The heart is located within the middle mediastinum, where it is surrounded by the pericardium. The pericardium is made up of two layers which are the outer, fibrous layer and the inner, serous layer. The serous layer is actually two layers as well which are referred to as the parietal and visceral layers of the serous pericardium. The parietal and visceral layers of the serous pericardium are separated by the pericardial cavity with the parietal layer being fused with the internal surface of the fibrous, outer layer. Separated by the pericardial cavity, the visceral layer of serous pericardium covers the external surface of the heart where it is also called the epicardium, the outermost of the three layers of the heart wall. Due to the fluid within the pericardial cavity, the heart is able to contract without oppositional friction.
Contraction of the heart is controlled by the impulses which are created and regulated by the conducting system of the heart. Nodal tissue and conducting fibers make up the conducting system. Nodal tissue is responsible for initiating the heart beat and coordinating the contraction of the chambers of the heart. The sinuatrial (SA) node is known as the "pacemaker" of the heart. Located anterolaterally, slightly deep to the epicardium, the SA node is responsible for initiating and controlling the heart contractions by sending a signal through the atrial musculature. Myogenic conduction is a term referring to such propagation of a signal through cardiac muscle and is what delivers this signal from to the atrioventricular (AV) node from the SA node. Located in the posteroinferior area of the interatrial septum near the inlet of the coronary sinus, the AV node sends the signal through the AV bundle (aka Bundle of His) and its branches. The AV bundle originates at the AV node and divides into the right and left bundles where the interventricular septum transitions from its membranous part to its muscular part. The right and left bundles continue to carry the signal through the muscular interventricular septum and branch into subendocardial branches (Purkinje fibers), which further branch into the ventricular walls where the signal stimulates contraction of the ventricles.
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| Figure 1.43 (D) from Clinically Oriented Anatomy |
Blood enters the left atrium via the four pulmonary veins and enters the right atrium via the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus. From the left and right atria, the blood is subsequently sent into the left and right ventricles, respectively. The contraction of the ventricles stimulated by the cardiac conduction system sends the blood out of the left ventricle and into the aorta while concurrently sending blood out of the right ventricle and into the pulmonary trunk. From the ascending portion of the aorta, blood is sent through the left and right coronary arteries to supply blood to the heart. From the aortic arch, blood is sent through the left brachiocephalic artery, left common carotid, and the left subclavian to supply the portions of the body superior to the diaphragm. From the third portion of the aorta, the descending aorta, there are no branches until it leaves the thoracic cavity and enters the abdominal cavity via the aortic hiatus of the diaphragm. Once the descending aorta reaches the abdominal cavity it sends out several branches to supply blood to the portion of the body inferior to the diaphragm. From the pulmonary trunk, blood is sent to the lungs to be oxygenated.
While functioning to supply blood throughout the body, the heart itself is in need of a blood supply as well. Supplying blood to the epicardium and myocardium of the heart is the function of the left and right coronary arteries and supplying blood to the endocardium is microvasculature from the heart chambers. The vessels of the heart are found just deep to the epicardium, over the surface of the myocardium. The surface covered by the left coronary artery (LCA) and its branches tends to be the left atrium and most of the left ventricle as well as part of the right ventricle. The LCA also supplies blood to the anterior 2/3 of the interventricular (IV) septum, the AV bundle of the conducting system of the heart by way of septal branches, and the SA node (in around forty percent of the population). In a similar way, the right coronary artery (RCA) typically supplies the right atrium and most of the right ventricle as well as part of the left ventricle (diaphragmatic surface), posterior 1/3 of the IV septum, AV node (in about eighty percent of the population), and the SA node (in those who do not have the SA node supplied by the LCA).
The left and right coronary arteries originate from the aorta just superior to the aortic valve with the left originating at the left aortic sinus and the right originating at the right aortic sinus. From its point of origin, the LCA courses between the left auricle and the left side of the pulmonary trunk and through the coronary sulcus where at the superior margin of the anterior IV groove it divides into the anterior IV branch and the circumflex branch. The anterior IV branch of the LCA runs between the two ventricles toward the apex of the heart, supplying blood to the ventricles and sending out septal branches to supply the anterior 2/3 of the IV septum. The anterior IV branch of the LCA continues until it anastomoses with the posterior IV branch of the RCA. In some people the anterior IV branch gives off a lateral branch which courses over the anterior surface of the heart. This lateral branch is also referred to as the diagonal artery and is found between the left marginal artery and the anterior IV artery.
Smaller than the anterior IV branch, the circumflex artery of the LCA gives off a left marginal branch and continues to run along the coronary sulcus around the left border of the heart to the posterior surface of the heart where it usually ends in the coronary sulcus prior to reaching the crux. The left marginal branch of the circumflex branch runs along the left margin of the heart, supplying the left ventricle. On the other hand, the RCA runs to the right side of the pulmonary trunk through the coronary sulcus and gives off a branch to the SA node, the SA nodal branch. The RCA continues through the coronary sulcus and gives off a second branch, the right marginal branch, which supplies the right border of the heart and runs toward the apex of the heart. The RCA still continues through the coronary sulcus to the posterior surface of the heart where at the crux of the heart it gives off a third branch, the atrioventricular nodal branch, which supplies the AV node. Usually the RCA gives off the posterior IV branch which runs between the two ventricles after which the RCA soon terminates in the coronary sulcus.
A. Robinson
Sources:
http://www.le.ac.uk/pa/teach/va/anatomy/case1/1_1.htmlSources:
Moore, Dalley, Agur: Clinically Oriented Anatomy. 6th ed. Baltimore: Lippincott Williams & Wilkins, 2010.
http://www.le.ac.uk/pa/teach/va/anatomy/case1/1_3.html
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