1) Circulation Circuits- An easier way to think of the cardiovascular system is as a set
of closed tubes (blood vessels) which are filled with fluid (blood) that is moved around by
a central pump (the heart). There are really two separate circulation circuits.
i) Pulmonary circulation: This circuit delivers blood to the lungs for it to get rid
of carbon dioxide and pick up a fresh supply of oxygen (more on this in Module
10). It then returns to the LEFT side of the heart.
ii) Systemic circulation: This circuit delivers the nicely oxygenated blood from
the left side of the heart out to all the tissues, including your brain, skeletal
muscles and many other tissues you have yet to learn about. Once it passes
through the tissues (picking up some carbon dioxide in exchange for delivering
oxygen), it returns to the RIGHT side of the heart.
This continues over and over. If our resting heart beat is 70 beats per minute,
and there are 60 minutes in an hour (70 x 60) and 24 hours in a day (x 24); that
is equal to 100,800 beats a day. That is a lot of work!
2) The heart structure:
You should know the basic structure of the heart. I like to think of it very simplistically as
a box with 4 compartments. Make sure you know where the valves/openings between
the compartments are. You may want to draw a very simple diagram and label these and
the route of the blood through the heart. These valves are very important in the heart to
ensure that the blood flows only in one direction through the heart and does
not slosh back and forth. The chordae tendineae keep the valves from collapsing
backwards when the heart contracts - much like the strings on a parachute.
The heart (myocardial) cells:
i) Contractile cells- similar to skeletal muscle cells, but there are a few
differences, make a chart to compare them- example- almost all cardiac
muscle cells can spontaneously generate action potentials without help
from any nerves
ii) Nodal (conducting) cells- important for the transmission of action
potentials through the heart, provide the heart with coordinated self-
3) What causes this self-excitation?
The Pacemaker Potential:
This is a different type of potential (dont confuse this with EPSPs or IPSPs, resting
membrane potential, receptor potential or equilibrium potentials)
- These potentials work a little differently than action potential generation in the neuron.
- Rather than just Na moving in through voltage-gated channels and then K leaving to
repolarize, there are many more ions that are involved in the heart.
(Throughout this module you will see that the sinoatrial (SA) node is associated with the
pacemaker potential. This isnt because it is the only area that spontaneously generates
action potentials (almost all cardiac cells can), but rather this is the fastest and therefore is considered the primary pacemaker of the heart. Essentially, it sets the pace.)
- The pacemaker potential occurs because of a combination of events.
- Na does enter (and the SA nodal cells are even a bit leakier or permeable to Na+ than
other cells) and causes a depolarizati+n.
- But in the SA node cells, LESS K leaves due to special channels preventing it from
leaking out. And to be sure we really depolarize these cells, we also add Ca to the mix.
Ca ++ also leaks in down its concentration gradient. And we have a spontaneous
depolarization to threshold (the pacemaker potential).
4) What happens after the threshold is reached in cardiac cells?
- Cardiac action potentials are not completely identical to neuron action potentials
- In brief, the depolarization phase of the cardiac action potential is caused by Ca ++
entering the cell through special Ca voltage-gated channels (rather than Na through its
voltage-gated channels). +
-Repolarization is similar to the action potential in the neuron, due to the movement of K
out through K voltage gated channels. But in the cardiac muscle this occurs much
slower (about 1 second long compared to 2-3 milliseconds in a neuron). This is a good
thing or else your heart would be pumping 500 times a second!
-In the neuron, resting membrane potential is about 70 mV and threshold is 55 mV. In
the SA node, there is no resting membrane potential (because it really doesnt REST).
Rather the membrane potential gets as low as 60 mV but then starts spontaneously
drifting slowly up towards threshold (a value of about 40 mV).
5) Where does the action potential go after being spontaneously generated in the
-First it spreads from the SA node through the atrial muscle (via those special gap
junctions between the cells) and makes it contract.
-Next the action potential makes its way to the atrioventricular (AV) node. The AV node
is an important structure because here the action potential slows down or gets held up a
bit. This pause is important so that the ventricles dont contract before the atria is still
contracting; blood wouldnt know where it had to go.
-After the AV node, the action potential spreads down the bundle of His quickly. This is
one of the speediest pathways of conduction in the heart. This takes the action potential
to the bottom (apex) of the ventricles of the heart. Then the action potential is
transmitted along the Purkinje fibers out to the ventricular muscle. The way the circuitry
of the conducting system of the heart is designed; the action potential then begins first at
the very bottom of the ventricles and works its way up to the top of the ventricles. This
allows blood to be squeezed out of your heart like a tube of toothpaste. Remember the
exit from the ventricles is actually more at the top of the heart.
6) Conduction and the ECG:
- Almost any electrical event in the body can be detected and recorded if you have the
-Electrical activity in the brain can be recorded in the form of an electroencephalogram
-The action potentials in muscles can also be recorded as an electromyogram (EMG)
- The electrical activity in the heart can be recorded by an electrocardiogram or ECG
- The ECG tells us a lot about the heart's conducting system, the timing of contraction
and can be used to diagnose various diseases or problems.-The ECG has a very specific waveform. Each part of this wave represents the electrical
activity in a particular area of the heart. While you read this explanation it might be best
to be looking at page 8.18.
i) The P wave- represents the electrical activity in the heart as the atrial muscle
depolarizes. Remember in order to have a muscle contraction, there must be an
action potential and therefore depolarization first. This depolarization then
leads to the contraction of the atrial muscle.
ii) The QRS complex- this next wave is the largest of all the waves in the ECG,
it represents the electrical activity caused by the depolarization of the ventricles.
This depolarization leads to contraction of the ventricles. It is the largest
waveform since the ventricles are the largest set of muscles in the heart.
iii)The T wave- This represents the repolarization of the ventricular muscle.
- The repolarization of the atria is a relatively small event (look at the size of the
QRS complex compared to the T wave) and it would be occurring at roughly the
same time as the QRS complex - so, it gets obscured.
7) Putting it together: The ECG and the cardiac cycle
-The cardiac cycle combines most of the events that take place in the heart during one
entire event of the heart. These events include all the pressure and volume changes in
the atria and ventricles as well as the ECG and valve activity from the beginning of one
contraction to the beginning of another.
Let's break the cardiac cycle into distinct events. But first, print out the whole animation.
You can do thi