Video Transcript
The diagrams provided show a
simplified outline of the cardiac cycle. The purple arrows indicate the
movement of an electrical impulse, and the purple shading indicates when an area is
contracting. Which diagram best demonstrates the
initial movement of the electrical impulse from the sinoatrial node?
As we know, the heart is the organ
that keeps us alive by pumping blood to the various parts of our body. The pumping action of the heart
involves cycles of contractions and relaxations of the chambers of the heart. During a cardiac cycle, which is
also called a heartbeat, the chambers of the heart contract in a certain sequence
then relax.
Let’s review this sequence of
events that occurs during a cardiac cycle. First, all the chambers of the
heart are relaxed and the atria are filled with blood arriving from the veins and
vena cava. Second, in the upper-right corner
of the right atrium of the heart, a structure called the sinoatrial node generates
an electrical impulse. This electrical signal propagates
through the muscular walls of both atria as shown by the purple arrows of the
diagram. As a result, both atria contract,
which is indicated by the purple shading on the second diagram.
The valves at the bottom of the
atria open and the contraction pushes the blood into the ventricles. The atrioventricular node found in
the lower-left corner of the right atrium gets stimulated by the electrical signal
generated by the sinoatrial node. The atrioventricular node then
generates an electrical signal that propagates through a bundle of muscle fibers
called the bundle of His.
The electrical signal then
progresses from the bundle of His to the Purkinje fibers, as shown by the purple
arrows in the diagram. These fibers transmit the
electrical signal in the muscular walls of the ventricles, causing them to
contract. This contraction of the ventricles
pumps the blood out of the heart and towards the aorta and pulmonary artery. And then, the heart returns to a
relaxed state that we described in the first diagram, ready to start a new
cycle.
Now that we have reviewed the
sequence of the cardiac cycle, we can answer our question. The diagram that best demonstrates
the initial movement of the electrical impulse from the sinoatrial node is the
second one.
