Sympathetic innervation plays a (major/minor) role in controlling the blood flow through the coronary circulation.
USMLE® Step 1 style questions USMLE
A 70 year old man was brought to the hospital by ambulance subsequent to chest pain that lasted 40 minutes. In order to assess the degree of stenosis of his coronary arteries, he underwent coronary angiography, a procedure during which a contrast dye was injected in his coronary vessels. To which phase of the ventricular action potential does visualization of the stenotic lesion correspond?
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With coronary circulation, coronary comes from the Latin word “coronarius,” meaning "crown."
This is because the coronary blood vessels surrounding the heart resembles a little crown!
And circulation, refers to “the flow of blood.” So, coronary circulation is the movement of blood throughout the vessels that supply the myocardium also known as the heart muscle.
As you will see, it is these coronary vessels and flow through them, coronary blood flow, that are essential in managing the delicate supply and demand balance of oxygen and nutrients in the cardiac muscle.
Now, the heart is a pump, primarily made up of cardiac muscle cells known as cardiomyocytes.
And like any other cell, they require a steady supply of oxygen, nutrients, and a way to eliminate wastes.
And although the heart is continually pumping blood throughout its chambers, the myocardium is too thick for the diffusion of blood to happen effectively.
So, instead, the coronary circulation provides an efficient way for the exchange of substances to occur.
Okay, the coronary circulation system is mainly made up of arteries and veins.
The aorta is like a superhighway that carries oxygenated blood from the heart to the rest of the body and the heart itself is the first exit off that highway!
Now, the left coronary artery originates from the left sinus of Valsalva in the aortic root and heads along the left coronary sulcus, a groove on the outer surface of the heart that marks the point of division between the ventricles and the atria, also known as the atrioventricular groove.
Not too far along the sulcus, the left coronary artery divides into two major branches.
The first is the left anterior descending artery or LAD.
It travels down the anterior interventricular sulcus, or groove, and it supplies the anterior 2/3 of the interventricular septum, the anterolateral papillary muscle, and the anterior surface of the left ventricle.
The second branch is the left circumflex artery or LCX. It goes along the coronary sulcus, around the left side of the heart and supplies the left atrium and the posterior walls of the left ventricle.
The right coronary artery originates from the right sinus of Valsalva in the aortic root and heads in the opposite direction, following the coronary sulcus, and along the way it supplies the right ventricular outflow tract, the SA node (66% of the time), and the right atrium.
It then sends multiple branches, marginal arteries, to supply the right ventricular muscle.
More distally at the crux of the heart, or the junction of the atrioventricular and interventicular grooves on the inferior surfaces of the heart, the RCA branches into the posterior descending artery or PDA, and the posterolateral ventricular branch or PLV.
The PDA goes down the posterior interventricular sulcus towards the heart’s apex while supplying the inferior 1/3 of the interventricular septum.
The PDA and PLV together supply the AV node, 2/3 of the inferolateral walls of the left ventricular, and the posteromedial papillary muscle.
Now, the anatomical layout of the coronary arteries can vary considerably.
Most of these are normal variants while some of them can be pathologic and even result in myocardial infarction or sudden cardiac death.
The most common variation is the origin of the posterior descending artery, as it can branch off from either the right or left coronary artery, sometimes even both!
So, if the posterior descending artery is supplied by the right coronary artery, then it's described as a right-dominant circulation.
And it's the most common. If it arises from the left circumflex, it's a left-dominant circulation, which is not so common.
And if it arises from both the right coronary and left circumflex artery, it's known as a codominant circulation, and it's way rarer than the others.
There are even more and stranger variations, like the left coronary artery arising from the right sinus of Valsalva or even from the pulmonary artery, which carry significant problems and risks as you can imagine.
The coronary circulation is also made up of veins, called cardiac veins.
They are responsible for returning deoxygenated blood and waste products like carbon dioxide, from the myocardium to the lungs.
The blood moves from the capillary beds of the myocardium into the cardiac veins. The cardiac veins usually follow the same path as the coronary arteries.
So, just like their arterial counterparts, there’s the great cardiac vein in the anterior interventricular sulcus, a middle cardiac vein in the posterior interventricular sulcus, and a small cardiac vein, running along the inferior margin of the right heart.
All three cardiac veins empty into one big vessel behind the heart called the coronary sinus which empties into the right atrium. Then there's also the anterior cardiac veins, which generally empty directly into the right atrium bypassing the coronary sinus.
The blood meets the rest of the deoxygenated venous blood delivered via the inferior and superior vena cava, then passes to to the right ventricle, through the pulmonary trunk and finally the lungs where the red blood cells can pick up oxygen and dump off carbon dioxide.
Interestingly, the blood in the coronary sinus has the lowest oxygen content in the body because the heart extracts the maximum amount of oxygen from the circulation at rest.
This sets the stage for our discussion on how changes in flow through the coronary circulation is the way in which the myocardial oxygen demands are met.
Now, the coronary arteries and their main branches lie in the epicardium, or the outer layer of the heart wall, and they send branches inward to supply the myocardium.
The epicardial vessels (R1) are normally free of resistance to flow, functionally conduit vessels, whereas the arterioles (R2) are the primary resistance vessels regulating the flow of blood to the myocardium.
The final components to resistance are the right atrium (usually negligible), and the forces imposed by the left ventricular chamber pressures.
It is this arteriolar regulation of flow (the R2 vessels) that allows the heart’s supply of oxygen to meet its demands in a linear relationship.
The left and right coronary arteries exhibit different flow patterns despite having identical starting pressures and this makes the heart unique, particularly the left ventricle.