The Heart
The heart is a muscular, fist-sized organ located in the centre of the chest and weighing 250-350 g. It is a remarkably effecient and reliable pump, propelling over 6000 litres of blood daily and beating more than 40 million times yearly, or over 3 billion times over a lifetime.
Anatomy
The heart lies centrally within the chest, at the levels of the x and x ribs.
The heart is contained in the pericardium, a tough, membranous sac bounded by the pleural sacs of the lungs and the diaphragm below. The pericardial cavity is filled with pericardial fluid, lubricating the heart and allowing it to beat smoothly.
The heart consists of three layers: the epicardium, a thin layer of epicardial and connective tissue; the myocardium, a thick layer of contractile cardiomyocytes, and the thin endocardium, composed of endothelial cells. Myocytes comprise only 25% of the number of heart cells but account for almost 90% of its volume. The remainder are mostly endothelial cells of the capillary network and fibroblasts. Extracellular matrix and leukocytes are rare.
Function
Embedded within the heart is an electrical conducting system, facilitating the coordinated contraction of the cardiac cycle. The heart rate and thereby cardiac output are controlled by internal pacemaker systems and external neural and endocrine systems, achieving its ultimate goal of maintaining appropriate blood pressure.
Blood returning from the body through the inferior and superior vena cavae enter the right atrium and pass through the tricuspid valve to enter the right ventricle. From there, blood is pumped through the pulmonary valve to the lungs, where blood is oxygenated. Blood returns to the left atrium and passes through the mitral valve into the muscular left ventricle, where is pumped across the aortic valve and into systemic circulation.
University of Utah WebPath images of the heart.
Coronary Arteries and Veins
Coronary Arteries and Veins (click for bigger picture)
The heart criticially depends on oxygen for its function. Altough it counts for only 1/200 of the body's weight, it requires almost 1/20 of total blood supply at rest. It therefore has a rich vasculature to keep it perfused - the coronary (Latin: crown) arteries, 5- to 10 cm long and 2-4 mm in diameter. Atherosclerosis of coronary vessels is the major cause of angina and the acute coronary syndromes.
The three major coronary arteries are the left anterior descending (LAD), left circumflex (LCX), and right coronary artery (RCA). Branches of the LAD are the diagonal and spetal perforators, while the LCX gives off obtuse marginals.
The LAD supplies most of the apex of the heart, the anterior wall of the left ventricle, and the anterior two-thirds of the septum.
Either the RCA or the LCX can perfuse the posterior third of the septum via the posterior descending branch, resulting in 'heart dominance'. 80% of people are right dominant.
The LCX perfuses the lateral wall of the left ventricle.
The right coronary artery perfuses the entire right ventricular free wall and the posterobasal left ventricle.
Most hearts have numerous coronary anastemoses, or collateral circulations.
There is nearly one capillary next to each cardiac cell.
Heart valves are composed of dense and loose connective tissue and are lined with endothelium. The AV valves depend on their tethering to the ventricular wall to maintain closure. This is accomplished by the cordae tendinae attached to papillary muscles.