Gross Anatomy

Objectives

When you have completed this section, you should be able to

• describe the relationship of the heart to the thoracic cavity and pericardium;
• identify the chambers and valves of the heart and the features of the heart wall;
• trace the flow of blood through the heart chambers and describe the blood supply to the myocardium itself; and
• explain how the structure and function of the adult heart relate to its evolutionary and embryonic developmental histories.

Think About It

How would prolapse of the mitral valve affect the amount of blood pumped into the aorta? How might this affect a person's physical stamina? Explain your reasoning.

Think About It

Which ventricle receives the greatest coronary blood supply? Why should it receive a greater supply than the other? List the vessels that supply it.

Key Point Review

1. Make a two-color sketch of the pericardium; use one color for the fibrous pericardium and another for the serous pericardium and show their relationship to the heart wall.

2. Trace the flow of blood through the heart, naming each chamber and valve in order.

3. Define pulmonary and systemic circuit.

4. Trace the flow of blood from the left coronary artery to the apex and then to the coronary sinus.

Cardiac Muscle and the Cardiac Conduction System

Objectives

When you have completed this section, you should be able to

• describe how the physiological properties of cardiac muscle differ from those of skeletal muscle and relate this to their structural differences;
• explain how the contraction of cardiac muscle is related to its electrophysiology; and
• describe the structure and function of the heart's conduction system.

Key Point Review

5. What organelle(s) are less developed in cardiac muscle than in skeletal muscle? What organelle(s) are more developed? What is the functional significance of these differences?

6. Name two types of cell junctions in the intercalated discs and explain their functional importance.

7. Why is the human heart described as myogenic? Where is its pacemaker and what is it called?

8. List the components of the cardiac conduction system in the order traveled by signals from the pacemaker.

Electrical and Contractile Activity of the Heart

Objectives

When you have completed this section, you should be able to

• explain why the SA node fires spontaneously and rhythmically;
• explain how the SA node excites the myocardium;
• describe the unusual action potentials of cardiac myocytes and relate them to the contractile behavior of this type of muscle; and
• interpret a normal electrocardiogram in relation to the electrical activities of the myocardium.

Think About It

With regard to the ions involved, how does the falling (repolarization) phase of a myocardial action potential differ from that of a neuron's action potential? (see p.441.)

Key Point Review

9. Define systole and diastole.

10. How does the pacemaker potential of the SA node differ from the RMP of skeletal muscle and neurons? Why is this important in creating the heart rhythm?

11. In what way is excitation-contraction coupling in cardiac muscle similar to that of skeletal muscle? In what way is it different?

12. What produces the plateau in the action potentials of myocytes? Why is this important with respect to the pumping ability of the heart?

13. Name the waves of the ECG and explain what myocardial events produce each wave.

Blood Flow, Heart Sounds, and the Cardiac Cycle

Objectives

When you have completed this section, you should be able to

• explain how pressure and resistance determine the flow of a fluid such as blood;
• explain what causes the sounds of the beating heart;
• describe what is occurring in the heart in each phase of the cardiac cycle; and
• relate the events of the cardiac cycle to the volume of blood entering and leaving the heart.

Think About It

Are the ventricular myocytes in isometric or isotonic contraction during the phase just described?

Key Point Review

14. Explain how a pressure gradient across a heart valve affects whether or not a ventricle will eject blood.

15. What factors are thought to cause the first and second heart sounds? When do these sounds occur?

16. What phases of the cardiac cycle are described as isovolumetric? Explain what this means.

17. If one of a child's ventricles has an EDV of 90 mL and an ESV of 60 mL, what is the stroke volume and what is the ejection fraction? Do you suspect normal health or a diseased heart?

Cardiac Output

Objectives

When you have completed this section, you should be able to

• define cardiac output and explain its importance;
• identify the factors that govern heart rate, contraction strength, and cardiac output;
• discuss some of the nervous and chemical factors that affect heart rate, stroke volume, and cardiac output; and
• explain how the right and left ventricles achieve balanced output.

Think About It

Suppose a person has a heart rate of 70 bpm and a stroke volume of 70 mL. A negative inotropic agent then reduces the stroke volume to 50 mL. What would the new heart rate have to be to maintain the same cardiac output?

Key Point Review

18. Define cardiac output in words and with a simple formula.

19. Describe the cardiac center and innervation of the heart.

20. Explain what is meant by positive and negative chronotropic and inotropic agents. Give two examples of each.

21. How do preload, contractility, and afterload influence stroke volume and cardiac output?

22. Explain the principle behind the Frank-Starling law of the heart. How does this mechanism normally prevent pulmonary or systemic congestion?

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