only has one nucleus. So, in summary,
endothelium is a very important epithelium
because it lines the entire vascular system.
And as we go through other lectures in this
histology course, I will emphasize all the
different sorts of functions that endothelium
has. Recall that the atria of the heart receive
blood, from the systemic circulation or from
the lungs. The ventricles are the pump of
the heart that sends blood to the lungs to
be oxygenated, and then it’s returned to
the left side of the heart where it’s pumped
by the left ventricle to the systemic circulation.
And the wall has got three distinct layers:
the outside layer, the epicardium, the inner
myocardium, which is where all the pumping
action occurs because that’s composed mostly
of cardiac muscle. And then an internal lining,
the endocardium, which includes the endothelium
supported by some connective tissue underlying
that epithelial layer. Cardiac muscle has
only got one nucleus. It’s striated.
It’s branched. It’s arranged and joined end to end
with other cardiac muscle fibres by intercalated
discs which contain adherent junctions to
make sure they’re bound very tightly together.
But also they contain gap junctions to enable
transmitters, messengers, ions, to go from
one cell to the other, and importantly, bring
about a sequential contraction of cardiac
muscle. The heart pumps because impulses are
generated at the sinoatrial node. And then
they are carried across, from the atria across
a fibrous skeleton to the ventricles, where
they initiate contraction again of the ventricles.
And that initiation of contraction of the
ventricles is acted upon by the presence of
Purkinje fibres. These Purkinje cells transmit
the impulse through the myocardium to all the
cardiac muscle cells within that myocardium.
And it’s important to know the difference between
the cardiac muscle fibres that contract and
these Purkinje fibres that just transmit the
impulse. Recall that the Purkinje fibres,
being just a conducting muscle cell, don’t
contract, and therefore, don’t have the
apparatus to contract. Therefore, they don’t
stain as heavily as normal cardiac muscle
cells stain. So I hope you’ve enjoyed this lecture.
I hope you now have a good understanding
of the histological structure of the heart
and the importance of the heart as being a
component of the cardiovascular system. Thank you.