A&P Ch. 11
The Circulatory System
The circulatory system has two major subdivisions,
-Cardiovascular system
-Lymphatic system
The Cardiovascular System
The Heart
 The heart, in one day pushes the body’s supply of blood (6 quarts or so) through
the blood vessels over 1000 times. Pumping about 6000 quarts of blood a day.
 The heart is approximately the size of a person’s fist, and weighs less than a
pound. It is located within the bony thorax and is flanked by the lungs.
 Its more pointed apex is directed toward the left hip and rests on the diaphragm,
at the 5th intercostal space.
 The broader base points toward the right shoulder and lies beneath the second rib,
the great vessels of the body emerge from the base.
 The heart is composed of 3 layers:
 Epicardium – (epi- outer) thin serous membrane that acts
as a sac enclosing the heart.
 Myocardium – (myo- middle) thick cardiac muscle twisted
and whorled into ring-like arrangements. This is the
layer that actually contracts.
 Endocardium – (endo- inner) sheet of endothelium that
lines the heart chamber and blood vessels. It decreases
friction, and help the blood flow smoothly through the
heart.
 The heart is enclosed in a double layer sac of serous membranes called the
Pericardium.
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 The pericardium is composed of the epicardium, which also is
considered to be the outer layer of the heart itself; and the parietal
pericardium, which is a dense connective tissue that protects and
anchors the heart to surrounding structures, to prevent jarring.
Chambers and Vessels of the Heart
 The heart has 4 chambers –2 atria and 2 ventricles
 Atria The superior atria are mainly receiving chambers. Blood flows into the
atria under low pressure fr4om the veins of the body and then continues on
to fill the ventricles below.
 Ventricles The inferior, thick-walled chambers are the discharging chambers, or the
actual pumps of the heart.
 The heart functions as a double pump.
 The right side works as the pulmonary circuit pump. ( Pulmonary Circulation) It
receives oxygen-poor blood from the veins of the body through the large superior
and inferior venae cavae and pumps it out through the pulmonary trunk.
 The pulmonary trunk splits into the right and left pulmonary arteries,
which carry blood to the lungs.
 Oxygen-rich blood drains from the lungs and is returned to the left side of
the heart through the 4 pulmonary veins.
 Blood returned to the left side of the heart is pumped out of the heart into the
aorta, from which the systemic arteries branch to supply all body tissue.
 The circulation from the left side of the heart through the body tissues and back
to the right side of the heart is called Systemic Circulation.
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 The heart is equipped with 4 valves that allow blood to flow in only one direction
through the heart chambers.
 The AV (atrioventricular) valve separates the atria and ventricle chambers.
 The semilunar valves control the flow of the two large arteries leaving the
heart.
 The blood supply that oxygenates and nourishes the heart it’s self is provided by
the right and left coronary arteries, which branch from the aorta and run down
the atrioventricular groove between the atria and ventricle.
Conduction and Cardiac Cycle
 Cardiac muscle cells can and do contract spontaneously and independently, even if
all nervous connections are severed.
 2 types of controlling systems act to regulate heart activity.
 One involves the nerves of the autonomic nervous system that act like brakes
and accelerators.
 The second is the intrinsic conduction system, or nodal system.
 The most important part of the system this called the SA (sinoatrial)
node. The SA node is often called the pacemaker.
 The nodal system causes heart muscle to depolarize in only one direction,
from the atria to the ventricles.
 It contains specialized tissue only found in this part of the body which
actually starts each heart beat and sets the pace for the whole heart.
 The cardiac cycle refers to the events of one complete heartbeat, during which
both atria and ventricles contract and then relax.
 Systole- means heart contraction.
 Diastole- means heart relaxation. This is usually referring to the ventricles.
 The detailed cardiac cycle is on pg. 321-322.
Cardiac Output / Blood flow
 Cardiac Output is the amount of blood pumped out by each side of the heart in
each minute. It is the product of the heart rate (HR) and the stroke volume (SV).
 Stroke volume is the volume of blood pumped out by a ventricle with each
heartbeat.
 Cardiac output varies with the demands of the body.
 As the heart beats, blood is propelled into the large Arteries leaving the heart. It
moves into the successively smaller and smaller arteries and then into the
arterioles, which feed the capillary beds.
 From the capillary beds within the tissue the blood is drained by venules, which
empty into veins and eventually empty into the great veins entering the heart.
Anatomy of a vessel
 Except for microscopic capillaries, the walls of blood vessels have three coats, or
tunics.
 Tunica intima- interior of the vessels, forms a slick surface that decreases
friction as blood flows through the vessel lumen.
 Tunica media- middle coat, smooth muscle, is active in changing the
diameter of the vessels.
 Tunica externa- outermost tunic, support and protect the vessels.
 Walls of arteries are usually much thicker than the walls of veins, especially the
tunica media.
 Arteries are closer to the pumping action of the heart and must be able to expand
when the blood is forced into them.
 Blood leaving the heart is refered to as Cardiac Output and blood entering is
referred to as venous return.