BIOC34 – Lecture I – Electrical Activity of the Heart
Some general principles
Cardiac Output (amount of blood pumped out of the heart per unit time) = heart rate x
stroke volume (volume of blood pumped per beat)
All the cardiovascular mechanisms, in summary, have one important function – to stop
blood pressure from dropping too low (low b.p. can kill in minutes; unlike high b.p.).
o Blood pressure = cardiac output x peripheral resistance (resistance to blood flow
within the circulation).
This resistance to flow occurs predominantly in small vessels or arterioles
o Therefore blood pressure = heart rate x stroke volume x peripheral resistance
This means that all three play a role in blood pressure
Heart Valves and Major Blood Vessels
The human heart is a four chambered.
Blood returns from the system circulation via the vena cava (inferior from the lower body
and the superior from the upper body). They empty into the right atria.
Blood flows from the right atria and through the tricuspid valve into the right ventricle.
When the right ventricle contracts, it pumps blood up through the pulmonary arteries and
to the lungs.
Oxygenated blood from the lungs returns to the heart via the pulmonary veins and enters
the left atria. Thiruvarangan
Blood will move from the left atria to the left ventricle and them pumped out into the aorta
and then into the systemic circulation.
The atria and the ventricles are separated by septa (interatrial and the interventricular septum).
The left ventricle is the one that pumps blood to the systemic circulation (high resistance
circuit) and the muscle in this ventricle is substantially greater than the right ventricle.
The right ventricle pumps against less pressure into the pulmonary artery and does not
need to contract with as much force as the left ventricle.
The pulmonary circuit is the only place where we see the artery carry deoxygenated blood and
the vein carrying oxygenated blood. This designation is based on whether the vessel is taking
blood away from the heart (as the pulmonary artery) or towards the heart (pulmonary vein).
The Conduction System of the Heart (pacemaker conduction fibres contractile fibres)
The electrical activity that will cause the heart to depolarize and contract is generated in the SA
node. This node consists of a group of cells with no resting membrane potential (i.e.
spontaneously depolarized) that send a wave of depolarization (AP) onto the contractile muscle
cells which causes the heart to contraction.
AP is fired in the SA node (pacemaker because it depolarizes quickest) cells which the
spreads throughout the atria through intermodal pathways.
o ** If the SA node fails, the AV node takes over as pacemaker as it depolarizes
second fastest. If the AV node fails, other regions will take over. **
o Because the SA node is the quickest to depolarize, it will send waves downward.
The AV node can also spontaneously depolarize, however, it is slower and it
cannot send depolarize upwards because it experiences a refractory period
following the depolarization waves from the SA node.
These waves of depolarization cause the atria to contract and also get transmitted from
the atria through the ventricle in a single pathway. Thiruvarangan
o All this electrical activity converges on the AV node. Through this node, these
waves of depolarization pass from the atria and onto the ventricles.
o The septa between the atria and ventricles prevent the flow of electrical activity
from simply flowing between the two (AV node opens the gate)
Then this wave of depolarization impinges on the AV bundle (bundle of His), through the
left and right branch bundles within the interventricular septum.
The activity then goes from the branch bundles and onto the Purkinje fibers which
originate at the bottom (apex) of the heart and move upwards to the contractile muscle.
o Thus the apex of the heart contracts before the upper regions and this helps to
squeeze blood efficiently from the ventricles and upwards into the pulmonary
artery or aorta.
Cells in these pathways (not contractile) are muscle-derived conduction cells whose purpose is
to pass waves of depolarization through the heart and allow it to spread to the contractile cells
which then contract and allow the heart