1. Respiratory Volumes

2. Respiratory volumes are the amount of air inhaled, exhaled and stored within the lungs at any given time.
There are a number of different measurements and terms which are often used to describe this including tidal volume, inspiratory reserve volume, residual volume, vital capacity and total lung capacity.

3. Tidal Volume
The amount of air which enters the lungs during normal inhalation at rest as opposed to exercising.
So if you are sitting normally and breathing quietly then the amount of air breathed in is the tidal volume. The average tidal volume is 500ml.

4. Inspiratory Reserve Volume
The amount of extra air inhaled (above tidal volume) during a deep breath.
This can be as high as 3000ml. So if you breath in the normal amount you would at rest, and then see how much additional air you can breath in before you simply cannot breathin any more, then this extra amount is the inspiratory reserve volume.

5. Expiratory Reserve Volume
The amount of extra air exhaled (above tidal volume) during a forceful breath out.
So if you breath out normally as you would, then try and breath out even more until you physically cannot breath out any more air, then this is the expiratory reserve volume.

6. Residual Volume
The amount of air left in the lungs following a maximal exhalation.
OK, lets assume you have breathed out as far and as much as you can so you physically cannot breath out any more. There is always some air remaining to prevent the lungs from collapsing. This air that is always in the lungs is called the residual volume.

7. Vital Capacity
The most air you can exhale after taking the deepest breath you can.
So if you breath in as much as you physically can and then measure the amount of air you can breath out then this is known as the vital capacity. It can be up to ten times more than you would normally exhale at rest.

8. Total Lung Capacity
This is the vital lung capacity plus the residual volume and is the total amount of air the lungs can hold. The average total lung capacity is 6000ml, although this varies with age, height, sex and health.