Heart
The heart is about the size of a clenched fist and beats approximately 100,000 times each day, pumping roughly 8,000 liters of blood. This is enough to fill 40 fifty-five gallon drums or 8,800 quart milk cartons. The normal cardiac output is about 5 liters (1.3 US gallon) per minute. Since the total blood volume of the body is about 5 liters, the entire volume passes through the heart every minute.
The heart lies directly behind the sternum, and is surrounded by the pericardial cavity, which is lined by a serous membrane called the pericardium. The pericardium is divided into three layers:
- visceral pericardium (epicardium), which covers the outer surface of the heart
- parietal pericardium, which lines the inner surface of the pericardial sac. This sac is reinforced by a dense network of collagen fibers that stablizes the positions of the heart, pericardium, and associated vessels in the mediastinum, along with the small amount of pericardial fluid which acts as a lubricant for the vessels.
The heart wall contains three distinct layers:
- epicardium: a serous membrane that covers the outer surface of the heart, consisting of epithelium and an underlying layer of loose connective tissue
- myocardium: the muscular wall, containing cardiac muscle tissue, connective tissues, blood vessels, and nerves. This tissue arrangement forms concentrated layers that wrap around the atria and spiral into the walls of the ventricles, resulting in squeezing and twisting contractions that increase the pumping efficiency.
- endocardium: the covering of the inner surface of the heart, including the valves. This is a simple squamous epithelium that is continuous with the epithelial lining of the attached blood vessels.
The heart has four chambers:
- left ventricle
- right ventricle
- left atria
- right atria
The ventricles are separated by the interventricular septum.
The atria are separated by the interatrial septum.
Each atrium communicates with the ventricle on the same side through an AV (atrioventicular valve), a flap of tissue that ensures blood flows only one way.
The right atrium receives unoxygenated blood from two large veins:
- superior vena cava, which collects and delivers blood from the head, neck, upper limbs, and chest.
- inferior vena cava, which collects and carries blood from the rest of the trunk, the viscera, and the lower limbs.
The right atrium delivers the collected blood to the right ventricle for distribution into the lungs.
The left atrium receives oxygenated blood from the lungs through four pulmonary veins (two from each lung) and discharges it into the left ventricle, which ejects it back into the system.
Pathway of blood through the heart:
- Right atrium -- Tricuspid valve -- Right ventricle -- Pulmonic semilunar valve -- Pulmonary artery (main) -- Pulmonary arteries (right and left) -- Pulmonary capillaries (within the lungs) -- Four pulmonary veins -- Left atrium -- Bicuspid valve (mitral valve) -- Left ventricle -- Aortic semilunar valve -- Aorta
Pathway of cardiac impulse:
- Sinoatrial (SA) node -- Atrial conducting fibers -- Atrioventricular (AV) node -- Bundle of His -- Bundle branches (right and left) -- Purkinje fibers.
This page was updated in November 2005.
