Chapter Notes for Lecture: E.N. Marieb, HUMAN ANATOMY & PHYSIOLOGY,5^TH Edition, , Benjamine Cummings Publisher, 2001

Prepare from : V.A. Austin’s PowerPpoint Presentation (ISBN: 0-8053-5469-7), CD ROM: Pearson Education, Inc. , 2003.ter 19

The Cardiovascular System: The Heart

Heart Anatomy

• Approximately the size of your fist

• Location

• Superior surface of diaphragm

• Left of the midline

• Anterior to the vertebral column, posterior to the sternum

Heart Covering

• Pericardial physiology

• Protects and anchors heart

• Prevents overfilling

Heart Covering

• Pericardial anatomy

• Fibrous pericardium

• Serous pericardium (separated by pericardial cavity)

• Epicardium (visceral layer)

Heart Wall

• Epicardium – visceral layer of the serous pericardium

• Myocardium – cardiac muscle layer forming the bulk of the heart

• Fibrous skeleton of the heart – crisscrossing, interlacing layer of connective tissue

• Endocardium – endothelial layer of the inner myocardial surface

External Heart: Major Vessels of the Heart (Anterior View)

• Returning blood to the heart

• Superior and inferior venae cavae

• Right and left pulmonary veins

• Conveying blood away from the heart

• Pulmonary trunk, which splits into right and left pulmonary arteries

• Ascending aorta (three branches) – brachiocephalic, left common carotid, and subclavian arteries

External Heart: Vessels that Supply/Drain the Heart (Anterior View)

• Arteries – right and left coronary (in atrioventricular groove), marginal, circumflex, and anterior interventricular

• Veins – small cardiac vein, anterior cardiac vein, and great cardiac vein

External Heart: Major Vessels of the Heart (Posterior View)

• Returning blood to the heart

• Right and left pulmonary veins

• Superior and inferior venae cavae

• Conveying blood away from the heart

• Aorta

• Right and left pulmonary arteries

External Heart: Vessels that Supply/Drain the Heart (Posterior View)

• Arteries – right coronary artery (in atrioventricular groove) and the posterior interventricular artery (in interventricular groove)

• Veins – great cardiac vein, posterior vein to left ventricle, coronary sinus, and middle cardiac vein

Gross Anatomy of Heart: Frontal Section

• Frontal section showing interior chambers and valves

• Major vessels leading to and from the heart

Gross Anatomy of Heart: Frontal Section

Atria of the Heart

• Atria are the receiving chambers of the heart

• Each atrium has a protruding auricle

• Pectinate muscles mark atrial walls

• Blood enters right atria from superior and inferior venae cavae and coronary sinus

• Blood enters left atria from pulmonary veins

Ventricles of the Heart

• Ventricles are the discharging chambers of the heart

• Papillary muscles and trabeculae carneae muscles mark ventricular walls

• Right ventricle pumps blood into the pulmonary trunk

• Left ventricle pumps blood into the aorta

Pathway of Blood through the Heart and Lungs

• Right atrium à tricuspid valve à right ventricle

• Right ventricle à pulmonary semilunar valve à pulmonary arteries à lungs

• Lungs à pulmonary veins à left atrium

• Left atrium à bicuspid valve à left ventricle

• Left ventricle à aortic semilunar valve à aorta

• Aorta à systemic circulation

Coronary Circulation

• Coronary circulation is the functional blood supply to the heart

• Collateral routes insure blood delivery to heart even if major vessels are occluded

Heart Valves

• Heart valves insure unidirectional blood flow through the heart

• Atrioventricular (AV) valves lie between the atria and the ventricles

• AV valves prevent backflow into the atria when ventricles contract

• Chordae tendineae anchor AV valves to papillary muscles

• Aortic semilunar valve lies between the left ventricle and the aorta

• Pulmonary semilunar valve lies between the right ventricle and pulmonary trunk

• Semilunar valves prevent backflow of blood into the ventricles

Microscopic Heart Muscle Anatomy

• Cardiac muscle is striated, short, fat, branched, and interconnected

• Connective tissue endomysium acts as both tendon and insertion

• Intercalated discs anchor cardiac cells together and allow free passage of ions

• Heart muscle behaves as a functional syncytium

Cardiac Muscle Contraction

• Heart muscle:

• Is stimulated by nerves and self-excitable (automaticity)

• Contracts as a unit

• Has a long (250 ms) absolute refractory period

• Cardiac muscle contraction is similar to skeletal muscle contraction

Heart Physiology: Intrinsic Conduction System

• Autorhythmic cells:

• Initiate action potentials

• Have unstable resting potentials called pacemaker potentials

• Use calcium influx (rather than sodium) for rising phase of the action potential

Heart Physiology: Intrinsic Conduction System

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)

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

Extrinsic Innervation of the Heart

• Heart is stimulated by the sympathetic cardioacceleratory center

• Heart is inhibited by the parasympathetic cardioinhibitory center

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

Electrocardiography

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

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

• Ventricular systole

• Atria relax

• Rising ventricular pressure results in closing of AV valves

• Isovolumetric contraction phase

• Ventricular ejection phase opens semilunar valves

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

Heart Sounds

• Heart sounds (lub-dup) are associated with closing of heart valves

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

Cardiac Output: Example

• CO (ml/min) = HR (75 beats/min) x SV (70 ml/beat)

• CO = 5250 ml/min (5.25 L/min)

Regulation of Stroke Volume

• SV = end diastolic volume (EDV) minus end systolic volume (ESV)

• EDV = amount of blood collected in a ventricle during diastole

• ESV = amount of blood remaining in a ventricle after contraction

Factors Affecting Stroke Volume

• Preload – amount ventricles are stretched by contained blood

• Contractility – cardiac cell contractile force due to factors other than EDV

• Afterload – back pressure exerted by blood in the large arteries leaving the heart

Frank-Starling Law of the Heart

• Preload, or degree of stretch, of cardiac muscle cells before they contract is the critical factor controlling stroke volume

• Slow heartbeat and exercise increase venous return to the heart, increasing SV

• Blood loss and extremely rapid heartbeat decrease SV

Preload and Afterload

Extrinsic Factors Influencing Stroke Volume

• Contractility is the increase in contractile strength, independent of stretch and EDV

• Increase in contractility comes from:

• Increased sympathetic stimuli

• Certain hormones

• Ca²⁺ and some drugs

• Agents/factors that decrease contractility include:

• Acidosis

• Increased extracellular potassium

• Calcium channel blockers

Contractility and Norepinephrine

• Sympathetic stimulation releases norepinephrine and initiates a cyclic AMP second-messenger system

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

Bainbridge Reflex

• Bainbridge (atrial) reflex – a sympathetic reflex initiated by increased blood in the atria

• Causes stimulation of the SA node

• Stimulates baroreceptors in the atria, causing increased SNS stimulation

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

Factors Involved in Regulation of Cardiac Output

Homeostatic Imbalances

• Hypocalcemia – reduced ionic calcium depresses the heart

• Hypercalcemia – dramatically increases heart irritability and leads to spastic contractions

• Hypernatremia – blocks heart contraction by inhibiting ionic calcium transport