Cardio Vascular System: Structue Of Human Heart

STRUCTURE OF HUMAN HEART It is a blunt conical organ of about 12 cm long and 9 cm broad, Its narrow apex is pointed downward and to the left. Heart is enclosed in a double walled sac called Pericardium outer membrane is parietal pericardial membrane (fibrous) and inner one is visceral pericardial membrane (serous). In between the two layers is a fluid-filled space, the fluid is called pericardial fluid & it prevents any friction between the heart walls and the surrounding tissues, protection from shock and mechanical injury. Heart-wall consists of three layers–epicardium (outermost), Myocardium (middle one) and endocardium (inner most). Among these three layers, myocardium consists of cardiac muscles, resembling the striated muscles structurally and smooth muscles functionally because of involuntary muscle. The human heart consists of four chambers two auricles (atria) and two ventricles. The auricles are located at the broader end and the ventricles are towards the posterior narrow end. There is an external transverse groove between the auricles and ventricles, it is called coronary sulcus.

Internally, the two (right and left) auricles are separated by an inter atrial septum. It has an oval depression called fossa ovalis, which is the remnant of foramen ovale of foetal heart. The two ventricles are separated by inter-ventricular septum. The atria and ventricles are separated by auriculo-ventricular septum (Av septum). It has an opening between the right auricle and right ventricle, guarded by tricuspid valve.

The opening in the A-v septum between the left auricle and left ventricle is guarded by biscuspid or mitral valves. The pointed ends of the flaps of A.V. valve are attached to the ventricular wall by chordae tendinae. The right auricle receives inferior and superior vena cava and the left auricle receives two pairs of pulmonary veins. The two major arteries leaving the heart are pulmonary artery (from right ventricle) and aorta from left ventricles) their openings are guarded by semilunar valves. The wall of the heart is made of cardiac muscle fibres which branch & connect with one another forming a network, this facilitates quick conduction of the cardiac impulse. Coronary arteries & vein supply blood to the cardiac muscles.

CONDUCTING SYSTEM OF HEART In mammals, the heart is myogenic i.e. the cardiac impulse is initiated by the cardiac muscle fibres. The cardiac impulse is initiated by the sino-atrial node (SA- node), situated in the upper lateral wall of the right atrium. The fibres of SA-node show the ability of self-excitation and by determining the rate of discharge of cardiac impulse, it determines the heart rate (hence called pacemaker). The SA node is in close association with the muscles of auricles & conducts the impulse to auricles. The impulse is conducted to the ventricles through the auriculo-ventricular node (Av-node) located in the posterior part of interatrial septum. From this node, the AV bundle (Bundle of His) arises and immediately branches in a right and a left bundle, that enter the wall of right & left ventricles respectively. These two bundle branches produce many branches called Purkinje fibres throughout the ventricular wall and conduct the impulse to the muscles of ventricles.

CARDIAC CYCLE The contraction (systole) and relaxation (diastole) of auricles & ventricles alternately resulting in one heart beat constitute a cardiac cycle. When the auricles contract (auricular systole) the ventricles relax (ventricular diastole). When the ventricles contract (ventricular systole), the auricle relax (auricular diastole). For a very brief period, both auricle and ventricles are in their diastole & relax simultaneously, which is called as joint diastole. During systole of a cardiac chamber, blood is pumped out from there and during diastole of a cardiac chamber, blood enters that chamber.