Download Document

:::Index>Profound Human World>Knowledge Acquisition>Material Circulation> The
Structure and Function of Heart
The Structure and Function of Heart
The blood circulates in the human body depending on the heartbeat. The heart,
locating to the left side of the thoracic cavity, is the center of the cardiovascular
system and a hollow organ formed with the muscles. Through the whole life, the heart
like a living muscular bump propelling the blood forward by beating average 60~70
times per minute, totally about 100,000 times a day and 20 billion times a life. The
human heart has four chambers, two above are atria and two below are ventricles. The
left and right two chambers form two separate pumps not communicating with each
other. And because ventricle contraction is the main power to propel the blood, so the
muscle layers of the ventricles are thicker than the atria.
Fig.1：The human heart
The heart has several blood vessels connecting to the atria and the ventricles
respectively. The vein connecting to the atrium conveys the blood into the heart. The
artery connecting to the ventricle conveys the blood out of the heart. The blood
circulating in the heart is directed by the valves. The valve is membranous structure
and has several kinds including bicuspid valve (between the left ventricle and atrium),
tricuspid valve (between the right ventricle and atrium) and semilunar valve (between
the ventricle and aorta), all of them play like the guarders of the heart and only can
open in one direction. When the atria contract, the bicuspid and tricuspid valves open
to the ventricles for blood flowing into the ventricles; when the ventricles contract, the
bicuspid and tricuspid valves close while the semilunar valve open for blood flowing
out of the heart to the artery.
How does the blood flow in and out of the heart? The heartbeat is not just a simple
All rights reserved by National Taiwan Science Education Center
action, actually it includes several phases to fill the heart with the blood and then
pump the blood out. First, the atria and ventricles relax to allow the blood to flow in
the ventricles; secondly, the atria contract along with the semilunar vale opening for
the blood flowing into the ventricles; thirdly, the ventricles contract along with the
semilunar vale closing for propelling the blood into the artery; finally, the ventricles
relax and the semilunar vale closes. Each heartbeat or cardiac cycle will continue
about 85 seconds and then repeat.
The physician uses the stethoscope to examine the heartbeat for understanding the
heart operation. Where does the “lub-dupp” sound come from when heart is beating?
It just results from the heart vales closing. When the bicuspid and tricuspid valves
close, the discharge of the blood against them makes the “lub” sound. Likewise, when
the semilunar vale closes, the discharge of the blood against it makes the “dupp”
sound.
Reviewed by：Zhang, Yong-Ta、Liu, Hsin professor
All rights reserved by National Taiwan Science Education Center