Blood Supply of the Heart
Blood Supply of the Heart
The heart has its own unique blood supply, more commonly known as the coronary circulation, which encircles the entire surface of the heart to supply its different regions. This begins with the coronary arteries arising from the ascending aorta, through openings called coronary ostia which are located above the aortic valves. These arteries run along the surface of the heart and so can also be reffered to as epicardial coronary arteries.
In fact, there are two main coronary arteries that branch from the ascending aorta, known as the left and right coronary arteries. In essence, the left main coronary artery supplies the left atrium, interventricular septum, left ventricle and the anterior wall of the right ventricle. On the other hand, the right coronary artery supplies the right atrium, the right ventricle as well as the sino-atrial node.
Interestingly, coronary blood flow can be described as a phasic flow. This is because during systole, extravacular compression occurs, meaning the contraction of the heart squeezes these arteries shut. It is only during diastole that these arteries become patent again, allowing blood to flow under aortic pressure into these coronary arteries, capillaries and finally into the coronary veins. This is known as coronary perfusion pressure (CPP) which is the difference between the diastolic pressure in the aorta and the diastolic pressure in the right atrium creating a pressure gradient, which results in coronary blood flow. This is important clinically, as in cases of tachycardia there is less time spent in diastole, implying a reduced time for coronary blood flow to occur and hence increased risk of ischaemia.
Left Coronary Artery
The left main coronary artery (LMCA) arises from the left aortic sinus, running towards the left hand side of the heart, just below the left auricle. It quickly divides into the anterior interventricular branch also known as the left anterior descending (LAD) artery and the circumflex artery.
The LAD runs along the anterior interventricular sulcus, a groove in the anterior surface of the heart that seperates the left and right ventricle, down towards the apex. From here, the LAD runs around to the posterior surface of the heart in another groove called the posterior interventicular sulcus. The LAD artery supplies blood to the walls of both the left and right ventricles.
The circumflex, follows a different course to the LAD. It runs along the atrioventricular groove, which seperates the atria from the ventricles, giving rise to the left marginal branch in the process. The circumflex then continues around the heart, terminating on its posteroinferior aspect, to supply blood to both the left and right atrium.
As an aside, in approximatly one-third of patients an intermediate artery will arise between the LAD and circumflex, to supply the anterolaterlal wall of the left ventricle.
The description thus far is a relativly simplistic explanation of the divisions of the left main coronary artery and in reality the LAD and circumflex arteries each give off multiple branches. For example, the LAD may give off one of several diagnonal branches, while the circumflex may give off one of several marginal branches.
Right Coronary Artery
The right coronary artery (RCA) arises from the right aortic sinus, running towards the right hand side of the heart, deep to the right auricle and along the right atrioventricular groove. From here, it curls around towards the inferior surface of the heart, forming the posterior interventricular branch, more commonly known as the posterior descending artery (PDA). The PDA runs along the posterior interventiculas sulcus, to supply blood to the walls of both the left and right ventricle. However, before turning towards the diaphragmatic surface of the heart the RCA gives rise to the right marginal branch, that runs along the right margin, to supply the wall of the right ventricle.
Again, the RCA gives rise to several more branches, other than the ones already discussed. Important branches from the RCA include, the conus branch and the sino-atrial node artery, passing upwards to the aurical wall to the junction between superior vena cava, sulcus termanalis and right auricle.
Communication between the Left and Right Coronary Arteries
Interestingly, there are four known communications between the left and right coronary arteries, at different sites within the heart. They are:
1. The artery of the conus which is a branch of the RCA, with the LAD.
2. At the interventricular septum, with septal perforators between the LAD and PDA .
3. At the apex of the heart, with the LAD and PDA.
4. At the crux, between the left circumflex and PDA.
As blood flows along the left and right coronary arteries it proceeds to travel down smaller arteries and arterioles, eventually reaching the coronary capillaries that run parallel with cardiac myocytes. This is a site for exchange, where oxygen and vital nutrients are released from the blood and carbon dioxide along with other important waste products are picked up. A high capillary to myocyte ratio as well as a short diffusion distance ensures that optimum exhange can occur. As an aside, compared to the left ventricle the right ventricle has a greater ratio of muscle fibres to capillaries. Consequently, it is more likely to suffer toxic damage but less likely to suffer ischaemic damage.
Small arteries and arterioles are key players in altering vascular resistance and thus regulating myocardial blood flow. Myocardial blood flow is closely linked with oxygen demand, at around 8-10 ml of O2/min/100g, with an increase in cardiac activity resulting in an increase in demand for oxygen. This is achieved by an increase in myocardial blood flow, involving an important process known as autoregulation. Whenever there is a change in coronary perfusion pressure, through changes in aortic pressure, the process of autoregulation ensures that myocardial blood flow is always maintained. Adenosine and nitric oxide (NO) are important mediators in the regulation of coronary blood flow as well as the involvement of the sympathetic and parasympathetic nervous system.
Once vital nutrients have been delivered and waste products of metabolism collected, in the coronary capillaries, coronary blood flows into the venules and then coronary veins. From here, most deoxygenated blood drains into the coronary sinus, located on the posterior surface of the heart, emptying into the right atrium. This includes:
The great cardiac vein, which drains areas of the heart supplied by the left coronary artery and lies in the anterior interventricular sulcus.
The middle cardiac vein, which drains areas of the heart supplied by the right coronary artery and lies in the postrior interventricular sulcus.
Small cardiac vein which drains the right atrium and ventricle
Other veins include the oblique vein draining the left atrium and posterior vein draining the left ventricle
As hinted at, some veins drain directly into the right atrium. This includes the anterior cardiac veins, which drain the right ventricle and the Venae cordis minime, also known as the thesbesian veins
The myocardium contains numerous anastomoses that form connections between the different coronary arteries. This is more commonly reffered to as collateral circulation, providing an alternate route for blood flow, if one artery becomes occluded. However, this collateral flow is often insufficient to maintain perfusion in the event an occlusion, in one coronary artery.
Within the patient population there are numerous anatomical variations in the coronary circulation. In the majority of individuals, around 65%, are often described as being right dominant. This means that it is the right coronary artery (RCA), which supplies the posterior descending artery. This coronary artery dominance is determined by the artery that supplies the PDA and the posterolateral artery. Of the remainder, around 25% are left dominant, meaning the left circumflex forms the PDA and 10% are co-dominant, which means that the PDA is supplied by both the right coronary and left circumflex arteries.
Other examples of anatomical variation include, the origin of the circumflex, which may arise from either the RCA or in turn, the RCA which could arise from part of the LMCA. This is important clinically, as this unusual course can compromise myocardial perfusion. For example, if one of these vessels runs between the aorta and pulmonary artery it can become compressed by the propogated pressure waves in these vessels, thus compromising blood flow.
In patients with coronary heart disease (CHD), coronary blood flow may be reduced, due to atherosclerosis and the presence of plaques within the coronary arteries. As a result, in these patients, supply can often fail to meet the increased demand for oxygen, causing angina pectoris. Thus, patients with symptoms of CHD, including chest pain, are often reffered for coronary angiography which can help identify possible stenoses within the coronary arteries creating an opportunity for percutanous coronary intervention (PCI).
Furthermore, as mentioned different areas of the myocardium are supplied by different coronary arteries. This has important clinical applications, especially in the use of a 12-lead ECG, where the evidence of ischaemia in certain leads can be linked to the specific coronary artery, that may contain an occlusion, supplying a certain region. Overall, this demonstrates that a firm understanding of the coronary circulation can assist in making the correct diagnosis and help guide the appropiate management of patients.
Coronary Arteries (1st Image)
Source: Wikimedia Commons Under Creative Commons Attribution ShareAlike 3.0 License. 2) Source: "By Coronary.pdf: Patrick J. Lynch, medical illustrator derivative work : Fred the Oyster (talk) adaption and further labeling: Mikael Häggström (Coronary.pdf) [CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons" "http://commons.wikimedia.org/wiki/File:Coronary_arteries.png"
Coronary Veins (2nd image)
Source: http://www.daviddarling.info/encyclopedia/C/coronary_vein.html The encyclopedia of science. Heart topics. Anatomy and Physiology.