The HeartThe Heart

OverviewOverview

IntroductionIntroduction The Heart and The Circulation SystemThe Heart and The Circulation System The HeartbeatThe Heartbeat

– Contractile cellsContractile cells– The conducting systemThe conducting system– The cardiac cycleThe cardiac cycle

Heart DynamicsHeart Dynamics– Factors controlling cardiac outputFactors controlling cardiac output

The HeartbeatThe Heartbeat

Atria and ventricles contract in Atria and ventricles contract in coordinated mannercoordinated manner– Ensures correct blood flowEnsures correct blood flow

2 types of cardiac muscle cells involved:2 types of cardiac muscle cells involved:– Contractile cellsContractile cells

Produce powerful contractions that propel bloodProduce powerful contractions that propel blood

– Conducting systemConducting system Control and coordinate activity of contractile Control and coordinate activity of contractile

cellscells

The Heartbeat: Contractile The Heartbeat: Contractile CellsCells

Differs from skeletal muscle:Differs from skeletal muscle:– Action potentialAction potential– Source of calcium ionsSource of calcium ions– Duration of resulting contractionDuration of resulting contraction

Cardiac vs. Skeletal Muscle Cardiac vs. Skeletal Muscle ContractionContractionCardiac Cardiac MuscleMuscle

Skeletal Skeletal MuscleMuscle

Action Action PotentialPotential

Duration: 250-300 Duration: 250-300 msecmsec

Duration: 10 msecDuration: 10 msec

Extracellular Extracellular Calcium IonsCalcium Ions

Delay Delay repolarization & repolarization & initiate contractioninitiate contraction

Initiate contractionInitiate contraction

Type of Type of contractioncontraction

Long refractory Long refractory period which period which continues while continues while relaxing = no relaxing = no tetanus!tetanus!

Tetanus occurs Tetanus occurs because short because short refractory periodrefractory period

The Conducting SystemThe Conducting System Cardiac muscle tissue contracts on its ownCardiac muscle tissue contracts on its own

– Does not need hormonal or neural stimulationDoes not need hormonal or neural stimulation These will change the forceThese will change the force

– Called automaticity or autorhythmicityCalled automaticity or autorhythmicity Atria contract followed by ventriclesAtria contract followed by ventricles

– Coordinated by conducting systemCoordinated by conducting system Network of specialized cardiac muscle cells Network of specialized cardiac muscle cells

– Initiate and distribute electrical impulsesInitiate and distribute electrical impulses– Made up of two types of cells that do not contract:Made up of two types of cells that do not contract:

Nodal cells (responsible for establishing rate of Nodal cells (responsible for establishing rate of contraction)contraction)

Conducting cells (distribute the contractile stimulus Conducting cells (distribute the contractile stimulus to general myocardium)to general myocardium)

Nodal CellsNodal Cells

Cell membranes depolarize Cell membranes depolarize spontaneously and generate APs at spontaneously and generate APs at regular intervalsregular intervals

Electrically coupled to each other, Electrically coupled to each other, conducting cells, and other cardiac cellsconducting cells, and other cardiac cells

Normal rate of contraction established Normal rate of contraction established by pacemaker cellsby pacemaker cells– Located in the SA (sinoatrial) nodeLocated in the SA (sinoatrial) node– Depolarize rapidly and spontaneouslyDepolarize rapidly and spontaneously– Generate 70-80 APs/min = HR of 70-80 bpmGenerate 70-80 APs/min = HR of 70-80 bpm

Conducting CellsConducting Cells

Stimulus for contraction generated by SA Stimulus for contraction generated by SA node, but must be distributed so that:node, but must be distributed so that:– The atria contract together before the The atria contract together before the

ventriclesventricles– The ventricles contract together The ventricles contract together

Wave must begin at apex and spread toward Wave must begin at apex and spread toward basebase

– This pushes blood toward base into the This pushes blood toward base into the aorta and pulmonary trunkaorta and pulmonary trunk

Now we will watch the CD!Now we will watch the CD!

The Cardiac CycleThe Cardiac Cycle

Cardiac cycle: period between the Cardiac cycle: period between the start of one heartbeat and the start of one heartbeat and the beginning to the nextbeginning to the next

Systole: contractionSystole: contraction Diastole: relaxationDiastole: relaxation Remember: Fluids move from high Remember: Fluids move from high

pressure to low pressure!pressure to low pressure! Now we will watch the CD!Now we will watch the CD!

Heart DynamicsHeart Dynamics Refers to the movements and forces Refers to the movements and forces

generated during cardiac contractionsgenerated during cardiac contractions Each time the heart beats the 2 ventricles Each time the heart beats the 2 ventricles

release = amounts of bloodrelease = amounts of blood Stroke Volume (SV)—the amount of blood Stroke Volume (SV)—the amount of blood

ejected by a ventricle during a single beatejected by a ventricle during a single beat– Can vary from beat to beatCan vary from beat to beat

Cardiac Output (CO)—the amount of blood Cardiac Output (CO)—the amount of blood pumped by each ventricle in 1 minpumped by each ventricle in 1 min

CO = SV x HRCO = SV x HR ml/min = ml x bpmml/min = ml x bpm

Factors Controlling Cardiac Factors Controlling Cardiac OutputOutput

Highly regulatedHighly regulated– Why?Why?

Major factors:Major factors:– Blood volume reflexesBlood volume reflexes– Autonomic innervationAutonomic innervation– Hormones Hormones

Secondary factors:Secondary factors:– Extracellular ion concentrationExtracellular ion concentration– Body temperatureBody temperature

Blood Volume ReflexesBlood Volume Reflexes

Contraction active, relaxation Contraction active, relaxation passivepassive

2 heart reflexes respond to changes 2 heart reflexes respond to changes in blood volin blood vol– One occurs in R atrium and affects HROne occurs in R atrium and affects HR

Atrial reflexAtrial reflex

– One occurs in the ventricles and affects One occurs in the ventricles and affects SVSV

Atrial ReflexAtrial Reflex

Involves adjustments in HRInvolves adjustments in HR– Triggered by increase in venous returnTriggered by increase in venous return

Walls of RA stretch Walls of RA stretch stimulate stretch stimulate stretch receptors in wall receptors in wall increase in sympathetic increase in sympathetic activity activity cells of SA node depolarize faster cells of SA node depolarize faster increase HR increase HR

Ventricular ReflexVentricular Reflex

Amount of blood pumped out of each Amount of blood pumped out of each ventricle each beat depends on:ventricle each beat depends on:– Venous returnVenous return– Filling timeFilling time

Frank-Starling principleFrank-Starling principle– Major effect is that the output of blood Major effect is that the output of blood

from both ventricles is balanced under a from both ventricles is balanced under a variety of conditionsvariety of conditions

Factors Controlling CO: Factors Controlling CO: Autonomic InnervationAutonomic Innervation

ANS can modify HRANS can modify HR– Innervated by both parasympathetic and Innervated by both parasympathetic and

sympathetic divisionssympathetic divisions Innervate SA and AV nodesInnervate SA and AV nodes

Factors Affecting CO: Factors Affecting CO: HormonesHormones

Adrenal medullaAdrenal medulla– NE and ENE and E– Result?Result?

Thyroid hormones and glucagonThyroid hormones and glucagon– Secreted by pancreasSecreted by pancreas– Produce similar effects to NE and EProduce similar effects to NE and E

Coordination of Autonomic Coordination of Autonomic ActivityActivity

Cardiac centers in medullaCardiac centers in medulla– Cardioacceleratory centerCardioacceleratory center

Which neurons are activated?Which neurons are activated?

– Cardioinhibitory centerCardioinhibitory center Which neurons are activated?Which neurons are activated?

Gets from heart to medulla through vagus Gets from heart to medulla through vagus nervenerve

Both respond to changes in bp and arterial Both respond to changes in bp and arterial concentrations of oxygen and carbon dioxideconcentrations of oxygen and carbon dioxide– Monitored by baroreceptorsMonitored by baroreceptors