8/12/2019 As 1 2 2 Control of the Cardiac Cycle

1/19

Control of the Cardiac Cycle

describe the cardiac cycle, with reference to

the action of the valves in the heart;

describe how heart action is coordinated with

reference to the sinoatrial node (SAN), the

atrioventricular node (AVN) and the Purkyne

tissue;

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

2/19

Control of the Cardiac Cycle

describe the cardiac cycle, with reference to

the action of the valves in the heart;

describe how heart action is coordinated with

reference to the sinoatrial node (SAN), the

atrioventricular node (AVN) and the Purkyne

tissue;

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

3/19

The Need for Co-ordination

Heart (cardiac) muscle is unusual as it can initiate its owncontraction

This is known as myogenic

The muscles can contract and relax rythmically even if its

not connected to the body The muscles of the atria and ventricles have their own

natural frequency of contraction- the atrial muscle has ahigher frequency (number of contractions) than theventricular muscle

The property of the muscle could cause inefficient pumping(fibrillation) if the contractions are not synchronised- so theheart needs a mechanism for control

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

4/19

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

5/19

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

6/19

Contraction of the Atria

The wave of excitation quickly spreads over

the walls of both atria

It travels along the membranes of the muscle

tissue and causes the cardiac muscles to

contract

This is Atrial Systole

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

7/19

Contraction of the Atria

At the base of the atria is a disc oftissue that cannot conduct theexcitation wave (electrical wave)

This means the wave cannot spreaddirectly to the ventricles

At the top of the inter ventricularseptum (separating the two ventricles)is another node- the atrio-ventricularnode (or AVN) position 2 on thediagram

This is the only route through the discof non-conducting tissue

The wave of excitation is delayed in thenode, this allows time for the atria tofinish contracting and for the blood toflow down into the ventricles beforethey contract

1 = SAN (pacemaker)

2 = AVN

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

8/19

Contraction of the Ventricles

After the delay the wave of excitationis carried away from the AVN anddown specialised conducting tissuecalled the Purkyne tissue which runsdown the inter ventricular septum(position 3 on diagram)

At the base of the septum theexcitation wave spreads out over thewalls of the ventricles

As it spreads upwards from the baseof the ventricles, it causes the musclesto contract

This means that that ventriclescontract from the base upwards,pushing blood up to the major arteries

the aorta and the pulmonary artery

3 = Bundle of His

4 & 5 = Purkyne Tissue

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

9/19

http://en.wikipedia.org/wiki/File:ECG_Principle_fast.gif

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

10/19

ElectroCardiograms

We can monitor theelectrical activity of theheart using anelectrocardiogram or ECG

Sensors must be attachedto the skin which pick up

the electrical signals fromthe heart

The trace of a healthyperson has a particularshape consisting of waveslabelled P, Q, R, S and T

P shows the excitation of the atria

QRS indicates excitation of the ventricles

T shows diastole (relaxing)

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

11/19

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

12/19

The Shape of an ECG

The shape of the ECG trace can sometimes be used to showwhich part of the heart is unhealthy

It can show irregular heart beat (arrhythmia) if it is infibrillation (the beat is not co-ordinated), if it has suffered aheart attack (myocardial infarction)

It can also indicate if the heart is enlarged or if the Purkynesystem is not conducting electrical activity properly

A heart block is when there is a problem with the electrical signals in the heart

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

13/19

Now try SAQ 6 on p71

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

14/19

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

15/19

In 1 second, 25 boxes travel past the machine

One box travels at 1 second/25 boxes = 0.04 seconds per box

One heart beat lasts for approximately 20 boxes

20 boxes x 0.04 seconds per box = 0.8 seconds for each heart beat

How many 0.8 seconds are in 1 minute?

60 seconds / 0.8 seconds = 75 beats per minute

20 boxes

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

16/19

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

17/19

b (i) This is the time during which the ventricles are contracting

B (ii) In 1 second, 25 boxes travel past the machine

One box travels at 1 second/25 boxes = 0.04 seconds per box

Contraction time (Q-T) lasts for approximately 7 boxes

7 boxes x 0.04 seconds per box = 0.28 seconds for each contraction time

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

18/19

8/12/2019 As 1 2 2 Control of the Cardiac Cycle

19/19

b (i) This is the time during which the ventricles are relaxed and filling with bloodB (ii) In 1 second, 25 boxes travel past the machine

One box travels at 1 second/25 boxes = 0.04 seconds per box

Contraction time (T-Q) lasts for approximately 13 boxes

13 boxes x 0.04 seconds per box = 0.52 seconds for each filling time