Embedded in the wall of the heart are four structures that conduct impulses through the cardiac muscle to cause first the atria then the ventricles to contract. These structures are the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, and the Purkinje fibres. On the rear wall of the right atrium is a barely visible knot of tissue known as the sinoatrial, or SA node. This tiny area is the control of the heartspacemaker mechanism. Impulse conduction normally starts in the SA node. It generates a brief electrical impulse of low intensity approximately 72 times every minute in a resting adult. From this point the impulse spreads out over the sheets of tissue which make up the two atria, exciting the muscle fibres as it does so. This causes contraction of the two atria and thereby thrusts the blood into the empty ventricles. The impulse quickly reaches another small specialized knot of tissue known as the atrioventricular, or AV node, located between the atria and the ventricles. This node delays the impulse for about 0. 07 seconds, which is exactly enough time to allow the atria to complete their contractions. When the impulses reach the AV node, they are relayed by way of the bundle of His and Purkinje fibres to the ventricles, causing them to contract. The electrical current races across the two ventricles within 0.06 seconds, causing the squeezing, thrusting motion of these powerful pumping chambers. The heart also has its own built in safety factors. The AV node, in an emergency situation, can take over the functions of the SA node by becoming the generator of the impulses. It is not quite as efficient, generating only a rate of 40 or 50 beats per minute. Research Atrioventricular Node
The left atrium is a small upper cavity of the heart. Oxygen rich blood returns from the lungs through the four pulmonary veins into the smooth chamber of the left atrium. The chamber is constructed of two overlapping layers of muscle: a superficial layer and an inner layer, composed of many small bundles. The wall of the chamber is slightly thicker and more powerful than the right atrium. As the heart contracts (ventricular systole), blood flows into the ascending aorta through the aortic arch. As the heart relaxes (ventriclediastole), the blood flows through the mitral valve to the left ventricle. The right atrium is a small upper cavity of the heart that holds about three-and-a-half tablespoons of blood. It serves as the receiving chamber for all the venous blood (short of oxygen and laden with carbon dioxide) returning through the superior and inferior vena cava, and from many minute blood vessels that drainblood from the walls of the chamber itself. The right atrium is slightly larger than the left atrium, which is slightly more powerful. The walls of the right atrium are less than an eighth of an inch thick. Two layers of muscle form the wall. The superficial layer spans both atria, and the inner layer, composed of many small bundles, arches over the atrial cavity at right angles to the superficial layer. As the heart contracts (ventricular systole), the blood is pushed through the pulmonary valve into pulmonary circulation. As the heart relaxes (ventricular diastole), the blood exits the right atrium through the tricuspid valve to the right ventricle. In the upper part of the right atrium there is a small patch of special hearttissue called the sinus node or the sinoatrial node. It is the heartspacemaker, triggering the heartbeat and establishes its rate. Research Atrium
Like all body organs, the heart needs a supply of blood to bring it oxygen. It cannot get oxygen from the blood within its chambers, which passes through too quickly and under too great a pressure, and in the right side is very low in oxygen. Instead, the muscle that makes up the wall of the heart, the myocardium, receives oxygen-rich blood from a system of small arteries that branch from the aorta. These are called the coronary arteries. They cross over the hearts surface, dividing and sending tiny branches into the heart muscle. The two coronary arteries are no wider than a drinking straw.
The right coronary artery lies in a groove between the right atrium and right ventricle and loops around the lower side and to the rear of the heart like a crown. Hence the name, coronary. This artery supplies blood to the thick muscle of the right ventricle.
On the other side, the left coronary artery divides almost immediately into two large branches, one of which (the anterior descending branch) passes over the front of the heart toward the tip. The other branch (the circumflex branch) lies in a groove between the left atrium and left ventricle. This artery supplies blood to the muscle of the left ventricle. The anterior descending branch supplies the front surface and tip of the heart and the front part of the septum. The circumflex branch supplies the portion of the left ventricular wall away from the septum. From the large coronary vessels, smaller branches arise, which divide and insert into the heart muscle, supplying its nutritional needs. If a blood clot occludes some part of the coronary artery, as in coronary thrombosis or coronary embolism, the cells of the heart are deprived of oxygen and soon die. This is called myocardial infarction. Research Coronary Artery
The coronary vein is often referred to as the great cardiac vein of the heart. It is a large vein with two branches, the left coronary vein and the right coronary vein. The vein commences at the apex of the heart and ascends along the heart to the base of the ventricles. It then curves left to the back part of the heart and opens into the left coronary sinus, which is about 2.5 centimeters in length and terminates in the right atrium near the inferior vena cava. Research Coronary Vein
The marginal vein, as its name implies, extends along the thick margin of the left ventricle. It is a large branch that receives blood from several smaller venous branches of the left atrium and ventricle. The blood is then carried to the great cardiac vein, or coronary vein. Research Marginal Vein
The cardiac pacemaker.a small area of specialized tissue within the wall of the right atrium of the heart whose spontaneous electrical activity initiates and controls the beat of the heart. An artificial pacemaker is an electronic device that stimulates the heartmuscles by delivery small electric shocks to it, thereby taking over the role of the cardiac pacemaker. Research Pacemaker
The posterior cardiac vein (often referred to as the middle cardiac vein) extends from the apex of the heart, receiving blood from both ventricles, and continues along the base of the heart to the coronary sinus, where the blood is then delivered to the right atrium of the heart. Research Posterior Cardiac Vein
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Atrioventricular Node
