Heart Anatomy
The heart is a cone-shaped, muscular organ about the size of a clenched fist. It is located in the thorax between the .ungs, anterior to the backbone and posterior to the sternum. Its apex is tilted to the left, and about two-thirds of :be heart is located to the left of the body's midline.
Pericardial Membranes
The heart lies within a sac formed by the pericardial membranes. First is the fibrous pericardium, a :ayer of fibrous connective tissue that adheres to the blood sels at the heart's base and the sternal wall of the thorax and the diaphragm below. Next comes the parietal pericardium, a serous membrane that is separated by a small space, called the pericardial cavity, from the epicardium (visceral pericardium), another serous membrane. These serous membranes produce a liquid called the pericardial fluid, which lubricates them and reduces friction as the heart beats.
The epicardium is a part of the heart wall that also has two other layers. The myocardium is the thickest part of the heart wall and is made up of cardiac muscle. When cardiac muscle fibers contract, the heart beats. The inner endocardium includes an endothelium formed from simple squamous epithelium that not only lines the heart but also continues into and lines the blood vessels. The endothelium's smooth nature helps prevent blood from clotting unnecessarily.
Chambers of the Heart
The heart has four chambers: The right and left atria (sing. atrium) are superior to the right and left ventricles. The atria are smaller and have thinner walls than the ventricles. Internally, the atria are separated by the interatrial septum, and the ventricles are separated by the interventricular septum. Thus, the heart has a right and left side.
Right Atrium
Three large openings are located in the wall of the right atrium. The superior vena cava enters superiorly, and the inferior vena cava enters inferiorly on the posterior side). There is also an opening between the right atrium and the right ventricle that is guarded by a valve appropriately called an atrioventricular (AV) valve. This valve, like other heart valves, directs the flow of blood and prevents any backflow. This particular AV valve is also known as the tricuspid valve because it has three cusps or flaps.
Aanother opening into the right atrium is the coronary sinus, a vein that carries oxygen-poor blood from the heart wall. The coronary sinus opens into the right atrium between the inferior vena cava and the tricuspid valve.
Right Ventricle
The wall of the right ventricle contains conical extensions of myocardium called papillary muscles. Fibrous cords from the papillary muscles, called the chordae tendineae, attach to the cusps of the tricuspid valve. The chordae tendineae support the valve and prevent the cusps from inverting (turning back into the right atrium) when the right ventricle fills with blood and begins to contract.
The pulmonary trunk leaves the right ventricle. This opening has a semilunar valve with cusps that resemble half-moons. This valve, called the pulmonary semilunar valve, prevents blood from flowing back into the right ventricle.
Left Atrium
The left atrium is smaller than the right atrium but has thicker walls. Four pulmonary veins, two from each lung, enter the left atrium. The openings do not have valves. An atrioventricular valve between the left atrium and the left ventricle is called the bicuspid or mitral valve because it has two cusps.
Left Ventricle
The cavity of the left ventricle is oval-shaped, while that of the right ventricle is crescent-shaped. It appears to be smaller in size because the walls are thicker.
The papillary muscles in the left ventricle are quite large, and the chordae tendineae are thicker and stronger
than those in the right ventricle. These chordae tendineae and papillary muscles keep the bicuspid valve from inverting into the left atrium when the left ventricle contracts.
The opening by which the aorta leaves the left ventricle is closed by a semilunar valve called the aortic semilunar valve. The semilunar cusps of this valve are larger and thicker than those of the pulmonary semilunar valve. Openings just beyond the aortic semilunar valve lead to the coronary arteries, blood vessels that lie on and nourish the heart itself.
The heart has a right and left side and four chambers, consisting of two atria and two ventricles. The heart valves are the tricuspid valve, the pulmonary semilunar valve, the bicuspid valve, and the aortic semilunar valve.
