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16: The Heart

  • Page ID
    12536
  • Skills to Develop

    • Describe the location and position of the heart within the body cavity
    • Describe the internal and external anatomy of the heart
    • Identify the tissue layers of the heart
    • Identify the veins and arteries of the coronary circulation system
    • Trace the pathway of oxygenated and deoxygenated blood thorough the chambers of the heart

    Location of the Heart

    The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum. Figure 16.1 shows the position of the heart within the thoracic cavity. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity. The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage, as seen in Figure 16.1. The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly. It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds, and also when looking at images taken from a midsagittal perspective. The slight deviation of the apex to the left is reflected in a depression in the medial surface of the inferior lobe of the left lung, called the cardiac notch.

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    Figure 16.1 Position of the Heart in the Thorax The heart is located within the thoracic cavity, medially between the lungs in the mediastinum. It is about the size of a fist, is broad at the top, and tapers toward the base.

    Chambers and Circulation through the Heart

    The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body.

    There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, we can initially view the circuits from the point of view of gases. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.

    The right ventricle pumps deoxygenated blood into the pulmonary trunk, which leads toward the lungs and bifurcates into the left and right pulmonary arteries. These vessels in turn branch many times before reaching the pulmonary capillaries, where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the post-natal body that carry relatively deoxygenated blood. Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only post-natal veins in the body that carry highly oxygenated blood. The pulmonary veins conduct blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries, where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.

    The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered. The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. Understanding the flow of blood through the pulmonary and systemic circuits is critical to all health professions (Figure 16.2).

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    Figure 16.2 Dual System of the Human Blood Circulation Blood flows from the right atrium to the right ventricle, where it is pumped into the pulmonary circuit. The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium. From here, blood enters the left ventricle, which pumps it into the systemic circuit. Following exchange in the systemic capillaries (oxygen and nutrients out of the capillaries and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated.

    Membranes, Surface Features, and Layers

    Our exploration of more in-depth heart structures begins by examining the membrane that surrounds the heart, the prominent surface features of the heart, and the layers that form the wall of the heart. Each of these components plays its own unique role in terms of function.

    Membranes

    The membrane that directly surrounds the heart and defines the pericardial cavity is called the pericardium or pericardial sac. It also surrounds the “roots” of the major vessels, or the areas of closest proximity to the heart. The pericardium, which literally translates as “around the heart,” consists of two distinct sublayers: the sturdy outer fibrous pericardium and the inner serous pericardium. The fibrous pericardium is made of tough, dense connective tissue that protects the heart and maintains its position in the thorax. The more delicate serous pericardium consists of two layers: the parietal pericardium, which is fused to the fibrous pericardium, and an inner visceral pericardium, or epicardium, which is fused to the heart and is part of the heart wall. The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the pericardium.

    In most organs within the body, visceral serous membranes such as the epicardium are microscopic. However, in the case of the heart, it is not a microscopic layer but rather a macroscopic layer, consisting of a simple squamous epithelium called a mesothelium, reinforced with loose, irregular, or areolar connective tissue that attaches to the pericardium. This mesothelium secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts. Figure 16.3 illustrates the pericardial membrane and the layers of the heart.

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    Figure 16.3 Pericardial Membranes and Layers of the Heart Wall The pericardial membrane that surrounds the heart consists of three layers and the pericardial cavity. The heart wall also consists of three layers. The pericardial membrane and the heart wall share the epicardium.

    Surface Features of the Heart

    Inside the pericardium, the surface features of the heart are visible, including the four chambers. There is a superficial leaf-like extension of the atria near the superior surface of the heart, one on each side, called an auricle—a name that means “ear like”—because its shape resembles the external ear of a human (Figure 16.4). Auricles are relatively thin-walled structures that can fill with blood and empty into the atria or upper chambers of the heart. You may also hear them referred to as atrial appendages. Also prominent is a series of fat-filled grooves, each of which is known as a sulcus (plural = sulci), along the superior surfaces of the heart. Major coronary blood vessels are located in these sulci. The deep coronary sulcus is located between the atria and ventricles. Located between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. The anterior interventricular sulcus is visible on the anterior surface of the heart, whereas the posterior interventricular sulcus is visible on the posterior surface of the heart. Figure 16.4 illustrates anterior and posterior views of the surface of the heart.

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    Figure 16.4 External Anatomy of the Heart Inside the pericardium, the surface features of the heart are visible.

    Internal Structure of the Heart

    Recall that the heart’s contraction cycle follows a dual pattern of circulation—the pulmonary and systemic circuits—because of the pairs of chambers that pump blood into the circulation. In order to develop a more precise understanding of cardiac function, it is first necessary to explore the internal anatomical structures in more detail.

    Septa of the Heart

    The word septum is derived from the Latin for “something that encloses;” in this case, a septum (plural = septa) refers to a wall or partition that divides the heart into chambers. The septa are physical extensions of the myocardium lined with endocardium. Located between the two atria is the interatrial septum. Normally in an adult heart, the interatrial septum bears an oval-shaped depression known as the fossa ovalis, a remnant of an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern.

    Between the two ventricles is a second septum known as the interventricular septum. Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development. It is substantially thicker than the interatrial septum, since the ventricles generate far greater pressure when they contract.

    The septum between the atria and ventricles is known as the atrioventricular septum. It is marked by the presence of four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve, a specialized structure that ensures one-way flow of blood. The valves between the atria and ventricles are known generically as atrioventricular valves. The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves. The interventricular septum is visible in Figure 16.5. In this figure, the atrioventricular septum has been removed to better show the bicupid and tricuspid valves; the interatrial septum is not visible, since its location is covered by the aorta and pulmonary trunk. Since these openings and valves structurally weaken the atrioventricular septum, the remaining tissue is heavily reinforced with dense connective tissue called the cardiac skeleton, or skeleton of the heart. It includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta, and serve as the point of attachment for the heart valves. The cardiac skeleton also provides an important boundary in the heart electrical conduction system.

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    Figure 16.5 Internal Structures of the Heart This anterior view of the heart shows the four chambers, the major vessels and their early branches, as well as the valves. The presence of the pulmonary trunk and aorta covers the interatrial septum, and the atrioventricular septum is cut away to show the atrioventricular valves.

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    LAB 16 EXERCISES 16-1

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    Label the following: Aorta, Inferior vena cava, L. ventricle, R. ventricle, Lungs, Interventricular septum

    Draw the pathway of blood through the heart

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    Visible human project, nlm.nih.gov

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    LAB 16 EXERCISES 16-2

    Label the following: Apex, Ascending aorta, Aortic arch, Anterior interventricular sulcus, L. Pulmonary artery, L. auricle, L. ventricle, Pulmonary trunk, Superior vena cava, R. ventricle, Coronary Sulcus

       

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    4 License: Anatomy & Physiology Lab Homework by Laird C. Sheldahl, under a Creative Commons Attribution-ShareAlike License 4.0

    R. Pumonary a.,

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    LAB 16 EXERCISES 16-3

    Label the following: Aorta, Coronary sulcus, L. atrium,

    L. ventricle, Inferior Vena Cava,

    Posterior interventricular sulcus, R. ventricle.

    R. Pulmonary v.,

    R. Pulmonary a.

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    LAB 16 EXERCISES 16-4

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    A Label the following: R. Coronary a. (RCA),

    R. Marginal branch of RCA, Anterior interventricular a., Great cardiac vein, Circumflex A.

    Label the following: Great cardiac vein, Posterior interventricular a., Middle cardiac vein, Right coronary a., Coronary sinus, Circumflex a., L. marginal vein

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    & Physiology Lab Homework by Laird C. Sheldahl, under a Creative Commons Attribution-ShareAlike License 4.0 License: Anatomy

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    LAB 16 EXERCISES 16-5

    Label the following: L. Pulmonary a., L. Pulmonary v., Interventricular septum, Mitral valve, Tricuspid valve, Pulmonary semilunar valve, Chordae tendinae, Papillary muscle, Endocardium, Myocardium, Epicardium, L. coronary a., Pulmonary trunk, Conus arteriosis.

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    The layers of the heart

    LAB 16 EXERCISES 16-6

    VALVES & CONDUCTION SYSTEM

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    Label the following: R. coronary a., L. coronary a., Mitral valve, Tricuspid valve, Pulmonary SL valve, Aortic SL valve

    On this picture, draw and label the following: SA Node, AV node, Bundle of His, Bundle branches, Purkinje fibers

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    y & Physiology Lab Homework by Laird C. Sheldahl, under a Creative Commons Attribution-ShareAlike License 4.0

    License: Anatom

    MODELS: Heart and Torsos

     Inferior vena cava

     Superior vena cava

     Right atrium

    • Pectinate muscles

    • Fossa ovalis

       Right atrioventricular valve (tricuspid)

    • Chordae tendineae

    • Papillary muscles

       Right ventricle

    • Trabeculae carneae

       Conus arteriosus

       Pulmonary SL valve

       Pulmonary trunk

       Pulmonary arteries

       Pulmonary veins

       Left atrium

       Left atrioventricular valve (mitral, bicuspid)

       Left ventricle

       Aortic SL valve

       Aorta

    • Ascending, arch, descending, abdominal

    • R. Brachiocephalic, L. Common Carotid a, L subclavian a.

       Apex

       Auricles

       Anterior interventricular sulcus

       Posterior interventricular sulcus

       Coronary sulcus

      Inter-atrial septum

       Interventricular septum

      Electrical conduits: (clear heart model)

    • Sinoatrial (SA) node

    • Atrioventricular (AV) node

    • Atrioventricular (AV) bundle (Bundle of His)

    • Right and left bundle branches

    • Purkinje fibers

    • Moderator band

       Tissue layers:

    • Parietal pericardium

    • Visceral pericardium (epicardium)

    • Myocardium

    • Endocardium

       Coronary circulation:

    • R. coronary a..

    • Anterior interventricular a.

    • Posterior interventricular a.

    • Circumflex a.

    • L. coronary a.

    • Middle cardiac v.

    • Great cardiac v.

    • Coronary sinus

    • Marginal arteries

    License: Anatomy & Physiology Lab Homework by Laird C. Sheldahl, under a Creative Commons Attribution-ShareAlike License 4.0

    Heart Dissection Walk Through

    The heart dissection is probably one of the most difficult dissections you will do. Part of the reason it is so difficult to learn is that the heart is not perfectly symmetrical, but it is so close that it becomes difficult to discern which side you are looking at (dorsel, ventral, left or right). Finding the vessels is directly related to being able to orient the heart correctly and figuring out which side you are looking at.

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    The heart is also difficult because the fatty tissue that surrounds the heart can obscure the openings to the vessels. This means that you really must experience the heart with your hands and feel your way to find the openings. Many people will be squeamish about this, and because the heart is slippery, it is easy to drop. Don't be shy with the heart, use your fingers to feel your way through the dissection.

    1. Step One: Orientation

      When you first remove your heart from the bag, you will see a lot of fatty tissue surrounding it. It is usually a waste of time to try to remove this tissue. Grab some colored pencils to help you identify and mark the vessels you find.

      There are a few clues to help you figure out the left and the right side, but often the packaging and preserving process can cause the heart to be misshapen. If you are lucky, the heart will be nicely preserved and you will see that the front (ventral) side of the heart has a couple of key features: 1) a large pulmonary trunk that extends off the top of it 2) the flaps of the auricles covering the top of the atria. 3) the curve of the entire front side, whereas the backside is much flatter.

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      The first image shows the front side of the heart, often identified by the anterior interventricular artery that runs cross it at an angle (yellow).

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      The auricle is the flap that covers the atrium, it looks like an ear. The pulmonary trunk is the located at the front of the heart and enters at an angle.

      Step 2: Locate the Aorta

      Use your fingers to probe around the top of the heart. Four major vessels can be found entering the heart: the pulmonary trunk, aorta, superior vena cava, and the pulmonary vein. Remember that if you are looking at the back of the heart, then the right and left sides are the same as your right and left hand. This picture was on the board the day of the dissection so that you could glance up and recall which vessel entered which part of the heart.

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      If you find the pulmonary vein, the aorta should be situated a little bit behind it. It may be covered by fat, so use your fingers to poke around until you find the opening. Push your finger all the way in and you will feel inside of the left ventricle. The left ventricle has a very thick wall, unlike the right ventricle. Insert your finger through the pulmonary vessel to feel the left ventricle and you will notice and feel that it is much thinner than the left side of the heart.

      With your fingers or probes in the aorta and the pulmonary trunk you should notice that they criss-cross each other, with the pulmonary trunk in the front. At this point, you may want to use your colored pencils to mark these vessels so that you don't get them confused when you are searching for the other two openings that top of the heart.

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      Step 3: Locate the Veins

      The two major veins that enter the heart can be found on the backside, as both enter the atria. On the left side, you should be able to find the opening of the pulmonary vein as it enters the left atrium. The superior vena cava enters the right atrium. In many preserved hearts, the heart was cut at these points, so you won't see the vessels themselves, you will just find the openings. Again, use your fingers to feel around the heart to find the openings. If you've marked the aorta and pulmonary then you won't mistake them for the veins you are looking for. This picture shows all of the vessels labeled.

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      Sometimes, the aorta still has its branches attached to it. There are three vessels that branch from the aorta: the brachiocephalic, left common carotid and the left subclavian. The majority of the time, these vessels are not visible because the aorta was cut too close to the main part of the heart when the heart was removed from the animal. Occassionally, you can find the brachiocephalic artery attached, as it is in this photo.

      Step 4: Make the Incisions

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      Now that you have all of the vessels located and marked, you can now open the heart to view the inner chambers. Use the superior vena cava and pulmonary vein as guides for where to cut. You are basically going to be cutting each side of the heart so that you can look inside. (Some dissections will ask you to make a coronal cut where a single cut opens the entire back side of the heart). The heart below is marked to show you where the two incisions should be made.

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      Cut the heart in half to expose the chambers. My students affectionally call these two variations the "hot dog cut" as pictured above because it looks like a hot dog bun, or the "hamburger cut, where the heart is cut into the front and the back half, as shown below.

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      Step 5: Viewing the Chambers

      At this point it is helpful to have two hands, one to hold the heart apart so you can take a peak inside of it and another to use a probe to locate the specific parts. Your colored pencils you used to mark the heart in step 2 can also now be used to see where those vessels connect within the heart. For instance, the aorta pencil can now be seen ending in the left ventricle.

      You can also now see how much thicker the walls of the left ventricle are compared to the right ventricle.

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      The other obvious structures seen within the heart are the chordae tendinae which are attached to papillary muscles. These tendons hold the heart valves in place, sometimes they are called the "heartstrings". The valves were probably cut when the heart was opened, but if you follow the "cords" they should lead you to a thin flap that is the atrioventricular (bicuspid) valve. You can find a similar valve on the right side of the heart (tricuspid).

      Image shows the left atrioventricular valve (bicuspid) and the chordae tendinae.

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