General functions of the CVS Structural organization of CVS cardiac autorhythmicity action potential of pace maker

1. Cardiovascular Physiology
Dr. Hussein Farouk Sakr

2. The primary function of cardiovascular system (CVS) is to ensure
that the tissues receive an adequate flow of blood to serve their
requirements:
 Homeostasis
• Oxygen
• Nutrients
• Hormones
• Elimination of waste products: CO2, H+,, etc
 The first priority of blood pressure homeostasis is to maintain
adequate perfusion to the brain & the heart

3. Components of Circulatory System
• Cardiovascular System (CV):
• Heart:
• Pumping action creates pressure head
needed to push blood through vessels.
• Blood vessels:
• Permits blood flow from heart to cells and
back to the heart.
• Arteries, arterioles, capillaries, venules,
veins.
• Lymphatic System:
• Lymphatic vessels transport interstitial
fluid.
• Lymph nodes cleanse lymph prior to return
in venous blood. The systemic & pulmonary circulation
Advantages of Parallel arrangement

4. The functions of the
heart
1. The pumping function of the
heart creates blood pressure
that determines the blood
flow from the left ventricle to
the right atrium through
systemic circulation.
2. It secretes atrial natriuretic
peptide
3. It contains receptors
regulating the secretion of
antidiuretic hormone from
the posterior pituitary.

5. The vascular system
• Arterial System
1. Elastic (conducting) arteries
2. Muscular arteries-
distributing arteries
3. Arterioles (resistance
vessels)
4. Capillaries (exchange
vessels)
Venous System
1. Venules
2. Veins (capacitant vessels
contains 60 % of total blood
volume)

6.  Cardiac muscle cells (fibers)
 Atrial muscle
 Ventricular muscle
 Specialized excitatory and
conductive muscle cells (1%) –
the conducting system of the
heart

7.  Like skeletal muscle, cardiac muscle
has striated appearance, which results
from the arrangement of numerous
thick and thin filaments
 The thick and thin filaments in each
myofibril are arranged in a repeating
pattern along the length of the
myofibril. One unit of this repeating
pattern is known as a sarcomere
 The thick filaments are composed
almost entirely of the protein myosin
 The thin filaments are principally
composed of the protein actin, as well
as two other proteins – troponin and
tropomyosin

8.  Cellular membranes include a T-tubule
system and associated calcium-loaded
sarcoplasmic reticulum. The mechanism by
which these membranes interact to release
calcium is different than in skeletal muscle
 Adjacent cells are joined end-to-end at
structures called intercalated disks, within
which are desmosomes that hold the cells
together and to which the myofibrils are
attached.
 Also found within the intercalated disks
are gap junctions that allow rapid diffusion
of ions

9. • Have two important functions
1. Act as a pacemaker (set
the rhythm of electrical excitation)
2. Form the conductive
system (network of specialized
cardiac muscle fibers that provide a
path for each cycle of cardiac
excitation to progress through the
heart)
Autorhythmic fibers
• Forms 1% of the cardiac muscle fibers

10. 1. Sinoatrial node (SA node)
Specialized region in the right atrial wall near
opening of superior vena cava.
2. Atrioventricular node (AV node)
Small bundle of specialized
cardiac cells located at base of right atrium
near septum
3. Bundle of His (atrioventricular bundle)
Cells originate at AV node and enters
interventricular septum
Divides to form right and left bundle branches
which travel down septum, curve around tip
of ventricular chambers, travel back toward
atria along outer walls
4. Purkinje fibers
Small, terminal fibers that extend from
bundle of His and spread throughout
ventricular myocardium
Cells with autorhythmicity

11. Cardiac properties
• AutoRhythmicity: the ability of the cardiac muscle to generate
action potential spontaneously and beat regularly.
• Contractility: the ability of the cardiac muscle to pump blood into
circulations.
• Conductivity: the ability of the cardiac muscle to conduct impulse
from one muscle fibre to the next.
• Excitability: the ability of the cardiac muscle to respond to stimuli.

12. Autorhythmicity
• the ability of the cardiac muscle to generate action
potential spontaneously and beat regularly.
• It is myogenic in origin.
• It is a property of some of the cardiac muscle fibres
like the conducting system.

13. Nature of automaticity
The ability of self excitation is confined to the nodal and
conducting system of the heart.
The SAN has the greatest rhythm and so called the pace maker of
the heart.
All the cardiac muscle fibers follow the SAN.

14. Rhythmicity of the different parts of the
heart
• The rhythmicity of the
different parts of the heart:
Sino-Atrial node: 110/min.
Atrio-Ventricular node:
70/min.
Bundle of His: 55/min.
Purkinje fibres: 45/min.
Ventricular muscle fibres: 25-
40/min.

15. Why SAN is the pace maker?
• The SAN is the pace-maker due to inherent permeability of the cell
membrane to Na that make the membrane potential unstable.
• If the SAN fails to generate the impulse, other backup node
become active and discharge through their inherent rate.

16. Self excitation of the SAN
•Resting membrane potential of the SAN is ranging from -
55 to -60 m.v., while the remaining cardiac muscle fibers
are ranging from -80 to -90.
•High permeability of the membrane of the SAN for Na+
(inherent leakiness of the membrane to Na+).

17. Action potential of SAN
• The action potential of the SAN is
formed of:
1- Diastolic prepotential (phase 4):
caused by the slow influx of Ca+2
through T- channels and Na+ through
funny channels with decreased K+ efflux
from delayed rectifier K+ channels
2- Rapid depolarization (phase 0):
Caused by Ca+2 influx through L type
Ca+2 channels
3- Repolarization (phase 3): caused by K
+ efflux from delayed rectifier K+
channels

18. Autonomic control of the SAN activity
• Autonomic influences alter the rate of pacemaker firing through the following mechanisms:
1) Changing the slope of phase 4
2) Altering the threshold for triggering phase 0
3) Altering the degree of hyperpolarization at the end of phase 3.
Effects of sympathetic and parasympathetic (vagal) stimulation on sinoatrial (SA) nodal pacemaker activity.
Sympathetic stimulation increases the firing rate by increasing the slope of phase 4 and lowering the threshold for
the action potential. Vagal stimulation has the opposite effects, and it hyperpolarizes the cell.

19. Sympathetic nervous system
Norepinephrine
Stimulation of B1 adrenergic
receptors
Increased formation of CAMP
Increased influx of Ca and Na
Increased rate of
depolarization Increased heart rate
Sympathetic stimulation: Noradrenaline stimulates B1
adrenergic receptors  increasing the membrane
permeability to Na+ and Ca++  rapid depolarization 
increasing heart rate. as occurs during exercise.
Positive Chronotropic

20. Parasympathetic nervous system
Acetylcholine
Stimulation of m2 cholinergic
receptors
Decreased CAMP
Decreased influx of Ca
Increased hyperpolarization
Increased CGMP
Increased efflux of K
Negative Chronotropic
•Parasympathetic stimulation (vagal tone): A.ch. Acts on muscrinic
receptors  increasing the membrane permeability to K+  increased
K+ efflux  hyperpolarization  decreases heart rate. decreases
rhythmicity from the SAN from 110 to 70/min.
Increased vagal stimulation  more decrease in heart rate.
Sever vagal stimulation  stoppage of SAN discharge and do atrial
arrest, while the ventricles continue to beat by idioventricular rhythm

21. Sympathetic stimulation
Catecholamines
Thyroxine
Hypokalemia
Anticholinergic drugs
Hyperthermia
Parasympathetic
stimulation
Acetylcholine
B blockers
Ca channels blockers
Ischemia
Hyperkalemia
Hypothermia
Positivechronotropic
Negativechronotropic
Factors affecting autorhythmicity (chronotropism)