Human Heart
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The human heart
The heart is a cone-shaped, muscular organ about the size of a fist which located between the lungs directly behind the sternum and is tilted so that the apex is oriented to the left.(Biology, Mader) The wall of the heart consists of three layers: the epicardium; external layer, the myocardium; middle layer and the endocardium; inner layer.(Principles of Anatomy and physiology) The heart lies within the pericardium that secretes a small quantity of lubricating liquid. Internally, a wall called the septum separates the heart into a right side and a left side. The heart has four chambers. The two superior chambers are the atria and the two inferior chambers are ventricles.(Biology Mader) The right atrium receives blood from three veins: the superior vena cava, inferior vena cava and coronary sinus. Between the right atrium and left atrium is a thin portion called interatrial septum. Right ventricle forms most of the anterior surface of the heart. The right ventricle is separated from the left ventricle by a partition called the interventricular septum. The left atrium form the most of the base of the heart. It receives blood from the lungs through four pulmonary veins. The left ventricle form the apex of the heart. Blood passes from the left ventricle to ascending aorta. Some of the blood in the aorta flows into the coronary arteries which branch from the ascending aorta and carry blood to the heart wall. The remainder of the blood passes into the arch of the aorta and descending aorta which carried blood throughout the body.(Anatomy and physiology) The heart also has four valves, which direct the flow of blood and prevent its backward movement. The two valves lie between the atria and the ventricles are called the atrioventricular valves. These valves supported by strong fibrous strings called chordae tendineae. The chordae, which are attached to muscular projections of the ventricular walls, support the valves and prevent them from inverting when the heart contracts. The atrioventricular valve on the right side is called the tricuspid valve it has three cusps. The valve on the left side is called the bicuspid (or mitral) because it has two cusps. The remaining two valves are the semilunar valve, whose cusps the resemble half-moons between the ventricles and their attached vessels. The pulmonary semilunar valve lies between the right ventricle and the pulmonary trunk. The aortic semilunar valve lies between the left ventricle and the aorta.
Even though the presence of the intercalated disks between cardiac muscle cells allows both atria and then both ventricles to contract simultaneously, the path of blood through the heart in the following manner can be travelled. Firstly, the superior vena cava and the inferior vena cava, which carry O2ˉˉ poor blood that is relatively high in carbon dioxide, enter the right atrium. Then, the right atrium sends blood through tricuspid valve to the right