The Lung Volumes and Capacities Page!      

The volumes and capacities listed here are normally determined using a spirometer!  A traditional "bell spirometer" is a canister of water with an inverted canister bell over it with air space inside the inverted canister bell.  Pulleys attach the canister bell to a pen which records volumes on paper on the outside of the canister.  A CO₂ absorber could be placed in the bell housing and used in experiments to determine O₂ consumption, working on the assumption that loss of volume of air in the bell housing correlated directly with the volume of CO₂ exhaled and absorbed.  The bell housing with the paper recording revolved electrically or was driven by a windup mechanism.  You could look at these old machines and quickly figure out how to use them!  A modern spirometer looks a lot like a little adding machine with a mouthpiece, air tube and a small printout or a cable that feeds data into a computer!  There are many brands available, each is a little different!  One model is displayed below.

Table 1 - Respiratory Volumes and Capacities (approximates which will vary somewhat between sources)

Resting Tidal Volume (V_T):  This is the volume of air taken into the lungs when you inhale.  Tidal volume increases with exercise or activity.

Inspiratory Reserve Volume (IRV):  Total lung capacity minus the volume of air in the lung at the end of a normal inspiration.  This means that we have a reserve volume that we can tap into as tidal volume increases with exercise or activity.

Inspiratory Capacity (IC): Sum of the tidal volume plus the inspiratory reserve volume (TV + IRV).

Expiratory Reserve Volume (ERV):  This is the difference between the volume of air left in the lung at the conclusion of normal expiration versus at the conclusion of maximal expiration.  That means that we have a "reserve" volume which we can tap into when our tidal volume increases with exercise or activity.

Residual Volume (RV):   The residual volume is the volume of air left in the lungs at the end of maximal expiration (ie. the volume of air which you cannot voluntarily exhale from your lungs).  As residual air cannot be exhaled, the volume can be estimated through gas dilution techniques and the use of helium in inspired air (we do not metabolize helium).

Functional Residual Capacity:  This is the total volume of air left in the lungs at the conclusion of normal, resting expiration.  This value includes the expiratory reserve volume plus the residual volume (ERV + RV).

Vital Capacity (VC):  Vital capacity is the total usable volume of the lungs which is under voluntary control.  This value does not include the entire lung volume as it is not possible to breath all of the air out of the lungs.

Total Lung Capacity (TLC):  The residual volume (air you cannot expire) + vital capacity (total volume available for use) = total lung capacity.  In other words, TLC is the total volume of the lungs!

Minimal Volume (MV):  Should the volume of the lungs fall below this value, the lungs will collapse.

Figure: Respiratory Volumes and Capacities

Minute Respiratory Volume:  the volume of air exchanged in 1 min (respiratory frequency is approximately 12-15 breaths/min X tidal volume = 12 to 15 X 500 mL = 6000 to 7500 mL/min) (the average is closer to 6 L/min)

Forced Expiratory Volume 1 (FEV1):  The volume of air that can be expired during the first second of expiration in a vital capacity determination.... generally about 80% of VC.

Forced Expiratory Volume 1 (%) or FEV1(%):   FEV1/VC

Anatomic Dead Space:  this is the volume of the air conducting pathways in which no gases are exchanged (approximately 150 mL in the average adult human being);  this volume is optimized for the fastest delivery of air to the exchange zone with the least airflow resistance possible; when this volume gets messed up by something like an asthma attack, the volume is no longer optimized for low resistance and breathing quickly becomes a major problem!  (see Poiseuille's Law).

Physiologic Dead Space:  sum of the anatomic dead space plus the volume of any non-functional areas of the lungs.  In young, health lungs, the volume of the anatomic dead space and physiologic dead space are equal.

Alveolar Gas:  this is the volume of air in the alveoli following normal expiration at rest; this value would be just slightly less than functional residual capacity as this volume would not include anatomic dead space whereas functional residual capacity does!  Alveolar gas, then, is close to 2.5L.

Alveolar Ventilation = (Tidal Volume - Anatomic Dead Space) X respiration rate = 5250 mL/min (at rest)

When we are resting quietly, our tidal volume drops so much that we only inhale enough air to fill our conducting zone as far as the distal ends of the terminal bronchioles (the border with the respiratory bronchioles)... and yet air still manages to reach our alveoli... can you guess by what mechanism gases are transferred between the ends of the terminal bronchioles and the alveoli?

Ventilation rate = total air flow/unit time

Perfusion rate = total blood flow through the lungs/unit time

Pressures are generally given in mmHg or cmH₂O (1.3 mmHg = 1 cmH₂O)

Pulmonary Capillary Blood Volume at normal rest = 70 mL

Pulmonary Blood Flow/min at normal rest = 5000-6000 mL/min (ie. 100% of cardiac output)

Systemic Blood Flow/min at normal rest = 5000-6000 mL/min (ie. 100% of cardiac output)