Before we talk about coronary artery disease and its treatment, it is necessary to understand what exactly the human coronary artery system is made up of. I will briefly explain the coronary arterial tree in man, and later we will see what disease of these arteries can cause.
Why do we need coronary arteries ?
As we have discussed earlier, the heart is a muscular pump that contracts to force blood to flow through arteries to each part of the body, providing nutrients and oxygen for their normal function. To perform such work, the heart muscle itself requires energy.
How is this energy provided? Nature has designed a special carrier system to supply oxygen and nutrient-rich blood directly to the heart muscle. This system is the coronary arteries.
Two Arteries, Three Systems
There are two coronary arteries – the left and right.
The left coronary artery begins as a main stem called the Left Main Coronary Artery (LMCA) which varies between 1 and 15 millimeters in length. This artery divides into two major branches, the Left Anterior Descending coronary artery (LAD) and the Left Circumflex Artery (LCX).
The Right Coronary Artery (RCA) is a single long vessel with a few branches.
The LAD and LCX by themselves supply large chunks of heart muscle with blood and oxygen. So the coronary artery tree is categorized into three systems based on the mass of heart muscle they supply.
The Left Anterior Descending Coronary Artery (LAD)
This major branch of the LMCA is fancifully called the “widow’s artery” since it produces many widows each year ! The LAD is a branch that runs on the front of the heart in a groove that demarcates the left and right ventricles. This artery provides blood supply, oxygen and nutrients to a large part of the inter-ventricular septum, and the front wall of the left ventricle. Obstruction of this artery causes damage to a large block of muscle in the left ventricle, and may be fatal.
The Left Circumflex Coronary Artery (LCX)
The LCX is the other major branch of the LMCA and turns backwards to run along a groove between the left atrium and ventricle. This artery gives off multiple smaller branches that supply blood to the left margin of the ventricle. Since this margin is obtuse angled, these branches are also called Obtuse Marginal (OM) branches, of which there may be a variable number (1-7). These OM branches also supply a considerable mass of ventricle muscle, and can cause serious damage if diseased.
The Right Coronary Artery (RCA)
The RCA is the other main coronary artery branch arising from the aorta and running in the groove between the right atrium and ventricle. This artery is usually smaller than the LMCA, and supplies a smaller mass of heart muscle, mainly the right ventricle. As it curves behind the heart, the RCA gives off two branches – the Posterior Descending Artery (PDA) and the Posterior Left Ventricular Branches (PLB).
The PDA supplies blood to the posterior portion of the interventricular septum and the PLB supplies a part of the back wall of the left ventricle.
Dominance of Coronary Systems
Depending on which coronary artery (left or right) crosses the midline of the heart posteriorly – the crux – dominance is determined. Mainly an issue of nomenclature, dominance sometimes is used to make decisions about the need to place bypass grafts to a particular diseased segment of coronary artery.
Other important coronary artery branches
While the “big three” are the major branches, some smaller ones may be quite important as well.
The sino-atrial node artery supplies the S-A node which is the pacemaker of the heart and sets its rhythm. This branch comes off the RCA in 55% and off the LCA in the other 45%.
The atrio-ventricular node artery supplies the A-V node, which is located between the atria and ventricles and controls spread of electrical impulses from the atrium to the ventricle. While in 90% of cases this branch comes off the RCA, in the other 10 % it may be a branch of the LCX. Damage of block of this branch may result in the serious arrhythmia called “heart block”.
Later I will discuss the impact of disease of these coronary arteries on the health of a person and the consequences of such arterial obstructions that are grouped under the category “Ischemic Heart Disease“.