The mechanics of breathing and

Respiratory VolumesPass and

Merit

Overview • Breathing is regulated by:• the respiratory centre,

located in the brain. • Receptors in the air

passages and lungs• Breathing in = Inspiration• Breathing out = Expiration• To breathe the thorax

must increase and then decrease in size

Inspiration• In order to breathe in the volume of the

chest cavity needs to increase. • This increase in size of the chest cavity,

causes a decrease in pressure within the lungs• Boyle’s Law states that a volume of gas is

inversely proportional to its pressure. • This means that the increase in volume in the

lungs causes a decrease in pressure. • Gases flow from a high pressure area to a low

pressure area• In this situation the ambient air is the high

pressure area and the lungs are the low pressure area, so the air flows into the lungs

Inspiration

• Inspiration – Breathing in• Diaphragm contracts

• It flattens and pulls down• This is an active process

• External intercostal muscles contract• The sternum moves up and out, with the

lungs following• The lungs are attached to the pleural sac

(containing pleural fluid), which in turn is attached to the thoracic cage

• As the chest expands, the surface tension, created by the film of pleural fluid causes the lungs to be pulled outwards, with the chest

• These two actions cause the volume of the thoracic cavity to increase• According to Boyles Law

this increase in volume causes a decrease in pressure

• Air flows into the lungs• As gas flows from high

pressure to low pressure.

Expiration• To breathe out – Expiration:• Diaphragm relaxes

• It moves back up and into the thoracic cavity• This is a passive process

• The external intercostal muscles relax• The ribs/sternum moves down. • The lungs, sternum and rib cage are elastic structures that naturally 'spring'

back to their resting positions once the forces of the inspiratory muscles are removed. So expiration is a passive process.

• The volume of the thoracic cavity decreases causing the air to move out of the lungs. • This is because air pressure in the lungs is now higher than atmospheric

pressure, according to Boyles Law, so the air is forced out of the lungs to equate the pressure in and out of the body.

The muscles of breathing

Respiratory volumes

Lung Volumes

• Lung volumes: refers to physical differences in lung volume, while lung capacities represent different combinations of lung volumes, usually in relation to inhalation and exhalation.

• The average pair of human lungs can hold about 6 litres of air, but only a small amount of this capacity is used during normal breathing.

Spirometer trace

Respiratory volumes

• Tidal Volume– The volume of air inspired

or expired per breath (Approx 500ml at rest)

• Expiratory Reserve Volume– The amount of space that is available to breathe out, once you have exhaled

normally

• Eg: Breathe out normally, then force out more air. This is your ERV.

• Inspiratory Reserve Volume– The amount of space that is

available to draw in more air• Eg; Breathe in normally, then

breathe in more. This extra capacity is your IRV

Respiratory volumes

• Total Lung Capacity– Take in as

much breath as possible

– This is your total lung capacity

– ERV+IRV+TV+RV (Approx 6000ml)

• Vital Capacity– Breathe in as much as you can,

and then force as much air out of your lungs as possible. • This is your IRV+ERV+TV, and

is your Vital Capacity

• Residual Volume– Breathe out as much as

possible• There is always

some air left in your lungs

• This is your RV (Approx 1200ml)

Spirometer trace