When physicians study the anatomy of the cardiovascular system, they need to understand the controlling nerve structures in the complex organ the heart as well as the cellular level.
The Heart’s Cell Structure
The heart is made up of three layers: the epicardium, the myocardium, and the endocardium.
The epicardium is the outermost layer of the heart and is made up of epithelial cells and connective tissue. It is part of the visceral pericardium, the innermost portion of the pericardial sac, an empty sac with a tiny amount of lubricating fluid in it which surrounds the heart. The outer most portion of the pericardial sac is the parietal pericardium.
Between the visceral and parietal pericardium is a potential space called the pericardial space which contains a trace of pericardial fluid to lubricate the continual motion of the heart.
The myocardium consists of the powerful cardiac muscle cells that contract to send blood throughout the body. The inner endocardium is a thin layer of epithelial cells and connective tissue, the same type of structure that lines the inside of most of the body’s organs.
The Heart’s Nerve Structure
Sympathetic and parasympathetic fibers provide contrl over the complete anatomy of the cardiovascular system. Sympathetic fibers descend from the brain in the spinal cord and when stimulated, increase heart rate and contractility. The parasympathetic fibers descend in the vagus nerve and when stimulated, decrease heart rate and contractility.
The actual contraction of the heart originates in impulses from the sinoatrial (SA) node at the juncture of the superior vena cava and the right atrium. These impulses travel to the atrioventicular (AV) node which sits in the interatrial septum.
From here, the impulse moves into the Bundle of His, which divides into right and left branches. The impulses now enter the Purkinje fibers from where they spread from the endocardial (inner) surface to the epicardial (outer) surface. The result is cardiac contraction: a heartbeat.
The sympathetic and parasympathetic innervation of the heart can slow or increase the heatbeat, but the initiation of impulses by the SA node is automatic. Also, baroreceptors in the carotid sinuses and aortic arch area can also affect heart rate.
With low blood pressure, these receptors decrease their impulses to the medullary cardiovascular center in the brain, resulting in increased sympathetic activity and decreased parasympathetic activity. This means an increase in strength and the rate of the heart beating.
The Blood Vessels
In addition to the heart, the anatomy of the cardiovascular system consists of a diffuse, branching network of blood vessels which both supplies the various organs of the body with blood and nutrients and also brings toxic materials to the kidneys, liver, and lungs for removal.
The arterial portion of circulation begins with arteries leaving the heart and decreasing in size to form arterioles. These vessels eventually become thin capillaries where the cells receive oxygen (O2) and eliminate carbon dioxide (CO2).
From the points of exchange, the venous portion of circulation returns the deoxygenated blood to the heart. Capillaries expand into venules and eventually veins as they approach the heart.
Two separate systems circulate blood:
systemic: arterial and venous transport to all organs of the body (high-pressure circuit)
pulmonary: the vessels that transport blood to the lungs to exchange CO2 for O2 and then back to the heart (low-pressure circuit)