HUMAN BIOLOGY

CirCulation: the heart and blood vessels 125

hoW does the heart pump blood?

• The heart works as a double pump, with each half divided into
  an atrium and a ventricle.


• In each cardiac cycle, the heart chambers contract and
  then relax.


• Contraction of the atria helps fill the ventricles. Contraction of
  the ventricles pumps blood out of the heart into the aorta.


• Heart valves keep blood flowing in one direction.

taKe-hoMe Message

Heart

sounds

Atria contract,

and fluid pressure

in ventricles rises

sharply.

Ventricles

contract; blood is

pumped into the

pulmonary artery

and the aorta.

Ventricles

relax even as the

atria begin to fill

and start another

cycle.

Fluid pressure in

filling atria opens AV

valves; blood flows

into ventricles.

2

1

3

4

Figure 7. 5 Animated! The heart beats in a sequence called the cardiac cycle. (© Cengage Learning)

fluid pressure inside them rises and the AV valves open.
Blood flows into the ventricles, which are 80 percent filled
by the time the atria contract. As the filled ventricles begin
to contract, fluid pressure inside them increases, forcing
the AV valves shut. The rising pressure forces the aortic
and pulmonary valves open—and blood flows out of the
heart and into the aorta and pulmonary artery. Now the
ventricles relax, and the valves close. For about half a
second the atria and ventricles are all in diastole. Then the
blood-filled atria contract, and the cycle repeats.
The amount of blood each ventricle pumps in a minute
is called the cardiac output. On average, it is about 5 liters—
nearly all the blood in the body. This means that in a year
each half of your heart pumps at least 2.5 million liters of
blood—more than 600,000 gallons!
The blood and heart movements during the cardiac
cycle generate audible sounds made by the forceful closing

of the heart’s one-way valves. A physician’s stethoscope

detects this “lub-dup.” At each “lub,” the AV valves are

closing simultaneously as the two ventricles contract.

At each “dup,” the aortic and pulmonary valves are closing

as the ventricles relax.

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