Home Articles 11th class Breathing and exchange of gases

Breathing and exchange of gases



oxygen (O2) is utilised by the organisms to
indirectly break down nutrient molecules like glucose and to derive energy
for performing various activities. Carbon dioxide (CO2) which is harmful is also released during the above catabolic reactions. It is, therefore, evident that O2 has to be continuously provided to the cells and CO2 produced by the cells have to be released out. This process of exchange of O2 from the atmosphere with CO2 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.


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 O2
with CO2
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 are used by most of the aquatic arthropods and
molluscs whereas vascularised bags called lungs are used by the
terrestrial forms for the exchange of gases. Among vertebrates, fishes
use gills whereas reptiles, birds and mammals respire through lungs.
Amphibians like frogs can respire through their moist skin also. Mammals have a well developed 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 . 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 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 O2 and CO2
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 CO2 rich alveolar air is released out.
(ii) Diffusion of gases (O2 and CO2)across alveolar membrane.
(iii) Transport of gases by the blood.
(iv) Diffusion of O2 and CO2 between blood and tissues.
(v) Utilisation of O2 by the cells for catabolic reactions and resultant release of CO2


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 sternum causing an increase in the volume of
the thoracic chamber in the dorso-ventral axis.
The overall increase in the thoracic volume
causes a similar increase in pulmonary
volume. An increase in pulmonary volume
decreases the intra-pulmonary pressure to less
than the atmospheric pressure which forces
the air from outside to move into the lungs,
i.e., inspiration . Relaxation of
the diaphragm and the inter-costal muscles
returns the diaphragm and sternum to their
normal positions and reduce the thoracic
volume and thereby the pulmonary volume.
This leads to an increase in intra-pulmonary
pressure to slightly above the atmospheric
pressure causing the expulsion of air from the
lungs, i.e., expiration .We have
the ability to increase the strength of
inspiration and expiration with the help of
additional muscles in the abdomen. On an
average, a healthy human breathes 12-16
times/minute. The volume of air involved in
breathing movements can be estimated by
using a spirometer which helps in clinical
assessment of pulmonary functions


  • Tidal Volume (TV): Volume of air inspired orexpired during a normal respiration. It is
    approx. 500 mL., i.e., a healthy man can
    inspire or expire approximately 6000 to 8000 mL of air per minute.
  • Inspiratory Reserve Volume (IRV):
    Additional volume of air, a person can inspireby a forcible inspiration. This averages 2500mL to 3000 mL.
  • Expiratory Reserve Volume (ERV):
    Additional volume of air, a person can expire
    by a forcible expiration. This averages 1000
    mL to 1100 mL
  • Residual Volume (RV): Volume of air remaining in the lungs even after a
    forcible expiration. This averages 1100 mL to 1200 mL. By adding up a few respiratory volumes described above, one can
    derive various pulmonary capacities, which can be used in clinical diagnosis.
  • Inspiratory Capacity (IC): Total volume of air a person can inspire after
    a normal expiration. This includes tidal volume and inspiratory reserve
    volume ( TV+IRV).
  • Expiratory Capacity (EC): Total volume of air a person can expire after
    a normal inspiration. This includes tidal volume and expiratory reserve
    volume (TV+ERV).
  • Functional Residual Capacity (FRC): Volume of air that will remain in
    the lungs after a normal expiration. This includes ERV+RV.
  • Vital Capacity (VC): The maximum volume of air a person can breathe in
    after a forced expiration. This includes ERV, TV and IRV or the maximum
    volume of air a person can breathe out after a forced inspiration.
  • Total Lung Capacity: Total volume of air accommodated in the lungs at
    the end of a forced inspiration. This includes RV, ERV, TV and IRV or
    vital capacity + residual volume.


Alveoli are the primary sites of exchange of gases. Exchange of gases also
occur between blood and tissues. O2
and CO2
are exchanged in these
sites by simple diffusion mainly based on pressure/concentration
gradient. Solubility of the gases as well as the thickness of the membranes
involved in diffusion are also some important factors that can affect the
rate of diffusion.
Pressure contributed by an individual gas in a mixture of gases is
called partial pressure and is represented as pO2 for oxygen and pCO2
for carbon dioxide. Partial pressures of these two gases in the atmospheric
air and the two sites of diffusion are given in  and in The data given in the table clearly indicates a concentration
gradient for oxygen from alveoli to blood and blood to tissues. Similarly.

(a gradient is present for CO2
in the opposite
direction, i.e., from tissues to blood and
blood to alveoli. As the 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).
.( The diffusion membrane is
made up of three major layers namely, the thin squamous epithelium ofalveoli, the endothelium of alveolar capillaries
and the basement substance in between
them. However, its total thickness is much
less than a millimetre. Therefore, all the
factors in our body are favourable for
diffusion of O2 from alveoli to tissues and that
of CO2 from tissues to alveoli.)


Blood is the medium of transport for O2
and CO2
. About 97 per cent of O2
is transported by RBCs in the blood. The remaining 3 per cent of O2 is carried in a dissolved state through the plasma. Nearly 20-25 per cent of CO2 is transported by RBCs whereas 70 per cent of it is carried as bicarbonate. About 7 per cent of CO2 is carried in a dissolved state through plasma.


Haemoglobin is a red coloured iron containing pigment present in the
RBCs. O2
can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of
four molecules of O2
. 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
. This curve is called the Oxygen
dissociation curve  and is highly
useful in studying the effect of factors like
, H+
concentration, etc., on binding of O2
with haemoglobin. In the alveoli, where there
is high pO2
, low pCO2
, lesser H+
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. This
clearly indicates that O2
gets bound to
haemoglobin in the lung surface and gets
dissociated at the tissues. Every 100 ml of
oxygenated blood can deliver around 5 ml of
to the tissues under normal physiological


CO2 is carried by haemoglobin as carbaminohaemoglobin (about 20-25 per cent). This binding is related to the partial pressure of CO2
pO2 is a major factor which could affect this binding. When pCO2
is high and pO2 is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO2 is low and pO2
is high as in the alveoli, dissociation of CO2
from carbamino-haemoglobin takes place, i.e., CO2 which is bound
to haemoglobin from the tissues is delivered at the alveoli. RBCs contain
a very high concentration of the enzyme, carbonic anhydrase and minute
quantities of the same is present in the plasma too. This enzyme facilitates
the following reaction in both directions.

At the tissue site where partial pressure of CO2
is high due to
catabolism, CO2
diffuses into blood (RBCs and plasma) and forms HCO3- and H+,. At the alveolar site where pCO2
is low, the reaction proceeds in
the opposite direction leading to the formation of CO2
and H2O. Thus, CO2
trapped as bicarbonate at the tissue level and transported to the
alveoli is released out as CO2 .Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO2 to the alveoli.


Human beings have a significant ability to maintain and moderate the
respiratory rhythm to suit the demands of the body tissues. This is done
by the neural system. A specialised centre present in the medulla region
of the brain called respiratory rhythm centre is primarily responsible for
this regulation. Another centre present in the pons region of the brain
called pneumotaxic centre can moderate the functions of the respiratory
rhythm centre. Neural signal from this centre can reduce the duration of
inspiration and thereby alter the respiratory rate. A chemosensitive area
is situated adjacent to the rhythm centre which is highly sensitive to CO2
and hydrogen ions. Increase in these substances can activate this centre,
which in turn can signal the rhythm centre to make necessary adjustments
in the respiratory process by which these substances can be eliminated.
Receptors associated with aortic arch and carotid artery also can recognise
changes in CO2 and H+ concentration and send necessary signals to the
rhythm centre for remedial actions. The role of oxygen in the regulation of
respiratory rhythm is quite insignificant.


  • Asthma is a difficulty in breathing causing wheezing due to inflammation
    of bronchi and bronchioles.
  • Emphysema is a chronic disorder in which alveolar walls are damaged
    due to which respiratory surface is decreased. One of the major causes of
    this is cigarette smoking.
  • Occupational Respiratory Disorders: In certain industries, especially
    those involving grinding or stone-breaking, so much dust is produced
    that the defense mechanism of the body cannot fully cope with the
    situation. Long exposure can give rise to inflammation leading to fibrosis
    (proliferation of fibrous tissues) and thus causing serious lung damage.
    Workers in such industries should wear protective masks.