125 14.3 Heart

Created by CK-12 Foundation/Adapted by Christine Miller

14.3.1 Stethoscope
Figure 14.3.1 Healthy hearts are happy hearts. What do you hear?



Lub, Dub

Lub dub, lub dub, lub dub… That’s how the sound of a beating heart is typically described. Those are also the only two sounds that should be audible when listening to a normal, healthy heart through a stethoscope, as in Figure 14.3.1.  If a doctor hears something different from the normal lub dub sounds, it’s a sign of a possible heart abnormality. What causes the heart to produce the characteristic lub dub sounds? Read on to find out.

Introduction to the Heart

The  is a muscular organ behind the sternum (breastbone), slightly to the left of the center of the chest. A normal adult heart is about the size of a fist. The function of the heart is to pump blood through blood vessels of the . The continuous flow of through the system is necessary to provide all the cells of the body with oxygen and nutrients, and to remove their metabolic wastes.

Structure of the Heart

The heart has a thick muscular wall that consists of several layers of tissue. Internally, the heart is divided into four chambers through which blood flows. Because of heart valves, blood flows in just one direction through the chambers.

Heart Wall

14.3.2 Layers of the Heart Wall
Figure 14.3.2 The wall of the heart is made up mainly of myocardium, which consists largely of cardiac muscle.

As shown in Figure 14.3.2, the wall of the heart is made up of three layers, called the endocardium, myocardium, and pericardium.

  • The  is the innermost layer of the heart wall. It is made up primarily of simple epithelial cells. It covers the heart chambers and valves. A thin layer of connective tissue joins the endocardium to the myocardium.
  • The  is the middle and thickest layer of the heart wall. It consists of surrounded by a framework of collagen. There are two types of cardiac muscle cells in the myocardium: cardiomyocytes — which have the ability to contract easily — and pacemaker cells, which conduct electrical impulses that cause the cardiomyocytes to contract. About 99 per cent of cardiac muscle cells are cardiomyocytes, and the remaining one per cent is pacemaker cells. The myocardium is supplied with blood vessels and nerve fibres via the pericardium.
  • The  is a protective sac that encloses and protects the heart. The pericardium consists of two membranes (visceral pericardium and parietal pericardium), between which there is a fluid-filled cavity. The fluid helps to cushion the heart, and also lubricates its outer surface.

Heart Chambers

As shown in Figure 14.3.3 the four chambers of the heart include two upper chambers called atria (singular, ), and two lower chambers called . The atria are also referred to as receiving chambers, because blood coming into the heart first enters these two chambers. The right atrium receives deoxygenated blood from the upper and lower body through the superior and inferior vena cava, respectively. The left atrium receives oxygenated blood from the lungs through the . The ventricles are also referred to as discharging chambers, because blood leaving the heart passes out through these two chambers. The right ventricle discharges blood to the lungs through the , and the left ventricle discharges blood to the rest of the body through the . The four chambers are separated from each other by dense connective tissue consisting mainly of .

Figure 14.3.3 Anatomy of the Heart
Figure 14.3.3 This cross-sectional diagram of the heart shows its four chambers and four valves. The white arrows indicate the direction of blood flow through the heart chambers.

Heart Valves

14.3.4 Heart Valves
Figure 14.3.4 If the veins and arteries of the heart were removed, a top-down view of the heart would reveal the four valves that are critical in preventing backflow of blood. Note the three cusps of the tricuspid AV valve and the 2 cusps of the bicuspid AV valve. Also note the size difference between the AV valves and the semilunar valves.

Figure 14.3.4 shows the location of the heart’s four valves in a top-down view, looking down at the heart as if the arteries and veins feeding into and out of the heart were removed. The heart valves allow blood to flow from the atria to the ventricles, and from the ventricles to the pulmonary artery and aorta. The valves are constructed in such a way that blood can flow through them in only one direction, thus preventing the backflow of blood. Figure 14.3.5 shows how valves open to let blood into the appropriate chamber, and then close to prevent blood from moving in the wrong direction and the next chamber contracts.  The four valves are the:

  1. , (can be shortened to tricuspid AV valve) which allows blood to flow from the right atrium to the right ventricle.
  2. (also know as the mitral valve), which allows blood to flow from the left atrium to the left ventricle.
  3. , which allows blood to flow from the right ventricle to the pulmonary artery.
  4. , which allows blood to flow from the left ventricle to the aorta.
14.3.4 Heart Animation
Figure 14.3.5 The valves of the heart prevent backflow of blood. The open when the chamber before them contracts (systole) and then close when that chamber relaxes (diastole).
14.3.6 Chordae Tendoneae
Figure 14.3.6 The chordae tendoneae, shown in this diagram in white, play a critical role in reinforcing the AV valves of the heart.

The two atrioventricular (AV) valves prevent backflow when the ventricles are contracting, while the semilunar valves prevent backflow from vessels.  This means that the AV valves must withstand much more pressure than do the semilunar valves.  In order to withstand the force of the ventricles contracting (to prevent blood from backflowing into the atria), the AV valves are reinforced with structures called — tendon-like cords of connective tissue which anchor the valve and prevent it from .  Figure 14.3.6 shows the structure and location of the chordae tendoneae.

The chordae tendoneae are under such force that they need special attachments to the interior of the ventricles where they anchor.  are specialized muscles in the interior of the ventricle that provide a strong anchor point for the chordae tendineae.

Coronary Circulation

The s of the muscular walls of the heart are very active cells, because they are responsible for the constant beating of the heart. These cells need a continuous supply of oxygen and nutrients. The carbon dioxide and waste products they produce also must be continuously removed. The blood vessels that carry blood to and from the heart muscle cells make up the . Note that the blood vessels of the coronary circulation supply heart tissues with blood, and are different from the blood vessels that carry blood to and from the chambers of the heart as part of the general circulation.  supply oxygen-rich blood to the heart muscle cells. Coronary veins remove deoxygenated blood from the heart muscles cells.

  • There are two — a right coronary artery that supplies the right side of the heart, and a left coronary artery that supplies the left side of the heart. These arteries branch repeatedly into smaller and smaller arteries and finally into capillaries, which exchange gases, nutrients, and waste products with cardiomyocytes.
  • At the back of the heart, small cardiac veins drain into larger veins, and finally into the great cardiac vein, which empties into the right atrium. At the front of the heart, small cardiac veins drain directly into the right atrium.

Blood Circulation Through the Heart

Figure 14.3.7 shows how blood circulates through the chambers of the heart. The right atrium collects blood from two large veins, the superior vena cava (from the upper body) and the inferior vena cava (from the lower body). The blood that collects in the right atrium is pumped through the tricuspid valve into the right ventricle. From the right ventricle, the blood is pumped through the pulmonary valve into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it enters the pulmonary circulation, gives up carbon dioxide, and picks up oxygen. The oxygenated blood travels back from the lungs through the pulmonary veins (of which there are four), and enters the left atrium of the heart. From the left atrium, the blood is pumped through the mitral valve into the left ventricle. From the left ventricle, the blood is pumped through the aortic valve into the aorta, which subsequently branches into smaller arteries that carry the blood throughout the rest of the body. After passing through capillaries and exchanging substances with cells, the blood returns to the right atrium via the superior vena cava and inferior vena cava, and the process begins anew.

Figure 14.3.7 Path of blood through the heart
Figure 14.3.7 The flow chart in this diagram summarizes the pathway blood takes as it flows into, through, and out of the heart. Trace the path of blood flow in the diagram of the heart as you follow it through the flow chart.

Cardiac Cycle

The cardiac cycle refers to a single complete heartbeat, which includes one iteration of the lub and dub sounds heard through a stethoscope. During the cardiac cycle, the atria and ventricles work in a coordinated fashion so that blood is pumped efficiently through and out of the heart. The cardiac cycle includes two parts, called diastole and systole, which are illustrated in the diagrams in Figure 14.3.8.

  • During , the atria contract and pump blood into the ventricles, while the ventricles relax and fill with blood from the atria.
  • During , the atria relax and collect blood from the lungs and body, while the ventricles contract and pump blood out of the heart.
14.3.8 Systole and Diastole
Figure 14.3.8 Diastole is referred to the filling stage, because this is when the ventricles fill with blood. Systole is referred to the pumping stage because this is when the ventricles pump blood out of the heart.

Electrical Stimulation of the Heart

The normal, rhythmical beating of the heart is called . It is established by the heart’s  cells, which are located in an area of the heart called the (shown in Figure 14.3.9). The pacemaker cells create electrical signals with the movement of electrolytes (sodium, potassium, and calcium ions) into and out of the cells. For each , an electrical signal rapidly travels first from the sinoatrial node, to the right and left atria so they contract together. Then, the signal travels to another node, called the (Figure 14.3.9), and from there to the right and left ventricles (which also contract together), just a split second after the atria contract.

14.3.9 SA and AV Nodes
Figure 14.3.9 The sinoatrial (SA) node causes the atria to contract and then signals the atrioventricular (AV) nodes to initiate the contraction of the ventricles.

The normal of the heart is influenced by the  through sympathetic and parasympathetic nerves. These nerves arise from two paired cardiovascular centers in the of the brainstem. The parasympathetic nerves act to decrease the heart rate, and the sympathetic nerves act to increase the heart rate. Parasympathetic input normally predominates. Without it, the pacemaker cells of the heart would generate a resting heart rate of about 100 beats per minute, instead of a normal resting heart rate of about 72 beats per minute. The cardiovascular centers receive input from receptors throughout the body, and act through the sympathetic nerves to increase the heart rate, as needed. Increased physical activity, for example, is detected by receptors in muscles, joints, and tendons. These receptors send nerve impulses to the cardiovascular centers, causing sympathetic nerves to increase the heart rate, and allowing more blood to flow to the muscles.

Besides the autonomic nervous system, other factors can also affect the heart rate. For example, hormones and hormones (such as epinephrine) can stimulate the heart to beat faster. The heart rate also increases when blood pressure drops or the body is dehydrated or overheated. On the other hand, cooling of the body and relaxation — among other factors — can contribute to a decrease in the heart rate.

Feature: Human Biology in the News

When a patient’s heart is too diseased or damaged to sustain life, a heart transplant is likely to be the only long-term solution. The first successful heart transplant was undertaken in South Africa in 1967. There are over 2,200 Canadians walking around today because of life-saving heart transplant surgery.  Approximately 180 heart transplant surgeries are performed each year, but there are still so many Canadians on the transplant list that some die while waiting for a heart. The problem is that far too few hearts are available for transplant — there is more demand (people waiting for a heart transplant) than supply (organ donors). Sometimes, recipient hopefuls will receive a device called a Total Artificial Heart (see Figure 14.3.10), which can buy them some time until a donor heart becomes available.

14.3.10 Total Artificial Heart
Figure 14.3.10 A Total Artificial Heart, shown here, can be used for short periods of time in order to maintain a patient until a donor heart becomes available.

Watch the video below “Total artificial heart option…” from Stanford Health Care to see how it works:

Total artificial heart option at Stanford (Includes surgical graphic footage), Stanford Health Care, 2014.

14.3 Summary

  • The is a muscular organ behind the sternum and slightly to the left of the center of the chest. Its function is to pump blood through the blood vessels of the .
  • The wall of the heart consists of three layers. The middle layer, the , is the thickest layer and consists mainly of . The interior of the heart consists of four chambers, with an upper and lower on each side of the heart. Blood enters the heart through the atria, which pump it to the ventricles. Then the ventricles pump blood out of the heart. Four valves in the heart keep blood flowing in the correct direction and prevent backflow.
  • The coronary circulation consists of blood vessels that carry blood to and from the heart muscle cells, and is different from the general circulation of blood through the heart chambers. There are two coronary arteries that supply the two sides of the heart with oxygenated blood. Cardiac veins drain deoxygenated blood back into the heart.
  • Deoxygenated blood flows into the right atrium through veins from the upper and lower body (superior and inferior , respectively), and oxygenated blood flows into the left atrium through four pulmonary veins from the lungs. Each atrium pumps the blood to the ventricle below it. From the right ventricle, deoxygenated blood is pumped to the lungs through the two pulmonary arteries. From the left ventricle, oxygenated blood is pumped to the rest of the body through the aorta.
  • The refers to a single complete heartbeat. It includes — when the atria contract — and , when the ventricles contract.
  • The normal, rhythmic beating of the heart is called . It is established by the heart’s in the . Electrical signals from the pacemaker cells travel to the atria, and cause them to contract. Then, the signals travel to the and from there to the ventricles, causing them to contract. Electrical stimulation from the  and hormones from the  can also influence heartbeat.

14.3 Review Questions

  1. What is the heart, where is located, and what is its function?
  2. Describe the coronary circulation.
  3. Summarize how blood flows into, through, and out of the heart.
  4. Explain what controls the beating of the heart.
  5. What are the two types of cardiac muscle cells in the myocardium? What are the differences between these two types of cells?
  6. Explain why the blood from the cardiac veins empties into the right atrium of the heart. Focus on function (rather than anatomy) in your answer.

14.3 Explore More

Noel Bairey Merz: The single biggest health threat women face, TED, 2012.

Watch a Transcatheter Aortic Valve Replacement (TAVR) Procedure at St. Luke’s in Cedar Rapids, Iowa, UnityPoint Health – Cedar Rapids, 2018.

A Change of Heart: My Transplant Experience | Thomas Volk | TEDxUWLaCrosse, TEDx Talks, 2018.

Heart Transplant Recipient Meets Donor Family For The First Time, WMC Health, 2018.

 

Attributions

Figure 14.3.1

Figure 14.3.2

Blausen_0470_HeartWall by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 14.3.3

Diagram_of_the_human_heart_(cropped).svg by Wapcaplet on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

Figure 14.3.4

Heart_Valves by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 14.3.5

CG_Heart Valve Animation by DrJanaOfficial on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.


Figure 14.3.6

Heart_tee_four_chamber_view by Patrick J. Lynch, medical illustrator from Yale University School of Medicine, on Wikimedia Commons is used under a CC BY 2.5 (https://creativecommons.org/licenses/by/2.5) license.

Figure 14.3.7

Circulation of blood through the heart by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license. [Original image in the bottom right is by Wapcaplet / CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/)]

Figure 14.3.8

Human_healthy_pumping_heart_en.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Common is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 14.3.9

Cardiac_Conduction_System by Cypressvine on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

 

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 19.12 Heart valves with the atria and major vessels removed [digital image].  In Anatomy and Physiology (Section 19.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/19-1-heart-anatomy#fig-ch20_01_04

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Heart and Stroke Foundation of Canada. (n.d.). https://www.heartandstroke.ca/

Sliwa, K., Zilla, P. (2017, December 7). 50th anniversary of the first human heart transplant—How is it seen today? European Heart Journal, 38(46):3402–3404. https://doi.org/10.1093/eurheartj/ehx695

Stanford Health Care. (2014, December 3). Total artificial heart option at Stanford (Includes surgical graphic footage). YouTube. https://www.youtube.com/watch?v=1PtxaxcPnGc&feature=youtu.be

TED. (2012, March 21). Noel Bairey Merz: The single biggest health threat women face. YouTube. https://www.youtube.com/watch?v=1bnzVjOJ6NM&feature=youtu.be

TEDx Talks. (2018, April 18). A change of heart: My transplant experience | Thomas Volk | TEDxUWLaCrosse. YouTube. https://www.youtube.com/watch?v=zU6mmix04PI&feature=youtu.be

UMagazine. (2015, Fall). The cutting edge: Patient first to bridge from experimental total artificial heart to transplant. UCLA Health. https://www.uclahealth.org/u-magazine/patient-first-to-bridge-from-experimental-total-artificial-heart-to-transplant

UnityPoint Health – Cedar Rapids. (2018, February 7). Watch a transcatheter aortic valve replacement (TAVR) Procedure at St. Luke’s in Cedar Rapids, Iowa. YouTube.  https://www.youtube.com/watch?v=jJm7zBcN6-M&feature=youtu.be

WMC Health. (2018, September 13). Heart transplant recipient meets donor family for the first time. YouTube. https://www.youtube.com/watch?v=biGuwQhuAsk&feature=youtu.be

 

 

 

License

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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