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 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).

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).

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).
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).

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).

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).

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).

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).

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 ).

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 )
Share on Google Plus


The publications and/or documents on this website are provided for general information purposes only. Your use of any of these sample documents is subjected to your own decision NB: Join our Social Media Network on Google Plus | Facebook | Twitter | Linkedin