Modifications among the vertebrate classes. Reptiles
Unlike lungfishes and amphibians, reptiles depend entirely
on their lungs for respiration. Gillsand skin do not
provide additional sources of oxygen. Only the crocodiles,
however, truly approach birds and mammals in their almost
complete “double” circulation. Because of thedevelopment
of a neck and relative elongation of that region of
the body, the heart may be displaced posteriorly and
the arrangement of arteries and veins may be altered
accordingly. In general, however, the circulatory system
resembles that in frogs.
Various changes can be seen in the reptilian heart.
The left atrium is smaller than the right and always
completely separate from it. The sinus venosus is present
but small. The ventricleis variously subdivided in different
groups. Three arterial trunks arise directly from the
ventricle, the conus having been partly incorporated
into it. The three trunks are the right and left systemic
arches and the pulmonary arch. The carotid arch is a
branch of the right systemic arch. When the ventricle
is actually beating, there is functional separation
of blood from the two atria so that most oxygenated
blood flows to the carotid arteries and hardly mixes
with deoxygenated blood going to the lungs.
Crocodiles are the only living representatives of the
archosaurian reptiles, the group that included the dinosaurs
and from which birds evolved. Crocodiles have a complete
ventricular septum, producing two equally sized chambers.
The blood from the right and left atria is not mixed;
despite this, there is an opening at the base of the
right and left systemic arches, and blood can be shunted
between the two. This is important during diving, when
blood flow to thelungs is decreased. The crocodile heart
is situated so posteriorly that the subclavian artery,
which would normally arise from the dorsal aorta at
the level of the systemic arch, arises fromthe carotid
artery.
