The heart is a muscle that relies on oxygen to work properly.
The delivery of this invaluable fuel is provided by the coronary arteries. Any compromise in oxygen delivery could have catastrophic effects.
The two coronary arteries (right and left) are located at the base of the aorta, just above the aortic valve.
To understand the problems related to coronary arteries (such as atherosclerotic plaques), we need to understand some basic anatomy concepts.
Formed by three layers
The coronary arteries, like all the other arteries of our body, are made of three layers. Each one of these layers plays a specific role, but the middle one is particularly important in the development of the cholesterol plaques.
Internal layer: intima
The internal layer called intima is like a thin layer of Teflon. It is responsible for the integrity of the vessel and protects it from the formation of blood clots.
It naturally produces substances used to dilate the artery when necessary. One could compare that to the nitroglycerin that certain patients must use to relieve chest pain. The intima thus helps keep the artery at a larger size where necessary.
Middle layer: media
The media, in the center, is thicker and made up of smooth muscle cells. It gives the artery the ability to contract and to dilate.
The nitroglycerin spray that some people use for angina acts on this layer. It induces a vessel relaxation. The vessel becomes larger, circulation improves and angina is relieved.
External layer: adventitia
The adventitia is the external coating. It is the most resistant part of the artery and offers a protective effect to the vessel.
Located on the surface of the heart
The coronary arteries are located on the surface of the heart and they divide into branches around and into the cardiac muscle.
Certain patients who benefit from a coronary artery repair using a stent are often interested to know the name of that artery. The illustration below shows the name given to branches of the coronary tree.
The left coronary artery divides in two branches almost immediately after its origin on the aorta. One calls this first segment the left main artery. The first branch goes down on the front of the heart. It is called the left anterior descending artery (LAD). It follows an anterior groove created by the junction of the right and left ventricles. The branches that are emerging from the LAD are called the diagonals, since they follow a diagonal trajectory towards the left side of the heart.
The second artery that leaves from the left main artery is called the circumflex artery. It goes around the heart to the left and reaches the back of the left ventricle. The branches that emerge from the circumflex artery are called the marginals.
The right coronary artery goes around the heart to the right and in nine people out of ten, it goes down to the bottom from the heart towards the tip (called apex). It follows a posterior groove created by the junction of the right and left ventricles. The artery sitting in the posterior groove is called the posterior descending artery.
In one person out of ten, the posterior descending artery emerges from a prolongation of the circumflex artery on the left side of the heart.
Oxygenation and circulation
Blood does not circulate in the same way in the heart muscle than in the rest of the body.
The small vessels that penetrate the cardiac muscle are crushed every time the heart contracts. Circulation, and consequently oxygenation of the heart occurs when the muscle is relaxing.
Strategic location at the base of the aorta
The coronary arteries are the first to emerge from the aorta, just above the aortic valve. It is a strategic place. Let’s see why!
With the relaxation of the heart, blood flow wants to go backwards
The contraction of the left ventricle ejects blood through the aortic valve into the aorta. The heart muscle then relaxes and rests for a short period of time.
At the time when heart is relaxing, the blood that was ejected into the aorta passively wants to return into the ventricle. It is an effect of suction caused by the relaxation of the heart regaining its initial shape.
This backward flow causes the opening of the aortic valve cusps, like a parachute opening once exposed to rapid airflow. The return of blood in the left ventricle is thus blocked.
The water hammer
The sudden closing of the aortic valve reproduces an effect well known in plumbing mechanics.
The sudden closing of a valve in a pipe with moving water is called a water hammer. It means that when fluid in motion is abruptly stopped, there is a short pressure increase of the valve and its vicinity.
In the case of the coronary arteries, this short pressure increase promotes blood flow towards the coronary arteries located at the base of the aorta, just above the aortic valve.
This all happens during the rest period of the heart, the strategic time where blood supplies the heart muscle with oxygen.
Well thought, isn’t it ?