Anatomy of the Heart

Transcription

1 Biology 212: Anatomy and Physiology II Anatomy of the Heart References: Saladin, KS: Anatomy and Physiology, The Unity of Form and Function 8 th (2018). Required reading before beginning this lab: Chapter 19 Introduction: In order for blood to properly circulate throughout your body, two things are required: a) Your heart must pump blood through the blood vessels, and it must do this at the correct volume and correct pressures to meet the needs of the other organs and tissues of your body; and b) Those blood vessels must distribute that blood to each tissue or organ at the correct volume and pressure, then return it to the heart. All mammals (in fact, all higher animals), have closed circulatory systems. That is, the blood is completely contained within blood vessels as it passes through the various organs of the body. This blood is under relatively high pressure in many of the blood vessels, allowing it to flow against gravity and against the resistance which the vessels normally provide. Although the heart is undoubtedly the best known organ of the circulatory system, the blood vessels are equally important. They must work together with the heart to move blood around the body in the most efficient manner possible, getting it to the correct organs at the correct time, the correct volume, and the correct pressure. Since all other tissues, organs, and systems depend on the circulatory system to deliver the materials they need and remove waste products they do not need, the circulatory system is one of the most vital systems in the body. Indeed, the classic definition of death, at least until brain activity could be evaluated, was failure of this system. Contraction of the heart, called systole, provides the propulsive force which moves blood through the blood vessels while relaxation of the heart, called diastole, allows its chambers to refill with blood for the next systole. It has a series of one-way valves which ensure that blood flow occurs in only one direction during both systole and diastole. However, it would be a mistake to view blood vessels as simply passive tubes through which the heart pumps blood. Vessels throughout the body also generate their own pressures, and they are constantly adjusting that pressure and flow to meet changing needs of different tissues. We will discuss that during our lab on the blood vessels. LEARNING OBJECTIVES: Upon completion of this lab exercise, students will be able to: Describe the position, gross anatomy, and histology of the heart Identify the great vessels and discuss their functions in carrying blood into and out of the heart. Describe the pathway of blood as is moves through the heart, and the roles of its valves in regulating this movement, Describe the coronary circulation which supplies blood to and from the myocardium

2 Exercise #1: Size and Position of the Heart Your heart is a cone-shaped muscular organ slightly larger than your closed fist. Examine Figure 19.2 in your Saladin text and note how it is located in the central region of the thorax known as the mediastinum surround by a membrane called the pericardium. During embryonic development, the heart rotates as it develops such that the two right chambers of the heart are actually facing anteriorly in the adult, while the two left chambers constitute most of the posterior surface of the heart. a) The heart is anterior to what bones? b) The heart is posterior to what bone? c) The heart is medial to what organs? d) The heart is superior to what broad, flat muscular structure? e) The heart is surrounded by a pericardial cavity between two layers of the pericardium. Which layer of the pericardium lies next to the heart and moves with it as the heart contracts and relaxes? Which layer of the pericardium lies superficial to the pericardial cavity and does not move as the heart contracts and relaxes? Exercise #2: Gross Anatomy of the Heart Examine the models of the heart using Figures 19.5 and 19.7 in your Saladin text as references. Note that there are different types of heart models which show somewhat different things, so be sure you examine all of them. Identify the following external structures of the heart on both the models and Figure Base Apex Right atrium Left atrium Right ventricle Left ventricle Right and left atrioventricular sulci (grooves) Anterior and posterior interventricular sulci (grooves) Identify the following internal structures of the heart on both the models and Figure Right atrium Left atrium Right ventricle Left ventricle Interventricular septum Interatrial septum Trabeculae carneae Papillary muscles Chordae tendineae ( tendinous cords )

3 Notice that the walls of the right and left atria are relatively thin. They do not need much cardiac muscle tissue to pump blood at low pressure into the ventricles. In contrast, the wall of the right ventricle is thicker since it needs to have enough cardiac muscle tissue to expel blood at moderate pressures to the lungs via the pulmonary trunk. The wall of the left ventricle, including the interventricular septum, is very thick because it contains enough cardiac muscle tissue to expel blood at high pressure into the aorta. Questions for discussion based on your reading of Chapter 19 prior to this lab: Which chamber of the heart forms its apex? Which chambers do have papillary muscles and chordae tendineae? Which chambers do not have these? Explain the function of papillary muscles and chordae tendineae to the other members of your lab group. What would happen if the chordae tendineae attached to the left atrioventricular valve were all cut? Exercise #3: Histology of the Heart Examine Figure 19.3 and 19.7 in your Saladin text and notice that the wall of the heart is formed by three layers of tissues. Although the thickness of these layers will vary in the different chambers, all four chambers of the heart have them. Identify the endocardium, myocardium, and epicardium. What types of tissues (there are two of them) form the endocardium? and What type of tissue forms the myocardium? What types of tissues (there are two of them) form the epicardium? and Examine slide #1. This slide shows the myocardium, although some sections may also show epicardium or endocardium. Under low power, notice that the myocytes are not oriented in the same direction, but gradually change orientation as you move through the layer of muscle.

4 Select a region of the slide where the cardiac myocytes were cut in longitudinal section, and switch to high magnification. Identify the nuclei within the myocytes and the intercalated disks that connect them end-to-end. The intercalated discs include gap junctions between the cardiac myocytes that permit all cells in the heart to depolarize together. You may be able to vaguely see the striated banding pattern of cardiac muscle, but this pattern is much less obvious than in skeletal muscle, so don t be concerned if you cannot do this easily. Compare what you see under the microscope with Figure and other pictures of cardiac muscle in your textbook and be sure you understand the three-dimensional structure of cardiac muscle. Exercise #4: The Great Vessels Examine the models of the heart using Figures 19.5 and 19.7 in your Saladin text as references. Note that there are large blood vessels which carry blood into and away from the base of the heart. These are called the great vessels. On those diagrams in your Saladin text and one the models of the heart, identify the Superior vena cava Inferior vena cava Pulmonary veins (usually four of them) Pulmonary trunk which divides into right and left pulmonary arteries Ascending Aorta Questions for discussion based on your reading of Chapter 19 prior to this lab: Blood enters the ascending aorta from which chamber of the heart? To which parts of the body is this blood going? Blood enters the pulmonary trunk from which chamber of the heart? To which parts of the body is this blood going? Blood in the superior vena cava enters which chamber of the heart? From which parts of the body is this blood coming? Blood in the inferior vena cava enters which chamber of the heart? From which parts of the body is this blood coming? Blood in the pulmonary veins enters which chamber of the heart? From which parts of the body is this blood coming?

5 Exercise #5: Blood Flow Through the Heart Examine Figure 19.9 of your Saladin text and trace the pathway which blood follows as it passes through the heart. The heart actually pumps blood through two circuits at the same time, called the pulmonary circuit and the systemic circuit. Blood returning to the heart from the peripheral circulation is low in oxygen and high in carbon dioxide and enters the right atrium of the heart through the superior vena cava and the inferior vena cava. Contractions of the right atrium send that blood to the right ventricle, which contracts to send it out the pulmonary trunk and pulmonary arteries to the lungs. As blood flows through the capillaries of the lungs it releases that carbon dioxide and picks up oxygen in its place. The blood, now high in oxygen and low in carbon dioxide, flows through pulmonary veins back to the left atrium of the heart. When this contracts the blood flows into the left ventricle, which then contracts to send the blood out through the aorta to eventually reach capillaries in all of the other organs of the body where it releases oxygen and picks up carbon dioxide before it returns once again to the heart. Note that all of the blood must flow in only one direction though the heart: from the right atrium right ventricle pulmonary arteries lungs pulmonary veins left atrium left ventricle peripheral circulation. This unidirectional flow of blood through the heart is ensured by the presence of four valves positioned at the entrance and exit of the ventricular chambers. These valves prevent blood from flowing in the wrong direction. Using the models and your textbook (Figures 19.7, 19.8, 19.9), identify the four heart valves: Right atrioventricular valve (tricuspid) Pulmonary valve (semilunar) Left atrioventricular valve (bicuspid or mitral) Aortic valve (semilunar) Blood flows out of which chamber through the right atrioventricular valve into which chamber Blood flows out of which chamber through the pulmonary valve into which chamber Blood flows out of which chamber through the left atrioventricular valve into which chamber Blood flows out of which chamber through the aortic valve into which chamber Note that there are no valves regulating the flow of blood into the right atrium from the superior vena cava or inferior vena cava, nor are there valves regulating flow of blood into the left atrium from the pulmonary veins.

6 Exercise #6: The Coronary Circulation More than five liters of blood normally flow through the chambers of your heart every minute. Even though the walls of the heart are in constant contact with that blood, the cardiac myocytes of the heart can t get oxygen or nutrients from it. Therefore the heart has its own vessels, called coronary arteries, which deliver blood into the capillaries surrounding its cardiac muscle cells of the myocardium, and then that blood returns to the circulation through cardiac veins. Blood flows into the coronary arteries from two branches of the ascending aorta just above the aortic valve. Like blood in all branches of the aorta, that blood is high in oxygen, low in carbon dioxide, and at high pressure. From there, the blood is distributed into the coronary arteries which lie in the epicardium on the surface of the heart, then through smaller arteries which distribute the blood to capillaries in the myocardium where it can provide oxygen and nutrients to the cardiac myocytes and remove waste from them. Small veins then carry that low oxygen / high carbon dioxide blood back toward the cardiac veins in the epicardium, which gather to return that blood into the right atrium of the heart to mix with the low oxygen, high carbon dioxide blood from the rest of the body. Examine Figure of your Saladin text and the models of the heart and identify the following: Right coronary artery in the right atrioventricular sulcus Marginal branch of the right coronary artery does not lie in a sulcus Left coronary artery between the beginnings of the aorta and the pulmonary trunk Anterior interventricular coronary artery in the anterior interventricular sulcus Circumflex coronary artery in the left atrioventricular sulcus Marginal branch of the left coronary artery does not lie in a sulcus Posterior interventricular coronary artery in the posterior interventricular sulcus Great cardiac vein in the anterior interventricular and left atrioventricular sulci Middle (posterior interventricular) cardiac vein in the posterior interventricular sulcus Left marginal cardiac vein does not lie in a sulcus Coronary sinus in the left atrioventricular sulcus Small cardiac vein: for some reason your Saladin book fails to show or discus this, but it is nearly universally recognized. It starts on the anterior wall of the right ventricle, runs alongside the marginal branch of the right coronary artery, and drains into the coronary sinus. While no two people have exactly the same pattern of coronary arteries and cardiac veins, the ones listed above the major ones which almost everyone has. Note also that venous blood from the anterior wall of the right ventricle can drain directly back into the right atrium through numerous thebesian veins.

7 Questions for discussion based on your reading of Chapter 19 prior to this lab: Which coronary arteries lie next to the small cardiac vein? Which coronary artery lies next to the middle cardiac vein? Which coronary arteries lie next to the great cardiac vein? Which coronary arteries lie next to the left marginal cardiac vein? What would happen if the left coronary artery were blocked just distal to its origin from the aorta? What would happen if the right coronary artery were blocked just distal to its origin from the aorta? What would happen if the circumflex coronary artery were blocked just distal to its origin from the left coronary artery? Exercise #7: Preserved Hearts In addition to the plastic models and the images in your textbook, we will also be studying cardiac anatomy on preserved human specimens. When examining these, of course, please wear disposable gloves. On the exterior of the preserved heart, you should be able to identify: Base Apex Right atrium Left atrium Right ventricle Left ventricle Aorta Pulmonary trunk On the interior of the preserved heart, you should be able to identify: Base Apex Right atrium Left atrium Interventricular septum Interatrial septum Papillary muscles and chordae tendineae in both ventricles Right atrioventricular valve Left atrioventricular valve (you might also be able to identify the aortic and pulmonary valve Note the differences in wall thicknesses of the four chambers: Thin for both atria, thicker for the right ventricle, thickest for the left ventricle. Note that the thickness of the interventricular septum matches that of the left ventricle. Note that the coronary arteries and cardiac veins can not be identified on preserved hearts.

8 Exercise #8: Heart of the Cadaver With gloves, examine the heart of the female cadaver in a prone (face up) position. It has been dissected in such a way that her heart has been exposed by incision of the pericardium, but otherwise has been left in place and not further dissected. Confirm that the heart is positioned as described in Exercise 1: Anterior to the bodies of vertebrae (you can feel these even if you can t see them) Posterior to the sternum (you will have to reposition the anterior thoracic wall) Surrounded by and medial to the lungs Most of the anterior surface of the heart is formed by the right atrium and right ventricle, Most of its posterior surface is formed by the left atrium and left ventricle, The inferior surface of the heart rests against the diaphragm. Identify the parietal pericardium. This has been cut to expose the heart, but most of it remains. Note the location of superior vena cava, pulmonary trunk, and aorta where they attach to the superior surface of the heart. Note the inferior vena cava as it passes through the diaphragm and enters the posterior part of the heart. Since the right atrium of the heart rests on the diaphragm, the inferior vena cava does not extend very far into the thoracic cavity. You can not see the pulmonary veins entering the left atrium since this is on the posterior surface of the heart, but you may be able to feel them. Gently lift up the apex of the heart. Observe that no blood vessels, nerves, or anything else enter this part of the heart it is free to move within the pericardial cavity Exercise #9: The Heart in the Living Human This exercise will require you to remove clothing, so you should do it at home. On yourself or another person, use a washable pen to mark the location of the heart on your anterior thoracic wall using Figure 19.2 in your Saladin text as a reference. The highest rib you can feel on the anterior surface of your chest is the 2 nd rib (the first rib is hidden under the clavicle), and you should be able to feel the xiphoid process of your sternum as references. Put an X where the apex of your heart is located. Mark where the superior vena cava, pulmonary trunk, and aorta attach to the heart. Show off your artwork to your roommate or mother if you wish, then go take a shower to wash it off.