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Human Body^135

Blood is driven around the body

by the heart, a powerful muscular

pump that never stops beating. The

heart is a dual pump. The left side

sends blood to the body, while the

right sends blood only to the lungs.

One complete contraction of the heart is called a heartbeat. Each

heartbeat involves a sequence of events, with different parts of the

heart contracting at different times. The rhythmic “lub-dub” noise

of the heartbeat is the sound of valves snapping shut.

WHAT IS THE HEART MADE OF?

The heart consists mostly of cardiac muscle, a special

type of muscle that contracts rhythmically of its own

accord. A tough membranous bag called

the pericardium surrounds the heart,

and a smooth membrane, called the

endocardium, lines the inner surface.

WHAT’S INSIDE THE HEART?

There are four chambers in the heart: two on the left

and two on the right. The top chambers, called atria,

are holding stations for incoming blood. The bottom

chambers, called ventricles, pump blood out of the

heart. It is the contractions of the atria and ventricles

that make the sound of the. HEARTBEAT.

HOW IS THE HEARTBEAT CONTROLLED?

A small patch of modified cardiac muscle, the sinoatrial

node, is the heart’s pacemaker. It sends out a wave of

electricity to make the heart’s walls contract about 70

times a minute. However, nerve signals from the brain

can speed it up during exercise or stress, if necessary.

FEEDING THE HEART 3

This angiogram shows the

network of blood vessels on

the surface of the heart

that keep it supplied

with oxygen and

nutrients. The large

vessels are the

coronary arteries, which

branch out from the

aorta to give the cardiac

muscle a direct supply of

oxygen-rich blood.

FIND OUT MORE. Lungs 137 • Muscle 132 • Respiration 136

HEARTBEAT

INSIDE THE HEART 3

A system of valves ensures that

blood follows a one-way route

through the heart, to the lungs

and back, and then out through

the aorta to the body.

Heart

1 STAGE 3 — THE VENTRICLES CONTRACT

The ventricles contract to force blood out. The valves

between the atria and ventricles shut to make the “lub” beat.

1 STAGE 1 — THE HEART RELAXES

As the heart relaxes and the atria fill up with blood, the

semilunar valves shut, making the “dub” beat.

1 STAGE 2 — THE ATRIA CONTRACT

The atria contract to push blood into the ventricles.

Valves between the atria and ventricles are pushed open.

Superior

vena cava carries

used blood into

the right atrium

Right ventricle

receives blood from

the right atrium and

pumps it to the lungs

Left ventricle

receives blood from

the left atrium and

pumps it to the body

CLOSED VALVE

The semilunar

valves close to

prevent backflow.

Right atrium

receives blood

from the

main veins

Pulmonary artery

takes blood from the

right atrium to the lungs

OPEN VALVE

The force of

blood rushing

through opens

the valve.

Left atrium

receives fresh blood

from the lungs

Right atrium fills

with used blood

from the body

Right atrium contracts

Left atrium contracts

Aorta

carries fresh

blood to

most of

the body

Inferior vena cava carries

blood from the lower body

Oxygen-rich

blood flows

to upper and

lower body

Tricuspid and

bicuspid valves open Tricuspid and bicuspid valves

snap shut

Semilunar valves open

Pulmonary

veins

Oxygen-rich blood

fills the left atrium

Full ventricles

Bicuspid

valve

Cardiac

muscle

Contracted

ventricles

Semilunar

valves

Tricuspid

valve

Semilunar valves

The septum is

the stout muscular

wall between the two

sides of the heart

Descending

aorta takes

blood to the

lower body

heart