Biology - Modelling the respiratory system / Bell jar model / Huff and puff apparatus

The bell jar model

The process of ventilation as a series of changes in pressure within the thorax can be modelled using the bell jar model

Glass tube represents -

Trachea and bronchi

Bell jar represents -

Chest cavity

Balloons represent -

Lungs

Rubber sheet represents -

Diaphragm

What happens to rubber sheet when pulled down

The volume of the jar increases, the pressure therefore decreases and air is drawn in through the glass tube inflating the balloons.

Limitations of bell jar model

• Balloons represent lungs but do not fill the jar

• Bell jar represent ribs but unable to move up and out

• Rubber sheet pulls downward unlike diaphragm that only flattens

The huff and puff apparatus

• When we breathe in, the air is drawn through boiling tube A

• When we breathe out, the air is blown into boiling tube B

• Lime water is colourless but becomes cloudy (or milky) when carbon dioxide is bubbled through it

• The lime water in boiling tube A will remain clear, but the limewater in boiling tube B will become cloudy

• This shows us that the percentage of carbon dioxide in exhaled air is higher than in inhaled air

The bell jar model shows inhalation:

• When the diaphragm (rubber sheet) moves down, the volume inside the glass jar (thorax) increases.

• This increase in volume causes a decrease in pressure.

• The lungs (balloons) inflate as air enters until the pressures inside and outside are equal.

The bell jar model shows exhalation:

• When the diaphragm (rubber sheet) moves up, the volume inside the glass jar (thorax) decreases.

• This decrease in volume causes an increase in pressure.

• The lungs (balloons) deflate as air exits until the pressures inside and outside are equal.