Cardiac

1. MBBS batch 2020
By: Aarti Sharma (03)

2. The sequence of changes in the pressure and flow in the
heart chambers and blood vessels in between the two
subsequent cardiac contractions is known as cardiac cycle.
Normal duration: 0.8 second at heart rate of 75 per
minute.
1. Atrial systole: 0.1 second
2. Ventricular systole: 0.3 second
3. Atrial diastole: 0.7 second
4. Ventricular diastole: 0.5 second

4.  Heart muscle cells cannot contract by their own natural rhythms.
 So, cardiac cycle is initiated in a specialised patch of muscle
1. Found in the right atrium called the sino-atrial node (SAN).
2. Located near the opening of vena cava.
3. Consists of a small number of cardiac muscle fibres and few
nerve endings.
4. Stimulus for contraction of the heart is originated in the SAN.
 However, there is a band of fibres between the atria and ventricles
which does not conduct the excitation wave of the SAN.

5. The only route through is via a patch of conducting
fibres called the atrio-ventricular node (AVN)
situated in the septum.
AVN picks up excitation wave as it spreads across
the atria and passes it on to a bunch of conducting
fibres called Bundle Of HIS.
This transmits the excitation wave rapidly down to
the base of the septum and then to the walls of the
ventricles through Purkinje Fibres.
Thus, causing the cardiac muscle in these walls to
contract. Squeezing blood upwards and into
arteries.

7. Bicuspid and tricuspid valves are closed.
Muscles in the atria relaxes and the pressure
decreases while volume increases inside.
Blood returns to the heart under low pressure in the
veins and enters the two atria.
The vena cava carry deoxygenated blood to the right
atrium.
The pulmonary veins carry oxygenated blood to the.
left atrium

8. It is the time when the heart is filled with blood.
The muscle in the atrial walls contracts with the help
of SAN.
The pressure increases and the atrio-ventricular
valves open so the blood found in atria is forced into
the ventricle.
Blood from atria not go back into the pulmonary veins
or the vena cava because of the semilunar valves
which prevent backflow.

9. 1. Period of isometric (isovolumetric or isovolumic)
contraction
Events:
ventricular contraction→ventricular pressure
rise →atrioventricular valve close →the
ventricular pressure increase sharply.

10. Period of ejection
Events:
ventricular contraction continuously→ the ventricular
pressure rise above the arterial pressure→ semilunar
valves open→ blood pours out of the ventricles.
Rapid ejection period (60% of the stroke volume)
Reduced ejection period (40% of the stroke volume)

11. Period of isometric (isovolumic) relaxation
Events:
Ventricular muscle relax→ the ventricular
pressure fall→ lower than the aortic pressure→
aortic valve close→ the ventricular pressure fall
sharply

12. Period of filling of the ventricles
Events:
Ventricular muscle relax continuously→ the
ventricular pressure is equal or lower than the
atrial pressure→ atrioventricular valve open→
blood accumulated in the atria rushes into the
ventricular chambers quickly from the atrium to
the ventricle.
Period of rapid filling. (amount of filling, 2/3)
Period of reduced filling (little blood fills into
the ventricle)