Childrens Illustrated Encyclopedia

250

OUR BODIES CONTAIN about 8 pints (4.5 liters) of blood. Throughout

life, the heart, an organ inside the chest, pumps blood to every part of the

body, keeping us alive. The heart is such a powerful pump that it takes

only about a minute for each blood cell to travel all the way around the

body and back to the heart. Traveling along tubes called blood vessels,

blood carries oxygen and nourishment from digested food to every part

of the body. Blood also carries away harmful waste products such as carbon

dioxide. Blood consists of red and white blood cells, platelets, and a watery

liquid called plasma. A drop of blood the size of a pinhead contains

millions of red cells and thousands of white cells. About once every

second, the muscular walls of

the heart contract, squeezing

blood out of the heart and into

blood vessels called arteries.

The arteries divide many times

until they form a network of

tiny blood vessels called

capillaries. The capillaries

gradually join up again to

form veins, which carry the

blood back to the heart.

ARTERIES

These blood vessels carry blood away

from the heart to the body. Arteries have

thick walls that can resist the high blood

pressure produced when the heart beats.

The coronary arteries deliver oxygen-rich

blood to the walls of the heart itself.

VEINS

These blood vessels have thinner walls than

arteries, as they are not subject to the high

blood pressure created by each heartbeat.

All veins, aside from the pulmonary veins,

carry oxygen-poor blood from the body to

the heart. Many have valves that prevent the

backflow of blood from the heart. Skeletal

muscles that surround veins help to squeeze

blood toward the heart when they contract.

CApIllARIES

The tiny blood vessels that

carry blood between the

smallest arteries (arterioles)

and the smallest veins

(venules) are called

capillaries. Capillaries allow

oxygen and nutrients to pass

through their walls to all

the body cells.

HOw BlOOD ClOTS

when you cut yourself and

blood flows out of the wound,

platelets in the blood stick

together and a fine meshwork

of fibers forms. This meshwork

traps more blood cells and

forms a clot to seal the wound.

BlOOD CEllS

There are three types of blood cells. Red

blood cells carry oxygen from the lungs

to the rest of the body. white blood

cells protect the body against

illnesses and fight infection.

platelets, which are actually

small cell fragments, help

the blood clot. All blood cells

are produced in the bone

marrow inside the bones.

INSIDE THE HEART

The heart consists of two pumps, left

and right, that work together. Each

side has two chambers, an upper

atrium and lower ventricle. Oxygen-

poor blood from the body enters the

right atrium through two big veins, the

superior and posterior venae cavae.

Blood passes into the right ventricle,

where it is pumped through the

pulmonary artery to pick up oxygen in

the lungs. The oxygen-rich blood returns

to the heart’s left atrium through the

pulmonary vein. From there, it passes to the

left ventricle, which pumps it along the aorta and its

branches to all parts of the body to deliver its oxygen

before returning to the right atrium. Valves inside the

heart ensure that blood flows in one direction only.

Heart and blood

Find out more

Brain and nerves

Human body

lungs and breathing

Muscles and movement

HEARTBEAT

On average, an adult’s heart beats 60 to

70 times each minute. This rises to more

than 150 beats after strenuous activity.

Each heartbeat has three phases. During

diastole (1) blood fills the two atria. They

contract during atrial systole (2) to push

blood into the ventricles that contract

together during ventricular systole (3) to

pump blood into the arteries.

Blood clot forms, sealing the cut.

Tiny meshwork of platelets begins to form.

Platelets stick together, and clotting begins.

Blood leaks out where blood vessel is cut.

3 4

Blood flows through

to ventricles (lower

chambers).

Blood enters atria

(upper chambers).

Atria refill

with blood.

Ventricles contract to

pump blood into arteries.

1 2

HUMAN HEART

The heart is protected by the rib cage.

An adult’s heart is the size of a clenched

fist and weighs about 9 oz (300 g).

Left ventricle

Valve

Muscle

Descending aorta

Aorta

(main artery)

Pulmonary

artery

Left atrium

Inferior

vena cava

Right

ventricle

Valve

Right

atrium

Valves

Superior

vena cava

White cell

Platelets

Red cell

Pulmonary veins

US_250_Heart_blood.indd 250 12/02/16 11:18 am