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Review of Cardiovascular Anatomy & Physiology
The heart is a cone shaped muscular organ located in the chest behind the sternum in the mediastinal cavity between the lungs and in front of the spine. The heart is approximately the size of a fist.
The heart’s wall (Pericardium) is made up of three layers:
Epicardium - inner surface
A skeleton of connective tissue called the fibrous pericardium surrounds the heart and acts a tough protective sac.
The heart has four chambers: two atria and two ventricles.
Right atrium receives deoxygenated blood from the body
Contraction of the atria forces the blood into the ventricles.
The right and left ventricles are the pumping chambers of the heart.
The right ventricle receives blood from the right atrium and pumps it through the pulmonary arteries to the lungs where it picks up oxygen and drops off carbon dioxide.
The heart has four valves:
Two atrioventricular (tricuspid and biscupid (mitral) valves and two semilunar (aortic and pulmonic valves). The valves open and close in response to changes in pressure within the chambers they connect and serve as one way doors that keep blood flowing through the heart in a forward direction. When the valves close they prevent backflow or regurgitation from one chamber to another. The closing of the valves creates the heart sounds heard through a stethoscope during a physical examination.
The heart needs an adequate supply of blood to survive. The coronary arteries which lie on the surface of the heart supply the heart muscle with blood and oxygen.
Left coronary artery originates off the aorta and supplies blood to the right atrium, the right ventricle, and part of the inferior/posterior surfaces of the left ventricle. The Bundle of His, the AV node and the SA node receive blood from this artery.
When two or more arteries supply the same region they usually connect through anastomoses junctions that provide alternative routes of blood flow. These alternate routes of blood are called collateral circulation and provide blood capillaries that directly feed the heart muscle. Collateral circulation becomes so strong that even if major coronary arteries become clogged with plaque collateral circulation can continue to supply blood to the heart.
Transmission of Electrical Impulses
The heart can’t pump unless an electrical stimulus occurs first. Generation and transmission of electrical impulses depend on the automaticity, excitability, conductivity and contractility of cardiac cells.
Automaticity refers to a cell’s ability to initiate an impulse. Pacemaker cells possess this ability.
As impulses are transmitted cardiac cells undergo cycles of depolarization and repolarization.
Polarized - cardiac cells are at rest meaning - no electrical activity takes place
Located in the upper right corner of the right atrium. Pacemaker of the heart. 60-100 times a minute.
Internodal tracts and Bachmann’s bundle to the
Responsible for (delaying the impulses that reach it.) The nodal tissue itself has no pacemaker ability, but the tissue around it (junctional tissue) has pacemaker ability. 40-60 times a minute.
Bundle of HIS, the Bundle branches
Resumes the rapid conduction of the impulse through the ventricles. The bundle divides into the right and left branches.
Network of nervous tissue that extends through the ventricles. Can serve as a pacemaker at a rate of 20-40 times a minute.