心 【名词】1. 心脏，心; 2. 胸，胸部；心胸，心地，心肠，胸怀; 3. 感情，热情，爱情；灵魂；良心; 4. 勇气，胆力；勇士; 5. 精神，气质；心境，心情; 6. 中心，核心；精华，要点，本质; 7. 意中人，情人；心爱的人，宝贝儿; 8. (主英)(土地的)肥沃程度; 9. 心脏形的东西
the hollow muscular organ lying slightly to the left of the midline of the chest. The heart serves as a pump controlling the blood flow in two circuits, the pulmonary and the systemic. Divisions of the Heart. The septum, a thick muscular wall, divides the heart into right and left halves. Each half is again divided into upper and lower quarters or chambers. The lower chambers are called ventricles and the upper chambers are called atria. The right side of the heart, consisting of the right atrium and right ventricle, receives deoxygenated blood and sends it into the pulmonary circuit. The left side, consisting of the left atrium and left ventricle, receives oxygenated blood and sends it into the systemic circuit. Valves of the Heart. The atrioventricular valves connect an atrium and a ventricle: between the right atrium and right ventricle is the tricuspid valve and between the left atrium and left ventricle is the mitral valve. The semilunar valves are valves at the blood's exit points from the heart: the pulmonary valve opens from the right ventricle into the pulmonary artery, and the aortic valve opens from the left ventricle into the aorta. These valves, both within the heart and leading out of it, open and shut in such a way as to keep the blood flowing in one direction through the heart's two separate pairs of chambers: fro atrium to ventricle and out through its appropriate artery. Layers of the Heart. The heart wall is composed of three layers of tissues. Its chambers are lined by a delicate membrane, the endocardium. The thick muscular wall essential to normal pumping action of the heart is called the myocardium. The thin but sturdy membranous sac surrounding the exterior of the heart is called the pericardium. The Heart's Pacemaker. The heart is made up of special muscle tissue, capable of continuous rhythmic contraction without tiring. The impulse that starts the contraction of the heart has its origin in an area of the right atrium called the sinoatrial node; it is this special tissue that acts as the normal pacemaker for the heart. The impulse is transmitted in a fraction of a second through the atria to another group of similarly sensitive fibers called the atrioventricular node, through the bundle of his, down the bundle branches, and to the Purkinje fibers, resulting in contraction of the ventricles.