HEART

Structure of the Heart Wall

The heart wall is composed of three layers of tissue, pericardium, myocardium and endocardium. The pericardium is the outermost layer and is made up of two sacs. The outer sac (the fibrous pericardium) consists of fibrous tissue and the inner (the serous pericardium) of a continuous double layer of serous membrane.

The fibrous pericardium is continuous with the tunica adventitia of the great blood vessels above and is adherent to the diaphragm below. Its inelastic, fibrous nature prevents overdistension of the heart. The outer layer of the serous pericardium, the parietal pericardium, lines the fibrous pericardium. The inner layer, the visceral pericardium, which is continuous with the parietal pericardium, is adherent to the heart muscle. The serous membrane consists of flattened epithelial cells. It secretes serous fluid, called pericardial fluid, into the space between the visceral and parietal layers, which allows smooth movement between them when the heart beats. The space between the parietal and visceral pericardium is only a potential space. In health the two layers lie closely together, with only the thin film of pericardial fluid between them.

The myocardium is the middle layer of the heart wall. It is composed of specialized cardiac muscle found only in the heart. It is striated, like skeletal muscle, but not under voluntary control. Each fiber has a nucleus and one or more branches. The ends of the cells and their branches are in very close contact with the ends and branches of adjacent cells. The myocardium is thickest at the apex and thins out towards the base. This reflects the amount of work each chamber contributes to the pumping of blood. It is thickest in the left ventricle, which has the greatest workload. Heart muscles are supplied by coronary circulation.
Endometrium is the innermost layer. This lines the chambers and valves of the hearts. It is a thin, smooth membrane to ensure smooth flow of blood through the heart. It consists of flattened epithelial cells, and it is continuous with the endothelium lining the blood vessels.
 

Arterial Blood Supply Of The Heart

Heart muscles (myocardium) needs blood. The heart is supplied with arterial blood by the right and left coronary arteries, which branch from the aorta immediately distal to the aortic valve. Right coronary artery consists of two arteries. They are posterior descending artery and right marginal artery. Left coronary artery consists 3 arteries. They are left circumflex artery, left marginal artery and left anterior descending artery. Right coronary artery supplies the SA node which invites the min pulses. The coronary arteries receive about 5% of the blood pumped from the heart, although the heart comprises a small proportion of body weight. This large blood supply, of which a large proportion goes to the left ventricle, highlights the importance of the heart to body function. The coronary arteries transverse the heart, eventually forming a vast network of capillaries.