The Cardiovascular System: The Heart

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1 The Cardiovascular System: The Heart
18 P A R T B The Cardiovascular System: The Heart

2 Cardiac Muscle Contraction
Heart muscle: Is stimulated by nerves and is self-excitable (automaticity) Contracts as a unit Has a long (250 ms) absolute refractory period (compared to 1-2 ms in skeletal muscle) Cardiac muscle contraction is similar to skeletal muscle contraction

3 Heart Physiology: Intrinsic Conduction System
Autorhythmic cells: Initiate action potentials Have unstable resting potentials called pacemaker potentials

4 Heart Physiology: Sequence of Excitation
Sinoatrial (SA) node generates impulses about 75 times/minute Atrioventricular (AV) node delays the impulse approximately 0.1 second Impulse passes from atria to ventricles via the atrioventricular bundle (bundle of His)

5 Heart Physiology: Sequence of Excitation
AV bundle splits into two pathways in the interventricular septum (bundle branches) Bundle branches carry the impulse toward the apex of the heart Purkinje fibers carry the impulse to the heart apex and ventricular walls

6 Cardiac Intrinsic Conduction
Figure 18.14a

7 Heart Excitation Related to ECG
SA node generates impulse; atrial excitation begins Impulse delayed at AV node Impulse passes to heart apex; ventricular excitation begins Ventricular excitation complete SA node AV node Bundle branches Purkinje fibers Figure 18.17

8 Heart Excitation Related to ECG
SA node generates impulse; atrial excitation begins SA node Figure 18.17

9 Heart Excitation Related to ECG
Impulse delayed at AV node AV node Figure 18.17

10 Heart Excitation Related to ECG
Impulse passes to heart apex; ventricular excitation begins Bundle branches Figure 18.17

11 Heart Excitation Related to ECG
Ventricular excitation complete Purkinje fibers Figure 18.17

12 Heart Excitation Related to ECG
SA node generates impulse; atrial excitation begins Impulse delayed at AV node Impulse passes to heart apex; ventricular excitation begins Ventricular excitation complete SA node AV node Bundle branches Purkinje fibers Figure 18.17

13 Extrinsic Innervation of the Heart
Heart is stimulated by the sympathetic cardioacceleratory center Heart is inhibited by the parasympathetic cardioinhibitory center Figure 18.15

14 Electrical activity is recorded by electrocardiogram (ECG)
Electrocardiography Electrical activity is recorded by electrocardiogram (ECG) P wave corresponds to depolarization of SA node QRS complex corresponds to ventricular depolarization T wave corresponds to ventricular repolarization Atrial repolarization record is masked by the larger QRS complex PLAY InterActive Physiology ®: Intrinsic Conduction System, pages 3–6

15 Electrocardiography Figure 18.16

16 ECG Tracings Figure 18.18

17 Heart Sounds Figure 18.19

18 Heart sounds (lub-dup) are associated with closing of heart valves
First sound occurs as AV valves close and signifies beginning of systole (when ventricles contract and the atria relax) Second sound occurs when semilunar (SL) valves close at the beginning of ventricular diastole (when ventricles relax)

19 Cardiac Cycle Cardiac cycle refers to all events associated with blood flow through the heart Systole – contraction of heart muscle Diastole – relaxation of heart muscle

20 Phases of the Cardiac Cycle
Ventricular filling – mid-to-late diastole Heart blood pressure is low as blood enters atria and flows into ventricles AV valves are open, then atrial systole occurs

21 Phases of the Cardiac Cycle
Ventricular systole Atria relax Rising ventricular pressure results in closing of AV valves Isovolumetric contraction phase Ventricular ejection phase opens semilunar valves

22 Phases of the Cardiac Cycle
Isovolumetric relaxation – early diastole Ventricles relax Backflow of blood in aorta and pulmonary trunk closes semilunar valves Dicrotic notch – brief rise in aortic pressure caused by backflow of blood rebounding off semilunar valves PLAY InterActive Physiology ®: Cardiac Cycle, pages 3–18

23 Figure 18.20

24 Blood flow through the heart is controlled by pressure changes.
Blood flows down a pressure gradient.

25 Cardiac Output (CO) and Reserve
CO is the amount of blood pumped by each ventricle in one minute CO is the product of heart rate (HR) and stroke volume (SV) HR is the number of heart beats per minute SV is the amount of blood pumped out by a ventricle with each beat Cardiac reserve is the difference between resting and maximal CO

26 Cardiac Output: Example
CO (ml/min) = HR (75 beats/min) x SV (70 ml/beat) CO = 5250 ml/min (5.25 L/min) Entire blood supply passes through each side of the heart once each minute Cardiac output varies directly with SV and HR

27 Regulation of Stroke Volume
SV = end diastolic volume (EDV) minus end systolic volume (ESV) EDV = amount of blood collected in a ventricle during diastole How long ventricular diastole lasts Venous pressure ESV = amount of blood remaining in a ventricle after contraction Arterial blood pressure Force of ventricular contraction

28 Factors Affecting Stroke Volume
Preload – amount ventricles are stretched by contained blood Affects EDV Contractility – cardiac cell contractile force Affects ESV Afterload – back pressure exerted by blood in the large arteries leaving the heart

29 Regulation of Heart Rate
Positive chronotropic factors increase heart rate Negative chronotropic factors decrease heart rate Autonomic Nervous System Chemical Regulation Other Factors Age, gender, exercise, body temperature

30 Regulation of Heart Rate: Autonomic Nervous System
Sympathetic nervous system (SNS) stimulation is activated by stress, anxiety, excitement, or exercise Parasympathetic nervous system (PNS) stimulation is mediated by acetylcholine and opposes the SNS PNS dominates the autonomic stimulation, slowing heart rate and causing vagal tone Slows SA node by 25 beats/min.

31 Chemical Regulation of the Heart
The hormones epinephrine and thyroxine increase heart rate Intra- and extracellular ion concentrations must be maintained for normal heart function PLAY InterActive Physiology ®: Cardiac Output, pages 3–9

32 Homeostatic Imbalance of Cardiac Output
Congestive heart failure (CHF) – when the pumping efficiency (CO) of the heart is so low that blood circulation is inadequate to meet tissue needs. Congestive heart failure (CHF) is caused by: Coronary atherosclerosis Persistent high blood pressure Multiple myocardial infarcts Dilated cardiomyopathy (DCM)

33 Developmental Aspects of the Heart
Begins as two simple endothelial tubes, fuse to form one chamber. Pumps by day 23 By day 25, four chambers are present

34 Developmental Aspects of the Heart
Fetal heart structures that bypass pulmonary circulation Foramen ovale connects the two atria Ductus arteriosus connects pulmonary trunk and the aorta At or shortly after birth, these shunts close

35 Examples of Congenital Heart Defects
Lead to mixing or oxygen-poor systemic blood with oxygenated pulmonary blood Involves narrowed valves or vessels Figure 18.25

36 Age-Related Changes Affecting the Heart
Sclerosis and thickening of valve flaps Usually mitral Decline in cardiac reserve Fibrosis of cardiac muscle Atherosclerosis