INTRODUCTION Essays (4543 words) - Blood Pressure, Angiology

INTRODUCTION Cardiac Location and Structures The heart is the driving force of the circulatory system, contracting about 70 times/minute to pump an adequate volume of blood with sufficient pressure to perfuse all body organs and tissues. The muscular organ, about the size of a clenched fist, weights from 300 to 400 g. It is located within the mediastinum of the thoratic cavity, above the diaphragm and between the lungs. This location subjects the heart's activity to influence from all pressure variances during respiration, Fassler, (1991). Intrathoracic pressure varies with the respiratory cycle. On inspiration, the heart moves slightly vertically, and the increased negative pressure generated in the thoracic cavity increases venous blood return to the heart and pulmonary blood flow. On respiration, the heart moves slightly horizontally as the diaphragm rises, and a decreased negative pressure is generated. The pericardial sac is a fibrous membrane that doubles over onto itself to form two surfaces. A small amount of pericardial fluid in the sac allows the two surfaces to slide over each other without friction as the heart beats. The pericardium performs several functions. First, it provides shock-absorbing protection. Second, it acts as a protective barrier against bacterial invasion from the lungs. Third, because of its fibrous nature, it protects the heart from sudden overdistention and increase in size, Fassler, (1991). The heart has three tissue layers: the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer). The epicardium is the thin inner layer of the pericardium. The myocardium, thickest of the three layers, is composed of muscle fibers that contract, creating the pumping effect of cardiac activity. The endocardium, a smooth, membranous layer that lines all cardiac chambers and valve leaflets, is continuous with the intima, or lining, of the aorta and arteries, Fassler, (1991). The heart's four chambers ? the right and left atria and left atria and the right and left ventricles ? are separated by the interatrial and interventricular septa. The atria are thin-walled, low-pressure chambers that serve primarily as reservoirs for blood flow into the ventricles. The ventricles are formed by muscle fibers that contract to eject blood to the pulmonary vasculature (right) and systemic circulation (left). Because the left ventricle must achieve the high pressure needed for systemic circulation, it is much thicker than the right ventricle, (Fig. #1), Fassler, (1991). The right atrium receives venous blood from the body via the venae cavae. The superior vena cava returns blood from the structures above the diaphragm, and the inferior vena cava drains venous blood from below the diaphragm. The coronary sinus returns venous blood to the right atrium. At the base of the right atrium is the tricuspid valve, which controls blood flow into the right ventricle and prevents back flow to the atrium during ventricular systole. The tight ventricle pumps blood through the pulmonary valve and the branches of the pulmonary artery to the lobes of the lungs, the pulmonary capillaries, and the alveolar capillaries that surround the alveoli, (Fig. #2), Fassler, (1991). At the alveolar capillaries, gas exchange occurs, that is, blood gives off carbon dioxide and receives oxygen. Then, oxygenated blood returns through the pulmonary veins to the left atrium. The mitral valve at the base of the left atrium controls blood flow to the left ventricle and prevents backflow to the left atrium. Both the mitral and the tricuspid valves are attached to the strong chorae tendineae, fibrous filaments that arise from the papillary muscles of the ventricle Fig. (#1) Location of cardiac structures, Fassler, (1991). Fig. (#2) Blood flow through the heart, Fassler, (1991). and work to prevent eversion of the valves when the ventricle contracts, The left ventricle pumps blood through the aortic valve into the aorta, (Fig. #3), Fassler, (1991). The basic contractile unit in the myocardium, the sarcomere, is composed of actin and myosin filaments, which are contractile proteins. The degree to which actin and myosin overlap depends on the length of the sarcomere, which is determined by muscle stretch. Less overlap occurs during diastole, as the ventricle fills and the muscle stretches; more overlap occurs during diastole, when the muscle contracts. Contraction occurs when the action potential stimulates movement of calcium with energy release, causes the filaments to slide past each other and shorten