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UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of
molecules, supra molecular assemblies, cells, tissues, organisms and
indeed, populations and communities, each create emergent
properties. In the chapters under this unit, major human physiological
processes like, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last
two chapters point to the coordination and regulation of body events
at the organismic level.
HUMAN PHYSIOLOGY
Chapter 14
Breathing and Exchange
of Gases
Chapter 15
Body Fluids and
Circulation
Chapter 16
Excretory Products and
their Elimination
Chapter 17
Locomotion and Movement
Chapter 18
Neural Control and
Coordination
Chapter 19
Chemical Coordination
and Integration
2024-25
Page 2


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of
molecules, supra molecular assemblies, cells, tissues, organisms and
indeed, populations and communities, each create emergent
properties. In the chapters under this unit, major human physiological
processes like, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last
two chapters point to the coordination and regulation of body events
at the organismic level.
HUMAN PHYSIOLOGY
Chapter 14
Breathing and Exchange
of Gases
Chapter 15
Body Fluids and
Circulation
Chapter 16
Excretory Products and
their Elimination
Chapter 17
Locomotion and Movement
Chapter 18
Neural Control and
Coordination
Chapter 19
Chemical Coordination
and Integration
2024-25
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2024-25
Page 3


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of
molecules, supra molecular assemblies, cells, tissues, organisms and
indeed, populations and communities, each create emergent
properties. In the chapters under this unit, major human physiological
processes like, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last
two chapters point to the coordination and regulation of body events
at the organismic level.
HUMAN PHYSIOLOGY
Chapter 14
Breathing and Exchange
of Gases
Chapter 15
Body Fluids and
Circulation
Chapter 16
Excretory Products and
their Elimination
Chapter 17
Locomotion and Movement
Chapter 18
Neural Control and
Coordination
Chapter 19
Chemical Coordination
and Integration
2024-25
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2024-25
183 BREATHING AND EXCHANGE OF GASES
As you have read earlier, oxygen (O
2
) is utilised by the organisms to
indirectly break down simple molecules like glucose, amino acids, fatty
acids, etc., to derive energy to perform various activities. Carbon dioxide
(CO
2
) which is harmful is also released during the above catabolic
reactions. It is, therefore, evident that O
2
  has to be continuously provided
to the cells and CO
2 
produced by the cells have to be released out. This
process of exchange of O
2
 from the atmosphere with CO
2
 produced by the
cells is called breathing, commonly known as respiration. Place your
hands on your chest; you can feel the chest moving up and down. You
know that it is due to breathing. How do we breathe? The respiratory
organs and the mechanism of breathing are described in the following
sections of this chapter.
14.1 RESPIRATORY ORGANS
Mechanisms of breathing vary among different groups of animals
depending mainly on their habitats and levels of organisation. Lower
invertebrates like sponges, coelenterates, flatworms, etc., exchange O
2
with CO
2
 by simple diffusion over their entire body surface. Earthworms
use their moist cuticle and insects have a network of tubes (tracheal
tubes) to transport atmospheric air within the body. Special vascularised
structures called gills (branchial respiration) are used by most of the
aquatic arthropods and molluscs whereas vascularised bags called lungs
(pulmonary respiration) are used by the terrestrial forms for the exchange
of gases. Among vertebrates, fishes use gills whereas amphibians, reptiles,
birds and mammals respire through lungs. Amphibians like frogs can
respire through their moist skin (cutaneous respiration) also.
BREATHING AND EXCHANGE OF GASES
CHAPTER  14
14.1 Respiratory
Organs
14.2 Mechanism of
Breathing
14.3 Exchange of
Gases
14.4 Transport of
Gases
14.5 Regulation of
Respiration
14.6 Disorders of
Respiratory
System
2024-25
Page 4


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of
molecules, supra molecular assemblies, cells, tissues, organisms and
indeed, populations and communities, each create emergent
properties. In the chapters under this unit, major human physiological
processes like, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last
two chapters point to the coordination and regulation of body events
at the organismic level.
HUMAN PHYSIOLOGY
Chapter 14
Breathing and Exchange
of Gases
Chapter 15
Body Fluids and
Circulation
Chapter 16
Excretory Products and
their Elimination
Chapter 17
Locomotion and Movement
Chapter 18
Neural Control and
Coordination
Chapter 19
Chemical Coordination
and Integration
2024-25
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2024-25
183 BREATHING AND EXCHANGE OF GASES
As you have read earlier, oxygen (O
2
) is utilised by the organisms to
indirectly break down simple molecules like glucose, amino acids, fatty
acids, etc., to derive energy to perform various activities. Carbon dioxide
(CO
2
) which is harmful is also released during the above catabolic
reactions. It is, therefore, evident that O
2
  has to be continuously provided
to the cells and CO
2 
produced by the cells have to be released out. This
process of exchange of O
2
 from the atmosphere with CO
2
 produced by the
cells is called breathing, commonly known as respiration. Place your
hands on your chest; you can feel the chest moving up and down. You
know that it is due to breathing. How do we breathe? The respiratory
organs and the mechanism of breathing are described in the following
sections of this chapter.
14.1 RESPIRATORY ORGANS
Mechanisms of breathing vary among different groups of animals
depending mainly on their habitats and levels of organisation. Lower
invertebrates like sponges, coelenterates, flatworms, etc., exchange O
2
with CO
2
 by simple diffusion over their entire body surface. Earthworms
use their moist cuticle and insects have a network of tubes (tracheal
tubes) to transport atmospheric air within the body. Special vascularised
structures called gills (branchial respiration) are used by most of the
aquatic arthropods and molluscs whereas vascularised bags called lungs
(pulmonary respiration) are used by the terrestrial forms for the exchange
of gases. Among vertebrates, fishes use gills whereas amphibians, reptiles,
birds and mammals respire through lungs. Amphibians like frogs can
respire through their moist skin (cutaneous respiration) also.
BREATHING AND EXCHANGE OF GASES
CHAPTER  14
14.1 Respiratory
Organs
14.2 Mechanism of
Breathing
14.3 Exchange of
Gases
14.4 Transport of
Gases
14.5 Regulation of
Respiration
14.6 Disorders of
Respiratory
System
2024-25
184 BIOLOGY
14.1.1 Human Respiratory System
We have a pair of external nostrils opening out above the upper lips.
It leads to a nasal chamber through the nasal passage. The nasal
chamber opens into the pharynx, a portion of which is the common
passage for food and air. The pharynx opens through the larynx region
into the trachea. Larynx is a cartilaginous box which helps in sound
production and hence called the sound box. During swallowing glottis
can be covered by a thin elastic cartilaginous flap called epiglottis  to
prevent the entry of food into the larynx. Trachea is a straight tube
extending up to the mid-thoracic cavity, which divides at the level of
5th thoracic vertebra into a right and left primary bronchi. Each bronchi
undergoes repeated divisions to form the secondary and tertiary bronchi
and bronchioles ending up in very thin terminal bronchioles. The
tracheae, primary, secondary and tertiary bronchi, and initial
bronchioles are supported by incomplete cartilaginous rings. Each
terminal bronchiole gives rise to a number of very thin, irregular-walled
and vascularised bag-like structures called alveoli. The branching
network of bronchi, bronchioles and alveoli comprise the lungs (Figure
14.1). We have two lungs which are covered by a double layered pleura,
with pleural fluid between them. It reduces friction on the lung-surface.
The outer pleural membrane is in close contact with the thoracic
Bronchus
Lung
heart
Diaphragm
Epiglottis
Larynx
Trachea
Cut end of rib
Pleural membranes
Alveoli
Pleural fluid
Bronchiole
Figure 14.1 Diagrammatic view of human respiratory system (sectional view of
the left lung is also shown)
2024-25
Page 5


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of
molecules, supra molecular assemblies, cells, tissues, organisms and
indeed, populations and communities, each create emergent
properties. In the chapters under this unit, major human physiological
processes like, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last
two chapters point to the coordination and regulation of body events
at the organismic level.
HUMAN PHYSIOLOGY
Chapter 14
Breathing and Exchange
of Gases
Chapter 15
Body Fluids and
Circulation
Chapter 16
Excretory Products and
their Elimination
Chapter 17
Locomotion and Movement
Chapter 18
Neural Control and
Coordination
Chapter 19
Chemical Coordination
and Integration
2024-25
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2024-25
183 BREATHING AND EXCHANGE OF GASES
As you have read earlier, oxygen (O
2
) is utilised by the organisms to
indirectly break down simple molecules like glucose, amino acids, fatty
acids, etc., to derive energy to perform various activities. Carbon dioxide
(CO
2
) which is harmful is also released during the above catabolic
reactions. It is, therefore, evident that O
2
  has to be continuously provided
to the cells and CO
2 
produced by the cells have to be released out. This
process of exchange of O
2
 from the atmosphere with CO
2
 produced by the
cells is called breathing, commonly known as respiration. Place your
hands on your chest; you can feel the chest moving up and down. You
know that it is due to breathing. How do we breathe? The respiratory
organs and the mechanism of breathing are described in the following
sections of this chapter.
14.1 RESPIRATORY ORGANS
Mechanisms of breathing vary among different groups of animals
depending mainly on their habitats and levels of organisation. Lower
invertebrates like sponges, coelenterates, flatworms, etc., exchange O
2
with CO
2
 by simple diffusion over their entire body surface. Earthworms
use their moist cuticle and insects have a network of tubes (tracheal
tubes) to transport atmospheric air within the body. Special vascularised
structures called gills (branchial respiration) are used by most of the
aquatic arthropods and molluscs whereas vascularised bags called lungs
(pulmonary respiration) are used by the terrestrial forms for the exchange
of gases. Among vertebrates, fishes use gills whereas amphibians, reptiles,
birds and mammals respire through lungs. Amphibians like frogs can
respire through their moist skin (cutaneous respiration) also.
BREATHING AND EXCHANGE OF GASES
CHAPTER  14
14.1 Respiratory
Organs
14.2 Mechanism of
Breathing
14.3 Exchange of
Gases
14.4 Transport of
Gases
14.5 Regulation of
Respiration
14.6 Disorders of
Respiratory
System
2024-25
184 BIOLOGY
14.1.1 Human Respiratory System
We have a pair of external nostrils opening out above the upper lips.
It leads to a nasal chamber through the nasal passage. The nasal
chamber opens into the pharynx, a portion of which is the common
passage for food and air. The pharynx opens through the larynx region
into the trachea. Larynx is a cartilaginous box which helps in sound
production and hence called the sound box. During swallowing glottis
can be covered by a thin elastic cartilaginous flap called epiglottis  to
prevent the entry of food into the larynx. Trachea is a straight tube
extending up to the mid-thoracic cavity, which divides at the level of
5th thoracic vertebra into a right and left primary bronchi. Each bronchi
undergoes repeated divisions to form the secondary and tertiary bronchi
and bronchioles ending up in very thin terminal bronchioles. The
tracheae, primary, secondary and tertiary bronchi, and initial
bronchioles are supported by incomplete cartilaginous rings. Each
terminal bronchiole gives rise to a number of very thin, irregular-walled
and vascularised bag-like structures called alveoli. The branching
network of bronchi, bronchioles and alveoli comprise the lungs (Figure
14.1). We have two lungs which are covered by a double layered pleura,
with pleural fluid between them. It reduces friction on the lung-surface.
The outer pleural membrane is in close contact with the thoracic
Bronchus
Lung
heart
Diaphragm
Epiglottis
Larynx
Trachea
Cut end of rib
Pleural membranes
Alveoli
Pleural fluid
Bronchiole
Figure 14.1 Diagrammatic view of human respiratory system (sectional view of
the left lung is also shown)
2024-25
185 BREATHING AND EXCHANGE OF GASES
lining whereas the inner pleural membrane is in contact with the lung
surface. The part starting with the external nostrils up to the terminal
bronchioles constitute the conducting part whereas the alveoli and their
ducts form the respiratory or exchange part of the respiratory system.
The conducting part transports the atmospheric air to the alveoli, clears
it from foreign particles, humidifies and also brings the air to body
temperature. Exchange part is the site of actual diffusion of O
2
 and CO
2
between blood and atmospheric air.
The lungs are situated in the thoracic chamber which is anatomically
an air-tight chamber. The thoracic chamber is formed dorsally by the
vertebral column, ventrally by the sternum, laterally by the ribs and on
the lower side by the dome-shaped diaphragm. The anatomical setup of
lungs in thorax is such that any change in the volume of the thoracic
cavity will be reflected in the lung (pulmonary) cavity. Such an
arrangement is essential for breathing, as we cannot directly alter the
pulmonary volume.
Respiration involves the following steps:
(i) Breathing or pulmonary ventilation by which atmospheric air
is drawn in and CO
2
 rich alveolar air is released out.
(ii) Diffusion of gases (O
2
 and CO
2
) across alveolar membrane.
(iii) Transport of gases by the blood.
(iv) Diffusion of O
2
 and CO
2
 between blood and tissues.
(v) Utilisation of O
2
 by the cells for catabolic reactions and resultant
release of CO
2
 (cellular respiration as dealt in the Chapter 12).
14.2 MECHANISM OF BREATHING
Breathing involves two stages : inspiration during which atmospheric
air is drawn in and expiration by which the alveolar air is released out.
The movement of air into and out of the lungs is carried out by creating a
pressure gradient between the lungs and the atmosphere. Inspiration
can occur if the pressure within the lungs (intra-pulmonary pressure) is
less than the atmospheric pressure, i.e., there is a negative pressure in
the lungs with respect to atmospheric pressure. Similarly, expiration takes
place when the intra-pulmonary pressure is higher than the atmospheric
pressure. The diaphragm and a specialised set of muscles – external and
internal intercostals between the ribs, help in generation of such gradients.
Inspiration is initiated by the contraction of diaphragm which increases
the volume of thoracic chamber in the antero-posterior axis. The
contraction of external inter-costal muscles lifts up the ribs and the
2024-25
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FAQs on NCERT Textbook: Breathing & Exchange of Gases - Biology Class 11 - NEET

1. What are the organs involved in the process of breathing and exchange of gases?
Ans. The organs involved in the process of breathing and exchange of gases are the nose, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.
2. How does oxygen from the air enter our bloodstream?
Ans. Oxygen from the air enters our bloodstream through the process of diffusion. When we inhale, oxygen-rich air enters our lungs and reaches the alveoli. From there, oxygen diffuses across the thin walls of the alveoli and enters the bloodstream, where it binds with hemoglobin in red blood cells.
3. What is the role of the diaphragm in the process of breathing?
Ans. The diaphragm plays a crucial role in the process of breathing. It is a dome-shaped muscle located below the lungs. When we breathe in, the diaphragm contracts and moves downwards, causing the volume of the chest cavity to increase. This creates a negative pressure, and air rushes into the lungs. When we breathe out, the diaphragm relaxes and moves upwards, reducing the volume of the chest cavity and pushing air out of the lungs.
4. How is carbon dioxide eliminated from our body during the process of respiration?
Ans. Carbon dioxide is eliminated from our body during respiration primarily through exhalation. When we breathe out, the carbon dioxide-rich air from our lungs is expelled. Additionally, a small amount of carbon dioxide is also dissolved in the blood and transported to the lungs, where it is released into the air during exhalation.
5. What is the purpose of the respiratory pigment hemoglobin?
Ans. The respiratory pigment hemoglobin plays a vital role in the transport of oxygen in our body. It is found in red blood cells and has a high affinity for oxygen. Hemoglobin binds with oxygen in the lungs and carries it to the body tissues. This ensures that oxygen is efficiently transported to cells where it is needed for various metabolic processes.
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