Video Transcript
When a heartbeat is initiated, a
wave of electrical activity spreads out from the sinoatrial node. Where does it first spread to? (A) The ventricles, (B) the
Purkinje fibers, (C) the atria, or (D) the aorta and vena cava.
We often talk about how the heart
beats and how this beating action is crucial to a person’s life. But what exactly is a
heartbeat? As you likely already know, the
heart is responsible for pumping blood around the body. It delivers oxygen to the other
organs and tissues and picks up carbon dioxide and other wastes so they can be
expelled. The heart performs these functions
by contracting and relaxing its chambers in a controlled, regular pattern that we
perceive as our heartbeat. These contractions and relaxations
are controlled by electrical activity.
We are being asked about the path
of this electrical activity through the heart, so let’s begin with a quick review of
its structure.
The heart has four chambers: two
upper chambers called the atria and two lower chambers called the ventricles. The chambers are numbered one
through four in the diagram. The atria and ventricles are
separated by flaps of connective tissue called valves. You’ve probably noticed that the
labels for the right side of the heart are on the left side of the diagram, and vice
versa. This is because when we study the
heart, we look at it as if it were the heart of a person facing us.
The right atrium receives
oxygen-poor, carbon dioxide-rich blood, shown in blue, through the superior and
inferior vena cava, while the left atrium receives oxygen-rich blood, shown in red,
through the pulmonary veins. As blood enters the atria, they
contract, pushing blood through the open valves and into the ventricles. The walls of the ventricles then
contract, pulling the valves into a closed position and pumping blood out through
the pulmonary artery and aorta.
The question mentions the
sinoatrial node, which is a cluster of specialized nerve and cardiac muscle cells
located in the upper right corner of the right atrium. The sinoatrial node generates an
electrical signal that spreads through the heart, causing the chambers to contract
to initiate a heartbeat. The signal first spreads across the
atria, causing them to contract, before reaching the atrioventricular node. From the atrioventricular node, the
electrical signal travels to specialized conducting fibers called the bundle of
His. The bundle of His branches into
left and right bundles. The electrical wave travels down
these branches to reach the Purkinje fibers, which conduct the electrical impulses
to the ventricles, causing them to contract.
Now, let’s return to our
question. We are being asked where the
electrical signal first spreads to after being initiated by the sinoatrial node. And we’ve learned that it travels
through the atria to reach the atrioventricular node. Therefore, the correct answer is
(C), the atria.
