Oct 26, 2009

The Coronary Arteries

The human heart is one of the most extraordinary tools employed for use in the body and has been perfected through evolution. Continuously pumping throughout its entire life, the heart supplies the body with a constant supply of blood and the payload which it carries. Oxygen, nutrients, hormones and countless other compounds are delivered via blood circulation throughout the body while cellular wastes are conversely deposited into returning blood flow for disposal. The muscular pumping action of the myocardium is dependent on a steady supply of oxygen just like any other muscle tissue and therefore requires a blood supply itself. The required distribution of blood to the myocardium is facilitated by the coronary arteries.


Despite the lumen of the heart being filled with blood for the majority of its life, nutrients and oxygen cannot be obtained by myocardial cells without the assistance of the coronary arteries. The cellular arrangement of the heart is very closely packed which does not permit blood flow directly among the myocardial muscle cells which therefore remain deoxygenated (Hill, 2008). Coronary circulation is carried out by the coronary arteries which branch from the base of the systemic aorta and carry freshly oxygenated blood to capillary beds within the myocardium before exiting to the right atrium via the coronary veins (Hill, 2008). This coronary circulation must deliver oxygen at a high resting rate in response to high myocardial demand as these cells typically require more than 20 times greater amounts of oxygen than resting skeletal muscle (Levick, 2003).

There are two coronary arteries, left and right, that originate from the right and left aortic sinuses (Fuster, 2004). The right artery travels along the coronary sulcus embedded in adipose tissue between the right atrium and towards the apex (Fuster, 2004). Branches from the right artery extend to the atrial myocardium, atrial septum, right ventricle, ventricular septum and the left ventricle (Little, 1989). The pulmonary conus and atrioventriuclar node of the heart may be supplied by either the right coronary artery or an alternate branch (Little, 1989). The left coronary artery divides into two branches termed the left circumflex artery and the left anterior descending artery which travel through adipose tissue with the latter manoeuvring through epicardial fat (Fuster, 2004)). The left anterior descending artery does as its name implies and travels downward towards the apex supplying blood to the left ventricle, ventricular septum and the right ventricle (Little, 1989). The left circumflex artery extends through the atrioventricular sulcus between the left atrium and ventricle which are both the main recipients of oxygenated blood from this branch (Little, 1989). Many structures of the heart are nourished by branches of both the left and right, unequally however, as dominance is displayed by the right coronary artery in 70% of all cases (Fuster, 2004). Despite the importance of the coronary arteries, circulation in this region is the shortest in the body with a mere 6-8 second cycles (Levick, 2003).

The walls of the coronary arteries have three layers termed the tunica intima, tunica media and tunica adventitia. The innermost layer, the intima, consists of a sheet of simple squamous endothelial cells which rests upon connective tissue and a thin basal lamina (Levick, 2003). This layer functions to secrete vasodilators such as nitric oxide and acts as a barrier which prevents the loss of plasma (Levick, 2003). The middle media layer contains circumferential smooth muscle cells spread through a matrix of elastin and collagen for contractile and mechanical strength (Levick, 2003). Signals may be conducted between the intima and media layers via endothelial cells that project through elastic lamina to contact smooth muscle cells (Levick, 2003). Fenestrations in the elastic lamina layers allow for the diffusion of nutrients and oxygen from the blood into the tunica media. The elastin content of the media layer allows the walls of the coronary arteries to stretch during cardiac ejection, resisting blood travelling at very high pressures (Mohrman, 2006). The outer connective tissue sheath that functions to keep the vessel loosely attached to surrounding tissue is termed the adventitia, consisting of mostly type one collagen, anchoring elastic fibres and a vasa vasorum network of smaller vessels (Levick, 2003). The coronary arteries consist of a relatively thick tunica media with a large amount of smooth muscle which functions to prevent collapse (Levick, 2003). This thick layer of muscle contracts and relaxes regularly due to autonomic innervations resulting in vasoconstriction or vasodilation respectively (Levick, 2003).


Angiogenesis, the formation of new vessels, occurs for many reasons including fetal development, tissue growth or adaptation and the growth of cancerous tumours (Levick, 2003). Endothelial growth is initiated by vascular endothelial growth factor as well as acidic and basic fibroblast growth factors (Levick, 2003). These growth factors are highly concentrated in tissues that form new vessels such as tumours and fetal tissue (Levick, 2003). Through a cascade of enzymatic reactions, these growth factors result in the activation of angiogenesis transcription genes (Levick, 2003). Endothelial basement membrane is degraded which is followed by the off-shooting of endothelium from existing vessels (Levick, 2003). The lumen of the vessel is created as vacuoles form within the cell interior as cells divide and migrate, lengthening the developing artery (Levick, 2003). Newly formed vessels are highly permeable and allow the vast influx of fibrinogen which forms an extensive fibrin vascular matrix termed the granulation tissue (Levick, 2003). Simple endothelial tubes are created which eventually differentiates into an artery, vein or capillary (Levick, 2003). In the developing embryo, both coronary endothelial and smooth muscle cells derive from a cluster of cells adjacent to the sinus venosus of the heart called the pro-epicardium organ (Fuster, 2004).

Problems may arise in the functioning of the coronary arteries and the results can accentuate how critical these vessels are to life. When the blood flow in the coronary arteries fails to meet the oxygen requirements of the myocardium, the pumping efficiency of the heart declines. The most common form of coronary artery disease is atherosclerosis in which lipid plaque deposits amass on the lumen of the vessel and potentially become calcified as well (Mohrman, 2006). The build up of plaque significantly narrows the lumen of the arteries which can increase the usually low vascular resistance, promote clot formation through aggregation of platelets and potentially close off the artery all together (Mohrman, 2006). Apart from restricted cardiac output, atherosclerosis can cause heart attacks or strokes when the hardened plaques fragment and metastasize in other important bodily vessels (American Heart Association, 2009). Damage to the endothelial lining caused by tobacco smoke, diabetes, physical inactivity, obesity, high blood pressure and high LDL blood cholesterol levels increase the risk of severe atherosclerosis (American Heart Association, 2009). When the strong contractions of the coronary tunica media are intense and prolonged or narrow arteries undersupply oxygen to myocardial cells, pain results as in the condition angina pectoris (Levick, 2003). While not a disease itself, angina is the most common symptom of coronary artery disease (National Heart Lung and Blood Institute, 2007).

There are several methods to combat the aforementioned pathologies. Behavioural and dietary changes can aid in the prevention or even reversal of coronary artery disease. As many of the causes of this disease are lifestyle choices, quitting smoking, regular exercise and decreasing blood lipid (especially LDL cholesterol) levels through diet or pharmaceuticals can result in improved coronary conditions. Drugs that alter the functioning of the vessels also act to treat coronary artery disease. Vasodilators, such as nitroglycerin or calcium channel inhibitors, expand the diameter of the vessel lumen and permit more blood flow (Mohrman, 2006). Other drugs, such as propranolol, may act to limit myocardial oxygen consumption and act on cardiac sympathetic nerves to prevent an increase in heart rate above what the coronary blood flow can sustain (Mohrman, 2006). Alternative surgical treatments are available as well in the management of coronary artery disease. In angioplasty, a balloon tipped catheter is fed through the vessels and rapidly inflated at strategic points as to compress plaque build up against the vessel wall (Mohrman, 2006). Often implanted at the same time as angioplasty is performed, a rigid device called a stent is left in the vessel and acts to prop it open which increases blood flow (Mohrman, 2006). A more invasive operation, coronary bypass surgery, may also be performed if the other methods are unsuccessful. In this surgery, blood vessels from the patient’s own body (such as a mammary artery) or a synthetic vessel are transplanted to provide an alternate route for blood flow (Mohrman, 2006). The artery originates prior to and inserts just after a blockage, providing a parallel low resistance passage for blood (Mohrman, 2006).

The coronary arteries form a critical lifeline for one of the most important organs in the body. These vessels are heavily depended upon by the heart but are highly specialized enough to keep up with the demand. When these nourishing structures do not receive the necessary care they require, problems may arise and a lifetime of perpetual service can suddenly cease.


References:

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http://www.nhlbi.nih.gov/health/dci/Diseases/stents/stents_placed.html


National Heart Lung and Blood Institute (2007). What is Angina?. Retrieved 15 October 2009 From: http://www.nhlbi.nih.gov/health/dci/Diseases/Angina/Angina_WhatIs.html

National Heart Lung and Blood Institute (2009). What Is Coronary Artery Disease?. Retrieved 15 October 2009 From: http://www.nhlbi.nih.gov/health/dci/Diseases/Cad/CAD_WhatIs.html

Society for Biomaterials Special Interest Group. Cardiovascular Biomaterials. Retrieved 13 November 2009 From: http://www.biomaterials.org/SIGS/Cardiovascular/Heart.htm