5.2 The cardiac cycle

Learning outcomesStudents should understand the following:Myogenic stimulation of the heart and

transmission of a subsequent wave of electrical activity.

Roles of the sinoatrial node (SAN), atrioventricular node (AVN) and bundle of His.

Cardiac output as the product of heart rate and stroke volume.

Pressure and volume changes and associated valve movements during the cardiac cycle.

Candidates should be able to analyse and interpret data relating to pressure and volume changes during the cardiac cycle.

The heartHeart muscle does not need to be

stimulated by a nerve before it will contract. The heart beat originates in the muscle itself and, for this reason, it is described as being myogenic.

There are 2 phases to the beating of the heart:Contraction (systole)Relaxation (diastole)

The Cardiac CycleThe Cardiac CycleCardiac cycle = sequence of events that

makes up one heart beat

Continuous but can be divided into 3 stages:Atrial systole  Ventricular systoleVentricular diastole

1. Atrial Systole1. Atrial Systole

• Heart is full of blood

• Contraction of both atria

• AV valves open • blood flows

ventricles• No backflow as

veins have valves

2. Ventricular Systole2. Ventricular Systole

• Atria relax• Ventricles contract (0.1s

after atria)• Increase in pressure in

ventricle– AV valves close– Semi-lunar valves

open – blood flows from

ventricles arteries• Lasts ~0.3 s

3. Ventricular Diastole3. Ventricular Diastole

• Ventricles relax

• Blood flows from veins (at low pressure) atria

• Some blood flows through atria ventricles

Atrial Systole

The walls of the atria contract. This reduces the

volume of the atria, increasing the pressure.

More blood is forced through the atrio-ventricular valves

into the ventricles.

Ventricular Systole

The walls of the ventricles now contract, reducing the

volume in the ventricles. The pressure increases and blood

is forced into the arteries.

Ventricular Diastole

The ventricle walls relax and the pressure in the ventricles

falls. Blood starts to flow from the atria into the

ventricles again.

Cardiac cycle

Cardiac cycle - narrated.swf

Control of HeartbeatControl of HeartbeatCardiac muscle is

myogenic – naturally contracts & doesn’t need to receive impulses 

Need to coordinate contractions with pacemaker = sino-atrial node (SAN)

Control of heartbeat

Cardiac cycle - SAN & AVN.swf

The Sino-atrial NodeThe Sino-atrial Node

• Patch of specialised muscle in wall of right atrium

• Sets rhythm for rest of heart

• The SAN has its own rhythm, but this may be modified by nerve impulses from the brain.

The Sino-atrial NodeThe Sino-atrial Node

• Contraction of SAN sends excitation wave (wave of depolarisation) through atrial walls

• Cardiac muscle in atrial walls contracts at same rhythm

• Atria contact simultaneously

The Atrio-ventricular NodeThe Atrio-ventricular Node

• Ventricles don’t contract until atria have finished contracting. 

• Delay due to band of fibres between atria & ventricles that does not conduct excitation wave

• Wave can only pass through patch of conducting fibres at top of septum = atrio-ventricular node (AVN).

Purkyne FibresPurkyne Fibres

• AVN picks up excitation wave and the signal now passes rapidly down the specialised conducting fibres (Purkyne fibres) which form the bundle of His in the wall or septum separating the two ventricles.

• wave spreads from base outwards & upwards

• ventricles contract from bottom up• squeezes blood upwards into

arteries

Ventricular diastole

Atrial systole

Ventricular systole

Control of heartbeat

Cardiac cycle - SAN & AVN.swf