Sheep Heart Dissection & Examination

I. Heart Dissection Procedures: (use fig. 20.6 as reference)

1. Obtain a preserved sheep heart , a dissecting tray, dissecting instruments, lab apron, & (optional: gloves C .25/pair). Rinse the sheep heart in cold water. Now you are ready to make your observations.

2. Observe the texture of the pericardium. Also, find its point of attachment to the heart. Where is it attached?

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3. If the serous pericardial sac is still intact, slit open the parietal pericardium & cut it from its attachments. Observe the visceral pericardium (epicardium). Using a sharp scalpel, carefully pull a little of this serous membrane away from the myocardium. Note the visceral pericardium is much thinner than the tough 2-layered sero-fibrous parietal pericardium. The visceral pericardium adhere tightly to the heart, while the parietal pericardium forms the outer sac surrounding the pericardial cavity.

4. Examine the external surface of the heart. Notice the accumulation of adipose tissue, which in many cases marks the separation of the chambers & the location of the coronary arteries. Carefully scrape away some of the fat w/a scalpel to expose the coronary blood vessels.

5. Identify the base & apex of the heart, & then identify the 2 wrinkled auricles, earlike flaps of tissue projecting from the atrial chambers. The balance of the heart muscle is ventricular tissue. To identify the left ventricle, compress the ventricular chambers on each side of the longitudinal fissures carrying the coronary blood vessels. The side that feels thicker & more solid is the left ventricle. The right ventricle is much thinner & feels somewhat flabby when compressed. This difference reflects the greater demand placed on the left ventricle, which must pump blood through the much longer system circulation. Hold the heart in its anatomical position (fig. 20.6a), with the anterior surface uppermost. In this position the left ventricle composes the entire apex & the left side of the heart.

6. Identify the pulmonary trunk & the aorta leaving the superior aspect of the heart. The pulmonary trunk is more anterior, & you may see its division into the right & left pulmonary arteries if it has not been cut too close to the heart. The thicker-walled aorta, which branches almost immediately, is located just beneath the pulmonary trunk. The first branch of the sheep aorta, the brachiocephalic artery, can be identified unless the aorta has been cut immediately as it leaves the heart. The brachiocephalic artery splits to form the right carotid & subclavian arteries, which supply the right side of the head & right forelimb, respectively.

Carefully clear away the fat b/w the pulmonary trunk & the aorta to expose the ligamentum arteriosum, a remnant of the ductus arteriosus. (in the fetus, the ductus arteriosus allows blood to pass directly from the pulmonary trunk to the aorta, thus bypassing the nonfunctional fetal lungs.)

7. Cut through the wall of the aorta until you see the aortic semilunar valve. Identify the two openings into the coronary arteries just above the valve. Insert a probe into one of these holes to see if you can follow the course of a coronary artery across the heart.

8. Turn the heart to view its posterior surface. The heart will appear as shown in Figure 20.6b. Notice that the right & left ventricles appear equal-sized in this view. Try to identify the 4 thin-walled pulmonary veins entering the left atrium. (It may or may not be possible to locate the pulmonary veins from this vantage point, depending on how they were cut as the heart was removed.) Identify the superior & inferior vena cava entering the right atrium. The diameter of the vena cava is larger than that of the aorta. The aorta has thicker walls. The aorta is capable of stretching & elastic recoiling, which helps to maintain pressure in the vessels. This requires strength & resilience. The vena cava is a low-pressure vessel for blood return to the heart, & is not subjected to large pressure fluctuations.

II. Exploring the Valve Action of the Heart

1. Insert a probe into the superior vena cava & use your scissors to cut through the walls of the SVC in order to open up the right atrium. Do not cut through the entire atrial wall or extend your cut entirely through the right atrium or into the ventricle. Only cut enough so you can see the interior of the chamber.

2. Observe the right A.V. valve (the right A.V. valve has “three flaps” or is “tricuspid” in structure).

3. Slowly pour water into the right atrium & allow it to flow into the right ventricle.

4. Slowly & Gently squeeze the right ventricle & watch the closing action of the right A.V. valve. WARNING: Do not squeeze the ventricle too roughly or too quickly. If you do then be prepared to have water squirted on your face, in your mouth, nose, eyes, etc.

5. Drain the water from the heart before continuing.

6. Now go to the pulmonary trunk & cut down (through) the front of its wall until you see the pulmonary semilunar valve.

7. Pour some water into the base of the pulmonary trunk so it runs towards the right ventricle. Observe the closing action of this valve. Note the pulmonary semilunar valve closes when fluid fills the collapsed cuplike valves, causing them to bulge out into the lumen. The AV valves are flaps that swing closed as pressure in the chamber increases. They are prevented from opening backwards into the atria by the chordae tendineae attached to the papillary muscles.

8. After observing semilunar valve action, drain the heart once again. Return to the superior vena cava, & continue the cut made in its wall through the right atrium & right atrioventriuclar valve into the right ventricle.

9. Next, make a longitudinal incision through the aorta & continue it into the left ventricle. Again notice how much thicker the myocardium of the left ventricle is than that of the right ventricle. Note the left ventricular cavity is much narrower than the right ventricular cavity. The papillary muscles & chordae tendineae are present in both cavities & the left AV valve has 2 cusps. In comparison the sheep valves are very similar to their human counterparts.

10. Continue your incision from the left ventricle superiorly into the left atrium. Reflect the cut edges of the atrial wall, & try to locate the entry points of the pulmonary veins into the left atrium. Follow the pulmonary veins to the heart exterior with a probe. Notice how thin-walled these vessels are.

11. Properly dispose of the organic debris, & clean the dissecting tray & instruments & lab table before leaving the laboratory.

Lab Questions:

1.  The heart is an organ of what body system?

  1. What is the thick, muscular layer of the heart is called?
  2. What is the name of the sac surrounding the heart?
  3. What type of tissue comprises the bulk of the myocardium?
  4. What is the function of the heart?
  5. What is the function of an artery?
  6. From outermost to innermost, what are the three layers of an artery?
  7. What is the function of a vein?
  8. What is the name of the space in a blood vessel where-in blood flows?
  9. What is the lining of the heart called?
  10. What is the primary brain stem structure that controls heart rate.
  11. What is the specific space in the thoracic cavity where the heart is located?
  12. What bone protects the heart anteriorly?
  13. The bulk of the heart rests on this side of the body.
  14. The pericardium attaches to this structure inferiorly.
  15. Which side of the heart as a thicker ventricular wall?
  16. What layer of an artery consists mostly of smooth muscle?
  17. What chambers of the heart function to receive blood from the veins?
  18. The tunica interna is continuos with this layer of the heart.
  19. What part of the heart rests just below the right second rib?
  20. What are the bottom two chambers of the heart called?
  21. What valves are located between the atria and the ventricles?
  22. The apex of the heart points to this side of the body.
  23. What is the branch of the aorta that divides into the right subclavian and right common carotid arteries?
  24. What is the scientific term for the "heart strings" that extend from the AV cusps to the papillary muscles?
  25. What structure divides the two ventricles of the heart?
  26. The superior vena cava attaches to this heart chamber.
  27. What is the largest artery of the human body?
  28. What are the "ear-like" structures that extend from the atria?

30.  The apex of the heart usually sits at the same approximate level as the space between these two ribs.


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