The basic vertebrate pattern. The plan. The blood vessels
Gill slits are a fundamental feature of all vertebrate
embryos, including humans. With few exceptions, there
are six gill slits on each side. Blood leaving the heart
travels from the conusarteriosus into the ventral aorta,
which branches to send six pairs of arteries between
the gill slits. The arterial branches join the dorsal
aorta above the alimentary canal. Anterior to the gill
slits, the ventral aorta branches again, forming two
external carotid arteries that supply the ventral part
of the head. Two internal carotids, which are the anterior
extensions of the dorsal aorta, supply the brain in
the dorsal part of the head.
Deoxygenated blood collects in capillaries and then
drains into larger and larger veins, whichtake it from
various parts of the body to the heart. Of these, the
anterior and posterior cardinal veins, each with left
and right components, take blood to the heart from the
front andrear of the body, respectively. They lie dorsal
to the alimentary canal, while the heart lies ventral
to it. There is a common cardinal vein on each side,
often called the duct of Cuvier, which carries blood
ventrally into the sinus venosus. Various other veins
join the cardinal veins from all over the body. The
ventral jugular veins drain the lower part of the head
and take blood directly into the common cardinal veins.
Lower vertebrates have two so-called portal systems,
areas of the venous system that begin in capillaries
in tissues and join to form veins, which divide to produce
another capillary network en route to the heart. They
are called the hepatic (liver) and renal (kidneys) portal
systems. The hepatic system is important because it
collects blood from the intestine and passes it to the
liver, the centre for many chemical reactions concerned
with the absorption of food into the body and the control
of substances entering the general circulation. The
function of the renal portal system is less clear, but
it involves two veins that pass from the caudal vein
to the kidneys, where they break up into capillaries.
The coronary circulation is that which supplies the
heart muscle itself. It is of crucial importance, for
the heart must never stop beating. Cardiac muscle needs
oxygen from early in embryonic development until death.
In mammals the coronary blood supply comes from the
aorta, close to the heart. In evolutionary terms, this
was not always so; many lower vertebrates, including
agnathans and amphibians, have no specialized coronary
arteries. Theheart obtains its oxygen from blood passing
through it. Fish have well-developed coronary vessels
that arise from various sources, but ultimately from
the efferent branchial system.
The introduction of lungs changed the site of oxygenation
of the blood. In lungfishes coronaryarteries arise from
those anterior arterial arches receiving the most oxygenated
blood from the heart. In reptiles coronary arteries
branch from the systemic arch, but their position of
origin varies. In some species they arise close to the
heart, as in birds and mammals. Coronary veins generally
run beside corresponding arteries but diverge from them
to enter the main venous supply to the right atrium,
or to the sinus venosus in fishes.
