It consists of a four chambered muscular heart, a network of closed branching blood vessels and blood.
Structure of the heart
The heart is mesodermal in origin. It is a thick walled, muscular and pulsating organ, situated in the mediastinum (the region in the thorax between the two lungs), and with its apex slightly turned to the left. It is of the size of a clinched fist.
The heart is covered by a double walled pericardium which consists of the outer fibrous pericardium and the inner serous pericardium- The serous pericardium is double-layered, formed of an outer parietal layer and an inner visceral layer. The parietal layer is fused with the fibrous pericardium, whereas the visceral layer adheres to the surface of the heart and forms its outer layer, the epicardium. The two layers are separated by a narrow pericardial space. that which was filled with the pericardial fluid. This can fluid reduces the friction between the two membranes and allows free movement of the heart.
The wall of the heart consists of three layers. They are the outer epicardium, the middle myocardium (a thick layer of cardiac muscles), and the inner most endocardium (a thin layer of endothelium). The endothelium covers the heart valves also and is continuous with the endothelial lining of the large blood vessels connected to the heart.
External structure Human heart has four chambers, with two relatively smaller upper chambers are called atria and the two larger lower chambers called as ventricles. Atria and ventricles are been separated by a deep transverse groove called coronary sulcus (atrio-ventricular groove). The muscular pouch like projection from each atrium is called auricular appendix (auricular appendage). The ventricles are separated by two inter
ventricular grooves (anterior and posterior), in which the coronary arteries and their branches are lodged.
Internal structure
i) Atria: Atria are thin walled 'receiving chambers' |upper chambers). The right one is larger than the left. The two atria are separated by thin inter-atrial septum. In the fetal heart, the atrial septum has a small pore called foramen ovale. Normally the foramen ovale closes at birth, when lungs become functional. It is represented by a depression in the septum between the right and left atria, called fossa ovalis (that marks the position of the foramen ovale in the fetus). If, the foramen ovale does not close properly. it was called as patent foramen ovale.
The right atrium receives deoxygenated blood from different parts of the body (except the lungs) through three caval veins viz. the two precavals (right and left) and a post caval vein. It also receives blood from the myocardium (wall of the heart) through the coronary sinus, whose opening into the right atrium is guarded by the valve of Thebesius. Opening of the postcaval vein
