Cardiac Cycle and Conduction System of the Heart

The Cardiac Cycle

The mechanical events that take place in one heartbeat.

The Cardiac Cycle

Contraction of a chamber - systole

Relaxation of a chamber - diastole

1 full contraction and relaxation - cardiac cycle

Systole and DiastolePeak pressure of a chamber or vessel

Stroke volumeamount of blood ejected by the heart (LV) in one contraction

• aprox. 70 cc (60-100 cc)

Relaxation of a chamber

Pressure at its lowest

Cardiac Output

Cardiac Outputamount of blood ejected every minute

• aprox. 5-6L

• SV X HR = CO

• The strength of contraction is directly related to venous return

Starling’s Law

The more a muscle is stretched the harder it will contract (up to a certain point)

Similar to a spring

Muscle fibers will stretch and contract harder when there is a increased blood return

Blood Pressure

The pressure exerted by blood against the walls of a given artery.

Factors Affecting Blood Pressure

1. Blood Volume

2. Strength of heart contractions

3. Heart rate

4. Blood viscosity – thickness of blood

Electrophysiology

Before the heart can contract it must be stimulated.

This is accomplished through a specialized network of cells called the conduction system.

There are numerous “pacemaker” cells to stimulate the heart to contract.

Sinoatrial Node (SA)

Located in high RA

Main pacemaker of heart

Sends out regular impulses at 60- 100 times per minute

Regulated by the Autonomic Nervous System (vagus nerve)

Atrioventricular Node (AV)

Located on floor of TA next to septum and behind CS os

Has 2 main jobsDelay the impulse from the SA node long enough so the ventricles can fill with blood

Protect the ventricles from rapid atrial rates

Inherent rate of 40-70 bpm

Bundle of His (HB)

A fibrous ring around the 4 valves

Only muscular connection between the atria and ventricles

AV node and HB collectively are called the AV junction

The AV junction can pace the heart at rate of 40-60 bpm

Bundle Branches

HB branches into rt.... And lt.. Branches

Lt.. Bundle branch (LBB) then divides into anterior and posterior sections

Purkinje Fibers

Terminal portion of the conduction system

Located in ventricular muscle

Can pace the heart at 20-40 bpm

Electrical Properties of Cardiac Muscle

AutomaticityProperty by which a cell can reach threshold potential (depolarize) and send an impulse without help from an external source

ExcitabilityAbility of cell to reach threshold potential in response to an outside stimulus

ConductivityThe propagation of an impulse form cell to cell through intercalated disks

ContractilityAbility of a muscle to shorten (contract) when stimulated

Electrophysiology cont.....

Electrophysiology Cont.....

The end result of contraction of the heart is electrochemical in nature.

The impulse is sent along the conduction system because of changes in cellular polarity.

Cardiac Emergencies

A thrombosis or embolism may plug the arteries causing a myocardial infarction or “heart attack”

Angina pectoris is used to describe the severe chest pain

infarct