Heart and Cardiac Muscle Essay

What is the heart? The heart is the organ that supplies blood and oxygen to all parts of the body. It is about the size of a clenched fist, weighs about 10.5 ounces and is shaped like a cone. The heart is located in the chest cavity just posterior to the breastbone, between the lungs and superior to the diaphragm. The heart is surrounded by a fluid filled sac called the pericardium. Blood is pumped away from the heart through arteries and returns to the heart through veins. The major artery of the body is the aorta and the major veins of the body are the vena cavae. Chambers of the Heart The heart is divided by a partition or septum into two halves. The halves are in turn divided into chambers. The upper two chambers of the heart are called atria and the lower two chambers are calledventricles. The atria receive blood returning to the heart from the body and the ventricles pump blood from the heart to the body. Valves allow blood to flow in one direction between the chambers of the heart. The Heart Wall The heart is composed of cardiac muscle which enable the heart to contract and allow the synchronization of the heart beat. The heart wall is divided into three layers: the epicardium, myocardium, and endocardium. * Epicardium – outer protective layer of the heart. * Myocardium – muscular middle layer wall of the heart. * Endocardium – inner layer of the heart that is continuous with the inner lining of blood * Cardiac Conduction * Cardiac conduction is the rate at which the heart conducts electrical impulses. Cardiac muscle cells contract spontaneously and are coordinated by nodal tissue, specifically the sinoatrial node. There are other factors that influence heart rate as well. These include endocrine hormones, body temperature and exercise. * Cardiac Cycle * The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle which are the diastole and systole phases. During the diastole phase, the atria and ventricles are relaxed and blood flows into the atria and ventricles. In the systole phase, the ventricles contract sending blood to the rest of the body. A heartbeat is a two-part pumping action that takes about a second. As bloodcollects in the upper chambers (the right and left atria), the heart’s natural pacemaker (the SA node) sends out an electrical signal that causes the atria to contract. This contraction pushes blood through the tricuspid and mitral valves into the resting lower chambers (the right and left ventricles). This part of the two-part pumping phase (the longer of the two) is called diastole. The second part of the pumping phase begins when the ventricles are full of blood. The electrical signals from the SA node travel along a pathway of cells to the ventricles, causing them to contract. This is called systole. As the tricuspid and mitral valves shut tight to prevent a back flow of blood, the pulmonary and aortic valves are pushed open. While blood is pushed from the right ventricle into the lungs to pick up oxygen, oxygen-rich blood flows from the left ventricle to the heart and other parts of the body. After blood moves into the pulmonary artery and the aorta, the ventricles relax, and the pulmonary and aortic valves close. The lower pressure in the ventricles causes the tricuspid and mitral valves to open, and the cycle begins again. This series of contractions is repeated over and over again, increasing during times of exertion and decreasing while you are at rest. The heart normally beats about 60 to 80 times a minute when you are at rest, but this can vary. As you get older, your resting heart rate rises. Also, it is usually lower in people who are physically fit. Your heart does not work alone, though. Your brain tracks the conditions around you—climate, stress, and level of physical activity—and adjusts your cardiovascular system to meet those needs. The human heart is a muscle designed to remain strong and reliable for a hundred years or longer. By reducing your risk factors for cardiovascular disease, you may help your heart stay healthy longer.