The mechanics of breathing and Respiratory Volumes Pass and Merit

Overview Breathing is regulated by: Breathing in = Inspiration 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) 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 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.

Respiratory volumes Residual Volume Vital Capacity Total Lung Capacity Take in as much breath as possible This is your total lung capacity ERV+IRV+TV+RV (Approx 6000ml) Residual Volume Breathe out as much as possible There is always some air left in your lungs This is your RV (Approx 1200ml) 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

Spirometer trace