The basic vertebrate pattern. The plan
All vertebrates have circulatory systems based on a
common plan, and so vertebrate systemsshow much less
variety than do those of invertebrates. Although it
is impossible to trace the evolution of the circulatory
system by using fossils (because blood vessels do not
fossilize as do bones and teeth), it is possible to
theorize on its evolution by studying different groups
of vertebrates and their developing embryos. Many of
the variations from the common plan are related to the
different requirements of living in water and on land.
The heart
The vertebrate heart lies below the alimentary canal
in the front and centre of the chest, housed in its
own section of the body cavity. During the development
of an embryo, the heart first appears below the pharynx,
and although it may also be in this position in adult
animals, the heart often moves posteriorly as the animal
grows and matures.
The heart is basically a tube made of special muscle
(cardiac muscle) that is not found anywhere else in
the body. This cardiac muscle beats throughout life
with its own automatic rhythm. Deoxygenated blood from
the body is brought by veins into the most posterior
part of the heart tube, the sinus venosus. From there
it passes forward into the atrium, the ventricle, and
the conus arteriosus (called the bulbus cordis in embryos),
and eventually to the arterial system. The blood is
pushed through the heart because the various parts of
the tube contract in sequence. As the heart develops
from embryo to adult, each part of the tube becomes
a chamber, separated from the others by valves, so that
blood can neither flow backward in the system nor reenter
the heart from the arteries. As the heart grows, it
bends into an “S” shape, so that the sinus venosus and
atrium lie above the ventricle and conus arteriosus.
