homework encyclopedia

The main organs of the respiratory system are the lungs

• two large organs that expand and contract. Each has

a network of airways which end in tiny pockets called

alveoli, where oxygen enters the blood and carbon

dioxide leaves – a process known as. GAS EXCHANGE.

HOW IS OUR BREATHING CONTROLLED?

The respiratory centre of the brain regulates

our breathing, even while we sleep, with the

help of receptors in some of the large

arteries. The receptors monitor the level

of carbon dioxide in the blood, which

rises when we are active, and tell the

brain if we need to breathe faster to

get rid of the carbon dioxide.

WHAT MAKES THE LUNGS EXPAND AND CONTRACT?

The lungs rely on surrounding muscles to make them

expand and contract. Muscles between the ribs pull

the ribcage up and out, making the chest expand and

drawing air in, or relax to squeeze the lungs and force

air out as the ribcage falls down and in. At the same

time, the diaphragm (a dome of muscle beneath the

lungs) contracts to suck air into the airway, or relaxes

and rises up to push air out.

HOW DOES OXYGEN GET INTO THE BLOOD?

Oxygen passes through the walls of the alveoli and

the surrounding blood capillaries into the blood. The

oxygen enters red blood cells and binds to a chemical

called haemoglobin. Carbon dioxide passes out of the

plasma (the watery part of blood) to enter the alveoli.

WHAT ARE ALVEOLI?

Alveoli are like tiny bags of air with walls so thin that

gases can pass through them. There are around 300

million alveoli in the lungs. Together they provide an

area about as big as a tennis court for gas exchange.

The passing of gases from the air into the

blood, and from the blood back to the air,

is called gas exchange. It takes place in the

lungs, in tiny air pockets called alveoli.

FIND OUT MORE. Brain 139 • Circulation 134 • Mouth 142 • Muscle 132 • Nose 143

1 INHALATION (BREATHING IN)

To inhale, the intercostal muscles contract, and the

diaphragm moves down, making the chest expand.

Air is sucked into the lungs, because the pressure in

the airways is less than it is outside.

1 EXHALATION (BREATHING OUT)

When the intercostal muscles and diaphragm relax,

we exhale. The ribs fall downwards and inwards,

and the diaphragm springs back into a dome shape,

gently squeezing the lungs and pushing air out.

THE BRONCHIAL TREE 3

In this X-ray, dye has been used

to outline the tree-like network

of airways in the lungs – the

trachea act as the trunk, the

bronchi as branches, and the

bronchioles as the twigs.

Lungs

ALVEOLI 3

Alveoli are clustered

together, like a bunch of grapes, at

the end of a bronchiole no wider

than a hair.

GAS EXCHANGE

Capillaries

(red and blue)

cover the alveoli,

bringing carbon

dioxide from

the body and

taking back

oxyen

Ribs provide a flexible

framework for the lungs,

protecting them, yet

allowing them to expand

and contract

HOW BREATHING WORKS

Intercostal muscles

move the ribs making

the lungs expand or

contract

Trachea

carries inhaled air

to the lungs

Exhaled air

is pushed out

through the

nostrils and lips

Muscles

contract to raise

the ribcage up and

outwards

Terminal bronchiole

surrounded by blood

capillaries

Muscles

relax and ribcage

moves downwards

and inwards

Diaphragm

contracts and pulls

downwards

Exhaled air

is pushed up

the trachea

to the throat

Lungs

shrink and

squeeze air out

Diaphragm

relaxes and

springs upwards

Lungs

expand, sucking

air in

Cluster of alveoli

lungs