Volumes and Capacities

Home Up

    Now that we have discussed how we breathe, it is time to learn how much we breathe!  How much air could be the total volume of air we can take in through one breath, how much of that air we use, or the amount of air we take in per minute!  In addition, sometimes we breathe in unplanned ways, and take in or expell air in other ways (sneezing, hiccupping, yawning, etc.).  I will cover each of these aspects here.

How much can we breathe in or out in one breath?

    We have a normal amount of air that we breathe in and out when we are at rest.  This normal amount of air is our tidal volume of air... typically around 500 mL or a half of a liter.

    Of course, if we want to, we can take a really deep breath in, and then blow out all the air we can.  This maximal amount of air that we can take in and breathe out is called our vital capacity.  It is approximately 3 liters!

    Throughout a normal day, we really never use our entire vital capacity.  We take our tidal volume of air in and then out throughout the entire day, though.  And, whenever we are active or speaking we take in and let out more air than the tidal volume.  The vital capacity and tidal volume can be shown graphically, as in the figure below.  This type of figure is obtained by measuring the air into and out of a person during breathing, a procedure called spirometry.  To see a picture of how this is done, visit this site from Johns Hopkins on respiration, and then follow the link to the spirometry page (I gave you the whole link so that you can look up other items in the respiratory unit there).

respiratory volumes shown on a spirometry reading    You should be able to see in this graph that our tidal volume is rather small compared to our vital capacity.   Terms are also given here for the extra amount we can breathe in (inspiratory capacity) or breathe out (expiratory capacity) beyond our tidal capacity.

    One more important concept is evident here as well.  You see, even when we breathe out as much as we possibly can, we can never eliminate all of the air from our respiratory tracts.  The absence of air is a vacuum, and that certainly cannot exist within us-- a vacuum would cause our respiratory tract to buckle in on itself.  Therefore, there is always bit of air that we can never exhale, approximately 1200 mL (1.2 liters) of air, in our respiratory system all the time.   This 1200 mL of air that remains in our respiratory tracts is called the residual volume.  Yet, we do not usually even exhale enough air to leave only a residual volume of air.  Instead, we normally only exhale through our tidal volume levels... so all the air that we could choose to exhale plus the residual volume normally remains in our respiratory tracts after a resting exhalation; this amount of air (residual + expiratory reserve) is called the functional residual capacity.

    If we add up all the air that always remains in our respiratory system (the residual volume) with the total amount of air we can ever inhale (our vital capacity), you can see that we can hold just under 6 liters of air within us!  That is a lot of air!  Think about pouring three 2 L bottles of air into yourself!   This total amount is referred to as total lung capacity.

How much of the air we breathe can we use?

    It would be nice to be able to tell you that our bodies are so efficient that all 6 liters of air could be used.  Unfortunately, our bodies aren't that efficient.  You see, we only use air when we can have gas exchange.  Gas exchange only takes place at the alveoli.  When we inhale, we take air into our entire respiratory system, and only that air that entered first makes it all the way into the alveoli.  Some air has to remain in the nasal cavity, trachea, bronchi, and bronchioles so that they do not create a vacuum!  The air that remains in these respiratory passageways cannot be used in our bodies and is exhale unchanged.   Because of this, our respiratory passageways can also be called anatomic dead space, since they are anatomically necessary to get air to the alveoli, but they have no way of using the air.

    How much air goes unused in the anatomic dead space?   Approximately 150 mL!  So, during normal resting breathing, 150 mL of the 500 mL is unused.  (150/500) * 100% = 30%.  That means that with every resting breath, we cannot use 30% of the air we draw in!

    There is another term to describe regions where air cannot be used:   physiologic dead space.  This term includes all the anatomic dead space.  But it also has more.  Let's say that a part of the lung is damaged and gas exchange cannot happen there-- air still goes there, but cannot be used.   That damaged area creates more dead space.  If we add it on to the anatomic dead space, we have a better idea of the total amount of dead space.  This total is the physiologic dead space.  It also includes dynamic changes in gas exchange abilities throughout the lungs.

Air movements in our respiratory system for purposes other than exchange

    These include coughing, sneezing, laughing, hiccupping, and yawning.   Your book describes each of these functions.  Certainly, we do not carry any of these functions out in order to get oxygen into our alveoli!  But they still affect our respiratory system, and so they need to be included.

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.