276 BIOLOGY
SUMMARY
Cells utilise oxygen for metabolism and produce energy along with substances
like carbon dioxide which is harmful. Animals have evolved different mechanisms
for the transport of oxygen to the cells and for the removal of carbon dioxide from
there. We have a well developed respiratory system comprising two lungs and
associated air passages to perform this function.
The first step in respiration is breathing by which atmospheric air is taken in
(inspiration) and the alveolar air is released out (expiration). Exchange of O
2
and
CO
2
between deoxygenated blood and alveoli, transport of these gases throughout
the body by blood, exchange of O
2
and CO
2
between the oxygenated blood and
tissues and utilisation of O
2
by the cells (cellular respiration) are the other steps
involved.
Inspiration and expiration are carried out by creating pressure gradients
between the atmosphere and the alveoli with the help of specialised muscles –
intercostals and diaphragm. Volumes of air involved in these activities can be
estimated with the help of spirometer and are of clinical significance.
Exchange of O
2
and CO
2
at the alveoli and tissues occur by diffusion. Rate of
diffusion is dependent on the partial pressure gradients of O
2
(pO
2
) and CO
2
(pCO
2
),
their solubility as well as the thickness of the diffusion surface. These factors in
our body facilitate diffusion of O
2
from the alveoli to the deoxygenated blood as
well as from the oxygenated blood to the tissues. The factors are favourable for the
diffusion of CO
2
in the opposite direction, i.e., from tissues to alveoli.
Oxygen is transported mainly as oxyhaemoglobin. In the alveoli where pO
2
is
higher, O
2
gets bound to haemoglobin which is easily dissociated at the tissues
where pO
2
is low and pCO
2
and H
+
concentration are high. Nearly 70 per cent of
carbon dioxide is transported as bicarbonate (HCO
3
–
) with the help of the enzyme
carbonic anhydrase. 20-25 per cent of carbon dioxide is carried by haemoglobin
as carbamino-haemoglobin. In the tissues where pCO
2
is high, it gets bound to
blood whereas in the alveoli where pCO
2
is low and pO
2
is high, it gets removed
from the blood.
Respiratory rhythm is maintained by the respiratory centre in the medulla
region of brain. A pneumotaxic centre in the pons region of the brain and a
chemosensitive area in the medulla can alter respiratory mechanism.
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.