Exercise 20

Gross Anatomy of the Heart

Laboratory Objectives

On completion of the activities in this exercise, you "'rill be able to: Describe the anatomical relations of the heart with other structures in the thoracic cavity. Provide details on the arrangement of the connective tissue layers (the pericardium) that surround the heart.

• Identify important structures on the surface of the heart. Locate the major internal structures of the heart . Identify the tissue layers of the heart wall.

• Describe the coronary circulation. Describe the flow of blood through the heart.

• Dissect a sheep heart and compare its structure with the human heart.

Materials Anatomical models or figures of the human heart

• Plastic bags Cotton or cheesecloth Colored pencils Preserved sheep hearts Dissecting trays

• Dissecting tools Dissecting gloves Protective eyewear

• Face mask

The heart is a two-sided, double-pumping organ. The left side (the left pump) controls the flow of blood to all tissues and cells in the body, where oxygen and nutrients are delivered

and metabolic wastes are taken away. The right side (the right pump) sends blood to the lungs, where oxygen stores in red blood «

cells are replenished and carbon dioxide, a metabolic waste, is re­leased. To keep blood circulating throughout the body, the heart beats approximately 100,000 times and pumps between 7000 and 9000 liters of blood each day. By any standard, this is an arduous workload, but the fact that the heart can maintain this level of ac­ti\-ity for decades, without stopping, is nothing short of remarkable.

In this exercise , you will examine the special anatomical fea­tures that reflect the enduring and efficient functioning of the hea rt. You will focus your examination on gross anatomical lruc ture. If you would like to review the light microscopic

st ru 'LUre of cardiac muscle, see Activity 10.2.

The Pericardium The heart is enclosed by a membranous sac called the peri­cardium (Figure 20.1c) . This structure consists of two parts. The outer fibrous pericardium is a tough, fibrous connective

tissue layer that is fused to adjacent structures (the diaphragm, sternum, costal cartilages of ribs, thoracic vertebrae, and the great vessels emerging from the heart). The inner serous peri­cardium is a delicate serous membrane that forms a double­layered sac around the heart. It consists of the parietal pericardium, which covers the deep or inner surface of the fi­brous pericardium, and the visceral pericardium, which forms the outer surface of the heart wall. The potential space between the parietal and visceral pericardial membranes is the pericardial cavity (Figure 20.1c). The cavity is filled with a wa­tery fluid produced by the epithelial cells lining the serous peri­cardium. The fluid helps to reduce friction when the two serous membranes rub against each other as the heart beats.

CLINICAL CORRELATION

Inflammation of the pericardiaI membranes, known as peri­carditis, increases the friction between the two membranes and causes an overproduction of fluid. As the fluid accumulates in the pericardial cavity it inhibits the normal movements of the heart wall and restricts cardiac output, leading to a condition called cardiac tamponade.

ACTIVITY 20.1 Examining the Organization of the Pericardium

1. Obtain a large, clear plastic bag and close off the open end .

2. With a heart model, push the inferior pOinted tip (the apex of the heart) into the wall of the closed plastic bag. This action is similar to pushing a fist into the bag as illus­trated in Figure 20.1c.

3. Notice that as you push the heart deeper into the closed bag, two layers of plastiC, separated by a space, cover the organ (Figure 20.1c).

4. Continue pushing the heart into the bag unti ll you reach the great vessels that are attached to the superior aspect of the heart (aorta, pulmonary trunk, superior vena cava).

5. The plastiC layers represent the serous pericardium (Figure 20.lc), as follows:

• The inner plastiC layer that is in contact with heart wall represents the visceral pericardium.

• The outer layer of plastiC represents the parietal pericardium.

• The space between the two plastic layers is the pericar­dial cavity.

Notice that the two layers of plastiC are continuous with each other at the great vessels. In other words, the visceral pericardium is continuous with the

355

------------ --------

356 EXERCISE TWENTY

parietal pericardium where the great vessels are con­What important function does the fibrous peri­nected to the heart (Figure 20.1c). cardium serve?

6. Wrap a layer of cotton or cheesecloth over the outer plas­tic laye r (the parietal pericardi u m). This layer represents the fibrous pericardium which is attached to the connec­tive tissue layers that surround adjacent structures.

Thyroid gland Right lung _ _ -- First rib (cut)

Aorta (segment removed)

(a)

Right pleural cavity

Rig ht pulmonary -1:-11k-------.~f":::9:">C-•..__~~..;f artery

Right pulmonary

Base of heart

Pericardial cavity

containing pericardial

attachment to diaphragm

fluid

Cut edge of

~~

, ,

Fibrous ---'- ­

vein

Superior vena cava

Right atrium

Parietal pericardium

parietal pericardium

Fibrous tissue of

w~pericardial sac

Areolar tissue

Mesothelium }

Cut edge of epicardium (visceral pericardium)

Apex of heart

(b)

Parietal pericardium

Outer wall (corresponds

to parietal pericardium)

Epicardium (visceral pericardium)

Wrist (corresponds to base of heart)

(c)

Figure 20.1 The anatomical relations of the heart. a) Anterior view of the thoracic cavity, showing the heart within the mediastinum and between the two lungs; b) transverse section of the thoracic cavity showing the position of the heart in relation to other structures; c) the relationship of the heart, pericardium, and pericar­dial cavity.

Left pleural cavity

Inner wall (corresponds to visceral pericardium)

Air space (corresponds to pericardia I cavity)

Balloon

Superior

Auricle of

RIGHT ATRIUM

Coronary sulcus

RIGHT

Pericardiu

GROSS ANATOMY OF THE HEART

Arch of aorta

Ligamentum arteriosum

Descending aortaAscending --#!!!jii!~I!

aorta Left pulmonary artery

___:--_ Pulmonary cava trunk

Auricle of ---"jl---­right atrium

Fat in anteriorFat in ....'--~,. interventricularcoronary

sulcus

\1I:1~ITiiHt'LE

(a)

Pericardium Ascending Pulmonary Auricle of

trunk left atrium

right atrium

VENTRICLE

to diaphragm

Left pulmonary artery

Left pulmonary ""':::::----:;;d'l!!!llllllll~to..

Fat in coronary

sulcus Coronary

sinus

interventricular LEFT sulcus VENTRICLE

(b)

~••~.""'~Fat in posterior interventricular

sulcus

(e)

e 20.2 External anatomy of the heart. a) Diagram, and b) dissec­-- ",~ :erior view; c) diagram, posterior view.

Gross Anatomy of the Heart The heart is a four-chambered organ that is shaped roughly like an inverted pear. On average, it is approximately 14 em long and 9 em wide, or slightly larger than a clenched fist. Its weight ranges from 230 to 280 grams in females and 280 to 340 grams in males. The heart and its surrolU)ding pericardial cavi ty are lo­cated within the mediastinum, a centrally located area within the thoracic cavity. Two thirds of the organ is pOSitioned to the left of the midline . It is bordered laterally by the pleural cavities , which surround the lungs, anteriorly by the sternum, and pos­teriorly by the esophagus and thoracic vertebrae (Figures 20.1a and b)

ACTIVITY 10.1 Examining the Gross Anatomy of tbe Human Heart

External Anatomy

1. Examine a model of the heart from an anterior view (Figures 20.2a and b) and make the following observations.

• The heart is divided into left and right sides. Each side contains two chambers: a superior atrium that receives blood and an inferior ventricle that discharges blood. Identify the four heart chambers: right atrium, right ventricle, left atrium (best observed from a posterior view), and left ventricle.

• The apex of the heart (Figure 20 .1c) is formed by the inferior tip of the left ventricle. It is located at the level of the fifth intercostal space, 7 to 9 cm to the left of the median plane. Locate this position on a skeleton.

• Extending off the main body of each atrium is a medial appendage known as an auricle. Locate the auricles of each atria.

WHATS IN A WORD The term auricle is derived from auricular, the Latin word for "external ear." The auricles of the atria were given that name because early anatomists noted their resem­blance to the external ear.

• Identify the atrioventricular (coronary) groove (sul­cus), which divides the atria, superiorly, from the ven­tricles, inferiorly.

• From the atrioventricular groove, identify the anterior interventricular groove as it travels toward the apex along the anterior surface of the heart. It forms a border bet\veen the left and right ventricles.

• Locate the ascending aorta, which receives blood from the left ventricle. The ascending aorta gives rise to the arch of the aorta. The aortic arch gives off three branches in the following order: the brachiocephalic artery, the left common carotid artery, and the left subclavian artery.

veins Superior vena cava

--7::.,;.z.==jIo.-:-' ~ ~..~~--i'T"-Right

pulmonary -j&---'+-----:::;=~ veins

Inferior vena cava

EXERCISE TWENTY

• Locate the pulmonary trunk. It is located anterior to the ascending aorta and receives blood from the right ventricle. The pulmonary trunk gives rise to the right and left pulmonary arteries.

• Locate the superior vena cava and inferior vena cava where they empty into the right atrium.

2. Examine a model of the heart from a posterior view (Figure 20.2c) and make the following observations.

• Identify the relative positions of the four hean cham­bers. Notice that the left atrium is more easily identified from a posterior view.

• The base of the heart is at the heart 's posterior and su­perior aspects, and is formed primarily by the left atrium. It is located beneath the second pair of ribs and the sternal angle (junction of the sternal body and manubrium), and extends from T6 to T9 vertebrae. Lo­cate this position on a skeleton.

• Once again, identify the atrioventricular (coronary) groove as it continues along the posterior surface of the heart. Notice that it forms a complete circle around the heart, and forms a border between the atria and vent rides.

• Locate the posterior interventricular groove. Like the anterior interventricular groove , the posterior groove de­scends toward the apex from the atrioventricular groove, and forms a border between the left and right ventricles.

• Observe the four pulmonary veins-two on each side-as they enter the left atrium.

• Once again, identify the superior and inferior vena cavae entering the right atrium.

Internal Anatomy

1. On the heart model, open the heart wall to expose the in­ternal structures (Figure 20.3).

2. Identify the two superior chambers, the right and left atria. With one hand , place your thumb on one side and your in­dex finger on the other side of the wall that separates the two atria. Your fingers are holding the interatrial septum. Identify the fossa ovalis, an oval depression along the in­teratrial septum within the right atrium.

CLINICAL CORRELATION

The wall that separates the right and left atria is called the interatrial septum. Inside the right atrium, along the interatrial septum, there is an oval depression called the fossa ovalis (Figure 20.3). This depression marks the site of the foramen ovaIe, an opening that connects the atria in the fetal heart. The foramen ovale has a valve that allows blood to travel from the right atrium to the left atrium but not in the reverse direction. This spe­cialization in the fetal circulation allows most of the oxygenated blood coming from the placenta to bypass the nonfunctional lungs and the pulmonary circulation, and to pass directly to other vital or­gans via the systemic circulation. At birth, the foramen ovale closes when the valve fuses with the interatrial septum. Incomplete clo­

sure of the foramen oval is, called an atrial septal defect, allows oxygenated blood in the left atrium to mix with deoxygenated blood in the right atrium. This malformat.ion can be repaired surgi­cally to prevent the two blood supplies from blending.

3. In the right atrium, identify the following distinct regions.

• The anterior wall is defined by the rough surface formed by the pectinate muscles. The pectinate muscles con­tinue onto the wall of the right auricle, the par-shaped, muscular pouch that extends medially from the atrial wall (Figure 20.2a). This portion of the atrium is de­rived from the embryonic heart.

• The posterior wall lacks pectinate muscles and is smooth. Verify that the openings for the superior vena cava and inferior vena cava are located along the smooth portion of the atrial wall. The posterior atrial wall is de­rived from embryonic veins. At the inferior end of the right atrium, the right atrioventricular (AV) orifice leads into the right ventricle.

4. Similar to the right atrium, the left atrium has two distinct regions with similar embryonic origins. The small anterior wall is dominated by the left auricle. Identify the pectinate muscles in this region. The posterior wall is smooth and relatively large. Verify that the four pulmonary veins drain into the left atrium from this region. Identify the left atri­oventricular (AV) orifice, at the inferior end of the left atrium, leading into the left ventricle.

5. Identify the two inrerior chambers, the right and left ventri­cles. Place the thumb and index finger of one hand on either side of the wall that separates the two ventricles. This struc­ture is the interventricular septum (Figure 20.3). Notice that this wall is much thicker than the interatrial septum. Once again, identify the anterior and posterior interventricu­lar grooves. Verify that these grooves delineate the anterior and posterior margins of the interventricular septum.

CLINICAL CORRELATION

The inferior portion of the interventricular septum is a thick mus­cular wall. The superior portion is a thin, membranous partition and, consequently, is a more likely site for ventricular septal defects. Because blood pressure in the left ventricle is higher than in the right ventricle, a ventricular septal defect will result in a left-to-right shunt of blood. This malformation, left unrepaired, can cause pulmonary disease and heart failure.

6. Within the right ventricle, the following distinct regions can be identified.

• The inferior portion receives the blood from the right atrium. Its walls are covered by an irregular network of muscular elevations called the trabeculae carneae.

• Superiorly, the right ventricle narrows into a cone­shaped chamber, the conus arteriosus, which leads to the pulmonary trunk. The wall of the conus arteriosus is smooth and, consequently, lacks trabeculae carneae.

GROSS ANATOMY OF THE HEART

Superior vena cava

Aortic arch

Ligamentum arteriosum

Pulmonary trunk

Ascending ----f--"'-' aorta

Figure 20.3 Internal anatomy of the heart. Internal structures of the heart chambers are revealed in a coronal section. The arrows indicate the direction of blood flow.

- -i"'I-- - --- Interatrial septum coronary sinus

r.:r--tt-+--.J~-_ Aortic semilunar valve

Pectinate Cusp of left AV (bicuspid) valve

Conus arteriosus LEFT VENTRICLE

Cusp of right AV (tricuspid) valve

Chordae tendineae --=::::::--Ir.-:~"--' - .........t-~~---::~- Interventricular septum

Papillary muscle ---+-"\.-"-'''':''I ...... ~

lI'.+,,=",,---,~-.:QI!:!L--"::"-I~:::"" Trabeculae carneae RIGHT VENTRICLE ---+--....._~'

Inferior vena cava -----!:~

Moderator band

~P-------- Descending aorta

7. Inside the left ventricle, the arrangement of rough- and smooth-walled sections is similar to what is found in the right ventricle. The wall along the inferior portion of the left ventricle is rough, due to the presence of the trabecu­lae carneae. The aortic vestibule is the smooth-walled, superior region that leads to the aorta .

S. Identify the two pairs of heart valves, which are strategi­cally located to regulate blood flow through the heart and into the great arteries (aorta and pulmonary trunk).

• The two atrioventricular (AV) valves are positioned between the atria and ventricles at the atrioventricular orifices. These valves open and close as a result of pres­sure differences between the atria and ventricles during the pumping action of the heart. The right valve is called the tricuspid valve because of its three cusps. The left valve is named the bicuspid (mitral) valve be­cause it possesses only two cusps. The cusps on the AV valves are membranous extensions of the endocardium that reach into the ventricular chambers. Fibrous cords, known as chordae tendinae, connect the inferior free margins of the cusps to papillary muscles located on the ventricular walls. The chordae tendinae and papil­lary muscles prevent the cusps from swinging back into

the atria when the ventricles contract. As a result, back­flow of blood into the atria is prevented.

• The two semilunar valves can be identified at the junc­tion of each ventricle to its respective great artery. The pulmonary semilunar valve is situated at the junction of the right ventricle and the pulmonary trunk, and the aortic semilunar valve is located at the junction of the left ventricle and the aorta (Figure 20.3). Each semilu­nar valve has three crescent-shaped cusps that are exten­sions of the great arterial walls. The operation of these valves is also controlled by changes in pressure, this time between the ventricles and great arteries. They open when the ventricles pump blood into the arteries. They close when the pumping action is complete to pre­vent backflow into ventricles.

WHAT'S IN A WORD The bicuspid valve is often referred to as the mitral valve, because when it is closed, the cusps resemble the tall pointed hat , with front and back peaks, worn by bishops and other members of the clergy. This ceremonial headdress is called a mitre (miter).

The term semilunar means "half moon. " The name refers ItO

the half-moon shape of the cusps in the semilunar valves.

EXERCISE TWENTY

The Heart Wall

1. Observe the left ventricular wall on a heart model and identify the three layers of the heart wall (Figure 20.4) .

• The inner endocardium is a thin, serous membrane of connective tissue and a simple squamous epithelium. It lines the internal walls of the heart chambers.

• The middle myocardium is the thickest layer and com­prises the bulk of the heart wall. It is composed prima­rily of cardiac muscle fibers, separated by connective tissue containing capillaries and nerves. Contractions of cardiac muscle fibers are responsible for the pumping action of the heart.

• Similar to the endocardium, the outermosl epicardium is a serous membrane of connective tissue and a simple squamous epithelium. This layer is also called the visceral pericardium.

2. Examine the relative thickness of the walls surrounding the heart chambers (Figure 20.3). Observe that the atrial walls are much thinner than the ventricular walls.

3. Compare the thickness of the two ventricular walls and observe that the left ventricular wall is thicker than the right ventricu llar wall.

CLINICAL CORRELATION

Microbial infection of the endocardium can cause tissue inflam­

mation or endocarditis. This condition often causes damage

to the heart valves and could impede normal blood flow

through the heart. In severe cases, blood clots can form along

the walls of the ventricles. These clots can break off and travel

to other blood vessels where they can bring about organ fail­

ure, heart attacks, or strokes.

Figure 20.4 Organization of the pericardium and heart wall. The re­lationship of the heart wall (epicardium, myocardium, and endocardium) with the pericardium and pericardial cavity is illustrated. Note that the epicardium and the visceral pericardium are the same structure.

Why are the walls of the ventricles much thicker than the walls of the atria?

Coronary Circulation Like any other organ, the heart must have an adequate blood supply that delivers sufficient amounts of oxygen and nutrients to cardiac muscle cells and carries away carbon dioxide and other metabolic wastes. The right and left coronary arteries branch directly off the ascending aorta. These arteries and their branches deliver blood to all regions of the heart. Blood is drained from the heart wall by a number of cardiac veins which empty, directly or indirectly, into the right atrium.

ACTIVITY 20.3 Identifying the Blood Vessels of the Coronary Circulation

1. Identify the following coronary arteries on a heart model (Figure 20.5).

• The right and left coronary arteries branch off the as­cending aorta just superior to the aortic semilunar valve.

• The left coronary artery travels to the left side, along the atrioventricular groove and posterior to the pulmonary trunk (Figure 20.Sa). Soon after it emerges from behind the pulmonary trunk, it gives rise to two main branches: the circumflex artery and the anterior interventricu­lar artery (Figure 20.Sa).

MesotheliUm} EPICARD. IUM Areolar (visceral tissue pericardium)

Dense fibrous layer

/ CO""""" l;""ffi

MYOCARDIUM ~-~--~-+,.,

(cardiac muscle tissue) ,.,..-_ _

{

Areolar tissue ENDOCARDIUM .

Endothelium --= .~

Aortic arch

Left coronary

artery

Atrial y-- ,.­ Great arteries cardiac

veinAnterjor cardiac

veins

Small cardiac (a)

vein

Marginal artery

Great Circumflex Coronarycardiac artery

Posterior cardiac

vein

Middle cardiac Marginal vein artery

(b)

Left pulmonary veins Left pulmonary artery

Circumflex Right pulmonary artery ~artery

/ _~ ./ Superior Great cardiac V vena cava

vein Right pulmonary veins

Posterior cardiac Left atrium

Right atrium

Inferior vena cava

Coronary sinus

Middle cardiac vein

Right ventricle

(c)

Figure 20.5 The coronary circulation. The major arteries and veins that 5 JPply and drain the heart wall are illustrated. a) Anterior view of the heart with :ne pulmonary trunk removed to reveal the left coronary artery; b) posterior . eo;! of the heart; c) dissection of the heart, posterior view.

GROSS A ATOMY OF T H E HEART

• The circumflex artery travels along the atrioventricular groove. It curves around the left side (Figure 20.5a) and continues onto the posterior surface (Figures 20.5b and c).

• The anterior interventricular artery descends toward the apex along the anterior interventricular groove (Figure 20.5a).

• The right coronary artery travels to the right along the atrioventricular groove (Figure 20.5a). It curves around the right side and continues onto the posterior surface (Figure 20.5b) .

The right coronary artery and the circumflex artery form an anastomosis (a natural connection between two blood vessels) on the posterior surface of the heart.

• Just before the right coronary artery curves around to the posterior surface, it gives off a branch called the marginal artery . The marginal artery descends along the right margin of the right ventricle (Figure 20.5a) .

• On the posterior surface of the heart, the right coronary artery gives off a second major branch, the posterior in­terventricular artery. This artery descends toward the apex along the posterior interventricular groove (Figures 20.5b and c).

• Near the apex of the heart, attempt to identify an anas­tomosis between the anterior and posterior interventric­ular arteries. This anastomosis may no t be demonstrated on the models that are available in your laboratory.

2. Identify the following cardiac veins on the heart model (Figure 20.5).

• On the posterior surface of the heart, the coronary si­nus is a large dilated sac that runs along the atrioven­tricular groove and empties into the right atrium (Figures 20.5b and c) . It drains most of the venous blood from the heart wall.

• The great cardiac vein ascends along the anterior inter­ventricular groove, running alongSide the anterior inter­ventricular artery (Figure 20.5a). At the atrioventricular groove it travels with the circumflex artery to the poste­rior surface, where it drains into the coronary sinus (Figures 20.5b and c).

• The middle cardiac vein ascends along the posterior in­terventricular groove, traveling with the posterior inter­ventricular artery. It drains into the coronary sinus as it approaches the base of the heart (Figures 20.5b and c).

• The smaU cardiac vein runs alongSide the marg.inal artery as it travels toward the atrioventricular groove (Figure 20 .5a). At the atrioventricular groove, it trav­els with the right coronary artery to the posterior sur­face and drains into the coronary sinus (Figure 20.5b).

• The anterior cardiac veins are small veins that travel a short distance along the anterior surface of the right ventricle. They drain directly into the right atrium (Figure 20.5a).

EX E .E TWE N n

Durin" the previous activity, you identified two ar­teria l "lOmoses in the coronary circulation.

I cio you think "the functional Significance of these arterial _ nnections in the heart? (Hint: Consider what must occur if blood flow to a part icular region of the heart is partially blocked.)

Blood Flow Through the Heart The heart functions as a two-sided muscular pump that regulates two separate blood circulatory loops. The right side of the heart controls the pulmonary circulation, which is the flow of blood to and from the lungs. The left side of the heart controls the systemic circulation, which is the flow of blood to and from all body tissues (Figure 20.6). The sequence of events that defines one pumping cycle of the heart is known as the cardiac cycle. During the cardiac cycle, the two atria relax and contract together and the t"vo ventri­cles relax and contract together. Thus , the flow of blood through pulmonary and systemic circuits is occurring simultaneously

o ~-

ure 20.6 Overview of the pulmonary and systemic circulations. ~- :: . ~ , ventricle pumps blood into the pulmonary trunk to begin the pul­- : - 3 . :ircu lation. The left ventricle pumps blood into the ascending aorta to - :: ~ - :-e systemic circuit.

ACTIVITY 20.4 Tracing the Flow of Blood Through the Heart Chambers

1. On a model of the heart, trace the pathway of blood through the pulmonary circuit by reviewing steps one through five in Figure 20.6.

2. On a model of the heart, trace the pathway of blood through the systemic circuit by reviewing steps six through nine in Figure 20.6.

If the left ventricle is damaged and is not pump­ing its normal volume of blood into the aorta , it

will lag behind the pace of the right ventricle and eventually blood will back up in the pulmonary circulation. This condi­tion is called congestive heart failure. Explain how restric­tions in blood flow from the left ventricle and into the systemic circulation can have a negative effect on blood flow in the pul­monary circuit.

Pulmonary circulation: Deoxygenated blood enters the right atrium via the venae cavae, coronary • sinus, and anterior cardiac veins.

Blood passes through the tricuspid valve and enters the right ventricle.

o Right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary trunk.

o Blood is transported to the lungs via the pulmonary arteries and their branches. In the lungs, the blood is oxygenated.

Oxygenated blood is transported to the left atrium of the heart via the pulmonary veins.

Systemic circulation:

Oxygenated blood passes through the bicuspid valve and enters the left ventricle.

G Left ventricle pumps blood through the aortic semilunar valve into the aorta.

o Oxygenated blood is transported by the aorta and its branches:

Blood is distributed to the head, neck and upper extremities via branches of the aortic arch.

Blood is distributed to the thorax, abdomen, pelvis and lower extremities via branches of the descending aorta.

Deoxygenated blood returns to the right atrium.

The Sheep Heart The sheep heart is remarkably similar to the human heart and thus represents an excellent model for studying cardiac struc­ture. As you dissect, have models , illustrations, or photographs of the human heart readily available so that you can make struc­lural comparisons.

ACTIVITY 10.5 Dissection of the Sheep Heart Organization of the Pericardium

l. Identify the pericardial sac if it is present. This structure includes the fibrous pericardium and parietal pericardium. The fibrous pericardium is a thick outer layer of fibrous connective tissue and fat that encloses the heart. With a pair of scissors , cut along the pericardial sac for a short distance (about 2.5 cm or 1 inch) and fold it back to ex­pose its inner surface. The thin, shiny layer along this sur­face is the parietal pericardium.

Right atrium

Right ventricle Anterior

interventricular sulcus

(al

Aorta

Aortic semilunar valve

Left atrium

Bicuspid valve

Chordae tendineae

Papillary muscle

Left ventricle

Tricuspid

(e)

GROSS A N ATOMY OF THE H EART

2. By making a cut in the pericardial sac, you have exposed the outside surface of the heart wall. . otice that the heart waIl is covered by a thin, translucent membrane. With forceps, lift a portion of this membrane off the heart's surface. This is the visceral pericardium. Recall that the visceral pericardium and epicardium are the same structure. The space between the parietal pericardium and visceral pericardium is the peri­cardial cavity. Identify this space by placing a probe into it.

External Anatomy of the Heart

1. Carefully remove the pericardial sac to expose the entire heart. This can be accomplished by continuing your initial scissors CUl toward the base of the heart and detaching the pericardial sac's attachments to the great vessels.

2. Observe the major sulci (grooves) that travel along the surface of the heart (Figures 20 .7a and b) . They can best be identified by the large amount of fat that is located along their paths.

• The atrioventricular groove travels around the heart's cir­cumference and separates the atria from the ventricles.

Right atrium

Pulmonary veins entering left atrium

Coronary sinus

Posterior interventricular sulcus

Right ventricle

Figure 20.7 Anatomy of the sheep heart. a) Anterior view; b) posterior view; c) corona l section.

(b)

EXERCISE TWENTY

• The ante.rior interventricular sulcus (groove) travels be­tween the left and right ventricles on the anterior surface.

• The posterior interventricular sulcus (groove) does the same on the posterior surface.

3. The atria on the sheep heart are quite small, and are com­parable to the auricles on the human heart. Often, during commercial preparation of the heart, these chambers are partially removed, leaving the internal structures exposed. Consequently, the venae cavae leading into the right atrium and the pulmonary veins to the left atrium are usu­ally absent. Identify the base of the heart by locating what remains of the atrial walls (Figure 20.7).

4. Hold the sheep heart so that the anterior surface is facing you (Figure 20.7a). At the inferior tip of the left ventricle, identify the apex of the heart. Identify the two great arter­ies-the pulmonary trunk and the aorta. From an anterior view, the pulmonary trunk is anterior to the aorta. Locate the anterior interventricular groove once again. Realize that this groove forms a boundary between the right and left ventricles (Figure 20.7a).

5. As described earlier, fat depOSits are found along the ma­jor grooves on the heart's surface. In order to identify the blood vessels that travel along the grooves, it is nec­essary to remove this fat. With forceps, carefully strip away the fat from a small section along one of the grooves to verify the presence of blood vessels. Review the names of the blood vessels that travel in each groove (Figure 20.5).

6. Hold the sheep heart so that the posterior surface is facing you (Figure 20. 7b). Along the atrioventricular groove, carefully remove the fat to reveal a thin walled, dilated blood vessel that empties into the right atrium. This vessel is the coronary sinus (Figure 20.7b).

Internal Anatomy of the Heart

1. Expose the interior of the right atrium and ventricle in the folloWing manner.

• Insert the blunt end of a pair of scissors into the supe­rior vena cava. H the superior vena cava is not present, insert the scissors into the opening where the blood ves­sd drains into the right atrium.

• Cut along the [ateral margin of the right atrium.

• Continue to cut along the lateral margin of the right ventricle to the apex. Be sure to cut through the entire thickness of the ventricular wall but avoid damaging in­ternal structures.

2. Expose the interior of the left atrium and left ventricle in the following manner.

• Using a scalpel or the sharp end of a pair of scissors, make a small incision in the lateral wall of the left atrium.

• Insert the blunt end of the scissors into the incision and cut along the lateral margin of the left atrium.

• Continue to cut along the lateral margin of the left ven­tricle to the apex.

3. Identify the interatrial septum that separates the left and right atria. The shallow, oval depression along the right atrial side of the septum is the fossa ovalis. Inside the atrial chambers, identify the pectinate muscles. Locate the atrioventricular orifices that lead into the ventricles. These openings mark the locations of the atrioventricular valves, which will be examined later.

4. Identify the interventricular septum that divides the two ventricles. Notice how the inferior portion is thick and mus­cular, and the superior portion is thin and membranous.

5. Beginning at the apex, cut through the interventricular septum with a scalpel or scissors. Continue cutting through the interatrial septum until you have completed a coronal section of the heart (Figure 20.7c).

6. In the coronal section, identify the following structures (Figure 20.7 c).

• The atrioventricular (AY) valves are located between the atria and ventricles. The tricuspid valve, with three cusps, is on the right side and the bicuspid valve, with two cusps, is on the left. For each valve, observe that the cusps are connected to the papillary muscles by the chordae tendinae.

• The trabeculae carneae are muscular elevations along the walls of both ventricles. Notice that they are found pre­dominately in the inferior portions of these chambers.

• The superior portions of the ventricles are narrow, smooth-walled corridors that lead to the great arteries. In the right ventricle, the conus arteriosus leads to the pulmonary trunk. In the left ventricle, the aortic vestibule leads to the aorta.

7. From the severed free margins of the aorta and pulmonary trunk, cut along the walls of these blood vessels toward the ventricles until you reach the semilunar valves. Observe that each valve is composed of three crescent-shaped cusps.

8. Along the wall of the aorta, just superior to the aortic semilunar valve, use a blunt probe to find the openings to the right and left coronary arteries.

The Heart Wall

1. Observe the left ventricular wall and identify the three lay­ers of the heart wall (Figure 20.7c).

• The inner endocardium is a thin serous membrane. It appears as a smooth, shiny surface lining the internal walls of the heart chambers.

• The middle myocardium is the thickest layer. It is com­posed primarily of cardiac muscle fibers.

• The outer epicardium, which is the visceral peri­cardium, is also a serous membrane.

2. Examine the relative thickness of the walls surrounding the heart chambers (Figure 20.7 c). Observe that the atrial '..valls are much thinner than the ventricular walls.

3. Compare the thickness of the two ventricular walls and observe that the left ventricular wall is thicker than the right ventricular wall.

GROSS ANATOMY O F THE H EART

2. Provide an explanation to account for the fact that the1. On a model of the human heart, observe the

heart wall is thicker in the left ventricle than in the rightthickness of the heart wall in the various cham­

ventricle.bers. In terms of relative thickness , how do the heart walls in the chambers of the human heart compare with the sheep heart?

Name ____________~________________________Exercise 20 Review Sheet Lab Section __________________________________

Gross Anatomy of the Heart Date ______________________________~______

Questions 1-4: Define the following terms.

1. Mediastinum

2. Atria versus ventricles

3. Apex of the heart

4. Base of the heart

5. Describe the circulatory pathways and the primary functions of the pulmonary and sys­temic circulations.

6. Describe the three tissue layers that comprise the heart wall.

7. Describe the organization of the pericardium.

Questions 8-11: Each heart valve is located at the junction of an atrium and ventricle, or a \'entricle and great artery. Use this concept to describe the location of the heart valves.

8. Bicuspid valve

9. Tricuspid valve

EXERCISE TWENTY

10. Pulmonary semilunar valve

1l. Aortic semilunar valve

12. Describe the function of the chordae tendinae and papillary muscles.

Questions 13-16: Complete the following table .

Artery Vessel from which Artery Branches Groove in which Artery Travels Regions Supplied by the Artery

13. Ascending aorta Right side of the heart

14. Anterior interventricular artery

15. Left coronary artery Left ventricle and left atrium

16. Posterior interventricular groove

Questions 17-20: Match the coronary artery in column A with the cardiac vein in column B that travels with it. The answers in column B may be used more than once or not at all.

A B

17. Marginal artery a. Great cardiac vein ~

18. Anterior interventricular artery b. Middle cardiac ve.in

19. Circumflex artery c. Small cardiac vein

20. Posterior interventricular artery d. Coronary sinus

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GROSS ANATOMY OF THE HEART

Questions 21-31: In the following diagram, identify the structures by labeling with the color that is indicated.

2l. Right atrium = yellow

22. Left ventricle = gray

23. Aorta = red

24. Left atrium = green

25. Pulmonary trunk = blue

26. Superior vena cava = purple

27. Right ventricle = orange

28. Inferior vena cava = pink

29. Coronary sinus = blue

30. Pulmonary arteries = brown

3l. Pulmonary veins = black

Questions 32-41: In the following diagram, identify the labeled structures. Select your an­swers from the follOWing list.

32. a. Apex of the heart

33. b. Cusp of tricuspid valve

34. c. Inferior vena cava 37

35.

36.

37.

d.

e.

f.

Chordae tendinae

Middle cardiac vein

Trabeculae carneae

33

34 39

38. g. Papillary muscle 35

39. h. Interventric.ular septum

40. i. Great cardiac vein 36

4l. j. Cusp of bicuspid valve

k. Ascending aorta

I. Coronary sinus

38

40

41