in your textbook for Biology in Standard 11. Our teachers have designed these concept notes for the benefit of Grade 11 students. You should use these chapter wise notes for revision on daily basis. These study notes can also be used for learning each chapter and its important and difficult topics or revision just before your exams to help you get better scores in upcoming examinations, You can also use Printable notes for Class 11 Biology for faster revision of difficult topics and get higher rank. After reading these notes also refer to MCQ questions for Class 11 Biology given our website

Breathing And Exchange Of Gases Class 11 Biology Revision Notes

Class 11 Biology students should refer to the following concepts and notes for Breathing And Exchange Of Gases in standard 11. These exam notes for Grade 11 Biology will be very useful for upcoming class tests and examinations and help you to score good marks

Breathing And Exchange Of Gases Notes Class 11 Biology

17.Breathing and Exchange of Gases

Points To Remeber :

• Breathing: (External respiration) the process of exchange of O2 from the atmosphere with CO2 produced by the cells.

Respiratory Organs :

• Direct respiration by diffusion from the environment – sponges, coelenterates, flat worms etc.

• Cutaneous or by skin – earthworm.

• Tracheal system – insects.

• Gills – aquatic arthropods mollusks

• Lungs – terrestrial forms.

Human Respirtory System:

• External nostril opens into the nasal chamber through nasal passage.

• The nasal chamber opens into the nasopharynx.

• Nasopharynx opens through glottis of the larynx into the trachea.

• Larynx is a cartilaginous box which produce sound hence called sound box.

• Cartilaginous epiglottis covers the glottis during swallowing to prevent entry of food into trachea.

• Trachea is a straight tube extending up to themed-thoracic cavity, which divides into right and left primary bronchi at the level of 5th thoracic vertebra.

• Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles.

• Trachea, primary, secondary and tertiary bronchi and initial bronchioles are supported by cartilaginous rings.

• Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bags like structures called alveoli.

• The branching network of bronchi, bronchioles and alveoli comprises the lungs.

• There are two lungs which are covered by a double layered pleura, with pleural fluid in them.

• Lungs are situated in the thoracic chamber which is anatomically a air tight chamber.

• The thoracic chamber is formed –

o Dorsally by vertebral column.

o Ventrally by sternum.

o Laterally by ribs.

o On the lower side by dome shaped diaphragm.

• Respiration involves in following steps –

o Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out.

o Diffusion of gases (O2 and CO2) across alveolar membrane.

o Transport of respiratory gases by blood.

o Diffusion of O2 and CO2 between blood and tissues.

o Utilization of O2 by the cells for catabolic reactions and resultant release of CO2.

Mechanism Of Breathing :

Inspiration :

• Intake of atmospheric air into the lungs.

• It occurs if the pressure within the lungs (intra-pulmonary pressure) is lower than the atmospheric pressure.

• Contraction of diaphragm which increases the volume of thoracic chamber in the anterior posterior axis.

• The contraction of external intercostals muscles lifts up the ribs and the sternum causing an increase in the volume of thoracic chamber in the dorso ventral axis.

• It causes an increase in pulmonary volume decrease the intra-pulmonary pressure to less than the atmospheric pressure.

• It forces the air out side to move in to the lungs, i.e, inspiration.

Expiration :

• Relaxation of diaphragm and inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic and pulmonary volume.

• It increases in intrapulmonary pressure slightly above the atmospheric pressure.

• It causes the expulsion of air from the lungs, i.e, expiration.

• A healthy man breathes 12-16 times/minutes.

• The volume of air involved in breathing is estimated by spirometer.

Respiratory Volumes and Capacities :

• Tidal volume: volume of air inspired or expired during a normal breathing. It is about 500 ml.

• Inspiratory reserve volume: Additional volume of air, a person inspire by a forceful inspiration. It is about 2500-3000 ml.

• Expiratory reserve volume: Additional volume of air, a person expires by a forceful expiration. It is about 1000-1100 ml.

• Residual volume: Volume of air remaining in the lungs even after a forceful expiration. It is about 1200 ml.

• Inspiratory capacity: it includes tidal volume and Inspiratory reserve volume.

• Expiratory capacity: it includes tidal volume and expiratory reserve volume.

• Functional residual capacity: This includes ERV+RV.

• Vital capacity: IRV + TV + ERV.

• Total lung capacity: RV + IRV + TV + ERV

Exchange Of Gases :

• Alveoli are the primary site of exchange of respiratory gases.

• Exchange of gases also takes place between blood and tissues.

• Exchange of O2 and CO2 take place in the pressure gradient, by simple diffusion.

• Pressure contributed by an individual gas in a mixture of gases is called the partial pressure and is represented by pO2 for oxygen and pCO2 for carbon dioxide.

• Diffusion of O2

o pO2 in alveolar air = 104 mm Hg.

o pO2 in venous blood = 40 mm Hg.

o O2 diffuses from alveoli to venous blood.

• Diffusion of CO2

o pCO2 is venous blood = 45 mm Hg.

o pCO2 is alveolar air = 40 mm Hg

o CO2 diffuses from venous blood to alveoli.

• Solubility of CO2 is 20-25 times higher than that of O2; the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O2.

• Respiratory membrane is formed by;

o Thin Squamous epithelium of the alveoli.

o Endothelium of alveolar capillaries

o Basement membrane between them.

Transport Of Gases :

• Blood is the medium of transport of O2 and CO2.

• About 97 per cent of O2 is transported by RBCs in the blood.

• 3 per cent of O2 is transported in the plasma in dissolved state.

• 20-25 per cent of CO2 transported in the RBC in the form of carbamino-haemoglobin.

• 70 percent CO2 carried as bicarbonate ion in plasma.

• 7 percent CO2 transported in dissolved state in plasm.

Transport of Oxygen :

• Haemoglobin is red coloured pigment present in the RBC.

• O2binds with hemoglobin reversibly to form oxy-hemoglobin.

• Each haemoglobin can binds maximum with four O2 molecules.

• Binding of Oxygen with haemoglobin is primarily related with partial pressure of O2.

• Partial pressure of CO2, hydrogen ion concentration (pH) and temperature are the factors that influence this binding.

• A sigmoid curve is obtained when percentage of saturation of hemoglobin with O2 is plotted against the partial pressure of O2 (pO2). This curve is called oxygen dissociation curve.

• Condition favourable for binding of Hemoglobin with O2 at alveolar level;

o High pO2

o Low H+ ion concentration.

o Low temperature.

• Condition favourable for dissociation of HbO2 into Hb and O2 at tissue level;

o Low pO2

o High H+ ion concentration.

o High temperature.

• Every 100 ml of oxygenated blood can deliver around 5 ml of O2 to the tissues under normal physiological conditions. Transport of Carbon dioxide:

• 20-25 percent of CO2 is carried out in the RBC by binding with the free amino group of haemoblobin by formation of carbamino-haemoglobin.


• When pCO2 is high and pO2 is low as in the tissues, more binding of CO2 occurs whereas, when the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from carbamino-haemoglobin takes place.

• 70 per cent of CO2 transported in the form of HCO3- in the plasma.

• CO2 from the tissue diffused into the plasma and along with the water it forms carbonic acid which dissociated into HCO3- and H+. This reaction is catalysed by an enzyme called carbonic anhydrase present in the plasma membrane of RBC and plasma.



Regulation Of Respiration :

• Specialized centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for regulation of breathing.

• Pneumotaxis centre of pons region of brain has moderate regulation.

• Neural signal from this centre can reduce the duration of inspiration and alter the rate of respiration.

• Chemosensitive area adjacent to rhythm centre is sensitive to CO2 and H+ ion.

• Receptors associated with aortic arch and carotid artery also can recognize changes in the CO2 and H+ concentration and send necessary signals to the rhythm centre for remedial actions.

Disorders Of Repiratory System :

• Asthma : is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.

• Emphysema : a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. It caused due to smoking.


MCQ Questions for NCERT Class 11 Biology Breathing and Exchange of Gases

Question. Wall of alveoli is composed of
(a) simple squamous epithelium
(b) simple cuboidal epithelium
(c) pseudostratified epithelium
(d) simple columnar epithelium.

Answer : A

Question. During inspiration, muscles of diaphragm
(a) contracts
(b) expands
(c) becomes convex shaped
(d) rises.

Answer : A

Question. Which one of the following statements is correct?
(a) Chest expands because air enters into the lungs.
(b) Air enters into the lungs because chest expands.
(c) The muscles of the diaphragm contracts because air enters into the lungs.
(d) All of these

Answer : B

Question. Residual volume is
(a) lesser than tidal volume
(b) greater than inspiratory volume
(c) greater than vital capacity
(d) greater than tidal volume.

Answer : D

Question. The oxidative breakdown of respiratory substrates with the help of oxygen is termed as
(a) fermentation
(b) aerobic respiration
(c) anaerobic respiration
(d) respiratory quotient.

Answer : B

Question. The enzyme that increases the reaction rate between CO2 and H2O in red blood cells is
(a) carbonic anhydrase
(b) adenylate cyclase
(c) carbonic synthetase
(d) alkaline phosphatase.

Answer : A

Question. During strenuous exercise, which of the following change occurs?
(a) Glucose is converted into glycogen.
(b) Glucose is converted into pyruvic acid.
(c) Starch is converted into glucose.
(d) Pyruvic acid is converted into lactic acid.

Answer : D

Question. The cells of respiratory tract that secrete mucus are
(a) goblet cells
(b) Kupffer cells
(c) oxyntic cells
(d) all of these.

Answer : A

Question. What percentage of CO2 is carried in a dissolved state through plasma?
(a) 3
(b) 7
(c) 20
(d) 97

Answer : B

Question. Breathing is controlled by
(a) hypothalamus
(b) lungs
(c) medulla oblongata
(d) trachea.

Answer : C

Question. Which respiratory disorder causes inflammation of lungs due to proliferation of fibrous connective tissue?
(a) Pulmonary tuberculosis
(b) Emphysema
(c) Silicosis
(d) Bronchitis

Answer : C

Question. Respiratory centre of brain is sensitive to
(a) more O2 concentration in blood
(b) more CO2 concentration in blood
(c) accumulation of blood in brain
(d) all of these.

Answer : B

Question. Breathing rate above normal (high) is called
(a) bradypnoea
(b) orthopnoea
(c) eupnoea
(d) tachypnoea.

Answer : D

Question. The serous membrane in contact with the lung is the
(a) parietal pleura
(b) pulmonary mesentery
(c) pulmonary peritoneum
(d) visceral pleura.

Answer : D

Question. Tidal volume is
(a) 5000 mL
(b) 1000 mL
(c) 500 mL
(d) 800 mL.

Answer : C

Ques. The partial pressure of oxygen in the alveoli of the lungs is
(a) equal to that in the blood
(b) more than that in the blood
(c) less than that in the blood
(d) less than that of carbon dioxide.

Answer: B

Ques. The exchange of gases in the alveoli of the lungs takes place by
(a) passive transport
(b) active transport
(c) osmosis
(d) simple diffusion.

Answer: D

Ques. In lungs, the air is separated from the venous blood through
(a) transitional epithelium + tunica externa of blood vessel
(b) squamous epithelium + endothelium of blood vessel
(c) squamous epithelium + tunica media of blood vessel
(d) none of the above. 

Answer: B

Ques. The alveolar epithelium in the lung is
(a) non-ciliated columnar
(b) non-ciliated squamous
(c) ciliated columnar
(d) ciliated squamous. 

Answer: B

Ques. Identify the wrong statement with reference to transport of oxygen.
(a) Binding of oxygen with haemoglobin is mainly related to partial pressure of O2.
(b) Partial pressure of CO2 can interfere with O2 binding with haemoglobin.
(c) Higher H+ conc. in alveoli favours the formation of oxyhaemoglobin.
(d) Low pCO2 in alveoli favours the formation of oxyhaemoglobin.

Answer: C

Ques. Reduction in pH of blood will
(a) decrease the affinity of haemoglobin with oxygen
(b) release bicarbonate ions by the liver
(c) reduce the rate of heartbeat
(d) reduce the blood supply to the brain.

Answer: A

Ques. Approximately seventy percent of carbon dioxideabsorbed by the blood will be transported to thelungs
(a) as bicarbonate ions
(b) in the form of dissolved gas molecules
(c) by binding to RBC
(d) as carbamino - haemoglobin.

Answer: A

Ques. A large proportion of oxygen remains unused in the human blood even after its uptake by the body tissues. This O2
(a) acts as a reserve during muscular exercise
(b) raises the pCO2 of blood to 75 mm of Hg
(c) is enough to keep oxyhaemoglobin saturation at96%
(d) helps in releasing more O2 to the epithelial

Answer: A

Ques. Bulk of carbon dioxide (CO2) released from body tissues into the blood is present as
(a) bicarbonate in blood plasma and RBCs
(b) free CO2 in blood plasma
(c) 70% carbamino-haemoglobin and 30% as bicarbonate
(d) carbamino-haemoglobin in RBCs. 

Answer: A

Ques. What is true about RBCs in humans?
(a) They carry about 20-25 percent of CO2.
(b) They transport 99.5 percent of O2.
(c) They transport about 80 percent oxygen onlyand the rest 20 percent of it is transported in dissolved state in blood plasma.
(d) They do not carry CO2 at all. 

Answer: A

Ques. The haemoglobin of a human fetus
(a) has only 2 protein subunits instead of 4
(b) has a higher affinity for oxygen than that of anadult
(c) has a lower affinity for oxygen than that of theadult
(d) its affinity for oxygen is the same as that of anadult. 

Answer: B

Ques. The majority of carbon dioxide produced by ourbody cells is transported to the lungs as
(a) attached to haemoglobin
(b) dissolved in the blood
(c) as bicarbonates
(d) as carbonates. 

Answer: C

Ques. Haemoglobin is a type of
(a) carbohydrate
(b) respiratory pigment
(c) vitamin
(d) skin pigment. 

Answer: B

Ques. How the transport of O2 and CO2 by blood happens?
(a) With the help of WBCs and blood serum
(b) With the help of platelets and corpuscles
(c) With the help of RBCs and blood plasma
(d) With the help of RBCs and WBCs 

Answer: C

Ques. At high altitude, the RBCs in the human blood will
(a) increase in number
 (b) decrease in number
(c) increase in size
(d) decrease in size.

Answer: A

Case I : Read the following passage and answer questions given below: (Img 100)
Binding of oxygen with haemoglobin is primarily related to partial pressure of O2. Partial pressure of CO2, hydrogen ion concentration and temperature are the other factors which can interfere with this binding. A sigmoid curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the pO2. This curve is called the oxygen dissociation curve and is highly useful in studying the effect of factors like pCO2, H+ concentration, etc., on binding of O2 with haemoglobin. In the alveoli, where there is high pO2, low pCO2, lesser H+ concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, whereas in the tissues, where low pO2, high pCO2, high H+ concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin.

Question. Select the correct statement regarding oxygen dissociation curve.
(a) The partial pressure at which the haemoglobin saturation is 50% is called P50.
(b) Decrease in O2 concentration resulting in increased dissociation of oxyhaemoglobin is known as Bohr effect.
(c) At 40 mmHg of PO2, the saturation is 95%.
(d) The lower part of the curve represents the binding or acceptance of oxygen by haemoglobin.

Answer : A

Question. Fetal haemoglobin has a sigmoid dissociation curve which is shifted to left relative to adult Hb because
(a) Fetal Hb has higher P50
(b) Fetal Hb has more affinity for carbon dioxide
(c) Fetal Hb has less affinity for oxygen than the adult Hb
(d) Fetal Hb has lower P50 (18-20 mm Hg) than adult Hb.

Answer : D

Question. Read the following statements and select the correct option. Statement A : The oxygen dissociation curve is ‘S’ shaped or sigmoid shaped. Statement B : The first oxygen molecule binding to haemoglobin increases the affinity of the Hb to subsequent oxygen molecules.
(a) Both statements A and B are true.
(b) Statement A is false but B is true.
(c) Statement A is true but B is false.
(d) Both statements A and B are false.
Answer : A

Important Questions for NCERT Class 11 Biology Breathing and Exchange of Gases

Very Short Answer Type Questions (VSA)

Question. Cigarette smoking causes emphysema. Give reason. 
Answer. Cigarette smoke contains various harmful chemicals like tar, nicotine, hydrogen cyanide and different metals. They damage alveolar walls due to which respiratory surface is decreased and it causes emphysema. Cigarette smoking is one of the major causes of emphysema. It is a chronic disorder.

Question. Name the term used for the volume of air that remains in the lungs, even after the most forceful expiration.
Answer. Residual volume (RV) is the volume of air that remains in the lungs even after a forcible expiration. It is about 1100 to 1200 mL.

Question. What percentage of CO2 is transported in the form of carbaminohaemoglobin?
Answer. About 20-25% of CO2 is transported by haemoglobin in the form of carbaminohaemoglobin.

Question. Where is the respiratory rhythm centre located?
Answer. Respiratory rhythm centre is located in the medulla region of the brain.

Question. A fluid filled double membranous layer surrounds the lungs. Name it and mention its important function. 
Answer. The fluid (pleural fluid) filled double membranous layer surrounding the lungs is called pleura. The pleural fluid lubricates the pleurae so that they may slide over each other without friction during breathing.

Short Answer Type Questions 

Question. Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
Answer. Oxygen dissociation curve is the graphic illustration of the relationship between the partial pressure of oxygen (pO2) and percentage saturation of the haemoglobin with oxygen (O2). The sigmoidal pattern of oxygen haemoglobin dissociation curve is the result of two properties that play significant role in the transport of oxygen. These two properties are: (i) Minimal loss of oxygen from haemoglobin occurs above pO2 of 70–80 mmHg despite significant changes in tension of oxygen beyond this. This is depicted by relatively flat portion of the curve. (ii) Any further decline in pO2 from 40 mmHg causes a disproportionately greater release of oxygen from the haemoglobin. It results in the steeper portion of the curve and causes the curve to be sigmoid.

Question. Define breathing. How is it different from respiration? 
Answer. Breathing is the exchange of oxygen from the atmosphere with CO2 produced by cells. Breathing is a physical process, it occurs outside the cells, hence an extracellular process. No energy is released or used during breathing. However, respiration is a biochemical process as it involves oxidation of food into carbon dioxide, water and energy. It occurs inside the cells and is an intracellular process.

Question. Complete the missing terms. (a) Inspiratory Capacity (IC) = _____ +IRV (b) _______ = TV + ERV (c) Functional Residual Capacity (FRC) = ERV + _______ .
Answer. (a) Inspiratory Capacity (IC) = Tidal volume (TV) + IRV (b) Expiratory Capacity (EC) = TV + ERV (c) Functional Residual Capacity (FRC) = ERV + Residual volume (RV).

Question. How is vital capacity different from total lung capacity?
Answer. Differences between vital capacity and total lung capacity are given below: (Table 108)

Question. What is the effect of pCO2 on oxygen transport?
Answer. An increase in carbon dioxide in the blood causes oxygen to be displaced from haemoglobin. Rise in pCO2 shifts oxygen haemoglobin dissociation curve to the right as it decreases the affinity of haemoglobin for oxygen and increases release of oxygen to the tissues.

Question. What do you understand by Bohr effect? Mention the factors that influence Bohr effect.
Answer. Shifting of the oxygen haemoglobin dissociation curve to the right by increasing carbon dioxide concentration in blood is known as Bohr effect. The presence of carbon dioxide decreases the affinity of haemoglobin for oxygen and increases release of oxygen to the tissues. The pH of the blood falls as its CO2 content increases so that when the pCO2 rises the curve shifts to the right and the p50 rises. The following factors influence the Bohr effect: (i) Decrease in partial pressure of oxygen (ii) Increase in partial pressure of carbon dioxide (iii) Decrease in pH (iv) Increase in H+ concentration (v) Increased body temperature (vi) Excess of 2, 3 - DPG in RBCs

Question. Distinguish between (a) Pharynx and larynx (b) Tracheoles and bronchioles.
Answer. (a) Differences between pharynx and larynx are as follows : (Table 107) 

Long Answer Type Questions 

Question. Explain the mechanism of breathing in humans with suitable diagrams.
Answer. Breathing refers to inflow (inspiration) and outflow (expiration) of air between atmosphere and alveoli of lungs. There are mainly two processes by which lungs are expanded or contracted. (i) Inspiration : It is the process by which fresh atmospheric air enters into the alveoli of the lungs. It is an active process and is brought about by activity of inspiratory muscles. The main muscles of inspiration in normal quiet breathing are the external intercostal muscles and muscles of diaphragm. Diaphragm becomes flat and gets lowered by the contraction of its muscle fibres thereby increasing the volume of the thoracic cavity in length. External intercostal muscles occur between the ribs. These muscles contract and pull the ribs and sternum upward and outward thus increasing the volume of the thoracic cavity. Abdominal muscles relax and allow compression of abdominal organs by the diaphragm. The abdominal muscles play a passive role in inspiration. Thus, overall volume of the thoracic cavity increases and as a result there is a decrease of the air pressure in the lungs. The greater pressure outside the body now causes air to flow rapidly into external nares (nostrils) and through nasal cavities into internal nares. Thereafter, the sequence of air flow is like this:- External → nares Nasal cavities → Internal nares → Pharynx → Glottis → Larynx → trachea → Bronchi → bronchioles → alveolar ducts alveoli. From the alveoli oxygen passes into the blood of the capillaries and carbon dioxide diffuses out from the blood to the lumen of the alveoli. (ii) Expiration : It is the process by which foul air is expelled out of the lungs. Expiration is normally a passive process and involves the relaxation of diaphragm. Diaphragm : When muscles of diaphragm relax, it becomes dome-shaped and, decreases the size of thoracic cavity. Internal intercostal muscles: Contraction of these muscles moves the ribs downward and inward and reduces the size thoracic cavity laterally and dorsoventrally. The abdominal and internal intercostal muscles are called expiratory muscles. Due to the action of these muscles the overall volume of thoracic cavity decreases and the intrapleural pressure increases. Due to this increased pressure in lungs, foul air is expelled out. Abdominal muscles : Contraction of abdominal muscles presses the abdominal viscera against the diaphragm, bulging it further upward and thus decreasing the thoracic cavity more vertically. The diagrammatic representation explaining the mechanism of breathing showing expiration and inspiration is given below :

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