The
cardiovascular system is composed of the heart, blood and vessels. (Mackenzie,2011).
The function of the cardiovascular system is to convey nutrients, oxygen,
hormones and other substances transported by blood to all cells of the body
from cells to the kidney and lungs for excretion (Ezeilo, 2002).
THE HEART
The
heart is a hollow muscular organ about the size of a fist and situated in the
middle mediastinum. It is responsible for blood circulation and pumps blood to
various part of the body to meet their nutritive requirement (Chummy 2005,
chaurasea, 2005).
The heart is placed obliquely behind
the body of the sternum and adjoining part of the costal cartilage. It has 4
chambers divided into 2 halves (right and left) by a central wall called septum.
Each
half is composed of an atrium which receives blood returning to the heart and a
ventricle that pumps blood out into the blood vessel that serve the body
(Silverthorn, 2011 and chaurasea, 2005). The heart measures about 12 x 9cm and
weighs about 300g in males and 250g in females. The heart presents borders,
surfaces, Apex, base and chambers (Chaurasea, 2005).
BORDERS OF THE HEART
1) Right
borders: Is formed by the right atrium, it extends from the right end of the
superior border above to a point on the right 6th costal cartilage
(Chaurasea, 2005).
2) Left
border: Is formed by the left ventricle and a very small part receives
contribution from the left atrium (Chummy, 2005).
3) Superior
border: Is formed by the two atria but chiefly the left atrium. It extends from
the inferior margin of the left 2 costal cartilage (Chaurasea, 2005).
4) Inferior
border: Is formed by the two ventricles, it extends from the end of the right
border to the end of the left border close to the 5th intercostal
space (Chaurasea, 2005).
SURFACES OF THE HEART
I) Inferior/Diaphragmatic
surface: Rests on the central tendon of the diaphragm. It is formed on its left
2/3rd by the left ventricle and in its right 1/3rd
by the right ventricle separated by the posterior interventricular branch of
the right coronary artery (Chaurasea, 2005 and Chummy, 2005).
2) Posterior
surface: Is also called the base of the heart and it is formed by the left
atrium, receiving 4 pulmonary veins (Chummy, 2005).
3) Anterior
surface: Is formed mainly by the right atrium and right ventricle and partly by
the left ventricle and left auricle. The top of the left auricular appendages
peeps over the top of this border (Chummy, 2005).
APEX OF THE HEART
The
apex of the heart is formed entirely by the left ventricle. It is directed
downwards forwards and to the left and is overlapped by the anterior of the
left lung. It is located in the left 5th intercostal space 9cm
lateral to the mid – sternal line medial to the mid – clavicular line
(Chaurasea, 2005).
CHAMBERS OF THE HEART
1) Right
atrium: Is the right chamber of the heart. It receives venous blood from
the whole body, pumps it to the right ventricle through the right atrio-ventricular
opening (Chaurasea, 2005).
The chamber is elongated vertically,
receiving the superior vena cava at the upper end and inferior vena cava at the
lower end. Along the right border of the atrium, there is a shallow vertical
groove which passes from superior vena cava above to the inferior vena cava
below (Chaurasea, 2005).
2) Left
atrium: Is a quadrangular chamber situated posterioly. Its appendages, left
auricle projects anteriorly to overlap the infundibulum of the right ventricle.
The left atrium form the left 2/3rd of the base of the heart, the
greater part of the upper border, parts of the sternocostal and left surface
and of the left border. It receives oxygenated blood from the lung through four
pulmonary veins and pumps it to the left ventricle through the left atrio-ventricular
(biscuspid) orifice which is guarded by the valve of the same name (Chaurasea,
2005).
3) Right
ventricle: Is a triangular chamber which receives blood from the right
atrium and pumps it to the lungs through the pulmonary trunk and pulmonary
arteries. It forms the inferior border and a large part of the sterncostal
surface of the heart. Their walls are thrown into a series of muscular ridges
called the trabeculae carnae which projects into the cavity of the ventricle.
Some of the trabeculi carnae are modified into papillary muscles and others
into modulator bands or septomarginal trabeculi (Chummy, 2005).
4) Left
Ventricle: The left ventricle receives oxygenated blood from the left
atrium and pumps it into the aorta. It forms the apex of the heart, a part of
the sternocostal surface, most of the left border and left surface, and the
left 2/3rd of the diaphragmatic surface.
The interior of the left ventricle
shows 2 drifice;
1) Left atrioventricular (Biscuspid) orifice
guarded by mitral valves.
2) Aortic orifice,
guarded by the aortic valve. The walls are 3 times thicker than those of the
right ventricle (Chaurasea, 2005).
CONTRACTION OF THE HEART
The
heart has an inherent rhythmicity (self exciting and does not depend on nerve
stimulation to contract unlike skeletal muscle. This inherent rhythmicity of
the heart is due to the presence of certain myocardial cells in well defined
areas of the heart, which can spontaneously depolarize causing action
potentials to be fired (Ezeilo, 2002).
This specialized area are the sino
atrial node (S.A node), atrioventricular (AV) node, bundle of His and purkinje fibres, all collectively
known as the special conductiong tissues of the heart (Ezeilo, 2002). The
myocardium is a specialized form of muscle consisting of individual cells
joined by electrical connections. The contraction of each cells is produced by
a rise in intracellular calcium concentration leading to spontaneous
depolarization, and as each cell is electrically connected to its neighbours,
contraction of one cell leads to a wave of depolarization and contraction
across the myocardium (Ruth et al,
2011).
This depolarization and contraction
of the hearts is controlled by a specialized group of cells localized in the
sino atrial node. The S.A node generate a rhythmical depolarization which
spread out over the atrial to the antioventricular (A.V) node. The atria then
contracts, pushing blood into the ventricle (Ruth et al, 2011).
The electrical conduction passes via
the A.V node to the bundle of His, which divides into right and left branches
and then spreads out from the base of the ventricle across the myocardium. This
leads to the contraction of the ventricle forcing blood into the pulmonary
artery (right) and aorta (left). The atria then refills as the myocardium relaxes.
The squeeze is called systole and normally lasts for about 250ms. The
relaxation period, when the atria and ventricle refills is called diastole, the
time given for diastole depends on the heart rate (Ruth et al, 2011).
DEVELOPMENT OF THE HEART
The
heart pericardium arises predominantly from the splanchnic mesoderm in the
cardiogenic region of the trilaminar embryo. The cardiogenic region can be
thought of as a bilateral fields that emerge cranially to form a horse shoe
shape fields (Sadlar, 2005).
During the 3rd week of
development (approximately day 18) angioblastic cords develop in the
cardiogenic mesoderm and canalize to form bilateral heart tube. Lateral folding
of the embryo bring the heart tubes into the ventral midline, allowing them to
fuse to form a single pericardial heart tube (Sadler, 2005). Fusion of the
heart tubes begins cranially and extends caudally and is facilitated by
apoptosis.. after fusion, constriction and dilation appears in the heart tubes
forming the following division from cranial to caudal portion..
(1) truncus arteriosus
(2) bulbus cordis
(3) Primordial ventricle
(4) Primordial atrium
(5) Sinus venosus
The
sinus venosus is divided into 2 parts: the right horn and left horn of the
sinus (Sadler, 2005 and Thaaer, 2010). The dorsal aorta develop concurrently
with the endocardial heart tubes and form a cranial connection with the
endocardial hearttubes prior to folding. As the embryo folds, the cranial ends
of the dorsal aorta are pulled ventrally until they form a dorsoventral loop
(Thaaer, 2010).
The myocardium forms from the
splanchnic mesoderm surrounding the pericardial coelom. The endocardium
develops from the endothelium of the heart tube while the epicardium develops
from the mesothelia cells arising over the myocardium (Sadler, 2005).
The straight heart tube begins to
elongate with simultaneous growth in the bulbus cordis and primitive ventricle.
This forces the heart to bend ventrally and rotate to the right, forming a
c-shaped loop with the convex side situated on the right. The ventricular end
moves caudally and the atrial outflow poles converge and myocardial cells are
added, forming the truncus arteriosus. Hence and s-shape is formed with the first
bend of the “S” being the large ventricular bend while the bend at the junction
of the atrium and sinus venosus forms the second “S” bend (Thaaer, 2010).
SEPTUM
FORMATION IN THE ATRIOVENTRICULAR CANAL
Two
endocardial cushions form the dorsal and ventral surfaces of the A.V canal (i.e.
superior and inferior cushion). The cushion fuses together, thus dividing the
common A.V canal into the right and left canals, hence partially separating the
primitive atrium and ventricle. Two smaller endocardial cushions are also formed
on the lateral walls of he AV canal, which later help to form the mitral and
tricuspid heart valve (Sadler, 2005 and Thaaer 2010).
SEPTUM
FORMATION IN THE ATRIA
Membraneous
tissue forming the septum primum grows from the roof of atrium, dividing it
into left and right halves. The space between the septum primum and the
endocardial cushion is referred to as foramen primum. Apoptosis (cell death)
occurs in the centre of the septum primum forming the foramen secundum. At this
time, the septum secundum grows immediately to the right of the septum primum
and gradually overlaps the foramen secundum during the 5th and 6th
week of development. This incomplete partition of the atrium by septum secundum
forms the foramen ovale. Blood flows from the right atrium via the foramen
ovale and foramen secundum to the left atrium, forming a right to left shunt
(Sadler, 2005 and Thaaer, 2010 ).
SEPTUM
FORMATION ON THE VENTRICULE
Minor
trabeculations appears during early development of the primordial ventricle.
However ventricular septation begins with the appearance of the primordial
muscular interventricular (I.V) ridge developing in the floor of the ventricle
near the apex. As either side of the ventricle grows and dilates, their medial
walls fuse forming the prominent intraventricular septum. The foramen located
between the cranial portion to the intraventricular septum and the endocardial
cushion(I.V foramen) closes by the end of the 7th week as the bulbar
ridges fuses with the endocardial cushion (Sadler 2005 and Thaaer, 2010 )