Download Lect 11 CV 3

• Last lecture
– Electrical activity of
CV 3
• ventricle myocytes
• Pacemaker cells
•Electrical coordination of the heart
•Cardiac cycle
Pacemaker Cells
Action potential of ventricular myocyte
Early repolarization
depolarization
Plateau
depolarization
repolarization
repolarization
Pacemaker potential
rest
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Coordination of contraction in the whole heart
1. SA node depolarization initiates
atrial contraction
2. atrial depolarization spreads
and activates AV node
3. Purkinje fibers carry excitation
to the bottom of the heart
• Important points about coordination
1. Spread through the atrial muscle is by
way of gap junctions
2. The only electrical connection between
atria and ventricles is the AV node and
conducting fibers
3. Conduction through the AV node is
slow→ delay between atrial and
ventricular excitation
4. Ventricular contraction sweeps
up from the bottom
Ventricle depolarization
• The electrocardiogram (ECG)
– Record of the electrical activity of the heart
measured from outside the body
Atrial depolarization
Ventricle repolarizatio
The relationship between
the electrocardiogram
(ECG), recorded as the
difference between currents
at the left and right wrists,
and
Lead I ECG
an action potential typical of
ventricular myocardial cells.
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Cardiac cycle
Normal
ECG: P waves (atrial depolarization) are followed faithfully by
QRS (ventricular depolarization) and T waves (ventricular repolarization).
Abnormal
ECG: every other P wave fails to evoke QRST (partial atrioventricular block).
Abnormal
ECG: P waves and QRST occur independently (full atrioventricular block).
Each phase is further subdivided to:
1. Systole
a) Isovolumetric ventricular contraction
b) Ventricle ejection
The rhythmic contraction & relaxation of the
heart
Cycle divided into 2 phases with respect to
ventricle action
1. Systole – ventricle contraction and blood
ejection
2. Diastole – ventricle relaxation and blood
filling
• Note
– Atria contract at the end of diastole, but most
blood (~80%) moves from the atria to the
ventricle prior to atrial contraction.
2. Diastole
a) Isovolumetric relaxation
b) Ventricle filling
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Heart Valves
Permit blood flow in only one direction
When right atrial
pressure > right
ventricle pressure,
blood fills ventricle
If right ventricle
pressure>right atrial
pressure, AV valve
closes – no flow back
into atria
• For blood ejection
– Pressure in ventricles must > pressure in
aorta and pulmonary artery
• After ventricle contraction, as ventricles
relax, backpressure from the vessels
closes the aortic and pulmonary valves
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ECG
Pressure and volume changes
110
in the left heart during a
contraction cycle.
Aortic
Pressure
mm Hg
Left Ventricle
Left Atria
0
End diastolic volume
130
End systolic volume
Volume
(ml)
AV valves open
65
D
Isovolumetric ventricle contraction
S
D
Aortic & pulmonary
valves open
Isovolumetric ventricle relaxation
Pressure changes in the right heart during a contraction cycle.
ECG
Aortic
When LV pressure > aortic pressure
Aortic valve opens, and blood leaves
Left Ventricle
Left Atria
When aortic pressure > LV pressure
Aortic valve closes
Volume
(ml)
D
S
D
5
Pressure-volume curve
120
Left Ventricle Pressure (mm Hg)
E
A–B
B–C
C
C–D
D–E
E
E–A
D
Diastolic filling
isovolumetric contraction
aortic valve opens
rapid ejection
slow ejection
aortic valve closes
isovolumetric relaxation
Cardiac Output
• Definitions:
Stroke Volume (SV) = amount of blood pumped by
the heart with each beat
C
End Diastolic volume (EDV) = volume of blood in
the ventricle after filling
End Systolic volume (ESV) = volume of blood
remaining in ventricle after heart beat
B
A
0
Left Ventricle Volume (ml)
200
Cardiac Output
Cardiac Output = Heart Rate X Stroke Volume
Stroke Volume = EDV – ESV
= 135 ml – 65 ml = 70 ml
To Understand cardiac output (CO)
• What controls HR?
• What controls SV?
At rest:
CO = 72 beats / min X 0.07 L/beat =5.0 L/min
i.e about ½ the volume remains in the left ventricle
Blood volume in most people = ~5L
CO in trained athletes can = 35 L/min
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