The Heartbeat
To move blood through the heart, the cardiac muscle needs to contract in an organized way.
Blood first enters the atria, as shown inFigure18.12. When the atria contract blood is
pushed into the ventricles. After the ventricles fill with blood, they contract and blood is
pushed out of the heart. Valves in the heart keep the blood flowing in one direction. You
can see some of the valves inFigure18.12. The valves do this by opening and closing in
one direction only. Blood moves only forward through the heart. The valves stop the blood
from flowing backward. There are four valves of the heart:
- The twoatrioventricular (AV) valvesstop blood from moving from the ventricles to
the atria. - The twosemilunar (SL) valvesare found in the arteries leaving the heart, and they
prevent blood flowing back from the arteries into the ventricles.
The “lub-dub” sound of the heartbeat is caused by the closing of the AV valves (lub), and
SL valves (dub), after blood has passed through them.
Figure 18.12: Blood flows in only one direction in the heart; blood enters the atria, con-
tracting and pushing blood into the ventricles, the atria relax, the ventricles fill with blood,
contract, and push blood around the body. ( 21 )
Control of the Heartbeat
The heart is a made up of cardiac muscle cells. Cardiac cells are able to contract by them-
selves. Theydonotneedhelpfromthenervoussystem. Thisisdifferentthanskeletalmuscle,
which needs messages from nerve to contract. But the contractions of cardiac muscle still
need to be coordinated to make sure the cells contract as a group.
The contraction rate of cardiac muscle is controlled by two small groups of cardiac muscle
cells called thesinoatrial (SA)andatrioventricular (AV) nodes.The SA node is found in the
wall of the right atrium. It starts the contraction of muscle cells in the atria. The contracting