1. The cardiovascular system consists of the heart and blood vessels. The heart has four chambers - two atria that receive blood and two ventricles that pump blood. 2. The right side receives deoxygenated blood from the body and pumps it to the lungs, while the left side receives oxygenated blood from the lungs and pumps it back out to the body. 3. The heart contracts automatically in a regular rhythm, alternating between systole where it contracts to pump blood and diastole where it relaxes and refills. It functions as two pumps - the right side pumps blood at low pressure to the lungs, while the left side pumps to the entire body at higher pressure.

1. Physiology of cardiovascular system
Prof.Dr. HEBA SHAWKY
Professor of Physiology, Faculty of Medicine,
Cairo University

2. Lecture 1

3. Introduction:
The cardiovascular system is composed of the heart and a closed system of blood vessels.
The heart is a muscular organ formed of 4 chambers.
The right side of the heart is formed of:
- Right atrium (RA): receives venous blood from systemic circulation through superior and
inferior vena cava (SVC & IVC) then delivers it to the right ventricle.
- Right ventricle (RV) which pumps blood to the pulmonary circulation through the pulmonary
artery (PA) that carries deoxygenated blood.
The left side of the heart is formed of:
- Left atrium (LA) which receives oxygenated blood from the pulmonary circulation through the
pulmonary veins then delivers it to the left ventricle.
- Left ventricle (LV) which pumps blood to the systemic circulation through the aorta.
The muscular wall of the left ventricle is therefore much thicker than that of the right ventricle.

5. It beats automatically & regularly before birth till death.
a- Systole: cardiac contraction to eject blood.
b- Diastole: cardiac relaxation to be filled with blood.
The heart is functionally divided into 2 separate pumps:
1- Right side, which pumps blood to lungs against low resistance and under low
pressure i.e. volume pump
2- Left side, which pumps blood to the whole body against high resistance and
under high pressure i.e. pressure pump

6. Each pump is formed of :
A. Atrium: thin wall- function mainly as reservoir of blood, 70% of
blood goes from atria to ventricles by pressure gradient. Atrial systole
(contraction) is to evacuate 30 % of venous return to ventricles.
B. Ventricle: thick wall- acting as a pump:
Right ventricle acts as a volume pump, it propels blood through
pulmonary vessels.
Left ventricle acts as a pressure pump, it propels blood through
peripheral circulation..

7. Cardiac properties:
[1]Excitability: i.e. Action potential (depolarization -repolarization).
[2]Contractility
[3]Rhythmicity: i.e. initiation of regular impulses independent of nerves.
[4]Conductivity i.e. spread of cardiac excitation.

8. [1] Excitability (Action Potential):
The cardiac muscle has the ability to respond to an adequate
stimulus by generating a propagating action potential, followed
by contraction.
The resting membrane potential of cardiac muscle is -90
mv due to selective permeability & Na-K pump

9. The action potential is composed of:
Depolarization (Phase 0): depolarization with
overshoot to + 20 mv due to opening of :
-Voltage gated fast Na+ channels
-Slow calcium-sodium channels

10. Repolarization which is slow & triphasic:
(1)Phase 1: small & fast due to:
a. Sudden closure of voltage gated fast Na+ channels.
b. Slow calcium-sodium channels still opened
c. Opening of chloride channels leading to influx of cl-.
d. Opening of K+ channels
(repolarization to + 10 mv)
(2)Phase 2: The Plateau
It is due to:
balance between Ca ++ & Na+ inflow through slow Ca-Na channels &
outflow of K+ through K+ channels
where the membrane potential is near zero.

11. Phase 3: Rapid repolarization to RMP
It is due to:
a- Closure of slow calcium-sodium channels.
b- The outflow of K+ is no longer compensated by slow
inward of Ca++.
The is continuous efflux of K+ until repolarization is complete
& return of RMP (Phase 4).

13. N.B.
The action potential of ventricular fibers shows
prolonged plateau & lasts 300 msec.
The action potential of atrial fibers shows less
prominent plateau & lasts 150 – 200 msec

14. Changes of excitability during the action potential:
1- Absolute refractory period:
the fiber does not response to any stimulus.
It coincides with phases 0-2 and about half of phase 3 i.e till
membrane potential reaches – 50 mv.
It coincides with the whole period of systole and the beginning of
diastole.

16. - So, during systole, the excitability of cardiac muscle is
completely lost and the muscle does not respond to any stimulus
whatever its strength
- This prevents the heart from developing sustained, tetanic
contractions like those that occur in skeletal muscle. Such sustained
contraction is not suitable for the pumping function of the heart.
i.e. cardiac muscle can not be tetanized
( NO tetanus)

17. 2- Relative refractory period:
the fiber respond to stronger stimulus.
It extend from the end of absolute refractory period till phase 4.
Ventricular muscle in vulnerable period
(dangerous period) till the end of action potential,
because stimulation of the heart at this time may produce a
fatal condition known as ventricular fibrillation.

18. [2] Contractility:
Contraction begins just after the beginning of action potential
lasts 1.5 times the action potential.
The systole reaches its maximum by the end of the plateau &
coincides with absolute refractory period( ARP ).
So, cardiac muscle can not be tetanized (tetanization is fatal )

19. Diastole begins with the rapid phase of repolarization.
-First half of diastole coincides with absolute refractory period (ARP).
-Second half of diastole coincides with relative refractory period (RRP).
* Excitation contraction coupling: same as skeletal muscle.
* But in cardiac muscle, extracellular calcium diffuses into sarcoplasm at
the time of action potential (mainly during plateau) because calcium of
sarcoplasmic reticulum is not enough.
So, the strength of contraction of cardiac muscle depends on the
concentration of calcium ions in the extracellular fluid