Heart the mesodermally derived organ, is situated in the thoracic cavity in between the lungs, slightly tilted to the left. Heart has size of clenched fist, weight 300 gm. Its triangular superior-broad portion is tilted slightly towards right (dorsal) side. Its lower narrow portion is tilted towards left side.
Heart is protected by a double layered bag called Pericardium. The narrow space in between these two membranes is called Pericardial cavity in which pericardial fluid is present. Pericardial fluid provides moisture to heart and reduces friction.
(i) Epicardium : Outermost layer, made up of simple squamous epithelium.
(ii) Myocardium : Middle layer, thickest, made up of cardiac muscles which are striated but involuntary.
(iii) Endocardium : Innermost layer, made up of simple squamous epithelium.
Thickness of wall of Heart depends on Myocardium.
The heart of human is four chambered. Two relatively small, upper chambers called atria and two larger, lower chambers called ventricles.
The atrium and the ventricle of the same side are separated by a thick fibrous tissue called the atrio-ventricular septum.
The right and left atria are separated by a thin muscular wall called Inter atrial septum which is shifted slighty towards left. So, right atrium is slightly bigger than left atrium.
Ventricular part is broad, muscular and of light colour. Ventricles have thicker walls than auricles.
The septa which divide the two ventricles is termed as Inter-ventricular septum.
Left ventricle is more muscular and thick walled than right because it has to pump blood into those arteries which take blood throughout the body while right ventricle has to pump blood only to the lungs.
Left ventricle is the largest chamber of heart.
Right Atrium: Receives opening of S.V.C., I.V.C. and one opening of coronary sinus in human. (SVC = superior vena cava; IVC= inferior vena cava). The SVC & IVC bring deoxygenated blood from the upper and lower body parts respectively. The Coronary sinus receives deoxygenated blood from the right & left coronary veins and drains it in the right atrium.
Right Ventricle: Receives deoxygenated blood through right AV foramen from right atrium. Drains the deoxygenated blood into pulmonary artery through which it reaches lungs for oxygenation.
Left atrium : Receives oxygenated blood from lungs via pulmonary veins.
This oxygenated blood is drained into left ventricle through left AV foramen.
In humans four pulmonary veins open into left atrium through separate openings.
Left Ventricle : Drains oxygenated blood into the Aorta from where it is supplied to systemic organs .
Atrium : The inner wall surface here presents a series of transverse muscular ridges called musculi pectinati. They run forward and downward towards AV foramen, giving appearance of the teeth of a comb (combed muscles).
Ventricles : The inner wall is rough due to presence of muscular ridges trabeculae carnae or columnae carnae. These continue as papillary muscles, whose one end is attached to the ventricular wall and the other end connected to the cusps of AV valves by chordae tendinae. These chordae tendinae are collagenous and inelastic chords, one end of which is inserted in the papillary muscles and other end is connected to the flaps of AV valves. These are meant for preventing the pushing of flaps into atrium during ventricular contraction.
Right Atrium : Superior vena cava, inferior vena cava and coronary sinus open in right atrium. The opening of IVC is guarded by a valve called Eustachian valve (during embryonic life the valve guides the inferior vena caval blood to the left atrium through foramen ovale). The opening of coronary sinus in right atrium is guarded by Thebesian valve.
Left Atrium : At its inlet is pulmonary vein (four veins in man and two in rabbit), these have no guarding valve.
AV foramen : The right AV foramen has a unidirectional valve called tricuspid valve (made of three flaps or cusps) which allows entry of blood from right atrium to right ventricle and prevents its backflow. The unidirectional valve present on left AV foramen is made of two cusps only, hence called bicuspid valve (also called as the Mitral valve).
Right Ventricle : Its outlet is in the pulmonary artery. It is guarded by a pulmonary semilunar valve.
Left Ventricle : Its outlet is in the systemic aorta. This opening is guarded by an aortic semilunar valve. Both of these semilunar valves are made of three cusps each and are unidirectional in nature.
The valves in the heart allow the flow of blood only in one direction from atria to the ventricles and from the ventricles to the pulmonary artery or aorta. These valves prevent any backward flow.
The oxygenated blood is supplied to the heart musculature for its consumption with the help of two coronary arteries, left and right. These arteries arise from the common origin at arch of aorta. The left and right coronary arteries then further subdivide into a number of branches carrying blood to different regions of heart. The deoxygenated blood from heart walls return back via coronary veins which drain into the coronary sinus. The coronary sinus opens in the right atrium.
(Session 2025 - 26)