1. The mediastinum is the medial cavity of the
thorax within which the heart, great vessels, thymus, and parts of the trachea, bronchi, and esophagus are found.

2. The layers of the heart wall are the endocardium, the myocardium, and the epicardium. The epicardium is also called the visceral layer of the serous pericardium. This is surrounded by the parietal
layer of the serous pericardium and the fibrous pericardium.

3. The serous fluid decreases friction caused by movement of the layers against one another.

4. Thestructures are (a) aorta, (b) pulmonary trunk, (c) left atrium, (d) right atrium,
(e) left ventricle, (f) right ventricle, (g) inferior vena cava. The right atrium (d) receives oxygen-poor blood. The left ventricle (e) pumps blood into the systemic circulation. The pulmonary trunk (b) transports blood toward the lungs.

5. The papillary muscles and chordae tendineae keep the AV valve flaps from everting into the atria as the ventricles contract.

6. The mitral (left atrioventricular)
valve has two cusps.

7. The right side of the heart acts as the pulmonary pump,
whereas the left acts as the systemic pump.

8. (a) True. The left ventricle wall is
thicker than the right.

(b) True. The left ventricle pumps blood at much higher
pressure than the right ventricle because the left ventricle supplies the whole body, whereas the right ventricle supplies only the lungs.

(c) False. Each ventricle pumps the same amount of blood with each beat. If this were not true, blood would back up in either the systemic or pulmonary circulation (because the two ventricles are in series).

9. The branches of the right coronary artery are the right
marginal artery and the posterior interventricular artery.

10. (a) The refractory period is almost as long as the contraction in cardiac muscle. (b) The source of Ca?+ for the contraction is only SR in skeletal muscle. (c) Both skeletal muscle
and cardiac muscle have troponin. (d) Only skeletal muscle has triads.

12. Cardiac muscle cannot go into tetany because the absolute refractory period is almost as long as
the contraction.

13. (a) The QRS wave occurs during ventricular depolarization.
(b) The T wave of the ECG occurs during ventricular repolarization. (c) The P-R interval of the ECG occurs during atrial depolarization and the conduction of the
action potential through the rest of the intrinsic conduction system.

14. It is not necessary for the intrinsic conduction system to contact each individual cardiac myocyte. This is because cardiac myocytes are connected to each other by gap junctions and so are depolarized when an adjacent myocyte depolarizes. The
subendocardial conducting network excites ventricular muscle fibers. The depo-larization wave travels upward from the apex toward the atria.

16. The second heart sound is associated with the closing of the semilunar valves.

17. The murmur of mitral insufficiency occurs during ventricular systole (because this is when the valve should be closed,
and the murmur is due to blood leaking through
Left ventricle the incompletely closed

19. Exercise activates the sympathetic
nervous system. sympathetic nervous system
activity increases heart
rate. It also directly AV valves
open increases ventricular con-
tactility, thereby increasing Josh's stroke volume.

20. If the heart is beating
very rapidly, the amount of time for ventricular
filling between contractions is decreased. This decreases the end
diastolic volume, decreases the stroke volume, and therefore decreases the cardiac output.