Page 309 - Biology_F5

Biology for Advanced Level Secondary Schools
for oxygen, since blood oxygenation
increases. It reduces systemic and
pulmonary blood pressures.
Adaptations to oxygen uptake for divers
Divers in deep water do not depend on
FOR ONLINE READING ONLY
lungs as a source of oxygen; instead, (f) Seals can hold their breath for about
they rely on enhanced oxygen stored in
their blood and muscles. Collapse of the
lungs forces air away from the alveoli,
where gas exchange between the lungs
and blood occurs. This blunting of gas
exchange is important in the deep diver
because it prevents the absorption of
nitrogen into the blood and the subsequent
development of high blood nitrogen
levels. High blood nitrogen pressure can
exert a narcotic effect (so-called nitrogen
narcosis) on the diver. It may also lead to
nitrogen bubble formation during ascent,
a phenomenon known as decompression
sickness or “the bends”. Collapse of the
lungs in deep divers helps to avoid these
two problems. Thus, diving mammals,
besides the collapse of their lungs, have
the following adaptive mechanisms to
oxygen uptake:
(a) They use oxygen more efficiently, that
is, they fill their lungs and exchange
90% of their air in each breath. Thus,
before a dive is taken, they take a deep
breath to accumulate oxygen in their
lungs.
(b) They have a high blood volume with
plenty of haemoglobin and myoglobin.
This allows long oxygen retention time.
(c) They have a high tolerance to lactic
acid and carbon dioxide, that is, their
muscles can work anaerobically while
holding their breath.
(d) They can tolerate tremendous
atmospheric pressure at great depths.
Their lungs and ribs are collapsible;
302
air spaces are minimised; and nitrogen
absorption is limited.
(e) Diving mammals slow their heart rate,
stop their breathing and shunt blood
flow from their extremities to the brain,
heart, and muscles when starting a dive.
two hours. They rely on internal oxygen
stores when they are down there.
(g) Myoglobin of the seals and dolphins is
more concentrated than that of humans,
almost ten times, this gives them a
chance for storing oxygen for a long
time when under water.
Adaptations of the mammalian foetus
to oxygen uptake
The foetus lives and develops inside the
maternal womb (uterus). It obtains nutrients,
exchange gases and waste products with
the maternal blood via the placenta. For
the foetus to obtain oxygen from maternal
blood, the oxygen has to diffuse from
maternal blood to foetus blood. There are
some adaptations to this, which are;
(a) Foetal haemoglobin has higher affinity
to oxygen; hence it can readily combine
with oxygen to form oxyhaemoglobin
(Figure.6.7). This allows the foetus
to extract oxygen from the maternal
blood supply. In adult hemoglobin
the protein subunits are identical
beta-chain subunits, while in foetal
hemoglobin the two subunits are
identical gamma subunits. This change
in structure of foetal hemoglobin also
leads to a change in function when
compared to adult hemoglobin. It has
been shown that the structural changes
in foetal hemoglobin caused by the
substitution of gamma subunits for
beta-chains, allows foetal hemoglobin
to have a higher affinity to oxygen.
Form Five Student’s Book