PinkMonkey Online Study Guide-Biology
The heart is enclosed in a tough, double walled membrane
called the pericardium. The outer tough layer of pericardium is
fibrous and forms a protective sheath. It is attached by ligaments to
the breastbone and spine, anchoring the heart in position. Inside the
fibrous pericardium is a thin but tough double membrane, the parietal
pericardium. The pericardial fluid between the two membranes prevents
friction, protects it from external shocks and over-distention of the
heart with blood in extreme conditions.
The heart (Figures 18.3 & 18.4) consist of four chambers,
two atria (auricles) and two ventricles.
The atria are thin walled and receive blood from four
pulmonary veins (auricles) and two ventricles. The atria are thin-walled
and receive blood from four pulmonary veins and two vena cavae.
The main tasks of the atria are
to prime (fill) the ventricles between "pumps", and to help
monitor blood pressure in the vascular system. The ventricles are thick
walled and muscular. The walls of the left ventricle are at least three
times thicker than those of the right, as it pumps blood to the whole
body through the aorta.
Two thick-walled muscular vessels arise from the
heart -- the aorta from the left side, which pumps blood
to the body, and the pulmonary artery from the right side
of the heart. The left and right coronary arteries arise from the
aorta and supply oxygenated blood to the atria and ventricles. The
coronary arteries divide several times to form a capillary network
over heart muscles. Blood is collected from the capillaries by the
coronary veins and returned to the right atrium. When defects in
coronary arteries prevent blood flow to the heart muscles, that
is when a heart attack occurs.
(B) Internal Structure of the Heart
Internally, the heart is divided into two sides
(right and left) by two muscular partitions, and these two sides
work in unison but independently as a double pump.
The auricles. The two atria or auricles
are separated by a thin inter atrial septum. The left
atrium receives oxygenated blood by four pulmonary veins, which
opens into its upper part. Their openings are without valves. The
left atrium opens into the left ventricle by atrio-ventricular
orifice (opening ), guarded by the bicuspid or mitral
The right atrium receives deoxygenated blood
from the body through the superior vena cava (opens into the upper
part), and inferior vena cava, which opens into the lower
part of the atrium. Its opening is guarded by a rudimentary valve.
A small opening of coronary sinus (vein), guarded
by a valve, is present between the orifice of the inferior vena
cava and atri-ventricular opening. The fossa ovalis is a
small oval depression above and to the left of the opening of the
inferior vena cava. The right atrium opens into the right ventricle,
and this opening is guarded by tricuspid valves to prevent
back-flow of the blood from the ventricle to the atrium.