The heart is a muscular organ enclosed in a fi brous sac, the pericardium, and located in the chest The
inner layer of the pericardium is closely affi xed to the heartand is called the epicardium.
The extremely narrow space between the outer wall of the pericardium and the epicardium is filled with a watery fluid that serves as a lubricant as the heart moves within the sac. The walls of the heart, the myocardium, are composed primarily of cardiac muscle cells. The inner surface of the cardiac chambers, as well as the inner wall of all blood vessels, is lined by a thin layer of cells known as endothelial cells,
anatomy of the heart
As noted earlier, the human heart is divided into right and left halves, each consisting of an atrium and a
ventricle.The two ventricles are separated by a muscular wall, the interventricular septum. Located between the atrium and ventricle in each half of the heart are the atrioventricular (AV) valves, which permit blood to
flow from atrium to ventricle but not from ventricle to atrium. The right AV valve is called the tricuspid valve
because it has three fibrous flaps, or cusps .
anatomy of the heart
The left AV valve has two fl aps and is thus called the bicuspid valve.
(Its resemblance to a bishop’s headgear has earned the left AV valve another commonly used name, mitral valve.)
The opening and closing of the AV valves is a passive process resulting from pressure differences across the valves.When the blood pressure in an atrium is greater than in the ventricle, the valve is pushed open and blood fl ows from atrium to ventricle.
In contrast, when a contracting ventricle achieves an internal pressure greater than that in its connected atrium, the AV valve between them is forced closed.
Therefore, blood does not normally move back into the atria, but is forced into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle.
To prevent the AV valves from being pushed up into the atria (a condition called prolapse), the valves are fastened to muscular projections (papillary muscles) of the ventricular walls by fibrous strands (chordae tendineae).The papillary muscles do not open or close the valves. They act only to limit the valves’ movements and prevent them from being everted.
The openings of the right ventricle into the pulmonary trunk and of the left ventricle into the aorta also contain valves, the pulmonary and aortic valves, respectively .
These valves are also referred to as the semilunar valves, due to the half-moon shape of the cusps.
These valves permit blood to flow into the arteries during ventricular contraction but prevent blood from moving in the opposite direction during ventricular relaxation. Like the AV valves,they act in a purely passive manner. Whether they are open or closed depends upon the pressure differences across them.
Vessels supplying the heart with oxygenated blood originate from behind the cusps of the aortic valve.
Another important point concerning the heart valves is that, when open, they offer very little resistance to
flow.Consequently, very small pressure differences across them suffifice to produce large flows.
In disease states, however, a valve may become narrowed so that it offers a high resistance to flow even when open. In such a state, the contracting cardiac must produce an unusually high pressure to cause
flow across the valve.
There are no valves at the entrances of the superior and inferior venae cavae (plural of vena cava) into the right atrium, and of the pulmonary veins into the left atrium.
However, atrial contraction pumps very little blood back into the veins because atrial contraction constricts their sites of entry into the atria, greatly increasing the resistance to backflow.
(Actually, a little blood is ejected back into the veins, and this accounts for the venous pulse that can often be seen in the neck veins when the atria are contracting).
this Figure summarizes the path of blood flow through the entire cardiovascular system.
this video explains the anatomy of the heart
