Embryonic development of the circulatory system
An embryo develops only with an adequate supply of
oxygen and metabolites. In its early stages these may
be provided by diffusion. Because the rate of diffusion
becomes limiting beyond a certain size, however, the
circulatory system becomes functional early in development,
often before other organs and systems are obvious.
The heart develops from the middle embryonic tissue
layer, the mesoderm, just below the anterior part of
the gut. It begins as a tube that joins with blood vessels
also forming in the mesoderm. Other mesodermal cells
form a coat around the heart tube and become the muscular
wall, or myocardium. The heart lies in its own section
of body cavity, called the pericardial coelom, formed
by partitions that cut it off from the main body cavity.
From an original tube shape, the heart bends back on
itself as it grows within the pericardial cavity. The
sinus venosus and atrium lie above the ventricle and
bulbus cordis (embryonic equivalent of the conus arteriosus).
Septa gradually partition the heart into chambers.
In mammalian and bird embryos, the lungs are not used
until birth. Oxygen is obtained in the former from the
placenta and in the latter from embryonic membranes
close to the porous eggshell.
The circulation has various modifications for diverting
oxygenated blood from sources outside the embryo to
the body of the embryo. In mammals blood from the placenta
travels tothe right auricle via the umbilical vein and
posterior vena cava. It passes through an opening, the
foramen ovale, into the left auricle, and then to the
left ventricle and around the body. Deoxygenated blood
entering the anterior vena cava fills the right ventricle;
however, instead of passing to the lungs, it is shunted
through the ductus arteriosus, between the pulmonary
and systemic arches, and into the dorsal aorta. From
the dorsal aorta the deoxygenated blood travels to the
placenta, bypassing the lungs completely. At birth the
foramen ovale closes, as does the ductus arteriosus,
and the lungs become functional.
The development of the circulatory system in higher
vertebrate embryos (i.e., those of birds and mammals)
generally follows a sequence of seven main events. Initially,
a tubular heart bends into an “S” shape. Blood then
flows from behind forward through the sinus venosus,
atrium, ventricle, and bulbus cordis. There is then
subdivision of the atrium and ventricle and of the opening
between them. The sinus venosus is incorporated into
the right atrium. The pulmonary veins are segregated
to open into the left atrium. The bulbus cordis is subdivided
into a pulmonary trunk from the right ventricle and
a systemic trunk from the left ventricle. Finally, an
embryonic set of six arterial arches is reduced to three
in adults, and their relationships are further complicated
by asymmetrical loss of some parts and development of
others.
