1 Cardiovascular System: The Heart Mary Christenson, PT, PhD DPT 732: Management Applications of Physiology II of Physiology II Spring 2009

Did You Know? Coronary circulation is the shortest circulation in the body Coronary circulation is the shortest circulation in the body “The longest cardiac arrest lasted four hours in the case of fisherman Jan Egil Refsdahl (Norway), who fell overboard off Bergen, Norway, on December 7, He was rushed to Haukeland Hospital after his body temperature fell to 75° F (24° C), and his heart stopped. He made a full recovery after being connected to a heart-lung machine.” “The longest cardiac arrest lasted four hours in the case of fisherman Jan Egil Refsdahl (Norway), who fell overboard off Bergen, Norway, on December 7, He was rushed to Haukeland Hospital after his body temperature fell to 75° F (24° C), and his heart stopped. He made a full recovery after being connected to a heart-lung machine.”

3 Objectives Describe the physiologic structure and function of the heart. Describe the physiologic structure and function of the heart. Describe the systemic & pulmonary blood flow circuits of the CV system. Describe the systemic & pulmonary blood flow circuits of the CV system. Describe the coronary circulation and compare and contrast to the pulmonary/systemic circulation. Describe the coronary circulation and compare and contrast to the pulmonary/systemic circulation. Describe the function of the different types of cardiac muscle and the microanatomy (striated, intercalated discs, gap junctions) of each. Describe the function of the different types of cardiac muscle and the microanatomy (striated, intercalated discs, gap junctions) of each. Describe the energy requirements of the heart. Describe the energy requirements of the heart. Compare/contrast the intrinsic and extrinsic regulation of heart pumping. Compare/contrast the intrinsic and extrinsic regulation of heart pumping.

4 Objectives (continued) Describe the cardiac cycle and compare and contrast the relationship between EKG tracing, heart sounds, atrial and ventricular pressure changes, atrial and ventricular volume changes, and valve actions that occur within the left chambers of the heart during a cardiac cycle. Describe the cardiac cycle and compare and contrast the relationship between EKG tracing, heart sounds, atrial and ventricular pressure changes, atrial and ventricular volume changes, and valve actions that occur within the left chambers of the heart during a cardiac cycle. Describe the specialized excitatory and conductive system of the heart. Describe the specialized excitatory and conductive system of the heart. Describe the cellular mechanisms of pacemaker potential and cardiac muscle contraction, and compare with action potentials and skeletal muscle contraction. Describe the cellular mechanisms of pacemaker potential and cardiac muscle contraction, and compare with action potentials and skeletal muscle contraction. Describe the characteristics and principle features of a normal EKG (waves, segments, complexes, polarization, depolarization, repolarization). Describe the characteristics and principle features of a normal EKG (waves, segments, complexes, polarization, depolarization, repolarization).

5 Heart Anatomy: A Review Functional anatomy Functional anatomy

6 Heart Anatomy: A Review External anatomy view External anatomy view Membrane coverings Membrane coverings Heart wall Heart wall Vessels entering/exiting Vessels entering/exiting Blood flow circuits: arteries versus veins Blood flow circuits: arteries versus veins Pulmonary circuit Pulmonary circuit Systemic circuit Systemic circuit Coronary arteries Coronary arteries

Pericardium 7

Heart Wall 8

9 Heart Anatomy: A Review Internal anatomy Internal anatomy Chambers Chambers Structures Structures

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11 Coronary Blood Flow

Systemic & Pulmonary Blood Flow

13 Cardiac Muscle & Microanatomy Muscle Muscle Atrial muscle Atrial muscle Ventricular muscle Ventricular muscle Specialized excitatory and conductive muscle fibers Specialized excitatory and conductive muscle fibers Muscle cells Muscle cells Intercalated discs Intercalated discs Gap junctions Gap junctions

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Electron Micrograph

Cell Connections

17 Compare/Contrast Cardiac and Skeletal Mechanism of Contraction Similarities Similarities Striated – myosin/actin mechanism Striated – myosin/actin mechanism T-tubule mechanism – acting on sarcoplasmic reticulum T-tubule mechanism – acting on sarcoplasmic reticulum Differences Differences T-tubule mechanism – direct diffusion of Ca++ T-tubule mechanism – direct diffusion of Ca++ Action potential Action potential Cardiac muscle “plateau” Cardiac muscle “plateau” Strength of contraction Strength of contraction

18 Conduction Ability of cardiac mm to depolarize and contract is intrinsic Ability of cardiac mm to depolarize and contract is intrinsic Intrinsic conduction system Intrinsic conduction system Components Components Sinus node = sinoatrial/S-A node Sinus node = sinoatrial/S-A node Internodal pathways Internodal pathways A-V node A-V node A-V bundle A-V bundle Left and right bundle branches of Purkinje fibers Left and right bundle branches of Purkinje fibers

Intrinsic Conduction System 19

Features of the S-A Node Smaller diameter muscle fibers Almost no contractile muscle fibers Connect directly with atrial muscle (mm) fibers Cell membranes naturally “leaky” to Na+ and Ca++ ions – therefore, less negative resting membrane potential than other cardiac mm cells Fast Na+ channels, at less negative potential, “inactivated” Self-excitation

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23 EKG Electrical impulses passing through the heart also spread into adjacent tissues and some to the surface of the body Electrical impulses passing through the heart also spread into adjacent tissues and some to the surface of the body Can be captured at surface of the body using electrodes Can be captured at surface of the body using electrodes

EKG Tracing 24

25 Cardiac Cycle Events that occur from the beginning of one heartbeat to the beginning of the next Events that occur from the beginning of one heartbeat to the beginning of the next Chamber and vessel blood volume changes Chamber and vessel blood volume changes Chamber and vessel blood pressures changes Chamber and vessel blood pressures changes Electrical activity noted Electrical activity noted Heart sounds occur Heart sounds occur Valves open and close Valves open and close Describe the relationships between the events Describe the relationships between the events

26 Cardiac cycle Consists of: Consists of: Diastole: period of relaxation; heart filling with blood Diastole: period of relaxation; heart filling with blood Systole: contraction period, heart ejects blood Systole: contraction period, heart ejects blood What would be: What would be: the definition of end-diastolic volume (EDV)? the definition of end-diastolic volume (EDV)? the definition of end-systolic volume (ESV)? the definition of end-systolic volume (ESV)? Ejection fraction: fraction of EDV ejected Ejection fraction: fraction of EDV ejected

Guyton Cardiac Cycle

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Relationship of Cardiac Cycle to ECG P wave: spread of depolarization through atrial tissue followed by contraction - atrial pressure P wave: spread of depolarization through atrial tissue followed by contraction - atrial pressure QRS complex: spread of depolarization through ventricular tissue followed by contraction - ventricular pressure QRS complex: spread of depolarization through ventricular tissue followed by contraction - ventricular pressure T wave: repolarization of the ventricles which represents ventricular relaxation T wave: repolarization of the ventricles which represents ventricular relaxation 29

Atria as “Pumps” Majority of returning venous blood flows directly from atrium to ventricle Majority of returning venous blood flows directly from atrium to ventricle Atrial contraction usually causes an additional 20% ventricle filling; “primer pump” Atrial contraction usually causes an additional 20% ventricle filling; “primer pump” Atrial function “unnecessary” except during vigorous exercise Atrial function “unnecessary” except during vigorous exercise Atrial pressure changes Atrial pressure changes 30

Ventricles as “Pumps” Ventricular filling: after systole, A-V valves open due to build up of pressure in atria during systole: period of rapid filling of ventricles followed by 2 additional phases Ventricular filling: after systole, A-V valves open due to build up of pressure in atria during systole: period of rapid filling of ventricles followed by 2 additional phases Period of Isovolumic Contraction Period of Isovolumic Contraction Period of Ejection Period of Ejection Period of Isovolumic Relaxation Period of Isovolumic Relaxation 31

Preload and Afterload Preload – End-diastolic pressure when the ventricle is filled; amount of tension on the muscle when it begins to contract Preload – End-diastolic pressure when the ventricle is filled; amount of tension on the muscle when it begins to contract Afterload – pressure in the artery leading from the ventricle; load against which the muscle exerts its contractile force Afterload – pressure in the artery leading from the ventricle; load against which the muscle exerts its contractile force Heart and/or circulation pathology can severely alter preload and/or afterload Heart and/or circulation pathology can severely alter preload and/or afterload 32

Chemical Energy Requirements for Cardiac Contraction Great dependency/almost exclusive reliance on O2 for energy metabolism (oxidative) compared to skeletal muscle which can utilize anaerobic metabolic sources as well Great dependency/almost exclusive reliance on O2 for energy metabolism (oxidative) compared to skeletal muscle which can utilize anaerobic metabolic sources as well Energy derived primarily from oxidative metabolism of fatty acids (food of choice: primary oxidative nutrient source), some lactate, glucose Energy derived primarily from oxidative metabolism of fatty acids (food of choice: primary oxidative nutrient source), some lactate, glucose Cardiac muscle can also use lactic acid generated by skeletal muscle activity Cardiac muscle can also use lactic acid generated by skeletal muscle activity 34

Intrinsic Regulation of the Cardiac Pump Heart pumps 4-6 liters of rest Heart pumps 4-6 liters of rest Frank-Starling Mechanism Frank-Starling Mechanism Heart automatically pumps incoming blood; i.e., amount of blood pumped determined primarily by rate of blood flow into heart Heart automatically pumps incoming blood; i.e., amount of blood pumped determined primarily by rate of blood flow into heart As cardiac muscle is stretched with returning blood volume, approach optimal length of actin and myosin fibers for contraction As cardiac muscle is stretched with returning blood volume, approach optimal length of actin and myosin fibers for contraction Stretch of R atrial wall Stretch of R atrial wall Increase HR by 10-20% Increase HR by 10-20% 35

Extrinsic Regulation of the Cardiac Pump (ANS) Sympathetic Nervous System (SNS) Norepinephrine released by sympathetic nerve fibers in response to stressors such as fright, anxiety, or exercise; threshold reached more quickly Increase cardiac output (CO) Pacemaker fires more rapidly Enhanced mm contractility Effects of inhibiting SNS

Extrinsic Regulation of the Cardiac Pump (ANS) Parasympathetic Nervous System Reduces HR when stressors removed Acetylcholine hyperpolarizes membranes of cells – opens K+ channels PNS fibers in Vagus nerves to heart can decrease CO Primarily affects HR rather than contractility

Autonomic Innervation of the Heart 38

Resting Conditions S-A node receives impulses from both autonomic divisions continuously Dominant influence is inhibitory – heart said to exhibit “vagal tone” “Disconnect” vagal nerves = HR increases ~25 bpm almost immediately