THE CARDIAC CYCLE

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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.
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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)

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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.

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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 semi-lunar
  valves.
• (the bicuspid and tricuspid valves are closed at
  this time)
• This lasts around 0.3 seconds at rest.

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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.

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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
atrio-ventricular node (between the right
atrium and right ventricle) continues
distribution of the electrical signal across the
ventricles.
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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.
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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.
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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.