Transcript Control of the heartbeat

Control of the Cardiac Cycle
Objectives
•To describe how heart action is
coordinated with reference to
the SA node, AV node and
Purkyne tissue
•Interpret and explain ECG
traces with reference to normal
and abnormal heart activity
• The heart is Myogenic so does not need
impulses from the nervous system
• This means that the cardiac muscle can
generate its own contraction
independently from the rest of the
body
• Contraction occurs in an organised
manner and the heart acts as a
functional unit
Atrial Systole
• The cardiac cycle is initiated by a specialised
area of the right atrium, near to where the
vena cava empties in - the Sinoatrial node
(the pacemaker)
• SAN sets the rhythm for all other cardiac
cells to contract (1/second)
• SAN sends out electrical impulses to the rest
of the atria
• This spreads across the both atrial walls as a
wave of depolarisation or excitation
• Cardiac muscle in the walls of both atria
contract in time with the SAN
Controlling the contractions
• Collagen fibres at the base of the atria
prevent the impulse from passing down
to the ventricles
• This delay ensures that atrial systole is
complete and that the ventricles don’t
contract until they are filled with blood
Ventricular Systole
• At the top of the interventricular septum is the
Atrioventricular node (AVN)
• The AVN picks up electrical impulses through the
atrial wall and then generates its own
• Impulses travel as a wave of excitation along
specialised conducting tissue in the septum Purkinje (purkyne) fibres
• Fibres from the RHS and LHS form a bundle of
His
• The wave of excitation spreads rapidly to the
apex of the ventricles then spreads up and out
over the ventricle walls
• Ventricles contract from the bottom up
Electrical Activity of the Heart
Annotate this
diagram to
show how the
cardiac cycle
is controlled
Electrical Activity of the Heart
Electrocardiograms (ECG)
• ECGs detect changes in the electrical
activity of the heart
• Electrodes are placed on the skin over
opposite sides of the heart and electrical
activity is recorded
A normal ECG trace
P is the wave
of excitation
sweeping
over the atrial
walls
T is the
relaxation
of the
ventricle
walls
QRS complex is the wave of excitation
spreading through the ventricular walls. It
immediately precedes ventricular systole
Electrocardiograms
Problems with the cardiac cycle
• Arrhythmia – irregular beats
• Fibrillation – loss of coordination of
contractions
• Myocardial Infarction – heart attack where
the coronary arteries are blocked by a clot
and the cardiac muscle cells do not receive
oxygen for respiring fatty acids
• Enlarged heart
• Poor electrical conduction by the Purkyne
tissue
Normal and
Abnormal ECG
traces