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17.4: Coronary Circulation

  • Page ID
    22377
  • By the end of this section, you will be able to:

    • Identify the veins and arteries of the coronary circulation system
    • Describe how atherosclerosis impacts blood flow in coronary arteries

    You will recall that the heart is a remarkable pump composed largely of cardiac muscle cells that are incredibly active throughout life. Like all other cells, a cardiomyocyte requires a reliable supply of oxygen and nutrients, and a way to remove wastes, so it needs a dedicated, complex, and extensive coronary circulation. And because of the critical and nearly ceaseless activity of the heart throughout life, this need for a blood supply is even greater than for a typical cell. However, coronary circulation is not continuous; rather, it cycles, reaching a peak when the heart muscle is relaxed and nearly ceasing while it is contracting.

    Coronary Arteries

    Coronary arteries supply blood to the myocardium and other components of the heart. The first portion of the aorta after it arises from the left ventricle gives rise to the coronary arteries. There are three dilations in the wall of the aorta just superior to the aortic semilunar valve. Two of these, the left posterior aortic sinus and anterior aortic sinus, give rise to the left and right coronary arteries, respectively. The third sinus, the right posterior aortic sinus, typically does not give rise to a vessel. Coronary vessel branches that remain on the surface of the heart and follow the sulci are called epicardial coronary arteries.

    The left coronary artery distributes blood to the left side of the heart, the left atrium and ventricle, and the interventricular septum. It runs posterior to the pulmonary trunk and almost immediately branches into two vessels: the circumflex and anterior interventricular arteries. The circumflex artery follows the coronary sulcus to the left and eventually fuses with the small branches of the right coronary artery. The larger anterior interventricular artery, also known as the left anterior descending artery (LAD), follows the anterior interventricular sulcus around the pulmonary trunk. Along the way it gives rise to numerous smaller branches that interconnect with the branches of the posterior interventricular artery, forming anastomoses. An anastomosis is an area where vessels unite to form interconnections that normally allow blood to circulate to a region even if there may be partial blockage in another branch. The anastomoses in the heart are very small. Therefore, this ability is somewhat restricted in the heart so a coronary artery blockage often results in death of the cells (myocardial infarction) supplied by the particular vessel.

    The right coronary artery proceeds along the coronary sulcus and distributes blood to the right atrium, portions of both ventricles, and the cardiac conducting system. Normally, one or more marginal arteries arise from the right coronary artery inferior to the right atrium. The marginal arteries supply blood to the superficial portions of the right ventricle. On the posterior surface of the heart, the right coronary artery abruptly dips inferiorly and becomes the posterior interventricular artery, also known as the posterior descending artery. It runs along the posterior portion of the interventricular sulcus toward the apex of the heart, giving rise to branches that supply the posterior aspects of the interventricular septum and portions of both ventricles. Figure \(\PageIndex{1}\) presents the major vessels of coronary circulation from both the anterior and posterior views.

    Labeled diagrams of anterior and posterior views of the major coronary vessels as they are positioned around the surface of the heart.
    Figure \(\PageIndex{1}\): Coronary Circulation. The anterior view of the heart shows the prominent coronary surface vessels. The posterior view of the heart shows the prominent coronary surface vessels. (Image credit: "Coronary Blood Vessels" by OpenStax is licensed under CC BY 3.0)

    DISORDERS OF THE...

    Heart: Coronary Artery Disease

    Coronary artery disease is the leading cause of death worldwide. It occurs when the buildup of plaque—a fatty material including cholesterol, connective tissue, white blood cells, and some smooth muscle cells—within the walls of the arteries obstructs the flow of blood and decreases the flexibility or compliance of the vessels. This condition is called atherosclerosis, a hardening of the arteries that involves the accumulation of plaque. As the coronary blood vessels become occluded, the flow of blood to the tissues will be restricted, a condition called ischemia that causes the cells to receive insufficient amounts of oxygen, called hypoxia. Figure \(\PageIndex{2}\) shows the blockage of coronary arteries highlighted by the injection of dye. Some individuals with coronary artery disease report pain radiating from the chest called angina pectoris, but others remain asymptomatic. If untreated, coronary artery disease can lead to MI or a heart attack.

    Coronary angiogram showing two blockages in coronary arteries that visibly restrict blood flow through the areas.
    Figure \(\PageIndex{2}\): Atherosclerotic Coronary Arteries. In this coronary angiogram (X-ray), the dye makes visible two occluded coronary arteries, with noticeable narrowing pointed out in the circumflex artery and the common trunk of the left coronary artery. Such blockages can lead to decreased blood flow (ischemia) and insufficient oxygen (hypoxia) delivered to the cardiac tissues. If uncorrected, this can lead to cardiac muscle death (myocardial infarction). (Image credit: "Occluded Coronay Arteries [sic]" by OpenStax is licensed under CC BY 3.0)

    The disease progresses slowly and often begins in children and can be seen as fatty “streaks” in the vessels. It then gradually progresses throughout life. Well-documented risk factors include smoking, family history, hypertension, obesity, diabetes, high alcohol consumption, lack of exercise, stress, and hyperlipidemia or high circulating levels of lipids in the blood. Treatments may include medication, changes to diet and exercise, angioplasty with a balloon catheter, insertion of a stent, or coronary bypass procedure.

    Angioplasty is a procedure in which the occlusion is mechanically widened with a balloon. A specialized catheter with an expandable tip is inserted into a superficial vessel, normally in the leg, and then directed to the site of the occlusion. At this point, the balloon is inflated to compress the plaque material and to open the vessel to increase blood flow. Then, the balloon is deflated and retracted. A stent consisting of a specialized mesh is typically inserted at the site of occlusion to reinforce the weakened and damaged walls. Stent insertions have been routine in cardiology for more than 40 years.

    Coronary bypass surgery may also be performed. This surgical procedure grafts a replacement vessel obtained from another, less vital portion of the body to bypass the occluded area. This procedure is clearly effective in treating patients experiencing a MI, but overall does not increase longevity. Nor does it seem advisable in patients with stable although diminished cardiac capacity since frequently loss of mental acuity occurs following the procedure. Long-term changes to behavior, emphasizing diet and exercise plus a medicine regime tailored to lower blood pressure, lower cholesterol and lipids, and reduce clotting are equally as effective.

    DISORDERS OF THE...

    Heart: Myocardial Infarction

    Myocardial infarction (MI) is the formal term for what is commonly referred to as a heart attack. It normally results from a lack of blood flow (ischemia) and oxygen (hypoxia) to a region of the heart, resulting in death of the cardiac muscle cells. An MI often occurs when a coronary artery is blocked by the buildup of atherosclerotic plaque consisting of lipids, cholesterol and fatty acids, and white blood cells, primarily macrophages. It can also occur when a portion of an unstable atherosclerotic plaque travels through the coronary arterial system and lodges in one of the smaller vessels. The resulting blockage restricts the flow of blood and oxygen to the myocardium and causes death of the tissue. MIs may be triggered by excessive exercise, in which the partially occluded artery is no longer able to pump sufficient quantities of blood, or severe stress, which may induce spasm of the smooth muscle in the walls of the vessel.

    In the case of acute MI, there is often sudden pain beneath the sternum (retrosternal pain) called angina pectoris, often radiating down the left arm in males but not in female patients. Until this anomaly between the sexes was discovered, many female patients suffering MIs were misdiagnosed and sent home. In addition, patients typically present with difficulty breathing and shortness of breath (dyspnea), irregular heartbeat (palpitations), nausea and vomiting, sweating (diaphoresis), anxiety, and fainting (syncope), although not all of these symptoms may be present. Many of the symptoms are shared with other medical conditions, including anxiety attacks and simple indigestion, so differential diagnosis is critical. It is estimated that between 22 and 64 percent of MIs present without any symptoms.

    An MI can be confirmed by examining the patient’s ECG, which frequently reveals alterations in the ST and Q components. Some classification schemes of MI are referred to as ST-elevated MI (STEMI) and non-elevated MI (non-STEMI). In addition, echocardiography or cardiac magnetic resonance imaging may be employed. Common blood tests indicating an MI include elevated levels of creatine kinase MB (an enzyme that catalyzes the conversion of creatine to phosphocreatine, consuming ATP) and cardiac troponin (the regulatory protein for muscle contraction), both of which are released by damaged cardiac muscle cells.

    Immediate treatments for MI are essential and include administering supplemental oxygen, aspirin that helps to break up clots, and nitroglycerine administered sublingually (under the tongue) to facilitate its absorption. Despite its unquestioned success in treatments and use since the 1880s, the mechanism of nitroglycerine is still incompletely understood but is believed to involve the release of nitric oxide, a known vasodilator, and endothelium-derived releasing factor, which also relaxes the smooth muscle in the tunica media of coronary vessels. Longer-term treatments include injections of thrombolytic agents such as streptokinase that dissolve the clot, the anticoagulant heparin, balloon angioplasty and stents to open blocked vessels, and bypass surgery to allow blood to pass around the site of blockage. If the damage is extensive, coronary replacement with a donor heart or coronary assist device, a sophisticated mechanical device that supplements the pumping activity of the heart, may be employed. Despite the attention, development of artificial hearts to augment the severely limited supply of heart donors has proven less than satisfactory but will likely improve in the future.

    MIs may trigger cardiac arrest, but the two are not synonymous. Important risk factors for MI include cardiovascular disease, age, smoking, high blood levels of the low-density lipoprotein (LDL, often referred to as “bad” cholesterol), low levels of high-density lipoprotein (HDL, or “good” cholesterol), hypertension, diabetes mellitus, obesity, lack of physical exercise, chronic kidney disease, excessive alcohol consumption, and use of illegal drugs.

    Coronary Veins

    Coronary veins drain the heart and generally parallel the large surface arteries (see Figure \(\PageIndex{1}\)). The great cardiac vein can be seen initially on the surface of the heart following the interventricular sulcus, but it eventually flows along the coronary sulcus into the coronary sinus on the posterior surface. The great cardiac vein initially parallels the anterior interventricular artery and drains the areas supplied by this vessel. It receives several major branches, including the posterior cardiac vein, the middle cardiac vein, and the small cardiac vein. The posterior cardiac vein parallels and drains the areas supplied by the marginal artery branch of the circumflex artery. The middle cardiac vein parallels and drains the areas supplied by the posterior interventricular artery. The small cardiac vein parallels the right coronary artery and drains the blood from the posterior surfaces of the right atrium and ventricle. The coronary sinus is a large, thin-walled vein on the posterior surface of the heart lying within the atrioventricular sulcus and emptying directly into the right atrium. The anterior cardiac veins parallel the small cardiac arteries and drain the anterior surface of the right ventricle. Unlike these other cardiac veins, it bypasses the coronary sinus and drains directly into the right atrium.

    Concept Review

    The right and left coronary arteries are the first to branch off the aorta and arise from two of the three sinuses located near the base of the aorta and are generally located in the sulci. Cardiac veins parallel the small cardiac arteries and generally drain into the coronary sinus. Coronary artery disease is caused by the build up of plaque in the walls of coronary arteries that causes ischemia and hypoxia in downstream tissues.

    Review Questions

    Q. Which of the following branches from the right coronary artery?

    A. coronary sinus

    B. anterior interventricular artery

    C. posterior interventricular artery

    D. circumflex artery

    Answer

    C

    Q. Which of the following drains blood most directly from the anterior of the heart?

    A. great cardiac vein

    B. middle cardiac vein

    C. small cardiac vein

    D. coronary sinus

    Answer

    A

    Critical Thinking Questions

    Q. How can coronary artery disease lead to a myocardial infarction?

    Answer

    Coronary artery disease is marked by plaques of atherosclerosis that harden the walls of one or more coronary arteries, reducing their flexibility and eventually blocking the passage of blood (a condition referred to as ischemia). When less oxygen and nutrients are supplied to myocardium of the heart, cardiac muscle cells may die (referred to as myocardial infarction).

    Glossary

    anastomosis
    (plural = anastomoses) area where vessels unite to allow blood to circulate even if there may be partial blockage in another branch
    anterior cardiac veins
    vessels that parallel the small cardiac arteries and drain the anterior surface of the right ventricle; bypass the coronary sinus and drain directly into the right atrium
    anterior interventricular artery
    (also, left anterior descending artery or LAD) major branch of the left coronary artery that follows the anterior interventricular sulcus
    circumflex artery
    branch of the left coronary artery that follows coronary sulcus
    coronary veins
    vessels that drain the heart and generally parallel the large surface arteries
    epicardial coronary arteries
    surface arteries of the heart that generally follow the sulci
    great cardiac vein
    vessel that follows the interventricular sulcus on the anterior surface of the heart and flows along the coronary sulcus into the coronary sinus on the posterior surface; parallels the anterior interventricular artery and drains the areas supplied by this vessel
    marginal arteries
    branches of the right coronary artery that supply blood to the superficial portions of the right ventricle
    middle cardiac vein
    vessel that parallels and drains the areas supplied by the posterior interventricular artery; drains into the great cardiac vein
    posterior cardiac vein
    vessel that parallels and drains the areas supplied by the marginal artery branch of the circumflex artery; drains into the great cardiac vein
    posterior interventricular artery
    (also, posterior descending artery) branch of the right coronary artery that runs along the posterior portion of the interventricular sulcus toward the apex of the heart and gives rise to branches that supply the interventricular septum and portions of both ventricles
    small cardiac vein
    parallels the right coronary artery and drains blood from the posterior surfaces of the right atrium and ventricle; drains into the great cardiac vein
    cardiac cycle
    period of time between the onset of atrial contraction (atrial systole) and ventricular relaxation (ventricular diastole)
    diastole
    period of time when the heart muscle is relaxed and the chambers fill with blood
    end diastolic volume (EDV)
    (also, preload) the amount of blood in the ventricles at the end of atrial systole just prior to ventricular contraction
    end systolic volume (ESV)
    amount of blood remaining in each ventricle following systole
    heart sounds
    sounds heard via auscultation with a stethoscope of the closing of the atrioventricular valves (“lub”) and semilunar valves (“dub”)
    isovolumetric contraction
    (also, isovolumic contraction) initial phase of ventricular contraction in which tension and pressure in the ventricle increase, but no blood is pumped or ejected from the heart
    isovolumetric ventricular relaxation phase
    (also, isovolumic ventricular relaxation phase) initial phase of the ventricular diastole when pressure in the ventricles drops below pressure in the two major arteries, the pulmonary trunk, and the aorta, and blood attempts to flow back into the ventricles, closing the two semilunar valves
    murmur
    unusual heart sound detected by auscultation; typically related to septal or valve defects
    preload
    (also, end diastolic volume) amount of blood in the ventricles at the end of atrial systole just prior to ventricular contraction
    systole
    period of time when the heart muscle is contracting
    ventricular ejection phase
    second phase of ventricular systole during which blood is pumped from the ventricle

    Contributors and Attributions