The cardiac cycle describes the sequence of events that occur with each heartbeat. It includes atrial systole, ventricular systole, and ventricular diastole. Atrial systole involves atrial contraction and pumping of blood into the ventricles. Ventricular systole has three phases: isometric contraction, ejection of blood from the ventricles, and reduced ejection. Ventricular diastole also has three phases: isometric relaxation, rapid filling of the ventricles, and reduced filling. The average duration of the cardiac cycle is 0.8 seconds.

1. Cardiac cycle (dental)
Def:
• “ The cardiac events that
occur from the beginning
of one heart beat to the
beginning of the next “.
Time:
• Average duration of each
cycle is 0.8 second (60
seconds/75 beats.min-)
Dr. Asaad Ahmed Elmileegy 1

2. cardiac cycle is divided into the
following phases:
I- Atrial systole:
II- Ventricular systole (0.3 sec),
which includes:
• a- Isometric (or isovolumetric)
contraction phase.
• b- Maximum ejection phase.
• c- Reduced ejection phase.
III- Ventricular diastole (0.5 sec),
which includes:
• a- Isometric (or isovolumetric)
relaxation phase.
• b- Rapid filling phase.
• c- Reduced filling phase.
Dr. Asaad Ahmed Elmileegy 2

3. I- Atrial systole:
Duration:
• 0.1 second.
During this phase:
• Atrial contraction  
the atrial pressure
(from zero to 2 mm Hg)
 pumping of the
blood into the
ventricles.
Dr. Asaad Ahmed Elmileegy 3

4. II- Ventricular systole:
Duration:
• 0.3 second.
During this phase:
1. The ventricle contract isometrically
(i.e., without change in the length
of the ms fibers)  
intraventricular pressure, from
zero to 80 mmHg in the left
ventricle, or from zero to 10 mm
Hg in the right ventricle.
2.  ventricular pressure  opens
the semilunar valves and the blood
is ejected at a high rate out of the
ventricles to the aorta and
pulmonary artery.
3. Then ejection of blood continues
but at a gradually slowing rate.
Dr. Asaad Ahmed Elmileegy 4

5. IV- Ventricular diastole:
Duration:
• 0.5 seconds.
During this phase:
• The ventricles relax isometrically
 the pressure   sudden
closure of the semilunar valves.
• The ventricular pressure falls
rapidly towards zero.
• Then, the atrial pressure  more
than the ventricular pressure 
opening of the AV valve and
rushing of blood by its weight
into the relaxed ventricle.
• This ventricular filling starts
rapidly and then continues
slowly.
Dr. Asaad Ahmed Elmileegy 5

6. Dr. Asaad Ahmed Elmileegy 6

7. HEART RATE (dental)
• In a normal adult male, the average heart beats
at rest is approximately 70 beats/ minute.
• Methods of counting the HR:
a) Counting the radial pulse or apex beats/min.
b) Counting the cardiac cycles or heart sounds by
stethoscope.
c) Recording the ECG.
By: Dr. Asaad Ahmed Elmileegy 7

8. HEART RATE (dental)
By: Dr. Asaad Ahmed Elmileegy 8

9. Regulation of the heart rate
• I- Nervous
– Impulses from higher centers: cerebral cortex,
hypothalamus, respiratory center
– Afferent impulses from right & left side of circulation
• II- Chemical:
– Blood gases, hormones, others
• III. Physical:
– Blood temperature
By: Dr. Asaad Ahmed Elmileegy 9

10. I- Nervous regulation of heart rate
The cardiac centers
[cardioinhibitory (CIC) &
cardioacceleratory (CAC)] in
the brain stem are
influenced by:
Impulses from higher centers:
1. Cerebral cortex,
2. Hypothalamus,
3. Respiratory center
Afferent impulses from:
1. Right &
2. left side of circulation
By: Dr. Asaad Ahmed Elmileegy 10

11. 1- Impulses from The cerebral cortex:
a- Conditioned reflex:
• Visual, smell or auditory stimuli → the cerebral
cortex →  or  heart rate .
b- Voluntarily:
• Some people can  or  their heart rate
voluntarily.
c- Emotions:
• Most emotions →  heart rate .
• Sudden shocking news →  heart rate .
By: Dr. Asaad Ahmed Elmileegy 11

12. 2- Impulses from The hypothalamus:
• Stimulation of anterior
hypothalamus (sleep) →
↑ parasympathetic
activities →  heart rate
due to stimulation of CIC.
• Stimulation of posterior
hypothalamus (ms
exercise) → ↑
sympathetic activities →
 heart rate due to
stimulation of CAC.
By: Dr. Asaad Ahmed Elmileegy 12

13. Parasympathetic (Vagal) Tone
Def:
• A continuous
parasympathetic
inhibitory impulses to
the heart at rest
It inhibits the inherited
high rhythm of the SAN
from 110 to 70 beats/
minute.
Dr. Asaad Ahmed Elmileegy 13

14. Mechanism of vagal tone: a reflex in
which:
1. Stimulus: The normal level of
arterial blood pressure.
2. Receptors: Baroreceptors in the
aortic arch and carotid sinus.
3. Afferent fibers: The impulses
pass along: a- glossopharyngeal
nerve (IX) from the carotid
sinus b- vagus nerve (X) from
the aortic arch
4. Center: Cardioinhibitory center
(CIC) in the medulla.
5. Efferent fibers: In the vagus
nerves.
6. Effect: Inhibitory impulses to
the SAN.
Dr. Asaad Ahmed Elmileegy 14

15. Vagal (ventricular) Escape
Phenomenon
1) Strong vagal stimulation
 marked inhibition of
the SAN   the heart
rate and then the heart
stops completely.
2) After that the ventricles
start to beat according to
their own rhythm i.e.
idioventricular rhythm
(25-40 beats/min).
• This proves that the
vagus does not supply
the ventricles.
Dr. Asaad Ahmed Elmileegy 15

16. 3- Impulses from The respiratory
center:
Respiratory sinus
arrhythmia:
• Inspiration   heart
rate,
• Expiration   heart
rate .
By: Dr. Asaad Ahmed Elmileegy 16

17. B- Impulses from the circulatory
system :
Impulses from the right side (Bainbridge
reflex):
• “ right atrial pressure  reflex  heart
rate ”.
Impulses from the left side:
• Marey's law: “The heart rate is inversely
proportional to the arterial blood pressure
provided that other factors remain constant”.
By: Dr. Asaad Ahmed Elmileegy 17

18. Bainbridge reflex:
• "  right atrial pressure
 stretch receptors 
stimulation of the CAC 
sympathetic fibers  
heart rate.
• Significance:
1. Pumping of excess
venous return
2. Prevents stagnation of
blood in veins.
By: Dr. Asaad Ahmed Elmileegy 18

19. Marey's law:
• The heart rate is inversely
proportional to the arterial
blood pressure provided that
other factors remain
constant.
• Mechanism:
•  arterial blood pressure
(ABP)  the arterial
baroreceptors in the carotid
sinus and aortic arch 
impulses by the IX and X
nerves  inhibition of the
CAC and stimulation of CIC
  heart rate.
By: Dr. Asaad Ahmed Elmileegy 19

20. Carotid sinus syndrome:
• A pathological condition in which the carotid
sinus is abnormally hypersensitive.
• Mechanism:
• A slight pressure on the carotid sinus 
baroreceptors  reflex  vagal tone and a 
of the sympathetic vasoconstrictor tone  
heart rate and hypotension  cerebral
ischemia and temporary loss of consciousness.
• Treatment:
• Denervation of the hypersensitive carotid
sinus.
By: Dr. Asaad Ahmed Elmileegy 20

21. II- Chemical regulation of heart rate
1) Changes in blood gases:
2) Changes in blood hormones:
3) Chemical substances affecting the H.R.:
By: Dr. Asaad Ahmed Elmileegy 21

22. 1) Changes in blood gases:
• Mild or moderate hypoxia (O2 lack): 
heart rate due to ++ of CAC and -- of CIC both:
• Severe hypoxia   heart rate, hypotension
and finally death due to:
• Damage of Medullary centers and Nodal
tissue of the heart.
By: Dr. Asaad Ahmed Elmileegy 22

23. 2) Changes in blood hormones:
Adrenaline:
• Small dose   HR due to direct stimulation
of the SAN.
• Large dose  reflex  HR (Marey's law).
Noradrenaline:
• Small or large doses  reflex  HR
(generalized VC   ABP).
Thyroxine:
•  HR
By: Dr. Asaad Ahmed Elmileegy 23

24. 3) Chemical substances affecting the
H.R.:
By: Dr. Asaad Ahmed Elmileegy 24
Substances that  HR Substances that  HR
1) Atropine:
Inhibits the vagus nerve 
 HR
1) Bile salts:
Inhibit the SAN
2) Histamine:
 VD   ABP  reflex 
HR (Marey's law).
2) Morphine:
Stimulates the CIC
3) Sympathomimetic drugs: 3) Parasympathomimetic
drugs:

25. III. Physical regulation of HR
↑ blood temperature:
• A rise of blood temperature 1°C   HR by
10 beats/min.
↓ blood temperature:
• Opposite effect.
By: Dr. Asaad Ahmed Elmileegy 25

26. Dr. Asaad Ahmed Elmileegy 26