Mechanical Events of the Cardiac Cycle

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I. The cardiac cycle is divided into systole (ventricular contraction) and diastole (ventricular relaxation).

a. At the onset of systole, ventricular pressure rapidly exceeds atrial pressure, and the AV valves close. The aortic and pulmonary valves are not yet open, however, and so no ejection occurs during this isovolumetric ventricular contraction.

b. When ventricular pressures exceed aortic and pulmonary trunk pressures, the aortic and pulmonary valves open, and ventricular ejection of blood occurs.

c. When the ventricles relax at the beginning of the diastole, the ventricular pressures fall significantly below those in the aorta and pulmonary trunk, and the aortic and pulmonary valves close. Because the AV valves are also still closed, no change in ventricular volume occurs during this isovolumetric ventricular relaxation.

d. When ventricular pressures fall below the pressures in the right and the left atria, the AV valves open, and the ventricular filling phase of diastole begins.

e. Filling occurs very rapidly at first so that atrial contraction, which occurs at the very end of diastole, usually adds only a small amount of additional blood to the ventricles.

II. The amount of blood in the ventricles just before systole is the end-diastolic volume. The volume remaining after ejection is the end-systolic volume, and the volume ejected is the stroke volume.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

III. Pressure changes in the systemic and pulmonary circulations have similar patterns, but the pulmonary pressures are much lower.

IV. The first heart sound is due to the closing of the AV valves, and the second to the closing of the aortic and pulmonary valves.

The Cardiac Output

I. The cardiac output is the volume of blood pumped by each ventricle and equals the product of heart rate and stroke volume.

a. Heart rate is increased by stimulation of the sympathetic nerves to the heart and by epinephrine; it is decreased by stimulation of the parasympathetic nerves to the heart.

b. Stroke volume is increased mainly by an increase in end-diastolic volume (the Frank-Starling mechanism) and by an increase in contractility due to sympathetic-nerve stimulation or to epinephrine. Afterload can also play a significant role in certain situations.

SECTION C REVIEW QUESTIONS

SECTION C KEY TERMS

pericardium myocardium endothelial cell endothelium atrioventricular (AV) valve tricuspid valve mitral valve papillary muscles pulmonary valve aortic valve intercalated disks conducting system coronary artery coronary blood flow sinoatrial (SA) node heart rate atrioventricular (AV) node bundle of His right and left bundle branches Purkinje fibers slow channel pacemaker potential automaticity electrocardiogram (ECG)

P wave QRS complex T wave refractory period (of cardiac muscle) cardiac cycle systole diastole isovolumetric ventricular contraction ventricular ejection stroke volume (SV) isovolumetric ventricular relaxation ventricular filling end-diastolic volume (EDV) end-systolic volume (ESV) heart sounds cardiac output (CO) afterload ventricular function curve Frank-Starling mechanism venous return contractility ejection fraction (EF)

List the structures through which blood passes from the systemic veins to the systemic arteries. Contrast and compare the structure of cardiac muscle with skeletal and smooth muscle. Describe the autonomic innervation of the heart, including the types of receptors involved. Draw a ventricular action potential. Describe the changes in membrane permeability that underlie the potential changes.

Contrast action potentials in ventricular cells with

SA-node action potentials. What is the pacemaker potential due to, and what is its inherent rate? By what mechanism does the SA node function as the pacemaker for the entire heart?

Describe the spread of excitation from the SA node through the rest of the heart.

Draw and label a normal ECG. Relate the P, QRS, and T waves to the atrial and ventricular action potentials.

Describe the sequence of events leading to excitation-contraction coupling in cardiac muscle. What prevents the heart from undergoing summation of contractions?

Draw a diagram of the pressure changes in the left atrium, left ventricle, and aorta throughout the cardiac cycle. Show when the valves open and close, when the heart sounds occur, and the pattern of ventricular ejection.

Contrast the pressures in the right ventricle and pulmonary trunk with those in the left ventricle and aorta.

What causes heart murmurs in diastole? In systole? Write the formula relating cardiac output, heart rate, and stroke volume; give normal values for a resting adult.

Describe the effects of the sympathetic and parasympathetic nerves on heart rate. Which is dominant at rest?

What are the two major factors influencing force of contraction?

Draw a ventricular function curve illustrating the Frank-Starling mechanism.

Describe the effects of the sympathetic nerves on cardiac muscle during contraction and relaxation. Draw a family of curves relating end-diastolic volume and stroke volume during different levels of sympathetic stimulation.

Summarize the effects of the autonomic nerves on the heart.

Draw a flow diagram summarizing the factors determining cardiac output.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

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Responses

  • terttu piirto
    What is the pacemaker potential due to, and what is its inherent rate?
    8 years ago
  • aryan jones
    How to draw events in a cardiac cycle?
    8 years ago
  • Selam
    What are the two major factors influencing force of contraction in heart?
    8 years ago
  • ARTO
    What is mechanical event in the heart?
    7 years ago
  • Liviana Panicucci
    How does the P, Qrs complex and T wave relate to events of the cardiac cycle?
    7 years ago
  • Lukasz
    Where does the cardiac cycle excitation wave terminate?
    7 years ago
  • blair docherty
    What are the mechanical events of the heart are?
    7 years ago
  • AMARANTH
    What is the cardiac cycle mechanism?
    6 years ago

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