Module 2Exchange and transport

1.2.8 Control of the cardiac cycle

Learning Objectives Success Criteria

• To explain the control of the cardiac cycle

• Explain the terms myogenic, sinoatrial node, atrioventricular node and purkyne tissue(Grade E - D)

• Describe how heart action is coordinated with reference to the sinoatrial node (SA), atrioventricular node (AVN) and Purkyne tissue (Grade C –B)

• Interpret and explain ECG traces, with reference to normal and abnormal heart activity (Grade B – A)

Lub dup

The sound heard through a stethoscope is described as a lub and a dup.

• Lub – caused by closing of atrioventricular valves

(bicuspid and tricuspid).• Dup – caused by closing of semilunar valves.

Heart Rate –beats/min

Sinoatrial node

Atrio – ventricular node

Purkyne

Tissue

Control of the Heart Beat• Cardiac muscle is myogenic.• However, have to be controlled.• Sinoatrial node: specialised patch of muscle• SAN is the pacemaker: each time they contract

send a wave of electrical activity over whole of atrial walls: both atria contract at same time.

•Explain the terms myogenic, sinoatrial node, atrioventricular node and purkyne tissue

(Grade E - D)

Control of Heart Beat

• There is a band of fibres between atria and ventricles which don’t conduct the wave.

• Only route through is the atrio-ventricular node. This picks up the excitation.

• AVN passes wave onto conducting fibres called the Purkyne Tissues (after a delay of 0.1 sec), causing the ventricle walls to contract from the base upwards

The heart has a natural pacemaker

SAN

AVN

1. The SAN sends waves of electrical activity which spread

through the muscle of the atria, causing it to contract.

(Blood is forced into ventricles)

ATRIAL SYSTOLE

2. The waves of electrical activity reach the AVN . Here there is a slight delay-making

sure the atria have emptied before

venticles contract

3. AVN passes activity onto the bundle of His, and these muscle fibres conducted the waves of

excitation along the Purkyne fibres. Once the impulses reach the walls

of the ventricles they cause the muscles to

contract, forcing blood out of the heart

VENTRICULAR SYSTOLE

4. The atria relax (atrial diastole), blood enters the

atria from the pulmonary veins or vena cava

5. The ventricles relax (ventricular diastole). Bicuspid and tricuspid valves open and

the next pumpful of blood enter the ventricles from the

atria

Complete cut and stickPage58-59 - help

•Describe how heart action is coordinated with reference to the sinoatrial node (SA), atrioventricular node (AVN) and Purkyne tissue (Grade C –B)

Heart relaxed Atrio-Ventricular valves are open

Deoxygenated blood from vena cava flows into Right AtriumOxygenated blood from pulmonary vein flows into Left Atrium

Blood passes into ventricles passively

Sino Atrial node sends impulses through atriaAtria contract (top downwards) – forces additional blood into ventricles through Atrio

Ventricular valvesBlood from Right Atrium to Right Ventricle; blood from LA to LV

Ventricles receive impulses from AVN via Purkyne fibres – ventricles contract (bottom upwards) – force of blood causes Atrio Ventricular valves to close (lub)– prevents blood

flowing back into atria

Blood is forced from Right Ventricle into Pulmonary Artery through pulmonary semilunar valves and from Left Ventricle into aorta through aortic semi lunar valves

Heart relaxes – semilunar valves close due to force of blood (dub)- prevent backflow from pulmonary artery and aorta into heart – Atrio Ventricular valves open

Atria fill with blood again to start cycle again

Cardiac Cycle

Electrocardiograms (ECG)Checks heart function using an electrocardiograph, it records

the electrical activity of the heart- The heart muscle depolarises (loses electrical charge) when it

contracts, and repolarises (regains charge) when it relaxes.- Patches with wires are placed on the patients chest and wires

are connected to a monitor.- Electric charges are recorded by an electrocardiograph, as a

electrocardiogram/ECG which shows the patients normal heart rhythm.

Electrocardiograms [ECGs](record the electrical activity of the heart)

The P wave is caused by contraction (depolarisation) of atria

The QRS complex is caused by contraction (depolarisation) of ventricles. Larger than P due to more muscle.

The ST wave is caused during ventricular diastole. (repolarisation of ventricles

http://anatimation.com/cardiac-cycle/cardiac-cycle.html

Terms not to be confused by• Tachycardia – fast heart rate

• Bradycardia – a slow heart rate

• Arrhythmia – abnormal heart rhythm

• Fibrillation – rapid contraction of heartmuscles

TASK – Identify each ECG and explanation

•Interpret and explain ECG traces, with reference to normal and abnormal heart activity (Grade B – A)

Sinus TachycardiaRapid fire of the SAN, the heart’s natural pacemaker, defined as a rate greater than 100 beats/min in an average adult.

It’s the normal response to exercise, excitement, anxiety and also stimulants, fever, dehydration, hypothermia & shock, hypoxia

Bradycardia

Bradycardia occurs when the heart is beating more slowly than fifty to sixty times a minute. The impulses that control a steady heartbeat, are blocked, delayed, or slowed down.

It can occur as a result of age, AVN or SVN problems, metabolic disturbances, as a result of taking certain medications, drug abuse, or as a result of a pre-existing heart disease.

Trained athletes tend to have slow resting heart rates, and resting bradycardia in athletes is not abnormal if no symptoms.

Sinus ArrhythmiaThe normal increase in heart rate that occurs during inspiration (when you breathe in).

Ventricular Fibrillation

Uncoordinated contraction of the cardiac muscle of the ventricles in the heart. As a result, the heart fails to adequately pump the blood,

During ventricular fibrillation, cardiac output drops to nil, and, unless terminated promptly, death usually ensues within minutes.

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Plenary - Multiple-choice quiz