Cardiovascular System Notes: Physiology of the Heart

Interesting Heart Fact Capillaries are so small it takes ten of them to equal the thickness of a human hair.

Heart Physiology: Electrical Events Heart depolarizes and contracts without nervous system stimulation Rhythm can be altered by autonomic nervous system © 2013 Pearson Education, Inc.

Heart Physiology: Setting the Basic Rhythm Coordinated heartbeat is a function of Presence of gap junctions Intrinsic cardiac conduction system Network of noncontractile (autorhythmic) cells Initiate and distribute impulses  coordinated depolarization and contraction of heart © 2013 Pearson Education, Inc.

PHYSIOLOGY OF THE HEART The Conducting System (electric) called an intrinsic conduction or “nodal” system of specialized tissue 1. Sinoatrial Node (SA) PACEMAKER OF HEART – starts each heartbeat located in wall of right atrium made of specialized myocardial cells sends impulse to both atria, causing them to contract

SA Node AV Node 2. Atrioventricular Node (AV node) located at base of right atrium – receives impulse from SA node

Atrioventricular Bundle Location: middle of septum Atrioventricular Bundle impulse from AV node goes down bundle to base of heart Direction of Heart Beat it then travels up the sides of the ventricles through the PURKINJE NETWORK causing the ventricles to contract from the bottom up

Superior vena cava Right atrium Internodal pathway Left atrium Figure 18.15a Intrinsic cardiac conduction system and action potential succession during one heartbeat. Slide 1 Superior vena cava Right atrium The sinoatrial (SA) node (pacemaker) generates impulses. 1 Internodal pathway The impulses pause (0.1 s) at the atrioventricular (AV) node. 2 Left atrium The atrioventricular (AV) bundle connects the atria to the ventricles. 3 Subendocardial conducting network (Purkinje fibers) The bundle branches conduct the impulses through the interventricular septum. 4 Inter- ventricular septum The subendocardial conducting network depolarizes the contractile cells of both ventricles. 5 Anatomy of the intrinsic conduction system showing the sequence of electrical excitation © 2013 Pearson Education, Inc.

RATE OF HEART BEAT Factors affecting heart rate age, sex, physical activity, temperature, thought processes, chemicals (natural and otherwise) rate high at birth (100 – 140 bpm) – then declines steadily until average is reached (70 – 80 bpm) heart rate faster in females – slower in trained athletes

Extrinsic Innervation of the Heart Heartbeat modified by ANS via cardiac centers in medulla oblongata Sympathetic   rate and force Parasympathetic   rate Cardioacceleratory center – sympathetic – affects SA, AV nodes, heart muscle, coronary arteries Cardioinhibitory center – parasympathetic – inhibits SA and AV nodes via vagus nerves © 2013 Pearson Education, Inc.

Homeostatic Imbalances Defects in intrinsic conduction system may cause Arrhythmias - irregular heart rhythms Uncoordinated atrial and ventricular contractions Fibrillation - rapid, irregular contractions; useless for pumping blood  circulation ceases  brain death Defibrillation to treat © 2013 Pearson Education, Inc.

Figure 18.16 Autonomic innervation of the heart. The vagus nerve (parasympathetic) decreases heart rate. Dorsal motor nucleus of vagus Cardioinhibitory center Cardioaccele- ratory center Medulla oblongata Sympathetic trunk ganglion Thoracic spinal cord Sympathetic trunk Sympathetic cardiac nerves increase heart rate and force of contraction. AV node SA node Parasympathetic fibers Sympathetic fibers Interneurons © 2013 Pearson Education, Inc.

CARDIAC CYCLE the events of one complete heartbeat length of cycle (heartbeat) is about 0.8 sec atria contract at the same time – as they relax, the ventricles contract SYSTOLE contraction of ventricle DIASTOLE relaxation of ventricle amount of blood pumped out of each side of the heart in 1 minute Cardiac Output heart rate X stroke volume

Heart Sounds Two sounds (lub-dup) associated with closing of heart valves First as AV valves close; beginning of systole Second as SL valves close; beginning of ventricular diastole Pause indicates heart relaxation Heart murmurs - abnormal heart sounds; usually indicate incompetent or stenotic valves © 2013 Pearson Education, Inc.

heard in 2nd intercostal space at right sternal margin Figure 18.20 Areas of the thoracic surface where the sounds of individual valves can best be detected. Aortic valve sounds heard in 2nd intercostal space at right sternal margin Pulmonary valve sounds heard in 2nd intercostal space at left sternal margin Mitral valve sounds heard over heart apex (in 5th intercostal space) in line with middle of clavicle Tricuspid valve sounds typically heard in right sternal margin of 5th intercostal space © 2013 Pearson Education, Inc.

Mechanical Events: The Cardiac Cycle Blood flow through heart during one complete heartbeat: atrial systole and diastole followed by ventricular systole and diastole Systole—contraction Diastole—relaxation Series of pressure and blood volume changes © 2013 Pearson Education, Inc.

Phases of the Cardiac Cycle 1. Ventricular filling—takes place in mid-to-late diastole AV valves are open; pressure low 80% of blood passively flows into ventricles Atrial systole occurs, delivering remaining 20% End diastolic volume (EDV): volume of blood in each ventricle at end of ventricular diastole © 2013 Pearson Education, Inc.

Phases of the Cardiac Cycle 2. Ventricular systole Atria relax; ventricles begin to contract Rising ventricular pressure  closing of AV valves Isovolumetric contraction phase (all valves are closed) In ejection phase, ventricular pressure exceeds pressure in large arteries, forcing SL valves open End systolic volume (ESV): volume of blood remaining in each ventricle after systole © 2013 Pearson Education, Inc.

Phases of the Cardiac Cycle 3. Isovolumetric relaxation - early diastole Ventricles relax; atria relaxed and filling Backflow of blood in aorta and pulmonary trunk closes SL valves Causes dicrotic notch (brief rise in aortic pressure as blood rebounds off closed valve) Ventricles totally closed chambers When atrial pressure exceeds that in ventricles  AV valves open; cycle begins again at step 1 © 2013 Pearson Education, Inc.

Figure 18.21 Summary of events during the cardiac cycle. Left heart QRS P T P Electrocardiogram 1st 2nd Heart sounds Dicrotic notch 120 80 Aorta Pressure (mm Hg) Left ventricle 40 Atrial systole Left atrium 120 EDV Ventricular volume (ml) SV 50 ESV Atrioventricular valves Open Closed Open Aortic and pulmonary valves Closed Open Closed Phase 1 2a 2b 3 1 Left atrium Right atrium Left ventricle Right ventricle Ventricular filling Atrial contraction Isovolumetric contraction phase Ventricular ejection phase Isovolumetric relaxation Ventricular filling 1 2a 2b 3 Ventricular filling (mid-to-late diastole) Ventricular systole (atria in diastole) Early diastole © 2013 Pearson Education, Inc.

PULSE & BLOOD PRESSURE pulse can be found at many sites on the body Pulse expansion & contraction of an artery as the left ventricle contracts normal is 70 – 80 bpm (there are exceptions to this)

Blood Pressure pressure blood exerts on the inner walls of blood vessels Measuring BP BP is a measure of the systolic pressure (ventricles contracting) OVER the diastolic pressure (ventricles relaxing) Example: 110/70 is in normal range Factors affecting BP nervous system, blood volume (kidneys), temp, chemicals, diet, exercise

The Electrocardiogram (EKG or ECG) amplifies electric current of heart producing distinct wave patterns

QRS Complex P wave T wave P wave depolarization of atria QRS Complex depolarization of ventricles T wave repolarization of ventricles

Electrocardiography Electrocardiogram (ECG or EKG) Three waves: Composite of all action potentials generated by nodal and contractile cells at given time Three waves: P wave – depolarization SA node  atria QRS complex - ventricular depolarization and atrial repolarization T wave - ventricular repolarization © 2013 Pearson Education, Inc.

Figure 18.17 An electrocardiogram (ECG) tracing. Sinoatrial node Atrioventricular node QRS complex R Ventricular depolarization Ventricular repolarization Atrial depolarization T P Q S-T Segment P-R Interval S Q-T Interval 0.2 0.4 0.6 0.8 © 2013 Pearson Education, Inc. Time (s)

Figure 18.18 The sequence of depolarization and repolarization of the heart related to the deflection waves of an ECG tracing. Slide 1 SA node R R P T P T Q S Q S Atrial depolarization, initiated by the SA node, causes the P wave. 1 Ventricular depolarization is complete. 4 R AV node R T P T P Q Q S S With atrial depolarization complete, the impulse is delayed at the AV node. Ventricular repolarization begins at apex, causing the T wave. 2 5 R R P T P T Q Q S S Ventricular repolarization is complete. 6 Ventricular depolarization begins at apex, causing the QRS complex. Atrial repolarization occurs. 3 Depolarization Repolarization © 2013 Pearson Education, Inc.

Homeostatic Imbalances Tachycardia - abnormally fast heart rate (>100 beats/min) If persistent, may lead to fibrillation Bradycardia - heart rate slower than 60 beats/min May result in grossly inadequate blood circulation in nonathletes May be desirable result of endurance training © 2013 Pearson Education, Inc.

Homeostatic Imbalance Congestive heart failure (CHF) Progressive condition; CO is so low that blood circulation inadequate to meet tissue needs Reflects weakened myocardium caused by Coronary atherosclerosis—clogged arteries Persistent high blood pressure Multiple myocardial infarcts Dilated cardiomyopathy (DCM) © 2013 Pearson Education, Inc.

Homeostatic Imbalance Pulmonary congestion Left side fails  blood backs up in lungs Peripheral congestion Right side fails  blood pools in body organs  edema Failure of either side ultimately weakens other Treat by removing fluid, reducing afterload, increasing contractility © 2013 Pearson Education, Inc.

Developmental Aspects of the Heart Embryonic heart chambers Sinus venosus Atrium Ventricle Bulbus cordis © 2013 Pearson Education, Inc.

Figure 18.24 Development of the human heart. Arterial end Arterial end Aorta Ductus arteriosus Superior vena cava 4a Pulmonary trunk 4 Tubular heart Ventricle Foramen ovale 3 Atrium 2 Ventricle 1 Inferior vena cava Ventricle Venous end Venous end Day 20: Endothelial tubes begin to fuse. Day 22: Heart starts pumping. Day 24: Heart continues to elongate and starts to bend. Day 28: Bending continues as ventricle moves caudally and atrium moves cranially. Day 35: Bending is complete. © 2013 Pearson Education, Inc.

Developmental Aspects of the Heart Fetal heart structures that bypass pulmonary circulation Foramen ovale connects two atria Remnant is fossa ovalis in adult Ductus arteriosus connects pulmonary trunk to aorta Remnant - ligamentum arteriosum in adult Close at or shortly after birth © 2013 Pearson Education, Inc.

Developmental Aspects of the Heart Congenital heart defects Most common birth defects; treated with surgery Most are one of two types: Mixing of oxygen-poor and oxygen-rich blood, e.g., septal defects, patent ductus arteriosus Narrowed valves or vessels  increased workload on heart, e.g., coarctation of aorta Tetralogy of Fallot Both types of disorders present © 2013 Pearson Education, Inc.

Figure 18.25 Three examples of congenital heart defects. Narrowed aorta Occurs in about 1 in every 500 births Occurs in about 1 in every 1500 births Occurs in about 1 in every 2000 births Ventricular septal defect. The superior part of the inter-ventricular septum fails to form, allowing blood to mix between the two ventricles. More blood is shunted from left to right because the left ventricle is stronger. Coarctation of the aorta. A part of the aorta is narrowed, increasing the workload of the left ventricle. Tetralogy of Fallot. Multiple defects (tetra = four): (1) Pulmonary trunk too narrow and pulmonary valve stenosed, resulting in (2) hypertrophied right ventricle; (3) ventricular septal defect; (4) aorta opens from both ventricles. © 2013 Pearson Education, Inc.

Age-Related Changes Affecting the Heart Sclerosis and thickening of valve flaps Decline in cardiac reserve Fibrosis of cardiac muscle Atherosclerosis © 2013 Pearson Education, Inc.