The Cardiovascular System at a Glance
The heart is positioned between the lungs and the mediastinum with about two-thirds of its bulk on the left side of the center of the body. The heart has a blunt cone shape and is about the size of a closed fist. The heart is encased in a loose fitting membrane called the pericardial sac that is made up of two layers. The outer layer of the pericardial sac is called the fibrous pericardium and is made up of tough fibrous connective tissue. It connects to the large vessels that enter and leave the heart, to the diaphragm muscle and to the inside of the sternal walls of the thorax. It also anchors the heart in the mediastinum. The inner layer of the pericardial sac is the serous pericardium. This is a thin and delicate membrane.
The heart itself has three layers called the epicardium, the myocardium and the endocardium. The epicardium is the outermost layer and is thin and transparent. The largest layer of the heart is the myocardium and is the layer of the cardiac muscle tissue. The myocardium is responsible for heart contractions. The inner layer is the endocardium and is also a thin layer. It acts as a covering for the myocardium and covers the heart valves and the chordae tendineae of the valves.
The heart is also composed of four chambers; the right and left atrium are the upper chambers of the heart and the right and left ventricles are the lower chambers of the heart. The right atrium receives blood from all parts of the body except the lungs. There are three veins that bring the blood into the right atrium. The three veins are known as the superior vena cava, the inferior vena cava, and the coronary sinus. The superior vena cava brings blood from the upper parts of the body. The inferior vena cava brings blood from the lower parts of the body. The coronary sinus drains the blood from most of the vessels that supply the blood to the heart. The blood is then squeezed from the right atrium into the right ventricle. The blood is then pumped by the right ventricle into a major vessel called the pulmonary trunk. The pulmonary trunk splits into the right pulmonary artery and the left pulmonary artery. These arteries carry blood to the lungs. Once this blood is in the lungs it releases carbon dioxide and picks up oxygen. The blood then returns to the heart by four pulmonary veins and empties into the left atrium where it is squeezed into the left ventricle. The left ventricle pumps the blood into the ascending aorta and the aortic blood goes to the coronary arteries, the arch of the aorta, and the descending thoracic aorta. The descending thoracic aorta becomes the abdominal aorta.
The valves of the heart are designed to prevent blood from flowing back into the pumping chamber. There are two atrioventricular valves between the atria and their ventricles. The tricuspid valve is located between the right atrium and the right ventircle and it contains three flaps or cusps, hence it’s name tricuspid. Between the left atrium and the left ventricle is the bicuspid valve also known as the mitral valve. The bicuspid valve is the only valve in the heart that contains two cusps.
The two arteries that leave the heart also contain valves that keep blood from flowing back into the heart called the semilunar valves. The pulmonary semilunar valve is at the opening where the pulmonary trunk exits the right ventricle. The aortic semilunar valve is at the opening where the ascending aorta leaves the left ventricle.
The heart is enervated by the autonomic nervous system, however it does not initiate a contraction but only increases or decreases the time it takes to complete a cardiac cycle. This is posible because the heart has its own intrinsic regulating systme called the conduction system. This system begins at the sinoatrial node which initiates the cardiac cycle and sets the pace for the heart rate. The sinoatrial node is the hearts pace-maker. It can be modified by nerve impulses from from the autonomic nervous system by having sympathetic impulses speeding it up and parasympathetic impulses slowing it down. Thyroid hormone and epinephrine carried by the blood will also effect the sinoatrial node. Once the electrical impulse is initiated by the sinoatrial node it is spread out over both the artia causing them to contract simultaneously. At the same time it depolarizes the atrioventricular node that is located in the lower part of the right atrium. From the atrioventricular node, a tract of conducting fibers known as the atrioventricular bundle runs through the cardiac mass to the top of the interventricular septum where it branches. This continues down both sides of the septum as the right and left bundle of branches. The bundle distributes the electrical charge over the medial surfaces of the ventricles. The contraction of the ventricles is stimulated by the Purkinje’s fibers that emerge from the bundle branches and pass into the cells of the myocardium of the ventricles.
During a cardiac cycle the two atria contract simultaneously while the two ventricles relax. Then when the two ventricles contract the two atria relax. Systole is the phase of contraction and diastole is the phase of relaxation. A complete cardiac cycle consists of the systole and diastole of both atria and the systole and diastole of both ventricles. When we listen to a heartbeat the first thump is the contraction and the second thump is the contraction of the ventricles, then there is a period of no sound.
As you can see the heart is a very complex organ with numerous things that need to work in sync for the whole thing to work correctly.
