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- The sinoatrial (SA) node, also known as the pace-
maker, initiates each cardiac cycle, and is found in
the superior wall of the right atrium. It spreads^
electrical impulses over both atria, causing them to
contract simultaneously while depolarizing the AV
node.^
- The atrioventricular (AV) node, in the lower part
of the right atrium, sends electrical impulses^
through the atrioventricular bundle or bundle of His to
the top of the interventricular septum.^
- The bundle of His branches down both sides of the
septum as the right and left bundle branches,
distributing the electrical impulses over the medial
surface of the ventricles.^
- Purkinje’s fibers emerge from the bundle branches
and distribute the impulses to the cells of the
myocardium- of the ventricle causing actual
contraction.
A Cardiac Cycle.
- In a cardiac cycle, the two atria contract simultane-
ously while the two ventricles relax, and the two
ventricles contract simultaneously while the two atria
relax.^
- The phase of contraction is called systole, and the
phase of relaxation is called diastole.^
- An average cardiac cycle takes 0.8 second: during-
the first 0.1 second, the atria contract and the^
ventricles relax and the atrioventricular valves are
open and the semilunars are closed; for the next 0.3
second, the atria relax while the ventricles-^
contract and all valves are closed at first and then
the semilunars open; the last 0.4 second is the
relaxation/quiescent period, during the first part of
which all valves are closed and then the
atrioventricular- valves open to start blood
draininginto theventricles.
Some Major Blood Circulatory Routes.
Routes
- Systemic circulation includes all the oxygenated blood
that leaves the left ventricle through the aortic
semilunar valve to the aorta and all the deoxygen-ated
blood that returns to the right atrium via the superior
and inferior venae cavae.^
- Systemic circulation has many subdivisions. Two
are the coronary circulation that goes to the heart
and the hepatic portal circulation that travels be-
tween the intestine and the liver.
Chapter 14- Pulmonary circulation includes the deoxygenated
blood that leaves the right ventricle through the
pulmonary semilunar valve to the pulmonary trunk,
which branches and goes to the lungs. In the lungs,
carbon dioxide gas is released and oxygen gas is
picked up to return to the left atrium via the four
pulmonary veins.^
4.^ Cerebral circulation is the route in the brain.^ - Fetal circulation exists only between the developing
fetus and its mother.
The Anatomy of Blood Vessels- Arteries and veins have walls made of three layers: the
innermost, tunica intima, made of a single layer of
endothelial cells; the middle, tunica media, made of
smooth muscle; and the outer, tunica adventitia, made
of white fibrous connective tissue.^
2.^ The cavity of blood vessels is called the lumen.^ - Arteries are thicker and stronger than veins. They
are elastic and can contract.^ - Arterioles are small arteries that deliver blood to
capillaries.^ - Capillaries are microscopic vessels made of a
single layer of endothelial cells with their basement^
membrane. They connect arterioles with venules and
because of their structure, allow the exchange of gases,
nutrients, and waste between blood and tissue^ cells.^ - Venules are small vessels that connect capillaries to
veins.^ - Veins have less elastic and smooth muscle than
arteries- but have more fibrous connective tissue.
They also have internal valves to ensure blood flow
in one direction.^ - Venous sinuses are veins with thin walls.
Major Arteries and Veins- The aorta is the largest artery of the body. It has
numerous branches named either according to the
region of the body or organ it goes to or according to
the bone its branch may^ follow.^ - Most of the arteries of the body are in deep and pro-
tected areas of the body.^ - Veins are found closer to the surface of the body,
and many can be seen superficially. Many of their
names are identical to the arteries.^ - The veins of the body converge with either the supe-
rior or inferior vena cava, the two largest veins of the
body, which empty into the right atrium of the heart.