VERTEBRATE CIRCULATORY SYSTEM
Transport gases, nutrients, waste products, hormones & various
other materials; thermoregulation, immunity.
Composed of:
1. BLOOD-VASCULAR SYSTEM- closed system consisting of
blood vessels where the enclosed fluid travels in a circuit

Arteries: carry blood away from the heart; have muscular,
elastic walls; terminate in capillary beds
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Veins: carry blood back to the heart; have less muscle in their
walls than arteries but the walls are very elastic; begin at the
end of capillary beds

Capillaries: have very thin walls (endothelium only); are the site
of exchange between the blood and body cells
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Heart: a muscular pump (cardiac muscle) containing a
pacemaker to regulate rate which can also be influenced by the
Autonomic Nervous System
2. LYMPHATIC SYSTEM - open system consisting of vessels
and sinuses which communicate with coelomic spaces of
the body. Found in all vertebrates and composed of:
 Lymph and lymph vessels
o Lymph is similar in composition to blood plasma. It
bathes all cells & contains WBC, but RBC are absent.
o Lymph empties into 1 or more veins (e.g., caudal, iliac,
subclavian, & posterior cardinal.
o Vessels are found in most soft tissues of the body &
begin as blind-end lymph capillaries that collect
interstitial fluid
o Valves (present in birds & mammals) prevent backflow
 Lymph nodes - located along lymph vessels; contain lots
of lymphocytes & macrophages
 Lymph hearts - consist of pulsating smooth muscle that
propels lymph fluid through lymph vessels; found in
fish, amphibians, & reptiles
 The entire circulatory system develops early from
mesoderm mesenchyme.
 In the mesenchyme, central patches of cells give rise
to blood cells, peripheral ones form blood vessels.
 Somatic vessels arise from somatic mesoderm;
visceral vessels arise from splanchnic mesoderm.
 The 1st blood vessels are the vitelline veins. They are
continued posteriorly as the subintestinal vein, and
supply the yolk sac.
 The splanchnopleure fuse on the ventral side of the embryo,
and the vitelline veins unite to form the heart.
 Dorsal & ventral mesocardia form but soon vanishes, leaving
the heart free in the coelom, but attached only at its ends.
 Heart then lies in a median ventral part of the body enclosed
by a portion of the ventral mesentery, the pericardial cavity.
HEART
 Originally a straight tube,
but since its 2 ends are
fixed, it assumes an Sshaped curve as it grows.
 Subdivided into 4
chambers: sinus
venosus, atrium,
ventricle, conus
arteriosus.
 The S-curvature brings
the sinus near the
ventricle, and the atrium
near the conus.
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In tetrapods, the heart becomes more compact,
bringing the ends of the great veins near the origins
of the great arteries.
Septa soon divide the atria and ventricles, making
possible double circulation of blood.
Sinus venosus becomes incorporated into wall of
right atrium, while the conus splits into 2 trunks:
systemic aorta and pulmonary trunk.
CHONDRICHTHYANS
Single-circuit, with 4 chambers:

Sinus venosus- receives
blood; filled by suction when
the ventricle contracts and
enlarges the pericardial
cavity

Atrium- a thin-walled
muscular sac; an A-V valve
regulates flow between
atrium & ventricle

Ventricle- muscular walls

Conus arteriosus- leads into
the ventral aorta; conal
valves in the conus arteriosus
prevent the backflow of blood
TELEOSTS
Heart is similar to that of cartilaginous fishes, except
that a bulbus arteriosus (a muscular extension of the
ventral aorta) is present rather than a conus arteriosus
(a muscular extension of the ventricle)
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LUNGFISH & AMPHIBIANS
Modifications are correlated
with the presence of lungs &
enable oxygenated blood
returning from the lungs to be
separated from deoxygenated
blood returning from
elsewhere.
2-circuit heart: partial or
complete interatrial septum
(complete in anurans and some
urodeles)
Partial interventricular septum
(lungfish) or ventricular
trabeculae (amphibians) to
maintain separation of
oxygenated & unoxygenated
blood
5 = ventricle, 11 = right
atrium, 12 = left atrium,
13 = conus arteriosus
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Formation of a spiral
valve in the conus
arteriosus of many
dipnoans and amphibians.
Spiral valve alternately
blocks & unblocks the
entrances to the left and
right pulmonary arches
(sending unoxygenated
blood to the skin & lungs).
Shortening of ventral
aorta, which helps ensure
that the oxygenated &
unoxygenated blook kept
separate in the heart
moves directly into the
appropriate vessels
AMNIOTES
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Consists of 2 atria & 2 ventricles &, except in adult birds
& mammals, a sinus venosus
Complete interatrial septum (a foramen ovale is an
embyronic hole in interatrial septum)
Auricle is an expansion of atria in mammals only
Complete interventricular septum only in crocodilians,
birds, & mammals; partial septum in other amniotes
In crocodilians, the
ventricular septum is
complete but a narrow
channel, the Foramen of
Panizza connects the
base of the right & left
systemic trunks
Fate of Conus
Arteriosus:
3 trunks in reptiles; 2
trunks in birds and
mammals
When a crocodilian is under water and not breathing, right
ventricular pressure increases due to vasoconstriction of
blood vessels supplying the lungs.
As a result, the semilunar valve in the right aorta is forced
open so blood from the right ventricle enters the right
aorta rather than the pulmonary artery.
Vital organs & tissues (skeletal muscles and the CNS) will
get an increased blood supply additional oxygen.
This allows crocodile to hunt by remaining underwater &
'ambush‘ prey.
BIRDS AND MAMMALS
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No mixing of oxygenated &
unoxygenated blood;
complete interventricular
septum + division of ventral
aorta into 2 trunks:
Pulmonary trunk takes
blood to the lungs
Aortic trunk takes blood to
the rest of the body
Result of modifications: All
blood returning to right side
of heart goes to the lungs;
blood returning from lungs
to the left side of heart goes
to systemic circulation.
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AV valves- bicuspid
& tricuspid in
mammals
Semilunar valvesbetween ventricles
and trunks
Sinus venosus- the
pacemaker
through reptiles;
becomes the
sinoatrial node in
birds & mammals
located in the wall
of the RA
Pacemaker- sets
the pace for all
heartbeats
low
pressure
to body
low
pressure
and low
O2
to body
high
pressure
& high O2
to body
high
pressure
& high O2
to body
 Supply most tissues with oxygenated blood but carry
deoxygenated blood to respiratory organs.
 Basic pattern:
1. ventral aorta emerges from heart & passes forward
beneath the pharynx
2. dorsal aorta (paired above the pharynx) passes
caudally above the digestive tract
3. six pairs of aortic arches connect the ventral aorta with
the dorsal aortae
FISHES
Cartilaginous fishes:
1. Ventral aorta extends forward below pharynx &
connects developing aortic arches. The 1st pair of
arches develop first.
2. Segments of first pair are lost & remaining sections
become efferent pseudobranchial arteries
3. Pairs 2-6 arches give rise to pre- & posttrematic
arteries
4. Arches 2 - 6 become occluded; dorsal segments =
efferent branchial arteries & ventral segments =
afferent branchial arteries; capillary beds develop
within nine demibranchs
Teleosts: same changes except that the pulmonary
artery branches off the 6th aortic arch to supply the
swim bladder
RESULT: Blood entering an aortic arch from
ventral aorta must pass through gill capillaries
before proceeding to dorsal aorta
TETRAPODS - embryos have 6 pairs of aortic
arches:
• 1st & 2nd arches are temporary & not found
in adults
• 3rd aortic arches & the paired dorsal aortae
anterior to arch 3 are called the internal
carotid arteries; common carotid artery is
from ventral aorta; external carotid artery is
from common carotid artery
• 4th aortic arches are called the systemic
arches or the aorta
• 5th aortic arch is usually lost
• pulmonary arteries branch off the 6th arches
& supply blood to the lungs
AMPHIBIANS
ANURANS- have 4 arches early in development (larval
stage); arches III, IV, V & VI supply larval gills. At
metamorphosis:
• Aortic arch 5 is lost
• Dorsal aorta between arches 3 & 4 is lost, so blood
entering arch 3 (carotid arch) goes to the head
• Ductus arteriosus of arch 6 is lost so blood entering
this arch goes to the skin & lungs
• Aortic arch 4 (systemic arch) on each side continue to
the dorsal aorta & distributes blood to the rest of the
body
• Oxygenated blood from the left atrium &
deoxygenated blood from the right are largely kept
separate in the ventricle by: ventricular trabeculae
and spiral valve in the conus arteriosus
REPTILES - similar to amphibians except:
• 2 aortic trunks from conus arteriosus send blood to
arch 3 and 4
• One pulmonary trunk from conus arteriosus sends
blood to 6th aortic arch
• Ventral aorta - no spiral valve but truncus arteriosus is
split into 3 Separate passages: 2 aortic trunks & a
pulmonary trunk. As a result:
1. pulmonary trunk emerges from the right ventricle &
connects with 6th aortic arches (deoxygenated
blood from right atrium goes to lungs)
2. one aortic trunk comes out of left ventricle &
carries oxygenated blood to the right 4th aortic
arch & to carotid arches
3. the other aortic trunk appears to come out of right
ventricle & leads to left 4th aortic arch.
• Turtles, snakes, & lizards - the interventricular
septum is incomplete where right & left systemic
arches (4th) leave the ventricle & trabeculae in that
region of the heart form a ‘pocket’ called the cavum
venosum.
• Oxygenated blood from the left ventricle is directed
into cavum venosum, which leads to the 2 systemic
arches. As a result, both the left & right systemic
arches receive oxygenated blood
BIRDS AND MAMMALS
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One aortic trunk sends blood
to arches 3 and 4
One pulmonary trunk sends
blood to arch 6
4TH Aortic Arch: Right side
stays in birds, Left side stays in
mammals
Right side of 4TH arch becomes
subclavian artery in mammals
Ductus arteriosus is seen in
fetus only; a bypass of blood
from the pulmonary trunk to
the aorta
Carotids have same general
pattern
AORTIC ARCHES
Dorsal aorta is the chief artery of the body
caudad of the heart in all vertebrates, and has
the same branches throughout.
• The pulmonary veins appear as new structures in
air breathing vertebrates. They enter the left auricle,
initiating the changes leading to double circulation
• Branches of dorsal aorta:
1. Somatic branches to skin, spine, muscles:
 Subclavian  Brachial arteries supply arm
 Iliac A  Femoral artery supplies leg
2. Lateral visceral branches: Urogenital
3. Ventral Visceral branches:
 Celiac artery- stomach, pancreas, liver
 Mesenteric artery- intestine (may be more
than one)
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Veins start as
capillaries & carry
blood towards the
heart
In early vertebrate
embryos, venous
channels conform to
a single basic
pattern.
As development
proceeds, these
channels are
modified by deletion
of some vessels &
addition of others.
The chief embryonic
somatic veins are the:
1. Anterior & posterior
cardinal veins, uniting at
the level of the heart to
a common cardinal vein
(duct of Cuvier)
2. Subintestinal veins
connects with one of the
vitelline veins & makes a
loop around the anus
3. Abdominal or umbilical
vein opening into the
common cardinal
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The vitelline veins break up
in the liver.
Their proximal ends remain
as hepatic veins, distal
portions form the hepatic
portal which is continuous
with the subintestinal, which
now loses its connection
with the caudal vein.
The subcardinals appear
between the kidneys, and
join the posterior parts of
the posterior cardinal veins
as the renal portal.
The caudal vein shifts to
open into the posterior parts
of the postcardinal veins, so
that blood now flows from
the postcardinals into the
subcardinals.
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Lateral abdominal vein join the renal portals and
unite anteriorly with the hepatic portal. Later they
unite to form the ventral abdominal vein.
The postcava arises from the hepatic veins, and fuse
with the posterior cardinals and subcardinals.
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In adult reptiles, the
anterior parts of the
posterior cardinals
disappear, leaving the
postcava as the sole
drainage for the
subcardinals and the
kidneys.
The lateral abdominals
become connected with
the renal portal veins
when the latter usurp
the veins from the legs.
Hence they constitute a
connection between the
renal portal and the
hepatic portal systems.
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In birds, the postcava
establishes direct
connection with the
renal portal veins so
that the renal portal
circulation thus become
greatly reduced.
In mammals the anterior
part of the postcava is
formed from the hepatic
and right subcardinal;
the rear part from the
renal portal
The supracardinal veins
form the part between
the subcardinals and
the renal portal veins.
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The supracardinal 1st
appears in reptiles,
where they are the
vertebral veins of the
adult.
Their anterior parts
persist as the paired
azygos vein of
monotremes and
marsupials; the right
one as the single
azygos vein of other
mammals.
Part of the left
posterior cardinal
persists as
hemiazygos.

With these connections,
veins from the legs, tail
and adjacent regions pass
directly into the
mammalian postcava.

The renal portal system
vanishes, and the
abdominal vein loses its
function; in embryos it
persists as the allantoic or
umbilical vein. It becomes
the round ligament of the
liver in the adult.

The hepatic portal system
remains unchanged
throughout.
 The anterior cardinal veins persist in all
vertebrates as the internal jugular vein, which
after receiving other veins from the body form
the PRECAVA
 Proximal portions of the precava are the
common cardinal veins (duct of Cuvier).
 In some mammals
the left precava
joins the right
precava in front
of the heart, so
that there is one
precaval stem
 The reduced
detached base of
the left precava
remains as the
coronary sinus of
the heart wall.
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Vitelline veins from yolk sac to heart
Subintestinal vein from digestive visceral
to vitelline vein
MAMMALIAN FETAL CIRCULATION:
 In a developing fetus, blood obtains oxygen (& gives up
carbon dioxide) via the placenta, not the lungs.
 Getting oxygenated blood from the placenta back to the
heart & out to the body as quickly and efficiently as
possible involves a series of vessels & openings found
only in a mammalian fetus:
a. blood (with oxygen & nutrients acquired in placenta)
passes into umbilical vein
b. blood largely bypasses the liver via the ductus
venosus
c. blood returns to the heart & enters right atrium, but
much of the blood then bypasses the right ventricle &
enters the left atrium via the foramen ovale
d. blood that does enter the right ventricle largely
bypasses the pulmonary circulation via the ductus
arteriosus
AT BIRTH:
1. Ductus
arteriosus
closes
2. Foramen
ovale sealed
off
3. Blood no
longer flows
through
umbilical
vein