To Calculate the Efficiency of the Performance of the Heart, We Looked at SV, HR and Q

Respiratory Volumes at Rest

Respiratory Volumes

To calculate the efficiency of the performance of the heart, we looked at SV, HR and Q. We used their average values to compare resting with exercice levels. In the same way, the respiratory system has three definitions and values which help to calculate the efficiency of the respiratory system.

We need to look at the link between the three definitions and values at rest.

Tidal Volume (TV) the volume of air inspired and expired per breath – this is approximately 500ml during breathing at rest.

Frequency (f) the number of breaths per minute – approximately 12-15 breaths per minute at rest.

Minute Ventilation (VE) the volume of air inspired and expired in one minute. VE can be calculated by multiplying the tidal volume with the frequency of breaths in one minute.

The link between TV, f and VE is shown in the following equation:

VE = TV x f

= 500ml x 15

= 7500ml/min

=7.5L/min

Inspiratory and expiratory reserve volumes

Take a normal resting breath in, and then immediately inspire further. You will notice that you are able to draw more air in above your resting TV volume – this is termed your inspiratory reserve volume (IRV – approx 3000ml)

Take a normal resting breath out and then immediately expire further. You will see that are able to actually expire above your resting TV volume – this is termed your expiratory reserve volume (ERV – approx 1100ml)

Residual or Reserve volumes

You are unable to fully expire all of the air in your lungs – there is always some air left. This is called your residual or reserve volume (RV – approx 1200ml)

Lung capacities

Lung capacities are calculated by adding together two or more of the lung volumes we have considered, and are measured by breathing into apparatus that produces a graph called a spirometer trace. This highlights three additional lung volume capacities and definitions that we need to learn.

Inspiratory capacity – take a normal breath, then take a maximal inspiration. This is your inspiratory capacity representing your total TV and IRV – approx 3500ml.

Vital capacity – take a maximal inspiration and then forcibly expire all of the air from your lungs. This volume represents the total of your TV + IRV + ERV and is called your vital capacity (VC) – approx 4600ml.

Total lung capacity – take a maximal breath in. The total volume of air in the lungs at this point is called your total lung capacity (TLC) and represents the sum total of your ERV + IRV + TV + RV – approx 5800ml.