Vital Capacity

What Is Vital Capacity?

Vital capacity is the greatest amount of air that can be moved out of the lungs in one breath, measured from the fullest possible inhalation to the most complete possible exhalation. It shows the usable range of the lungs and chest together, and it is one of the four standard capacities described in our overview of lung volumes and capacities.

Vital capacity definition in anatomy

In anatomy and physiology, vital capacity is defined as the sum of three lung volumes: the inspiratory reserve volume, the tidal volume and the expiratory reserve volume. It does not include the residual volume, the air that stays in the alveoli and airways even after the hardest exhalation. Because it depends on the lungs, the rib cage and the breathing muscles at once, it measures the whole breathing apparatus rather than any one organ.

How school biology defines it

High school and NCERT biology textbooks describe vital capacity in two equivalent ways: the most air a person can breathe in after a forced exhalation, or the most air a person can breathe out after a forced inhalation. Both describe the same range, from completely full to as empty as the lungs can be made. Those courses also list it as expiratory reserve volume plus tidal volume plus inspiratory reserve volume.

Explained simply

Think of the lungs as a balloon inside a box that can never be squeezed totally flat. Vital capacity is the amount of air the balloon can take in from its emptiest to its fullest. The small bit of air that can never be pushed out, the residual volume, keeps the balloon from collapsing completely.

What Is the Vital Capacity Formula?

The vital capacity formula is VC = IRV + TV + ERV: inspiratory reserve volume plus tidal volume plus expiratory reserve volume. Because these three volumes lie end to end, the formula can be written in three equivalent ways.

• VC = IRV + TV + ERV, adding the three volumes that can be moved by breathing.
• VC = IC + ERV, where the inspiratory capacity (IC) is tidal volume plus inspiratory reserve volume.
• VC = TLC - RV, total lung capacity minus the residual volume that cannot be breathed out.

Worked example

Using standard textbook values for a young adult man, an inspiratory reserve volume of about 3,100 mL, a tidal volume of about 500 mL and an expiratory reserve volume of about 1,200 mL give a vital capacity of 3,100 + 500 + 1,200 = 4,800 mL, or 4.8 liters. For a young adult woman, about 1,900 + 500 + 700 = 3,100 mL. Checking the other route, a total lung capacity of 6,000 mL minus a residual volume of 1,200 mL also gives 4,800 mL.

Why residual volume is left out

Residual volume is left out because no breath can move it. The chest wall cannot shrink past a certain point, and the smallest airways close before the alveoli empty, trapping the last 1 to 1.2 liters of air. That trapped air keeps the alveoli from collapsing and keeps gas exchange going between breaths, so it matters to the body even though it never enters or leaves.

What Is the Vital Capacity of the Lungs in a Healthy Adult?

The vital capacity of the lungs in a healthy young adult is typically about 4 to 5 liters in men and about 3 to 4 liters in women, with textbook averages of about 4.8 and 3.1 liters. Individual values vary widely, so a result is judged against a predicted value for the same age, height and sex rather than against a single number.

Predicted values

Predicted vital capacity comes from reference equations built from measurements of large groups of healthy, non-smoking people. The equations use age, height and sex, and they produce both an average value and a lower limit of normal, the level below which only about 5% of healthy people fall. Results are often reported as a percent of predicted; a value of about 80% of predicted or more has long been treated as within the normal range, although modern reports prefer the lower limit of normal.

Peak and decline

Vital capacity grows through childhood and the teenage years with the size of the chest, reaches its highest level in the early to mid-20s, and then declines slowly for the rest of life. The decline comes from stiffening of the rib cage, a gradual loss of elastic recoil in the lungs and some weakening of the muscles of respiration. At the same time residual volume rises, so total lung capacity changes much less than vital capacity does.

What Is Vital Capacity Measured In?

Vital capacity is measured in liters, or sometimes milliliters, of air, and it is recorded with a spirometer during a lung function test. Volumes are corrected to body conditions, meaning body temperature and full humidity, because air expands as it warms inside the chest.

How the test is done

For the test, the person sits upright, wears a nose clip and seals the lips around a mouthpiece connected to the spirometer. After a few normal breaths, they breathe in as deeply as possible and then breathe out until no more air will come. The test is repeated to make sure the results agree, and the best values are kept. Our spirometry page explains the trace, called a spirogram, in detail.

Slow and forced vital capacity

There are two versions of the test. In slow vital capacity (SVC), the person breathes out steadily at a relaxed pace. In forced vital capacity (FVC), the person blows out as hard and fast as possible, and the machine also records how much air leaves in the first second, the FEV1. In healthy lungs the two values are nearly the same, but in narrowed airways the forced value is often smaller, because fast exhalation squeezes the airways shut and traps air.

What Is the Function of Vital Capacity?

Vital capacity has no single job of its own; it is the full working range of breathing, and its function is to provide the reserve that lets ventilation rise far above the resting level. A quiet breath uses only about a tenth of it, so the rest is available whenever the body needs more air.

Room for exercise

During hard exercise, tidal volume can grow to around half of the vital capacity, while the breathing rate also climbs. A larger vital capacity therefore means more room to deepen each breath before the breathing rate has to carry the extra load. The effect of training on this reserve is described on how exercise affects the respiratory system.

Coughing, speech and singing

An effective cough starts with a deep breath in, so a cough needs a good share of the vital capacity to build enough pressure and airflow to clear mucus. Long sentences, shouting and singing also draw on the reserve, since the vocal cords are powered by a steady flow of exhaled air.

A window on lung health

Because it depends on the lungs, the chest wall, the breathing muscles and the nerves that drive them, vital capacity is a simple overall test of the breathing system. A reduced value does not say which part is affected, but it shows that the usable range has shrunk, and doctors then use other tests to find out why.

What Affects Vital Capacity?

Vital capacity is affected mainly by body size, age and sex, and to a lesser degree by posture, fitness, the environment people grow up in and health conditions of the lungs and chest. Height is the strongest single predictor, because taller people have longer chests and larger lungs.

• Height: vital capacity rises steadily with standing height in both sexes.
• Sex: at the same height, men have larger lungs and a larger vital capacity on average than women.
• Age: it peaks in early adulthood and declines slowly after that.
• Posture: it is a little larger sitting or standing than lying flat, because lying down lets the abdominal organs push the diaphragm upward and draws more blood into the chest.
• Fitness and sport: some athletes, especially swimmers, have above-average values, partly because of body build; in adults, training changes vital capacity only modestly.
• Altitude: people born and raised at high altitude tend to have larger lungs for their size, an adaptation described on breathing at high altitude.
• Pregnancy: vital capacity stays about the same, because the rib cage widens while the diaphragm is pushed upward.
• Body weight: severe obesity lowers vital capacity by limiting how far the chest and diaphragm can move.

Vital Capacity vs Total Lung Capacity and Forced Vital Capacity

Vital capacity differs from total lung capacity by the residual volume: total lung capacity is all the air the lungs hold when full, about 6 liters in a young adult man, while vital capacity is only the part of it that can be breathed out, about 4.8 liters.

Total lung capacity

Total lung capacity (TLC) equals vital capacity plus residual volume. A spirometer cannot measure TLC on its own, because it only records air that moves in and out; the trapped residual air has to be measured with body plethysmography or a gas dilution test. When people talk loosely about "lung capacity," they usually mean vital capacity, the part that can actually be measured with a mouthpiece.

Forced vital capacity and FEV1

Forced vital capacity is the vital capacity blown out as hard as possible, and the FEV1 is the share of it that leaves in the first second. Healthy young adults blow out about 75 to 85% of their FVC in that first second, so the FEV1/FVC ratio is about 0.75 to 0.85. A ratio below about 0.70 in an adult points to airway narrowing, while a low FVC with a normal or high ratio points to lungs or a chest that cannot expand fully.

Inspiratory capacity

Inspiratory capacity is the part of the vital capacity above the resting level: tidal volume plus inspiratory reserve volume, about 3.6 liters in a young man. The rest of the vital capacity, the expiratory reserve volume, lies below the resting level, the functional residual capacity.

Common Conditions That Affect Vital Capacity

Vital capacity falls in conditions that stop the lungs or chest from expanding fully, called restrictive conditions, and it can also fall in airway diseases that trap air, called obstructive conditions. Our respiratory diseases section explains these in more depth, and anyone with breathing symptoms should see a doctor.

• Pulmonary fibrosis: scar tissue makes the lungs stiff, so they cannot fill as far, and vital capacity and total lung capacity both fall.
• Chest wall problems: a severe curve of the spine (scoliosis), a rigid rib cage or severe obesity limit how far the chest can move.
• Neuromuscular conditions: weakness of the diaphragm and chest muscles, as in some nerve and muscle diseases, lowers vital capacity, so it is often tracked over time to follow breathing muscle strength.
• Pleural conditions: fluid in a pleural effusion or air in a pneumothorax takes up space that the lung would otherwise fill.
• COPD and asthma: narrowed airways close early during exhalation, trapping air, so residual volume rises and vital capacity can fall even though total lung capacity may be normal or high.

The two patterns are compared in the diagram below and in more detail on obstructive vs restrictive lung disease.