Chapter 18
Anatomy of the
Cardiovascular System
Location of the heart
– Lies in the mediastinum, behind the body of the sternum
between the points of attachment of ribs 2 through 6;
approximately two thirds of its mass is to the left of the
midline of the body and one third to the right
– Posteriorly the heart rests on the bodies of thoracic
vertebrae 5 through 8
– Apex lies on the diaphragm, pointing to the left
– Base lies just below the second rib
– Boundaries of the heart are clinically important as an aid
in diagnosing heart disorders
Coverings of the heart
– Structure of the heart coverings
• Pericardium
– Fibrous pericardium—tough, loose-fitting inextensible
sac
– Serous pericardium—parietal layer lies inside fibrous
pericardium, and visceral layer (epicardium) adheres
to outside of the heart; pericardial space with
pericardial fluid separates the two layers
– Function of the heart coverings—provides
protection against friction
Structure of the heart
– Wall of the heart—made up of three distinct
layers:
• Epicardium—outer layer of heart wall
• Myocardium—thick, contractile middle layer of heart
wall; compresses the heart cavities, and the blood
within them, with great force
• Endocardium—delicate inner layer of endothelial
tissue
Structure of the heart (cont.)
– Chambers of the heart—divided into four cavities with the right and
left chambers separated by the septum:
• Atria
– Two superior chambers, known as “receiving chambers,” because they
receive blood from veins
– Atria alternately contract and relax to receive blood and then push it into
ventricles
– Myocardial wall of each atrium is not very thick, because little pressure is
needed to move blood such a small distance
– Auricle—earlike flap protruding from each atrium
• Ventricles
– Two lower chambers, known as “pumping chambers,” because they push
blood into the large network of vessels
– Ventricular myocardium is thicker than myocardium of the atria, because
great force must be generated to pump blood a large distance;
myocardium of left ventricle is thicker than the right, because it must push
blood much further (throughout body, not just to lungs)
Structure of the heart (cont.)
– Valves of the heart—mechanical devices that
permit the flow of blood in one direction only
• Atrioventricular (AV) valves—prevent blood from
flowing back into the atria from the ventricles when
the ventricles contract
– Tricuspid valve (right AV valve)—guards the right
atrioventricular orifice; free edges of three flaps of
endocardium are attached to papillary muscles by
chordae tendineae
– Bicuspid, or mitral, valve (left AV valve)—similar in
structure to tricuspid valve except only two flaps present
Structure of the heart (cont.)
• Valves of the heart (cont.)
• Semilunar (SL) valves—half moon–shaped flaps
growing out from the lining of the pulmonary artery
and aorta; prevent blood from flowing back into
ventricles from aorta and pulmonary artery
– Pulmonary semilunar valve—at entrance of pulmonary
artery
– Aortic semilunar valve—at entrance of aorta
Structure of the heart (cont.)
– Blood supply of heart tissue
• Coronary arteries—myocardial cells receive blood from
right and left coronary arteries
– First branches to come off aorta
– Ventricles receive blood from branches of both right and
left coronary arteries
– Each ventricle receives blood only from a small branch of
corresponding coronary artery
– Most abundant blood supply goes to myocardium of left
ventricle
– The right coronary artery is dominant in approximately 50%
of all hearts and the left in about 20%; in approximately
30%, neither coronary artery is dominant
– Few anastomoses exist between the larger branches of the
coronary arteries
Structure of the heart (cont.)
– Blood supply of heart tissue (cont.)
• Veins of the coronary circulation
– As a rule, veins follow a course that closely parallels that
of coronary arteries
– After going through cardiac veins, blood enters coronary
sinus to drain into right atrium
– Several veins drain directly into right atrium (superior and
inferior vena cava)
Structure of the heart (cont.)
– Conduction system of the heart—comprising the
sinoatrial (SA) node, atrioventricular (AV) node, AV
bundle, and Purkinje fibers; made up of modified cardiac
muscle (Figure 18-11)
• Sinoatrial node (SA node or pacemaker)— hundreds of cells in
right atrial wall near opening of superior vena cava
• Atrioventricular node (AV node)—small mass of special cardiac
muscle in right atrium along lower part of interatrial septum
• Atrioventricular bundle (AV bundle or bundle of His) and
Purkinje fibers
– AV bundle originates in AV node, extends by two branches down
the two sides of the interventricular septum, and continues as
Purkinje fibers
– Purkinje fibers extend out to papillary muscles and lateral walls of
ventricles
Structure of the heart (cont.)
– Nerve supply of the heart
• Cardiac plexuses—located near arch of aorta,
made up of the combination of sympathetic and
parasympathetic fibers
• Fibers from cardiac plexus accompany right
and left coronary arteries to enter the heart
• Most fibers end in the SA node, but some end
in the AV node and in the atrial myocardium
• Sympathetic nerves—accelerator nerves
• Vagus fibers—inhibitory, or depressor, nerves
Blood Vessels
• Types of blood vessels
– Arteries
• Carry blood away from heart—all arteries
except pulmonary artery carry oxygenated
blood
• Elastic arteries—largest in body
– Examples: aorta and its major branches
– Able to stretch without injury
– Accommodate surge of blood when heart contracts
and able to recoil when ventricles relax
Blood Vessels
– Arteries (cont.)
• Muscular (distributing) arteries
– Smaller in diameter than elastic arteries
– Muscular layer is thick
– Examples: brachial, gastric, superior mesenteric
• Arterioles (resistance vessels)
– Smallest arteries
– Important in regulating blood flow to end-organs
• Metarterioles
– Short connecting vessel between true arteriole and 20 to 100
capillaries
– Encircled by precapillary sphincters
– Distal end called thoroughfare channel, which is free
of precapillary sphincters
Blood Vessels
• Types of blood vessels (cont.)
– Capillaries—primary exchange vessels
• Microscopic vessels
• Carry blood from arterioles to venules—together,
arterioles, capillaries and venules constitute the
microcirculation
• Not evenly distributed—highest numbers in tissues
with high metabolic rate; may be absent in some
“avascular” tissues such as cartilage
Blood Vessels
• Types of capillaries
– True capillaries—receive blood flowing from metarteriole with
input regulated by precapillary sphincters
– Continuous capillaries
» Continuous lining of endothelial cells
» Openings called intercellular clefts exist between adjacent
endothelial cells
– Fenestrated capillaries
» Have both intercellular clefts and “holes” or fenestrations
through plasma membrane to facilitate exchange functions
– Sinusoids
» Large lumen and tortuous course
» Absent or incomplete basement membrane
» Very porous—permit migration of cells into or out of vessel
lumen
Blood Vessels
• Types of blood vessels (cont.)
– Veins
• Carry blood toward the heart
• Act as collectors and as reservoir vessels;
called capacitance vessels
Blood Vessels
• Structure of blood vessels
– Layers
• Tunica adventitia—found in arteries and veins
• Tunica media—found in arteries and veins
• Tunica intima—found in all blood vessels; only
layer present in capillaries
Blood Vessels
• Structure of blood vessels (cont.)
– “Building blocks” commonly present
• Lining endothelial cells
– Only lining found in capillary
– Line entire vascular tree
– Provide a smooth luminal surface—protects against
intravascular coagulation
– Intercellular clefts, cytoplasmic pores, and
fenestrations allow exchange to occur between blood
and tissue fluid
– Capable of secreting a number of substances
– Capable of reproduction
Blood Vessels
– “Building blocks” commonly present (cont.)
• Collagen fibers
–
–
–
–
Exhibit woven appearance
Formed from protein molecules that aggregate into fibers
Visible with light microscope
Have only a limited ability to stretch (2% to 3%) under
physiological conditions
– Function to strengthen and keep lumen of vessel open
Blood Vessels
– “Building blocks” commonly present (cont.)
• Elastic fibers
– Composed of insoluble protein called elastin
– Form highly elastic networks
– Fibers can stretch over 100% under physiological
conditions
– Play important role in creating passive tension to help
regulate blood pressure throughout cardiac cycle
• Smooth muscle fibers
– Present in all segments of vascular system except
capillaries
– Most numerous in elastic and muscular arteries
– Exert active tension in vessels when contracting
Major Blood Vessels
• Circulatory routes
– Systemic circulation—blood flows from the left
ventricle of the heart through blood vessels to
all parts of the body (except gas exchange
tissues of lungs) and back to right atrium
– Pulmonary circulation—venous blood moves
from right atrium to right ventricle to pulmonary
artery to lung arterioles and capillaries where
gases are exchanged; oxygenated blood
returns to left atrium via pulmonary veins; from
left atrium, blood enters left ventricle
Major Blood Vessels
• Systemic circulation
– Systemic arteries
• Main arteries give off branches, which continue to
rebranch, forming arterioles and then capillaries
• End-arteries—arteries that eventually diverge into
capillaries
• Arterial anastomosis—arteries that open into other
branches of the same or other arteries; incidence of
arterial anastomoses increases as distance from the
heart increases
• Arteriovenous anastomoses or shunts occur when
blood flows from an artery directly into a vein
Major Blood Vessels
• Systemic circulation (cont.)
– Systemic veins
• Veins are the ultimate extensions of capillaries; unite into vessels
of increasing size to form venules and then veins
• Large veins of the cranial cavity are called dural sinuses
• Veins anastomose as do arteries
• Venous blood from the head, neck, upper extremities, and thoracic
cavity (except lungs) drains into superior vena cava
• Venous blood from thoracic organs drains directly into superior vena
cava or azygos vein
• Hepatic portal circulation
– Veins from the spleen, stomach, pancreas, gallbladder, and intestines send
their blood to the liver via the hepatic portal vein
– In the liver, venous blood mingles with arterial blood in the capillaries and is
eventually drained from liver by hepatic veins that join the inferior vena cava
• Venous blood from lower extremities and abdomen drains into inferior
vena cava
Major Blood Vessels
• Fetal circulation
– Basic plan of fetal circulation—additional vessels needed to allow fetal
blood to secure oxygen and nutrients from maternal blood at the
placenta
(Figure 18-31)
• Two umbilical arteries—extensions of internal iliac arteries; carry fetal
blood to placenta
• Placenta—attached to uterine wall, where exchange of oxygen and other
substances between the separated maternal and fetal blood occurs
(Figure 18-30)
• Umbilical vein—returns oxygenated blood from placenta to fetus; enters
body through umbilicus and goes to undersurface of liver, where it gives
off two or three branches and then continues as ductus venosus
• Ductus venosus—continuation of umbilical vein, drains into inferior vena
cava
• Foramen ovale—opening in septum between right and left atria
• Ductus arteriosus—small vessel connecting pulmonary artery with
descending thoracic aorta
Major Blood Vessels
• Fetal circulation (cont.)
– Changes in circulation at birth
(compare Figures 18-31 and 18-32)
• When umbilical cord is cut, the two umbilical arteries, the
placenta and the umbilical vein no longer function
• Umbilical vein within the baby’s body becomes the round
ligament of the liver
• Ductus venosus becomes the ligamentum venosum of the liver
• Foramen ovale—functionally closed shortly after a newborn’s
first breath and pulmonary circulation is established; structural
closure takes approximately 9 months
• Ductus arteriosus—contracts with establishment of respiration,
becomes ligamentum arteriosum
Cycle of Life:
Cardiovascular Anatomy
• Birth—change from placenta-dependent system
• Heart and blood vessels maintain basic structure and function
from childhood through adulthood
– Exercise thickens myocardium and increases supply of
blood vessels in skeletal muscle tissue
• Adulthood through later adulthood—degenerative changes
– Atherosclerosis—blockage or weakening of critical arteries
– Heart valves and myocardial tissue degenerate—reduces
pumping efficiency