{"id":5060,"date":"2015-10-28T15:56:24","date_gmt":"2015-10-28T15:56:24","guid":{"rendered":"https:\/\/opentextbc.ca\/biology\/chapter\/21-3-mammalian-heart-and-blood-vessels\/"},"modified":"2021-03-04T00:09:01","modified_gmt":"2021-03-04T00:09:01","slug":"21-3-mammalian-heart-and-blood-vessels","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/biology\/chapter\/21-3-mammalian-heart-and-blood-vessels\/","title":{"raw":"21.3.\u00a0Mammalian Heart and Blood Vessels","rendered":"21.3.\u00a0Mammalian Heart and Blood Vessels"},"content":{"raw":"<div class=\"titlepage\">\n<div class=\"abstract\">\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\nBy the end of this section, you will be able to:\n<div class=\"itemizedlist\">\n<ul class=\"itemizedlist\">\n \t<li class=\"listitem\">Describe the structure of the heart and explain how cardiac muscle is different from other muscles<\/li>\n \t<li class=\"listitem\">Describe the cardiac cycle<\/li>\n \t<li class=\"listitem\">Explain the structure of arteries, veins, and capillaries, and how blood flows through the body<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44805-fs-idm49489856\"> <\/span>The heart is a complex muscle that pumps blood through the three divisions of the circulatory system: the coronary (vessels that serve the heart), pulmonary (heart and lungs), and systemic (systems of the body), as shown in <a class=\"xref target-figure\" title=\"Figure\u00a040.10.\u00a0\" href=\"#attachment_1324\">Figure 21.10<\/a>. Coronary circulation intrinsic to the heart takes blood directly from the main artery (aorta) coming from the heart. For pulmonary and systemic circulation, the heart has to pump blood to the lungs or the rest of the body, respectively. In vertebrates, the lungs are relatively close to the heart in the thoracic cavity. The shorter distance to pump means that the muscle wall on the right side of the heart is not as thick as the left side which must have enough pressure to pump blood all the way to your big toe.\n<div id=\"m44805-fs-idm48341552\" class=\"note art-connection\">\n<div class=\"title\">\n<div class=\"body\">\n<div id=\"m44805-fig-ch40_03_01\" class=\"figure\" title=\"Figure\u00a040.10.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\"><\/div>\n<div class=\"mediaobject\">\n\n[caption id=\"attachment_1324\" align=\"aligncenter\" width=\"450\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_01ab.png\"><img class=\"wp-image-5052\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_40_03_01ab.png\" alt=\"Figure_40_03_01ab\" width=\"450\" height=\"413\"><\/a> Figure 21.10.\u00a0 The mammalian circulatory system is divided into three circuits: the systemic circuit, the pulmonary circuit, and the coronary circuit. Blood is pumped from veins of the systemic circuit into the right atrium of the heart, then into the right ventricle. Blood then enters the pulmonary circuit, and is oxygenated by the lungs. From the pulmonary circuit, blood re-enters the heart through the left atrium. From the left ventricle, blood re-enters the systemic circuit through the aorta and is distributed to the rest of the body. The coronary circuit, which provides blood to the heart, is not shown.[\/caption]\n\n<\/div>\n<\/div>\n<div class=\"title\"><\/div>\n<span id=\"m44805-fs-idm88596752\"> <\/span>Which of the following statements about the circulatory system is false?\n<div class=\"orderedlist\">\n<ol class=\"orderedlist\">\n \t<li class=\"listitem\">Blood in the pulmonary vein is deoxygenated.<\/li>\n \t<li class=\"listitem\">Blood in the inferior vena cava is deoxygenated.<\/li>\n \t<li class=\"listitem\">Blood in the pulmonary artery is deoxygenated.<\/li>\n \t<li class=\"listitem\">Blood in the aorta is oxygenated.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Structure of the Heart\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44805-fs-idm125746512\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Structure of the Heart<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44805-fs-idp58983376\"> <\/span>The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in <a class=\"xref target-figure\" title=\"Figure\u00a040.11.\u00a0\" href=\"#attachment_1325\">Figure 21.11<\/a>. In humans, the heart is about the size of a clenched fist; it is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the <span id=\"m44805-autoid-cnx2dbk-id1770127\"> <\/span><strong>superior vena cava<\/strong>, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the <strong><span id=\"m44805-autoid-cnx2dbk-id1770131\"> <\/span>inferior vena cava<\/strong> which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the <span id=\"m44805-autoid-cnx2dbk-id1770137\"> <\/span><strong>atrioventricular valve<\/strong> or the <strong><span id=\"m44805-autoid-cnx2dbk-id1770141\"> <\/span>tricuspid valve<\/strong>, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the biscuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, by-passing the <strong><span id=\"m44805-autoid-cnx2dbk-id1767724\"> <\/span>semilunar valve<\/strong> (or pulmonic valve) to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the <span id=\"m44805-autoid-cnx2dbk-id1767730\"> <\/span><strong>bicuspid valve<\/strong> or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through <span id=\"m44805-autoid-cnx2dbk-id1767735\"> <\/span><strong>aorta<\/strong>, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.\n<div id=\"m44805-fs-idm2331712\" class=\"note art-connection\">\n<div class=\"title\">\n<div class=\"body\">\n<div id=\"m44805-fig-ch40_03_02\" class=\"figure\" title=\"Figure\u00a040.11.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\">\n\n[caption id=\"attachment_1325\" align=\"aligncenter\" width=\"500\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_02ab.png\"><img class=\"wp-image-5053\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_02ab-824x1024-1.png\" alt=\"Figure_40_03_02ab\" width=\"500\" height=\"621\"><\/a> Figure 21.11.\u00a0 (a) The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers. (b) Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated.[\/caption]\n\n<\/div>\n<div class=\"mediaobject\"><\/div>\n<\/div>\n<span id=\"m44805-fs-idm34396288\"> <\/span>Which of the following statements about the heart is false?\n<div class=\"orderedlist\">\n<ol class=\"orderedlist\">\n \t<li class=\"listitem\">The mitral valve separates the left ventricle from the left atrium.<\/li>\n \t<li class=\"listitem\">Blood travels through the bicuspid valve to the left atrium.<\/li>\n \t<li class=\"listitem\">Both the aortic and the pulmonary valves are semilunar valves.<\/li>\n \t<li class=\"listitem\">The mitral valve is an atrioventricular valve.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44805-fs-idm123004624\"> <\/span>The heart is composed of three layers; the epicardium, the myocardium, and the endocardium, illustrated in <a class=\"xref target-figure\" title=\"Figure\u00a040.11.\u00a0\" href=\"#attachment_1325\">Figure 21.11<\/a>. The inner wall of the heart has a lining called the <strong><span id=\"m44805-autoid-cnx2dbk-id1767595\"> <\/span>endocardium<\/strong>. The <span id=\"m44805-autoid-cnx2dbk-id1767598\"> <\/span><strong>myocardium<\/strong> consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the <span id=\"m44805-autoid-cnx2dbk-id1767604\"> <\/span><strong>epicardium<\/strong>, of which the second layer is a membranous layered structure called the <span id=\"m44805-autoid-cnx2dbk-id1767607\"> <\/span><strong>pericardium<\/strong> that surrounds and protects the heart; it allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.\n\n<span id=\"m44805-fs-idm202630448\"> <\/span>The heart has its own blood vessels that supply the heart muscle with blood. The <span id=\"m44805-autoid-cnx2dbk-id1767619\"> <\/span><strong>coronary arteries<a id=\"id833341\" class=\"indexterm\" href=\"\"><\/a><\/strong> branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the <span id=\"m44805-autoid-cnx2dbk-id1767625\"> <\/span><strong>coronary veins<\/strong><a id=\"id833356\" class=\"indexterm\" href=\"\"><\/a> to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. <span id=\"m44805-autoid-cnx2dbk-id1767630\"> <\/span><strong>Atherosclerosis<\/strong><a id=\"id833371\" class=\"indexterm\" href=\"\"><\/a> is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain, known as <span id=\"m44805-autoid-cnx2dbk-id1767636\"> <\/span><strong>angina<\/strong><a id=\"id833387\" class=\"indexterm\" href=\"\"><\/a>, and complete blockage of the arteries will cause <strong><span id=\"m44805-autoid-cnx2dbk-id1767640\"> <\/span>myocardial infarction<\/strong><a id=\"id833401\" class=\"indexterm\" href=\"\"><\/a>: the death of cardiac muscle tissue, commonly known as a heart attack.\n<div class=\"section\" title=\"The Cardiac Cycle\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44805-fs-idp5274160\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">The Cardiac Cycle<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44805-fs-idm116254208\"> <\/span>The main purpose of the heart is to pump blood through the body; it does so in a repeating sequence called the cardiac cycle. The <span id=\"m44805-autoid-cnx2dbk-id1767661\"> <\/span><strong>cardiac cycle<\/strong> is the coordination of the filling and emptying of the heart of blood by electrical signals that cause the heart muscles to contract and relax. The human heart beats over 100,000 times per day. In each cardiac cycle, the heart contracts (<span id=\"m44805-autoid-cnx2dbk-id1767667\"> <\/span><strong>systole<\/strong>), pushing out the blood and pumping it through the body; this is followed by a relaxation phase (<span id=\"m44805-autoid-cnx2dbk-id1767671\"> <\/span><strong>diastole<\/strong>), where the heart fills with blood, as illustrated in <a class=\"xref target-figure\" title=\"Figure\u00a040.12.\u00a0\" href=\"#attachment_1326\">Figure 21.12<\/a>. The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Closing of the atrioventricular valves produces a monosyllabic \u201clup\u201d sound. Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs (via the pulmonary artery). Closing of the semilunar valves produces a monosyllabic \u201cdup\u201d sound.\n\n<\/div>\n\n[caption id=\"attachment_1326\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_03.jpg\"><img class=\"wp-image-5054\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_03-1024x562-1.jpg\" alt=\"Figure_40_03_03\" width=\"600\" height=\"329\"><\/a> Figure 21.12.\u00a0 During (a) cardiac diastole, the heart muscle is relaxed and blood flows into the heart. During (b) atrial systole, the atria contract, pushing blood into the ventricles. During (c) atrial diastole, the ventricles contract, forcing blood out of the heart.[\/caption]\n\n<div id=\"m44805-fig-ch40_03_03\" class=\"figure\" title=\"Figure\u00a040.12.\u00a0\">\n<div class=\"title\"><\/div>\n<span id=\"m44805-fs-idm170442304\"> <\/span>The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart muscle. <span id=\"m44805-autoid-cnx2dbk-id1769846\"> <\/span><strong>Cardiomyocytes<\/strong>, shown in Figure 21.13, are distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle; they are connected by intercalated disks exclusive to cardiac muscle. They are self-stimulated for a period of time and isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.\n\n[caption id=\"attachment_1327\" align=\"aligncenter\" width=\"400\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_04.jpg\"><img class=\"wp-image-5055\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_04.jpg\" alt=\"Figure_40_03_04\" width=\"400\" height=\"320\"><\/a> Figure 21.13.\u00a0 Cardiomyocytes are striated muscle cells found in cardiac tissue. (credit: modification of work by Dr. S. Girod, Anton Becker; scale-bar data from Matt Russell)[\/caption]\n\n<div id=\"m44805-fig-ch40_03_04\" class=\"figure\" title=\"Figure\u00a040.13.\u00a0\">\n<div class=\"title\"><\/div>\n<span id=\"m44805-fs-idm94701888\"> <\/span>The autonomous beating of cardiac muscle cells is regulated by the heart\u2019s internal pacemaker that uses electrical signals to time the beating of the heart. The electrical signals and mechanical actions, illustrated in <a class=\"xref target-figure\" title=\"Figure\u00a040.14.\u00a0\" href=\"#attachment_1328\">Figure 21.14<\/a>, are intimately intertwined. The internal pacemaker starts at the <span id=\"m44805-autoid-cnx2dbk-id1769901\"> <\/span><strong>sinoatrial (SA) node<\/strong>, which is located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node causing the two atria to contract in unison. The pulse reaches a second node, called the atrioventricular (AV) node, between the right atrium and right ventricle where it pauses for approximately 0.1 second before spreading to the walls of the ventricles. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, and then the ventricles contract. This pause allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes. This information can be observed as an <span id=\"m44805-autoid-cnx2dbk-id1769913\"> <\/span><strong>electrocardiogram (ECG)<\/strong>\u2014a recording of the electrical impulses of the cardiac muscle.\n\n[caption id=\"attachment_1328\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_05.jpg\"><img class=\"wp-image-5056\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_05-1024x623-1.jpg\" alt=\"Figure_40_03_05\" width=\"600\" height=\"365\"><\/a> Figure 21.14.\u00a0 The beating of the heart is regulated by an electrical impulse that causes the characteristic reading of an ECG. The signal is initiated at the sinoatrial valve. The signal then (a) spreads to the atria, causing them to contract. The signal is (b) delayed at the atrioventricular node before it is passed on to the (c) heart apex. The delay allows the atria to relax before the (d) ventricles contract. The final part of the ECG cycle prepares the heart for the next beat.[\/caption]\n\n<div id=\"m44805-fs-idm190521904\" class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44805-fs-idp35661232\"> <\/span><img src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/electric_heart.png\" alt=\"QR Code representing a URL\" width=\"120\"><\/div>\n<span id=\"m44805-fs-idm356726752\"> <\/span>Visit <a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/electric_heart\" target=\"\" rel=\"noopener noreferrer\">this site<\/a> to see the heart\u2019s \u201cpacemaker\u201d in action.\n\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Arteries, Veins, and Capillaries\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44805-fs-idp11676624\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Arteries, Veins, and Capillaries<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44805-fs-idm128248512\"> <\/span>The blood from the heart is carried through the body by a complex network of blood vessels (<a class=\"xref target-figure\" title=\"Figure\u00a040.15.\u00a0\" href=\"#attachment_1330\">Figure 21.15<\/a>). <span id=\"m44805-autoid-cnx2dbk-id1768892\"> <\/span><strong>Arteries<\/strong> take blood away from the heart. The main artery is the aorta that branches into major arteries that take blood to different limbs and organs. These major arteries include the carotid artery that takes blood to the brain, the brachial arteries that take blood to the arms, and the thoracic artery that takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidney, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then smaller vessels called <span id=\"m44805-autoid-cnx2dbk-id1768900\"> <\/span><strong>arterioles<\/strong>, to reach more deeply into the muscles and organs of the body.\n\n[caption id=\"attachment_1330\" align=\"aligncenter\" width=\"400\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_06.jpg\"><img class=\"wp-image-5058\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_06.jpg\" alt=\"Figure_40_03_06\" width=\"400\" height=\"602\"><\/a> Figure 21.15.\u00a0 The major human arteries and veins are shown. (credit: modification of work by Mariana Ruiz Villareal)[\/caption]\n\n<div id=\"m44805-fig-ch40_03_06\" class=\"figure\" title=\"Figure\u00a040.15.\u00a0\">\n\n<span id=\"m44805-fs-idm204731312\"> <\/span>Arterioles diverge into capillary beds. <span id=\"m44805-autoid-cnx2dbk-id1768944\"> <\/span><strong>Capillary beds<\/strong><a id=\"id833854\" class=\"indexterm\" href=\"\"><\/a> contain a large number (10 to 100) of <span id=\"m44805-autoid-cnx2dbk-id1768947\"> <\/span><strong>capillaries<\/strong><a id=\"id833868\" class=\"indexterm\" href=\"\"><\/a> that branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into <span id=\"m44805-autoid-cnx2dbk-id1762819\"> <\/span><strong>venules<\/strong><a id=\"id833885\" class=\"indexterm\" href=\"\"><\/a> that connect to minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart. <span id=\"m44805-autoid-cnx2dbk-id1762824\"> <\/span><strong>Veins<\/strong><a id=\"id833900\" class=\"indexterm\" href=\"\"><\/a> are blood vessels that bring blood back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system.\n\n<span id=\"m44805-fs-idm69642496\"> <\/span>The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels (<a class=\"xref target-figure\" title=\"Figure\u00a040.16.\u00a0\" href=\"#attachment_1331\">Figure 21.16<\/a>). The first tunic is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by <span id=\"m44805-autoid-cnx2dbk-id1762848\"> <\/span><strong>vasoconstriction<\/strong>, narrowing of the blood vessels, and <span id=\"m44805-autoid-cnx2dbk-id1762851\"> <\/span><strong>vasodilation,<\/strong> widening of the blood vessels; this is important in the overall regulation of blood pressure.\n\n<span id=\"m44805-fs-idm46223136\"> <\/span>Veins and arteries both have two further tunics that surround the endothelium: the middle tunic is composed of smooth muscle and the outermost layer is connective tissue (collagen and elastic fibers). The elastic connective tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different than arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.\n\n[caption id=\"attachment_1331\" align=\"aligncenter\" width=\"500\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_07.jpg\"><img class=\"wp-image-5059\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_07.jpg\" alt=\"Figure_40_03_07\" width=\"500\" height=\"313\"><\/a> Figure 21.16.\u00a0 Arteries and veins consist of three layers: an outer tunica externa, a middle tunica media, and an inner tunica intima. Capillaries consist of a single layer of epithelial cells, the tunica intima. (credit: modification of work by NCI, NIH)[\/caption]\n<h2>Summary<\/h2>\nThe heart muscle pumps blood through three divisions of the circulatory system: coronary, pulmonary, and systemic. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The pumping of the heart is a function of cardiomyocytes, distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle. The internal pacemaker starts at the sinoatrial node, which is located near the wall of the right atrium. Electrical charges pulse from the SA node causing the two atria to contract in unison; then the pulse reaches the atrioventricular node between the right atrium and right ventricle. A pause in the electric signal allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The blood from the heart is carried through the body by a complex network of blood vessels; arteries take blood away from the heart, and veins bring blood back to the heart.\n<div class=\"section\">\n<div class=\"body\">\n<div class=\"textbox exercises\">\n<h3>Exercises<\/h3>\n<ol>\n \t<li>The heart\u2019s internal pacemaker beats by:\n<ol>\n \t<li>an internal implant that sends an electrical impulse through the heart<\/li>\n \t<li>the excitation of cardiac muscle cells at the sinoatrial node followed by the atrioventricular node<\/li>\n \t<li>the excitation of cardiac muscle cells at the atrioventricular node followed by the sinoatrial node<\/li>\n \t<li>the action of the sinus<\/li>\n<\/ol>\n<\/li>\n \t<li>During the systolic phase of the cardiac cycle, the heart is ________.\n<ol>\n \t<li>contracting<\/li>\n \t<li>relaxing<\/li>\n \t<li>contracting and relaxing<\/li>\n \t<li>filling with blood<\/li>\n<\/ol>\n<\/li>\n \t<li><span id=\"m44805-fs-idm52992688\">Cardiomyocytes are similar to skeletal muscle because:<\/span>\n<ol>\n \t<li>they beat involuntarily<\/li>\n \t<li>they are used for weight lifting<\/li>\n \t<li>they pulse rhythmically<\/li>\n \t<li>they are striated<\/li>\n<\/ol>\n<\/li>\n \t<li>How do arteries differ from veins?\n<ol>\n \t<li>Arteries have thicker smooth muscle layers to accommodate the changes in pressure from the heart.<\/li>\n \t<li>Arteries carry blood.<\/li>\n \t<li>Arteries have thinner smooth muscle layers and valves and move blood by the action of skeletal muscle.<\/li>\n \t<li>Arteries are thin walled and are used for gas exchange.<\/li>\n<\/ol>\n<\/li>\n \t<li>Describe the cardiac cycle.<\/li>\n \t<li><span id=\"m44805-fs-idm83165984\">What happens in capillaries?<\/span><\/li>\n<\/ol>\n<strong>Answers<\/strong>\n<ol>\n \t<li>B<\/li>\n \t<li>A<\/li>\n \t<li>D<\/li>\n \t<li>A<\/li>\n \t<li>The heart receives an electrical signal from the sinoatrial node triggering the cardiac muscle cells in the atria to contract. The signal pauses at the atrioventricular node before spreading to the walls of the ventricles so the blood is pumped through the body. This is the systolic phase. The heart then relaxes in the diastole and fills again with blood.<\/li>\n \t<li>The capillaries basically exchange materials with their surroundings. Their walls are very thin and are made of one or two layers of cells, where gases, nutrients, and waste are diffused. They are distributed as beds, complex networks that link arteries as well as veins.<\/li>\n<\/ol>\n<\/div>\n&nbsp;\n\n<\/div>\n<div class=\"body\">\n<div class=\"section\" title=\"Blood Pressure Regulation\">\n<div class=\"glossary\" title=\"Glossary\">\n<div class=\"titlepage\">\n<div class=\"bcc-box bcc-success\">\n<h3>Glossary<\/h3>\n<div class=\"glossary\" title=\"Glossary\">\n<dl>\n \t<dt><strong>angina<\/strong><\/dt>\n \t<dd>pain caused by partial blockage of the coronary arteries by the buildup of plaque and lack of oxygen to the heart muscle<\/dd>\n \t<dt><strong>aorta<\/strong><\/dt>\n \t<dd>major artery of the body that takes blood away from the heart<\/dd>\n \t<dt><strong>arteriole<\/strong><\/dt>\n \t<dd>small vessel that connects an artery to a capillary bed<\/dd>\n \t<dt><strong>artery<\/strong><\/dt>\n \t<dd>blood vessel that takes blood away from the heart<\/dd>\n \t<dt><strong>atherosclerosis<\/strong><\/dt>\n \t<dd>buildup of fatty plaques in the coronary arteries in the heart<\/dd>\n \t<dt><strong>bicuspid valve<\/strong><\/dt>\n \t<dd>(also, mitral valve; left atrioventricular valve) one-way membranous flap between the atrium and the ventricle in the left side of the heart<\/dd>\n \t<dt><strong>capillary bed<\/strong><\/dt>\n \t<dd>large number of capillaries that converge to take blood to a particular organ or tissue<\/dd>\n \t<dt><strong>capillary<\/strong><\/dt>\n \t<dd>smallest blood vessel that allows the passage of individual blood cells and the site of diffusion of oxygen and nutrient exchange<\/dd>\n \t<dt><strong>cardiac cycle<\/strong><\/dt>\n \t<dd>filling and emptying the heart of blood by electrical signals that cause the heart muscles to contract and relax<\/dd>\n \t<dt><strong>cardiac output<\/strong><\/dt>\n \t<dd>the volume of blood pumped by the heart in one minute as a product of heart rate multiplied by stroke volume<\/dd>\n \t<dt><strong>cardiomyocyte<\/strong><\/dt>\n \t<dd>specialized heart muscle cell that is striated but contracts involuntarily like smooth muscle<\/dd>\n \t<dt><strong>coronary artery<\/strong><\/dt>\n \t<dd>vessel that supplies the heart tissue with blood<\/dd>\n \t<dt><strong>coronary vein<\/strong><\/dt>\n \t<dd>vessel that takes blood away from the heart tissue back to the chambers in the heart<\/dd>\n \t<dt><strong>diastole<\/strong><\/dt>\n \t<dd>relaxation phase of the cardiac cycle when the heart is relaxed and the ventricles are filling with blood<\/dd>\n \t<dt><strong>electrocardiogram (ECG)<\/strong><\/dt>\n \t<dd>recording of the electrical impulses of the cardiac muscle<\/dd>\n \t<dt><strong>endocardium<\/strong><\/dt>\n \t<dd>innermost layer of tissue in the heart<\/dd>\n \t<dt><strong>epicardium<\/strong><\/dt>\n \t<dd>outermost tissue layer of the heart<\/dd>\n \t<dt><strong>inferior vena cava<\/strong><\/dt>\n \t<dd>drains blood from the veins that come from the lower organs and the legs<\/dd>\n \t<dt><strong>myocardial infarction<\/strong><\/dt>\n \t<dd>(also, heart attack) complete blockage of the coronary arteries and death of the cardiac muscle tissue<\/dd>\n \t<dt><strong>myocardium<\/strong><\/dt>\n \t<dd>heart muscle cells that make up the middle layer and the bulk of the heart wall<\/dd>\n \t<dt><strong>pericardium<\/strong><\/dt>\n \t<dd>membrane layer protecting the heart; also part of the epicardium<\/dd>\n \t<dt><strong>semilunar valve<\/strong><\/dt>\n \t<dd>membranous flap of connective tissue between the aorta and a ventricle of the heart (the aortic or pulmonary semilunar valves)<\/dd>\n \t<dt><strong>sinoatrial (SA) node<\/strong><\/dt>\n \t<dd>the heart\u2019s internal pacemaker; located near the wall of the right atrium<\/dd>\n \t<dt><strong>superior vena cava<\/strong><\/dt>\n \t<dd>drains blood from the jugular vein that comes from the brain and from the veins that come from the arms<\/dd>\n \t<dt><strong>systole<\/strong><\/dt>\n \t<dd>contraction phase of cardiac cycle when the ventricles are pumping blood into the arteries<\/dd>\n \t<dt><strong>tricuspid valve<\/strong><\/dt>\n \t<dd>one-way membranous flap of connective tissue between the atrium and the ventricle in the right side of the heart; also known as atrioventricular valve<\/dd>\n \t<dt><strong>vasoconstriction<\/strong><\/dt>\n \t<dd>narrowing of a blood vessel<\/dd>\n \t<dt><strong>vasodilation<\/strong><\/dt>\n \t<dd>widening of a blood vessel<\/dd>\n<\/dl>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"cnx-eoc summary\">\n<div class=\"section\"><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>","rendered":"<div class=\"titlepage\">\n<div class=\"abstract\">\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<div class=\"itemizedlist\">\n<ul class=\"itemizedlist\">\n<li class=\"listitem\">Describe the structure of the heart and explain how cardiac muscle is different from other muscles<\/li>\n<li class=\"listitem\">Describe the cardiac cycle<\/li>\n<li class=\"listitem\">Explain the structure of arteries, veins, and capillaries, and how blood flows through the body<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44805-fs-idm49489856\"> <\/span>The heart is a complex muscle that pumps blood through the three divisions of the circulatory system: the coronary (vessels that serve the heart), pulmonary (heart and lungs), and systemic (systems of the body), as shown in <a class=\"xref target-figure\" title=\"Figure\u00a040.10.\u00a0\" href=\"#attachment_1324\">Figure 21.10<\/a>. Coronary circulation intrinsic to the heart takes blood directly from the main artery (aorta) coming from the heart. For pulmonary and systemic circulation, the heart has to pump blood to the lungs or the rest of the body, respectively. In vertebrates, the lungs are relatively close to the heart in the thoracic cavity. The shorter distance to pump means that the muscle wall on the right side of the heart is not as thick as the left side which must have enough pressure to pump blood all the way to your big toe.<\/p>\n<div id=\"m44805-fs-idm48341552\" class=\"note art-connection\">\n<div class=\"title\">\n<div class=\"body\">\n<div id=\"m44805-fig-ch40_03_01\" class=\"figure\" title=\"Figure\u00a040.10.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\"><\/div>\n<div class=\"mediaobject\">\n<figure id=\"attachment_1324\" aria-describedby=\"caption-attachment-1324\" style=\"width: 450px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_01ab.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-5052\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_40_03_01ab.png\" alt=\"Figure_40_03_01ab\" width=\"450\" height=\"413\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_40_03_01ab.png 532w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_40_03_01ab-300x275.png 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_40_03_01ab-65x60.png 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_40_03_01ab-225x206.png 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_40_03_01ab-350x321.png 350w\" sizes=\"auto, (max-width: 450px) 100vw, 450px\" \/><\/a><figcaption id=\"caption-attachment-1324\" class=\"wp-caption-text\">Figure 21.10.\u00a0 The mammalian circulatory system is divided into three circuits: the systemic circuit, the pulmonary circuit, and the coronary circuit. Blood is pumped from veins of the systemic circuit into the right atrium of the heart, then into the right ventricle. Blood then enters the pulmonary circuit, and is oxygenated by the lungs. From the pulmonary circuit, blood re-enters the heart through the left atrium. From the left ventricle, blood re-enters the systemic circuit through the aorta and is distributed to the rest of the body. The coronary circuit, which provides blood to the heart, is not shown.<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<div class=\"title\"><\/div>\n<p><span id=\"m44805-fs-idm88596752\"> <\/span>Which of the following statements about the circulatory system is false?<\/p>\n<div class=\"orderedlist\">\n<ol class=\"orderedlist\">\n<li class=\"listitem\">Blood in the pulmonary vein is deoxygenated.<\/li>\n<li class=\"listitem\">Blood in the inferior vena cava is deoxygenated.<\/li>\n<li class=\"listitem\">Blood in the pulmonary artery is deoxygenated.<\/li>\n<li class=\"listitem\">Blood in the aorta is oxygenated.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Structure of the Heart\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44805-fs-idm125746512\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Structure of the Heart<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44805-fs-idp58983376\"> <\/span>The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in <a class=\"xref target-figure\" title=\"Figure\u00a040.11.\u00a0\" href=\"#attachment_1325\">Figure 21.11<\/a>. In humans, the heart is about the size of a clenched fist; it is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the <span id=\"m44805-autoid-cnx2dbk-id1770127\"> <\/span><strong>superior vena cava<\/strong>, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the <strong><span id=\"m44805-autoid-cnx2dbk-id1770131\"> <\/span>inferior vena cava<\/strong> which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the <span id=\"m44805-autoid-cnx2dbk-id1770137\"> <\/span><strong>atrioventricular valve<\/strong> or the <strong><span id=\"m44805-autoid-cnx2dbk-id1770141\"> <\/span>tricuspid valve<\/strong>, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the biscuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, by-passing the <strong><span id=\"m44805-autoid-cnx2dbk-id1767724\"> <\/span>semilunar valve<\/strong> (or pulmonic valve) to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the <span id=\"m44805-autoid-cnx2dbk-id1767730\"> <\/span><strong>bicuspid valve<\/strong> or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through <span id=\"m44805-autoid-cnx2dbk-id1767735\"> <\/span><strong>aorta<\/strong>, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.<\/p>\n<div id=\"m44805-fs-idm2331712\" class=\"note art-connection\">\n<div class=\"title\">\n<div class=\"body\">\n<div id=\"m44805-fig-ch40_03_02\" class=\"figure\" title=\"Figure\u00a040.11.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\">\n<figure id=\"attachment_1325\" aria-describedby=\"caption-attachment-1325\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_02ab.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-5053\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_02ab-824x1024-1.png\" alt=\"Figure_40_03_02ab\" width=\"500\" height=\"621\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_02ab-824x1024-1.png 824w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_02ab-824x1024-1-241x300.png 241w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_02ab-824x1024-1-768x954.png 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_02ab-824x1024-1-65x81.png 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_02ab-824x1024-1-225x280.png 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_02ab-824x1024-1-350x435.png 350w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-1325\" class=\"wp-caption-text\">Figure 21.11.\u00a0 (a) The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers. (b) Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated.<\/figcaption><\/figure>\n<\/div>\n<div class=\"mediaobject\"><\/div>\n<\/div>\n<p><span id=\"m44805-fs-idm34396288\"> <\/span>Which of the following statements about the heart is false?<\/p>\n<div class=\"orderedlist\">\n<ol class=\"orderedlist\">\n<li class=\"listitem\">The mitral valve separates the left ventricle from the left atrium.<\/li>\n<li class=\"listitem\">Blood travels through the bicuspid valve to the left atrium.<\/li>\n<li class=\"listitem\">Both the aortic and the pulmonary valves are semilunar valves.<\/li>\n<li class=\"listitem\">The mitral valve is an atrioventricular valve.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44805-fs-idm123004624\"> <\/span>The heart is composed of three layers; the epicardium, the myocardium, and the endocardium, illustrated in <a class=\"xref target-figure\" title=\"Figure\u00a040.11.\u00a0\" href=\"#attachment_1325\">Figure 21.11<\/a>. The inner wall of the heart has a lining called the <strong><span id=\"m44805-autoid-cnx2dbk-id1767595\"> <\/span>endocardium<\/strong>. The <span id=\"m44805-autoid-cnx2dbk-id1767598\"> <\/span><strong>myocardium<\/strong> consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the <span id=\"m44805-autoid-cnx2dbk-id1767604\"> <\/span><strong>epicardium<\/strong>, of which the second layer is a membranous layered structure called the <span id=\"m44805-autoid-cnx2dbk-id1767607\"> <\/span><strong>pericardium<\/strong> that surrounds and protects the heart; it allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.<\/p>\n<p><span id=\"m44805-fs-idm202630448\"> <\/span>The heart has its own blood vessels that supply the heart muscle with blood. The <span id=\"m44805-autoid-cnx2dbk-id1767619\"> <\/span><strong>coronary arteries<a id=\"id833341\" class=\"indexterm\" href=\"\"><\/a><\/strong> branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the <span id=\"m44805-autoid-cnx2dbk-id1767625\"> <\/span><strong>coronary veins<\/strong><a id=\"id833356\" class=\"indexterm\" href=\"\"><\/a> to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. <span id=\"m44805-autoid-cnx2dbk-id1767630\"> <\/span><strong>Atherosclerosis<\/strong><a id=\"id833371\" class=\"indexterm\" href=\"\"><\/a> is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain, known as <span id=\"m44805-autoid-cnx2dbk-id1767636\"> <\/span><strong>angina<\/strong><a id=\"id833387\" class=\"indexterm\" href=\"\"><\/a>, and complete blockage of the arteries will cause <strong><span id=\"m44805-autoid-cnx2dbk-id1767640\"> <\/span>myocardial infarction<\/strong><a id=\"id833401\" class=\"indexterm\" href=\"\"><\/a>: the death of cardiac muscle tissue, commonly known as a heart attack.<\/p>\n<div class=\"section\" title=\"The Cardiac Cycle\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44805-fs-idp5274160\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">The Cardiac Cycle<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44805-fs-idm116254208\"> <\/span>The main purpose of the heart is to pump blood through the body; it does so in a repeating sequence called the cardiac cycle. The <span id=\"m44805-autoid-cnx2dbk-id1767661\"> <\/span><strong>cardiac cycle<\/strong> is the coordination of the filling and emptying of the heart of blood by electrical signals that cause the heart muscles to contract and relax. The human heart beats over 100,000 times per day. In each cardiac cycle, the heart contracts (<span id=\"m44805-autoid-cnx2dbk-id1767667\"> <\/span><strong>systole<\/strong>), pushing out the blood and pumping it through the body; this is followed by a relaxation phase (<span id=\"m44805-autoid-cnx2dbk-id1767671\"> <\/span><strong>diastole<\/strong>), where the heart fills with blood, as illustrated in <a class=\"xref target-figure\" title=\"Figure\u00a040.12.\u00a0\" href=\"#attachment_1326\">Figure 21.12<\/a>. The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Closing of the atrioventricular valves produces a monosyllabic \u201clup\u201d sound. Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs (via the pulmonary artery). Closing of the semilunar valves produces a monosyllabic \u201cdup\u201d sound.<\/p>\n<\/div>\n<figure id=\"attachment_1326\" aria-describedby=\"caption-attachment-1326\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_03.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-5054\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_03-1024x562-1.jpg\" alt=\"Figure_40_03_03\" width=\"600\" height=\"329\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_03-1024x562-1.jpg 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_03-1024x562-1-300x165.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_03-1024x562-1-768x422.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_03-1024x562-1-65x36.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_03-1024x562-1-225x123.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_03-1024x562-1-350x192.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-1326\" class=\"wp-caption-text\">Figure 21.12.\u00a0 During (a) cardiac diastole, the heart muscle is relaxed and blood flows into the heart. During (b) atrial systole, the atria contract, pushing blood into the ventricles. During (c) atrial diastole, the ventricles contract, forcing blood out of the heart.<\/figcaption><\/figure>\n<div id=\"m44805-fig-ch40_03_03\" class=\"figure\" title=\"Figure\u00a040.12.\u00a0\">\n<div class=\"title\"><\/div>\n<p><span id=\"m44805-fs-idm170442304\"> <\/span>The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart muscle. <span id=\"m44805-autoid-cnx2dbk-id1769846\"> <\/span><strong>Cardiomyocytes<\/strong>, shown in Figure 21.13, are distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle; they are connected by intercalated disks exclusive to cardiac muscle. They are self-stimulated for a period of time and isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.<\/p>\n<figure id=\"attachment_1327\" aria-describedby=\"caption-attachment-1327\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_04.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-5055\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_04.jpg\" alt=\"Figure_40_03_04\" width=\"400\" height=\"320\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_04.jpg 800w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_04-300x240.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_04-768x614.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_04-65x52.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_04-225x180.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_04-350x280.jpg 350w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><\/a><figcaption id=\"caption-attachment-1327\" class=\"wp-caption-text\">Figure 21.13.\u00a0 Cardiomyocytes are striated muscle cells found in cardiac tissue. (credit: modification of work by Dr. S. Girod, Anton Becker; scale-bar data from Matt Russell)<\/figcaption><\/figure>\n<div id=\"m44805-fig-ch40_03_04\" class=\"figure\" title=\"Figure\u00a040.13.\u00a0\">\n<div class=\"title\"><\/div>\n<p><span id=\"m44805-fs-idm94701888\"> <\/span>The autonomous beating of cardiac muscle cells is regulated by the heart\u2019s internal pacemaker that uses electrical signals to time the beating of the heart. The electrical signals and mechanical actions, illustrated in <a class=\"xref target-figure\" title=\"Figure\u00a040.14.\u00a0\" href=\"#attachment_1328\">Figure 21.14<\/a>, are intimately intertwined. The internal pacemaker starts at the <span id=\"m44805-autoid-cnx2dbk-id1769901\"> <\/span><strong>sinoatrial (SA) node<\/strong>, which is located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node causing the two atria to contract in unison. The pulse reaches a second node, called the atrioventricular (AV) node, between the right atrium and right ventricle where it pauses for approximately 0.1 second before spreading to the walls of the ventricles. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, and then the ventricles contract. This pause allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes. This information can be observed as an <span id=\"m44805-autoid-cnx2dbk-id1769913\"> <\/span><strong>electrocardiogram (ECG)<\/strong>\u2014a recording of the electrical impulses of the cardiac muscle.<\/p>\n<figure id=\"attachment_1328\" aria-describedby=\"caption-attachment-1328\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_05.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-5056\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_05-1024x623-1.jpg\" alt=\"Figure_40_03_05\" width=\"600\" height=\"365\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_05-1024x623-1.jpg 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_05-1024x623-1-300x183.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_05-1024x623-1-768x467.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_05-1024x623-1-65x40.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_05-1024x623-1-225x137.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_05-1024x623-1-350x213.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-1328\" class=\"wp-caption-text\">Figure 21.14.\u00a0 The beating of the heart is regulated by an electrical impulse that causes the characteristic reading of an ECG. The signal is initiated at the sinoatrial valve. The signal then (a) spreads to the atria, causing them to contract. The signal is (b) delayed at the atrioventricular node before it is passed on to the (c) heart apex. The delay allows the atria to relax before the (d) ventricles contract. The final part of the ECG cycle prepares the heart for the next beat.<\/figcaption><\/figure>\n<div id=\"m44805-fs-idm190521904\" class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action<br \/>\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44805-fs-idp35661232\"> <\/span><img decoding=\"async\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/electric_heart.png\" alt=\"QR Code representing a URL\" width=\"120\" \/><\/div>\n<p><span id=\"m44805-fs-idm356726752\"> <\/span>Visit <a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/electric_heart\" target=\"\" rel=\"noopener noreferrer\">this site<\/a> to see the heart\u2019s \u201cpacemaker\u201d in action.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Arteries, Veins, and Capillaries\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44805-fs-idp11676624\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Arteries, Veins, and Capillaries<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44805-fs-idm128248512\"> <\/span>The blood from the heart is carried through the body by a complex network of blood vessels (<a class=\"xref target-figure\" title=\"Figure\u00a040.15.\u00a0\" href=\"#attachment_1330\">Figure 21.15<\/a>). <span id=\"m44805-autoid-cnx2dbk-id1768892\"> <\/span><strong>Arteries<\/strong> take blood away from the heart. The main artery is the aorta that branches into major arteries that take blood to different limbs and organs. These major arteries include the carotid artery that takes blood to the brain, the brachial arteries that take blood to the arms, and the thoracic artery that takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidney, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then smaller vessels called <span id=\"m44805-autoid-cnx2dbk-id1768900\"> <\/span><strong>arterioles<\/strong>, to reach more deeply into the muscles and organs of the body.<\/p>\n<figure id=\"attachment_1330\" aria-describedby=\"caption-attachment-1330\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_06.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-5058\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_06.jpg\" alt=\"Figure_40_03_06\" width=\"400\" height=\"602\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_06.jpg 512w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_06-199x300.jpg 199w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_06-65x98.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_06-225x339.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_06-350x527.jpg 350w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><\/a><figcaption id=\"caption-attachment-1330\" class=\"wp-caption-text\">Figure 21.15.\u00a0 The major human arteries and veins are shown. (credit: modification of work by Mariana Ruiz Villareal)<\/figcaption><\/figure>\n<div id=\"m44805-fig-ch40_03_06\" class=\"figure\" title=\"Figure\u00a040.15.\u00a0\">\n<p><span id=\"m44805-fs-idm204731312\"> <\/span>Arterioles diverge into capillary beds. <span id=\"m44805-autoid-cnx2dbk-id1768944\"> <\/span><strong>Capillary beds<\/strong><a id=\"id833854\" class=\"indexterm\" href=\"\"><\/a> contain a large number (10 to 100) of <span id=\"m44805-autoid-cnx2dbk-id1768947\"> <\/span><strong>capillaries<\/strong><a id=\"id833868\" class=\"indexterm\" href=\"\"><\/a> that branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into <span id=\"m44805-autoid-cnx2dbk-id1762819\"> <\/span><strong>venules<\/strong><a id=\"id833885\" class=\"indexterm\" href=\"\"><\/a> that connect to minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart. <span id=\"m44805-autoid-cnx2dbk-id1762824\"> <\/span><strong>Veins<\/strong><a id=\"id833900\" class=\"indexterm\" href=\"\"><\/a> are blood vessels that bring blood back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system.<\/p>\n<p><span id=\"m44805-fs-idm69642496\"> <\/span>The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels (<a class=\"xref target-figure\" title=\"Figure\u00a040.16.\u00a0\" href=\"#attachment_1331\">Figure 21.16<\/a>). The first tunic is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by <span id=\"m44805-autoid-cnx2dbk-id1762848\"> <\/span><strong>vasoconstriction<\/strong>, narrowing of the blood vessels, and <span id=\"m44805-autoid-cnx2dbk-id1762851\"> <\/span><strong>vasodilation,<\/strong> widening of the blood vessels; this is important in the overall regulation of blood pressure.<\/p>\n<p><span id=\"m44805-fs-idm46223136\"> <\/span>Veins and arteries both have two further tunics that surround the endothelium: the middle tunic is composed of smooth muscle and the outermost layer is connective tissue (collagen and elastic fibers). The elastic connective tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different than arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.<\/p>\n<figure id=\"attachment_1331\" aria-describedby=\"caption-attachment-1331\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_40_03_07.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-5059\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_07.jpg\" alt=\"Figure_40_03_07\" width=\"500\" height=\"313\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_07.jpg 800w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_07-300x188.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_07-768x480.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_07-65x41.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_07-225x141.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_40_03_07-350x219.jpg 350w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-1331\" class=\"wp-caption-text\">Figure 21.16.\u00a0 Arteries and veins consist of three layers: an outer tunica externa, a middle tunica media, and an inner tunica intima. Capillaries consist of a single layer of epithelial cells, the tunica intima. (credit: modification of work by NCI, NIH)<\/figcaption><\/figure>\n<h2>Summary<\/h2>\n<p>The heart muscle pumps blood through three divisions of the circulatory system: coronary, pulmonary, and systemic. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The pumping of the heart is a function of cardiomyocytes, distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle. The internal pacemaker starts at the sinoatrial node, which is located near the wall of the right atrium. Electrical charges pulse from the SA node causing the two atria to contract in unison; then the pulse reaches the atrioventricular node between the right atrium and right ventricle. A pause in the electric signal allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The blood from the heart is carried through the body by a complex network of blood vessels; arteries take blood away from the heart, and veins bring blood back to the heart.<\/p>\n<div class=\"section\">\n<div class=\"body\">\n<div class=\"textbox exercises\">\n<h3>Exercises<\/h3>\n<ol>\n<li>The heart\u2019s internal pacemaker beats by:\n<ol>\n<li>an internal implant that sends an electrical impulse through the heart<\/li>\n<li>the excitation of cardiac muscle cells at the sinoatrial node followed by the atrioventricular node<\/li>\n<li>the excitation of cardiac muscle cells at the atrioventricular node followed by the sinoatrial node<\/li>\n<li>the action of the sinus<\/li>\n<\/ol>\n<\/li>\n<li>During the systolic phase of the cardiac cycle, the heart is ________.\n<ol>\n<li>contracting<\/li>\n<li>relaxing<\/li>\n<li>contracting and relaxing<\/li>\n<li>filling with blood<\/li>\n<\/ol>\n<\/li>\n<li><span id=\"m44805-fs-idm52992688\">Cardiomyocytes are similar to skeletal muscle because:<\/span>\n<ol>\n<li>they beat involuntarily<\/li>\n<li>they are used for weight lifting<\/li>\n<li>they pulse rhythmically<\/li>\n<li>they are striated<\/li>\n<\/ol>\n<\/li>\n<li>How do arteries differ from veins?\n<ol>\n<li>Arteries have thicker smooth muscle layers to accommodate the changes in pressure from the heart.<\/li>\n<li>Arteries carry blood.<\/li>\n<li>Arteries have thinner smooth muscle layers and valves and move blood by the action of skeletal muscle.<\/li>\n<li>Arteries are thin walled and are used for gas exchange.<\/li>\n<\/ol>\n<\/li>\n<li>Describe the cardiac cycle.<\/li>\n<li><span id=\"m44805-fs-idm83165984\">What happens in capillaries?<\/span><\/li>\n<\/ol>\n<p><strong>Answers<\/strong><\/p>\n<ol>\n<li>B<\/li>\n<li>A<\/li>\n<li>D<\/li>\n<li>A<\/li>\n<li>The heart receives an electrical signal from the sinoatrial node triggering the cardiac muscle cells in the atria to contract. The signal pauses at the atrioventricular node before spreading to the walls of the ventricles so the blood is pumped through the body. This is the systolic phase. The heart then relaxes in the diastole and fills again with blood.<\/li>\n<li>The capillaries basically exchange materials with their surroundings. Their walls are very thin and are made of one or two layers of cells, where gases, nutrients, and waste are diffused. They are distributed as beds, complex networks that link arteries as well as veins.<\/li>\n<\/ol>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<div class=\"body\">\n<div class=\"section\" title=\"Blood Pressure Regulation\">\n<div class=\"glossary\" title=\"Glossary\">\n<div class=\"titlepage\">\n<div class=\"bcc-box bcc-success\">\n<h3>Glossary<\/h3>\n<div class=\"glossary\" title=\"Glossary\">\n<dl>\n<dt><strong>angina<\/strong><\/dt>\n<dd>pain caused by partial blockage of the coronary arteries by the buildup of plaque and lack of oxygen to the heart muscle<\/dd>\n<dt><strong>aorta<\/strong><\/dt>\n<dd>major artery of the body that takes blood away from the heart<\/dd>\n<dt><strong>arteriole<\/strong><\/dt>\n<dd>small vessel that connects an artery to a capillary bed<\/dd>\n<dt><strong>artery<\/strong><\/dt>\n<dd>blood vessel that takes blood away from the heart<\/dd>\n<dt><strong>atherosclerosis<\/strong><\/dt>\n<dd>buildup of fatty plaques in the coronary arteries in the heart<\/dd>\n<dt><strong>bicuspid valve<\/strong><\/dt>\n<dd>(also, mitral valve; left atrioventricular valve) one-way membranous flap between the atrium and the ventricle in the left side of the heart<\/dd>\n<dt><strong>capillary bed<\/strong><\/dt>\n<dd>large number of capillaries that converge to take blood to a particular organ or tissue<\/dd>\n<dt><strong>capillary<\/strong><\/dt>\n<dd>smallest blood vessel that allows the passage of individual blood cells and the site of diffusion of oxygen and nutrient exchange<\/dd>\n<dt><strong>cardiac cycle<\/strong><\/dt>\n<dd>filling and emptying the heart of blood by electrical signals that cause the heart muscles to contract and relax<\/dd>\n<dt><strong>cardiac output<\/strong><\/dt>\n<dd>the volume of blood pumped by the heart in one minute as a product of heart rate multiplied by stroke volume<\/dd>\n<dt><strong>cardiomyocyte<\/strong><\/dt>\n<dd>specialized heart muscle cell that is striated but contracts involuntarily like smooth muscle<\/dd>\n<dt><strong>coronary artery<\/strong><\/dt>\n<dd>vessel that supplies the heart tissue with blood<\/dd>\n<dt><strong>coronary vein<\/strong><\/dt>\n<dd>vessel that takes blood away from the heart tissue back to the chambers in the heart<\/dd>\n<dt><strong>diastole<\/strong><\/dt>\n<dd>relaxation phase of the cardiac cycle when the heart is relaxed and the ventricles are filling with blood<\/dd>\n<dt><strong>electrocardiogram (ECG)<\/strong><\/dt>\n<dd>recording of the electrical impulses of the cardiac muscle<\/dd>\n<dt><strong>endocardium<\/strong><\/dt>\n<dd>innermost layer of tissue in the heart<\/dd>\n<dt><strong>epicardium<\/strong><\/dt>\n<dd>outermost tissue layer of the heart<\/dd>\n<dt><strong>inferior vena cava<\/strong><\/dt>\n<dd>drains blood from the veins that come from the lower organs and the legs<\/dd>\n<dt><strong>myocardial infarction<\/strong><\/dt>\n<dd>(also, heart attack) complete blockage of the coronary arteries and death of the cardiac muscle tissue<\/dd>\n<dt><strong>myocardium<\/strong><\/dt>\n<dd>heart muscle cells that make up the middle layer and the bulk of the heart wall<\/dd>\n<dt><strong>pericardium<\/strong><\/dt>\n<dd>membrane layer protecting the heart; also part of the epicardium<\/dd>\n<dt><strong>semilunar valve<\/strong><\/dt>\n<dd>membranous flap of connective tissue between the aorta and a ventricle of the heart (the aortic or pulmonary semilunar valves)<\/dd>\n<dt><strong>sinoatrial (SA) node<\/strong><\/dt>\n<dd>the heart\u2019s internal pacemaker; located near the wall of the right atrium<\/dd>\n<dt><strong>superior vena cava<\/strong><\/dt>\n<dd>drains blood from the jugular vein that comes from the brain and from the veins that come from the arms<\/dd>\n<dt><strong>systole<\/strong><\/dt>\n<dd>contraction phase of cardiac cycle when the ventricles are pumping blood into the arteries<\/dd>\n<dt><strong>tricuspid valve<\/strong><\/dt>\n<dd>one-way membranous flap of connective tissue between the atrium and the ventricle in the right side of the heart; also known as atrioventricular valve<\/dd>\n<dt><strong>vasoconstriction<\/strong><\/dt>\n<dd>narrowing of a blood vessel<\/dd>\n<dt><strong>vasodilation<\/strong><\/dt>\n<dd>widening of a blood vessel<\/dd>\n<\/dl>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"cnx-eoc summary\">\n<div class=\"section\"><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"author":90,"menu_order":57,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":"cc-by"},"chapter-type":[],"contributor":[],"license":[57],"class_list":["post-5060","chapter","type-chapter","status-publish","hentry","license-cc-by"],"part":5037,"_links":{"self":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/5060","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/users\/90"}],"version-history":[{"count":1,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/5060\/revisions"}],"predecessor-version":[{"id":5061,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/5060\/revisions\/5061"}],"part":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/parts\/5037"}],"metadata":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/5060\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/media?parent=5060"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapter-type?post=5060"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/contributor?post=5060"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/license?post=5060"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}