THE CARDIAC CYCLE
What is the cardiac cycle.
The cardiac cycle is the sequence
of events that occur when the
heart beats. There are two
phases of this cycle:
Diastole - Ventricles are relaxed.
Systole - Ventricles contract.
The cardiac cycle
Exercise increases blood flow through the heart so that the
cardiac cycle accelerates to accommodate the increased
demand for oxygen.
The normal cycle is around 0.8 seconds. This accelerates with
faster and more powerful atrial and ventricular contraction,
which is stimulated by the cardiac centre in the brain.
Heart rate:- is defined as the number of heart contractions in
each minute.
# There are two distinct periods in the cardiac cycle- one of the
heart muscle relaxation (cardiac diastole), the other of
contraction (cardiac systole)
Cardiac diastole
During cardiac diastole
• The bicuspid and tricuspid
valves are closed and the
atrium is full.
• Once full with blood, the atria
forces the bicuspid and
tricuspid valves to open and
fill the ventricles.
• This lasts for around 0.5
seconds at rest.
Cardiac systole
Cardiac systole
• The atria contract and send
blood via the bicuspid and
tricuspid valves into the
ventricles.
• When full, these contract
causing blood to be expelled
from the heart via the semilunar valves.
• (the bicuspid and tricuspid
valves are closed at this time)
• This lasts around 0.3 seconds
at rest.
What are heart valves?
Valves are flap-like structures
that allow blood to flow in one
direction. The heart has two kinds
of valves, atrioventricular and
semilunar valves.
Heart sounds.
* The audible sounds that can be
heard from the heart are made by
the closing of the heart valves.
These sounds are referred to as
the “lub-dupp” sounds. The “lub”
sound is made by the contraction
of the ventricles and the closing
of the atria-ventricular valves.
The “dupp” sound is made by the
semi-lunar valves closing.
Stimulation of the heart originates in
the cardiac centre, in the “medulla
oblongata.”
The “sympathetic and
parasympathetic nervous systems”
work antagonistically and provide
the stimulation for acceleration and
deceleration of the heart rate.
Cardiac systole (contraction) is
initiated by the electrical cardiac
impulse from the “sinu-atrial node”
(the pace-maker found in the right
atria wall.) This distributes electrical
stimulus through the “myocardial”
(heart muscle) wall between the
heart chambers, where the “atrioventricular node” (between the right
atrium and right ventricle) continues
distribution of the electrical signal
across the ventricles.
The SA node
•
In the upper part of the right atrium of the heart is a specialized
bundle of neurons known as the sinoatrial node (SA node). Acting as the
heart's natural pacemaker, the SA node "fires" at regular intervals to
cause the heart of beat with a rhythm of about 60 to 70 beats per
minute for a healthy, resting heart. The electrical impulse from the SA
node triggers a sequence of electrical events in the heart to control the
orderly sequence of muscle contractions that pump the blood out of the
heart.
The AV node
The AV node (AV stands for atrioventricular) is an electrical relay
station between the atria (the upper) and the ventricles (the lower
chambers of the heart). Electrical signals from the atria must pass
through the AV node to reach the ventricles.
AV node (bundle of his)
The bundle of His is located in the proximal intraventicular septum. It
emerges from the AV node to begin the conduction of the impulse from
the AV node to the ventricles.
Purkinje fibers
Purkinje fibers are heart muscle tissues that are
specialized to conduct electrical impulses to ventricular
cells, which induce the lower chambers of the heart to
contract.
Impulses from the upper chambers of the heart are
relayed by this node to large bundles of Purkinje fibers
referred to as the Bundle of His. These bundles branch
into smaller elements and eventually form terminal ends
that burrow into left and right ventricular chamber
muscles. As the impulse is passed to the ventricles, the
muscles contract and pump blood. The contraction
caused by the specialized fibers begins from the bottom
of the ventricles and move upwards so that the blood
leaves the lower chambers through the pulmonary
arteries and the aorta.