4.2 The mechanism of breathing

Learning outcomesStudents should understand the following:

The mechanism of breathing.

Pulmonary ventilation as the product of tidal volume and ventilation rate.

The body separates the procedure of breathing in and breathing out.

 

By separating these two processes, the body can concentrate on the two tasks in turn.

Breathing in is one process and is known as…

Inhalation(When we breathe in we inhale)

Breathing out is a separate process and is known as…

Exhalation(When we breathe out we exhale)

It has the ability to move, whilst remaining enclosed within the protection of the ribcage.

ribs

rib muscles

Right Lung

diaphragm

Left Lung

trachea

right bronchus

Breathtaking featuresThe breathing system does not have a fixed shape.

This means that the rib cage must also be able to change position.

Take your hands and place them flat on your chest just above your hips on each side of your body. Now breathe in and out very deeply. Whilst you do this, watch to see what happens to your hands.

You should notice the following things…..

A mobile ribcage?

When you breathe in (inhale), your hands move up and outwards.

When you breathe out (exhale), your hands move down and inwards..

Take a breath

When we inhale, our lungs fill with air.

As they fill, they become enlarged.

The ribs must then move upwards and outwards to make more room in the thorax.

The overall effect of this is that our chest expands.

Inhaling: chest expansion

Pressure changesThe pressure changes in the lungs are brought

about by the movement of 2 sets of muscles:

The diaphragm – a sheet of muscle that separates the thorax and the abdomen.

The intercostal muscles:Internal intercostal muscles – contraction leads

to expirationExternal intercostal muscles – contraction

leads to inspiration

Your diaphragm is located beneath the lungs, which means that it separates the thorax from the abdomen.

It is a sheet of muscle that spans the width of the body.

inhaling

The diaphragm

Before we inhale, it is found in a domedome shape.

As we inhale, it contracts and flattensflattens.

The result of this change in shape is a change in the volume of the thorax.

As the volume of the thorax increases, the internal air pressure drops.

This means that the air pressure outside the lungs is greater than the air pressure inside the lungs.

Low

High

Diaphragm flattens

Air pressure drops

Thorax volume increases

High air pressure outside

Low air pressure inside

Air diffuses into the lungs

Pressure regulation

During exerciseInspiration is always an active process. When

the body is at rest expiration is passive ( the diaphragm muscles relax and the elastic recoil of the lungs and chest wall return the thorax to it’s original shape.)

During exercise expiration is boosted by contraction of the internal intercostal muscles

Key definitionsTidal Volume - The volume of air inhaled and

exhaled at each breath, this is normally equivalent to only 10% of the total 5L volume of the trachea and lungs.

Residual Volume - the amount of air still remaining in the lungs after the most forceful expiration.

Vital Capacity - the maximum amount of air that can be exhaled after a maximum inhalation.

Training can increase the strength of the diaphragm and intercostal muscles and can thus raise the vital capacity and hence improve performance.

Pulmonary ventilationThis is the total volume of air that is moved into the

lungs during one minute. To calculate it we can multiply 2 factors together:

Tidal volume – the volume of air normally taken in in each breath when the body is at rest (normally 0.5dm3).

Ventilation rate – the number of breaths taken in one minute (normally 12-20 breaths).

Pulmonary Ventilation = Tidal Volume x Ventilation Rate (dm3min-1) (dm3) (min-1)

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Learning outcomesStudents should understand the following:

The mechanism of breathing.

Pulmonary ventilation as the product of tidal volume and ventilation rate.