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Control of the Cardiac Cycle




I). Cardiac Cycle

The cardiac cycle refers to all of the events associated with the flow of blood through the heart during one complete heartbeat.
Systole: Contraction of the heart muscle
Diastole: Relaxation of heart muscle
A).  First the atriums go through systole and diastole together
B).  Then the ventricles go through systole and diastole together
Atriale Systole
Atrial  Diastole


Events of the cardiac cycle

II).  Control of the cardiac cycle

Intrinsic:  Internal control

Extrinsic:  External control
The heart functions in response to a combination of  factors both within the  heart muscle itself and
outside of the cardiovascular system.


III).  Intrinsic Control of the Cardiac Cycle

Intrinsic control is due to specialized cells that initiate and distribute

electrical impulses in an orderly manner  through out the heart.

A). Unique Characteristics of Cardiac Muscles

1).  automaticity.

2). The heart contracts as a unit

3). Long refractory period


B).  Action Potential of Pacemaker Cells

1).Decrease in K+ permeability but a continuous Na+ permeability that upsets the Na+ and K+ balance.

2).  Ca++ channels open allowing Ca++  in until the action potential is reached.

3). K+ cannel open allowing for repolarization and than the K+ channels are closed.

cardiac conduction system

C).  Sequence of Excitation

1). The SA (sinoatrial) node

2)  AV (atrioventricular) node

3). AV node to the  bundle of His

4).  left and right branches.

5).   Purkinje fibers to the  ventricular muscles.

Cardiac conduction system


Check this out

D).  Electrocardiogram



1).  There are 3 distinguishable deflection waves.

i).  P wave

depolarization from SA node

ii). QRS wave

from ventricular depolarization

iii).T wave

repolarization or relaxation of the ventricles




2).  ECG Tracings

i). Normal sinus rhythm.


ii). Atrial Flutter



iii). Prolonged QRS Complex


iv). Ventricular fibrillation


v). Tachycardia


vi).  Bradycardia




IV).  Extrinsic Control of the Cardiac Cycle

A).  autonomic nervous system.

1).  sympathetic system enhances activity

2).  The parasympathetic system inhibits activity

Thus extrinsic control is the result of a balance between both the parasympathetic and the sympathetic nervous system.


B).  During times of physical or emotional stress:

The sympathetic system  releases norepinephrine and epinephrine allowing the heart to reach threshold faster and the SA node to fire faster.


C).  When the stress is over or during depression:

The parasympathetic system  releases acetylcholine which hyperpolarizing the heart and thus the SA node does not fire as often.


D).  Other extrinsic controls on the cardiac cycle:

1).  Pressoreceptors that detect blood pressure.
2).  Hormones such as epinephrine and thyroxine
3).  Age
4).  Exercise
5).  Body temperature