Download The Heart: Day 2

Heart Review from Day 1
On a whiteboard:
1. Draw and label a diagram
of the heart including:
•The 4 chambers
•Associated blood vessels
•Valves
•The path of blood flow
2. Be prepared
to verbally state
the path for:
• Pulmonary
circulation
• Systemic
circulation
The Heart:
Day 2
Outline the control of the heart beat
• The heart utilizes myogenic muscle
contraction –the cardiac muscle contracts
and relaxes without the use of the nervous
system
*Note: it is very important to control this
process and the next slides will tell us how
the heart keeps correct timing
Details of Control Mechanism:
• sinoatrial node (SA
node) aka “pacemaker”
– the mass of tissue
within the wall of the
right atrium (near
where the superior
vena cava enters the
right atrium)
• It keeps the correct
rhythm of the heart
1. The SA node sends out an
electrical impulse (similar to those
produced by the nervous system)
which causes the contraction of
both atria
*Note: for a person with a resting heart rate
of 72 bpm, the signal from the SA node is
sent out about every .8 seconds
2. Next, the signal is received by
the atrioventricular node (AV
node)
3. The AV node then
delays its signal by
about .1 seconds; this is
to ensure that the atria
will contract first and
empty completely before
the ventricles contract
NOTE: atria empty b/c pressure
is lower in ventricles, therefore
valves open and blood empties
4. The AV node sends
it’s signal to the
Purkinje fibers which
results in the
contraction of both
ventricles
simultaneously
NOTE: the impulses created in the heart
can be read by electrodes placed on
the skin and connected to an
electrocardiogram (EKG)
Factors that Affect Heart Rate
1. Nerves – two sets of nerves
oppose each other in adjusting the
heart rate; one speeds it up and the
other slows it down (signals sent to
SA node)
Example:
• You start exercising
• Decreased O2 and increased CO2
result in medulla sensing changes
• Accelerator nerve increases
impulses sent to SA node
• Heart rate increases
Factors that Affect Heart Rate
2. Hormones – during periods of high stress or
excitement, chemicals like adrenaline (aka
epinephrine) can speed up the rate
Example:
• You enter a “fight or flight” situation
• Epinephrine is produced by adrenal glands
• Hormone travels through bloodstream to SA node
• Epinephrine increases impulses sent by SA node
• Heart rate accelerates
Real Life Application:
What is the difference
between a pacemaker and
an implantable
defibrillator?
• What a pacemaker does is keep the heart
beating at the proper rate and from beating too
slow. It also will only work if it is needed, it
doesn't work all the time. Low-energy electrical
impulses for minor irregularities
• An implanted defibrillator is used for more lifethreatening rhythm disturbance often from the
lower chambers of the heart; high-energy
electrical impulses