The cardiovascular systemThe cardiovascular system is made up of the heart, blood vessels and blood and its main functions include transporting blood around the body, protecting against invading microorganisms and preventing blood loss blood through hemostasis. (Widamaier, et al 2011:5)One of the main organs of the cardiovascular system is the heart; the heart is made up of four chambers. Blood enters the right atrium of the heart from the body through the vena cava, then travels through the tricuspid atrioventricular valve into the right ventricle. Blood is then pumped through the pulmonary semilunar valve from the heart to the lungs using the pulmonary arteries. It is then oxygenated and returns to the left atrium into the pulmonary veins, travels through the mitral atrioventricular valve into the left ventricle, and is then pumped out of the heart into the systematic circulatory system passing through the aortic semilunar valve into the aorta. (Widamaier, et al 2011:359) The heart is located in the thoracic cavity protected by a sac called the pericardium. The inner wall of the heart is made up of three layers, the outer part of the heart wall is called the epicardium and this lubricates the heart and prevents friction. The middle and thickest layer of the heart, called the myocardium, is a thick muscular layer made up of heart muscle responsible for pumping blood. The thickness of the myocardium varies throughout the heart, the atria do not need as much myocardium as the ventricles as the atria only need enough pressure to pump blood to the ventricles while the ventricles need enough pressure to pump blood to the lungs and to the body. The left ventricle has a thicker layer of myocardium than the right because it is located to the left of the center of the lesion. (Widamaier, et al 2011:354) The cardiovascular system consists of the heart which pumps blood to the lungs through the pulmonary circulation, the cardiac muscle through the cardiac circulation and the body systems through the systematic circulation. It is made up of various chambers and valves and its cycle is controlled by an electrical signal from the heart itself. There are several different vessels that control blood flow throughout the body. Which include the high-pressure arteries and arterioles that come from the heart. Capillaries that have a semipermeable membrane that allows the exchange of gases, nutrients and waste between the blood and tissues. Even veins that are low pressure and contain valves that prevent backflow. All together they perform the main functions of the cardiovascular system; protection, transportation and regulation.