Circulatory System

•    Pulmonary circuit

•    Systemic circuit

–  Coronary circulation

•    Arteries

•    Veins

•    Capillaries

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 Anatomy

Coverings of the Heart

•    Pericardium

–  Two-layer serous pericardium

–  The parietal layer

–  The visceral layer or epicardium lines the surface of the heart

–  They are separated by the fluid-filled pericardial cavity

•    The pericardium:

–  Protects and anchors the heart

–  Prevents overfilling of the heart with blood

–  Allows for the heart to work in a relatively friction-free environment

Pericardial Layers of the Heart

Heart Wall

•    Epicardium

–  Visceral layer of the serous pericardium

•    Myocardium

–  Cardiac muscle layer forming the bulk of the heart

•    Endocardium

–  Endothelial layer of the inner myocardial surface

Cardiac Anatomy

•    Atria

–  Receiving chambers of the heart

•    Ventricles

–  Discharging chambers of the heart

Cardiac Anatomy

•     Interatrial septum

–    Foramen ovale/fossa ovalis

•     Interventricular septum

•     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

Cardiac Anatomy

•     Right atrium

–   Superior & inferior vena cava

•   Vessels returning blood to the heart

–   Coronary sinus

–   Right AV valve (tricuspid)

•     Right ventricle

–   Pumps blood into the pulmonary trunk

–   Pulmonary semilunar valve

–   Pulmonary trunk

•   Which splits into right and left pulmonary arteries

–   Pulmonary arteries

Cardiac Anatomy

•     Left atrium

–   Pulmonary veins

•   Vessels returning blood to the heart

–   Left AV valve (bicuspid)

•   A.k.a. mitral valve

•     Left ventricle

–   Pumps blood into the aorta

–   Aortic (semilunar) valve

–   Ascending aorta (three branches)

•   Brachiocephalic, left common carotid, and subclavian arteries

–   Coronary arteries

–   Arch of aorta

–   Descending aorta

–   Ductus arteriosus/ligamentum arteriosum

Heart Valves

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

Cardiac Anatomy

•    Histology of Cardiac Muscle Tissue

–  Cells are shorter and branched

•  Possess sarcomeres (striated)

•  More mitochondria

•  Less Ca⁺²

•  Intercalated discs

–  Anchor cardiac cells together and allow free passage of ions

HEARTBEAT

•    Coordinated contraction of atria and ventricles

–  Valves must be working correctly as well

•    Conducting system

•    Contractile fibers

Cardiac Muscle Contraction

•     Autorhythmic fibers (stimulated by nerves)

–    Automaticity (self-excitable)

–    Contracts as a unit

•     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

•     Cardiac muscle contraction is similar to skeletal muscle contraction

–    Resting potential ~ -90 mV

–    Threshold = -75 mV

•     Has a long (250 ms) absolute refractory period

Pacemaker and Action Potentials of the Heart

Conducting System

•     Specialized cells initiate and distribute electrical signals

–   Sinoatrial (SA) node

•   Generates impulses about 75 times/minute

•   SA node (pacemaker) determines the heart rate

–   Atrioventricular (AV) node

•   Delays the impulse approximately 0.1 second

–   Conducting system

•   Internodal pathways

•   Impulse passes from atria to ventricles via the atrioventricular bundle (bundle of His)

•   Bundle branches

–   Carry the impulse toward the apex of the heart

•   Purkinje fibers

–   Carry the impulse to the heart apex and ventricular walls

Cardiac Intrinsic Conduction

Heart Excitation Related to ECG

Heart Excitation Related to ECG

Electrocardiogram (ECG)

•    Electrical signals are large

•    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 (ECG)

ECG Tracings

Extrinsic Innervation of the Heart

•    Heart is stimulated by the sympathetic cardioacceleratory center

•    Heart is inhibited by the parasympathetic cardioinhibitory center

Heart Sounds

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

–    First sound occurs as AV valves close and signifies beginning of systole

–    Second sound occurs when SL valves close at the beginning of ventricular diastole

Cardiac Cycle

•     Refers to all events associated with blood flow through the heart

•     Systole

–   Contraction of heart muscle

•     Diastole

–   Relaxation of heart muscle

•     Blood flow: high à low pressure

Cardiodynamics

•    Cardiac output

–  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

•     Maximum SV occurs when:

–   EDV @ max

–   ESV @ min

Control of Stroke Volume

•     EDV

–    Filling time

–    Venous return

–    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

•   Amount ventricles are stretched by contained blood

•     ESV

–    Contractility

•   Cardiac cell contractile force due to factors other than EDV

–    Afterload

•   Back pressure exerted by blood in the large arteries leaving the heart

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

•     Blood loss and extremely rapid heartbeat decrease SV

Preload and Afterload

Control of Heart Rate

•     Autonomic innervation

–    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

–    Cardiovascular center

–    Atrial (Bainbridge) reflex is 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

•     Chemicals

–    Hormones

•   Epinephrine and thyroxine increase heart rate

–    Cations

•   Intra- and extracellular ion concentrations must be maintained for normal heart function