How the Heart Works: A Basic Overview by David Yee
(Originally Written 10-5-1994)
A. the heart is located in the chest; it lies left of the body's midline, above and in contact with the diaphragm
B. situated immediately behind the breastbone, or sternum, and between the lungs, with apex tiled to the body cavity's left side
II. Heart has two cavities, divided by the cardiac septum
A. Right cavity takes in oxygen poor blood from the body and pumps it to the lungs
1. has an atrium (collecting chamber) and a ventricle (pumping chamber); atrium is on top of ventricle
a. atrium draws blood from veins, and ventricle pushes blood into arteries
b. thin walls for atrium and thick walls (=3x walls for atrium) for ventricle
2. has tricuspid (an atrioventricular valve) and pulmonary (a semilunar valve) valves
B. Left cavity takes in oxygen rich blood from the lungs and pumps it to the body
1.Same as A1, except walls are 2x as thick
2. has mitral (an atrioventricular valve), 2 flaps, also called bicuspid valve and aortic (a semilunar valve) valves
III. Two stages in each heartbeat cycle: diastole and systole
1. Heart muscle relaxes, blood is thus drawn into the two atria from veins.
a. oxygen-poor blood into the right atrium from major veins superior and inferior vena cava; each through a separate opening
b. oxygen-rich blood into the left atrium from four pulmonary veins (2 from each lung)
2. Rising pressure in each atrium opens the tricuspid and mitral valve and blood flows into the ventricles.
1. Sinoatrial (SA) node fires impulses, stimilating atria to contract.
2. All the blood are forced into the ventricles
3. Mitral and tricuspid valves close due to rising pressure in ventricles.
4. Ventricles fully contract.
5. Aotric and pulmonary valves are forced open.
6. Blood pushes out to the arteries (aorta and pulmonary arteries).
7. Heart relaxes, and the aortic and pulmonary valves close. Return to diastole.
V. Additional information
A. Amount of blood pumped
1. At rest, the heart pumps about 59 cc (2 oz) of blood per beat and 5 l (5 qt) per minute 2. During exercise, 120-220 cc (4-7.3 oz) per beat and 20-30 l (21-32 qt) per minute B. Size
1. The adult human heart is about the size of a fist and weighs about 250-350 gm (9 oz).
2. Thickness of heart muscles varies from 2 mm to 20 mm
C. Heart's wall has three layers
1. The outer layer of the heart is called the epicardium.
a. is in intimate contact with the pericardium (a serous membrane that is a closed sac covering the heart muscle's outside wall). Within the sac, a small amount of fluid reduces the friction between the two layers of tissue.
2. The middle layer is the myocardium (heart muscle)
3. The inner layer is the endocardium
a. consists of a thin layer of endothelial tissue overlying a thin layer of vascularized connective tissue