bypass the lungs, and allow blood to flow from the right
ventricle directly out to the body again via the left aorta,
so that more blood reaches the vital organs. To achieve
this, a valve with cog-like teeth closes off the entry to
the pulmonary artery from the right ventricle, and the
pressure in the right ventricle causes blood to be pushed
through the valve previously limiting blood flow into the
left aorta.
This cog-like valve differs
from the valves in all
other vertebrates, which
are thin, leaf-like, and
work passively. In con-
trast, the cog-like valve of
crocodiles is made of
nodules of connective
tissue, and moreover is
actively controlled. The
opening and closing of the
cog-like teeth is regulated by the amount of the
hormone adrenalin in the bloodstream. When relaxed,
such as when a crocodile is submerged, adrenalin levels
are low, and the cog-toothed valve is closed, thus
preventing blood flow to the lungs and instead shunting
oxygen-poor blood out through the left aorta. The role
of this unique valve in actively regulating blood flow
between pulmonary and systemic circulations was
revealed by experiments on the Estuarine Crocodile,
Crocodylus porosus, by Franklin and Axelsson as recently
as 2000. Described as an ‘evolutionary novelty’, these
findings were impressive enough to be accepted for
publication in the renowned journal Nature.
where in the body, which promotes effective unloading
of oxygen to respiring tissues. The duration of dives is
generally 10-15 minutes, but if pressed, crocodilians
have the capacity to stay submerged for incredible
periods. Calculations suggest that a one-tonne, six-
meter crocodile is capable of remaining submerged at
rest for almost an hour without factoring in any reduc-
tion in oxygen consumption, and remarkably, for almost
nine hours if oxygen consumption is reduced to one-
eighth that at rest! Yet when
breathing air above water,
crocodiles can be remarkably
active, able to reach speeds
of 35km/hr.
‘A six-meter crocodile
may be capable of
remaining submerged for
almost nine hours, but
can also reach speeds of
35km/hr!’
This active shunt bestows incredible cardiovascular
flexibility. By-passing the pulmonary circulation when
submerged keeps blood high in carbon dioxide away
from the lungs, prolonging the efficient uptake of
oxygen. Instead, carbon dioxide is sequestered else-
Although other reptiles can
also divert blood from the
pulmonary circuit into the
systemic circulation, this is
by means of an intracardiac
shunt, across the two ventricles; only crocodilians have
an extra-cardiac shunt. This enables them to have
entirely separate left and right ventricles, which in turn
means they can completely separate oxygenated and
deoxygenated blood. Crocodiles’ complex cardiovascular
system also has the advantage that, with the shunt in
operation, low-oxygen blood enters the systemic circula-
tion (mostly) downstream of the carotid circuit, which
delivers blood to the brain. The majority of the low-
oxygen, shunted blood is instead directed to the gut,
where oxygen is less in demand, but the blood can pick
up nutrients from digestion. Hence, oxygen levels in the
blood that is pumped to the brain can still be maintained
at a relatively high concentration - certai