Lung Volumes and Capacities

What Are Lung Volumes and Capacities?

Lung volumes are the four separate, non-overlapping amounts of air that make up the total air in the lungs, and lung capacities are combinations of two or more of those volumes. Together they describe how much air moves in a quiet breath, how much extra can be drawn in or pushed out, and how much always stays behind.

Lung volumes and capacities definitions

A volume is a single slice of the lungs' air that cannot be divided further in a standard breathing test. A capacity is a sum, so it always includes at least two volumes. For example, vital capacity is the tidal volume plus both reserve volumes, and total lung capacity is all four volumes added together.

Why they matter

These measurements show how well the lungs fill and empty, and they change in typical ways with age, body size and lung disease. Doctors read them as part of pulmonary function tests, and physiologists use them to explain how breathing, gas exchange and the work of the breathing muscles fit together.

Lung volumes and capacities explained simply

Imagine the lungs as a large jug that is never completely empty. In a normal breath you pour a small cup of air in and out at the middle of the jug. You can pour in much more if you try, or squeeze out extra, but a layer of air always stays at the bottom. The volumes are the separate layers, and the capacities are what you get when you add layers together.

What Are the Four Lung Volumes?

The four lung volumes are tidal volume, inspiratory reserve volume, expiratory reserve volume and residual volume, and added together they equal total lung capacity.

Tidal volume (TV)

Tidal volume is the amount of air moved in or out during one normal, quiet breath, about 500 mL in a resting adult. About 150 mL of it fills the airways and never reaches the alveoli. Our tidal volume page covers how it is measured and how it changes with body size and activity.

What is inspiratory reserve volume?

Inspiratory reserve volume (IRV) is the extra air that can be breathed in with maximum effort after a normal, quiet inhalation, about 3,100 mL in an adult man and about 1,900 mL in an adult woman. It is the largest of the four volumes and the main reserve the body draws on during exercise, when deeper breaths eat into it.

Expiratory reserve volume (ERV)

Expiratory reserve volume is the extra air that can be forced out after a normal, quiet exhalation, about 1,200 mL in an adult man and about 700 mL in an adult woman. Pushing it out takes active work by the abdominal muscles and the internal intercostals, as described on the inhalation and exhalation page.

Residual volume (RV)

Residual volume is the air that remains in the lungs after the most forceful possible exhalation, about 1,200 mL in an adult man and about 1,100 mL in an adult woman. It cannot be breathed out, so it cannot be measured with a simple spirometer.

What Are the Four Lung Capacities?

The four lung capacities are inspiratory capacity, functional residual capacity, vital capacity and total lung capacity, each made by adding two or more lung volumes.

• Inspiratory capacity (IC) = TV + IRV. The most air that can be breathed in starting from the end of a quiet exhalation, about 3,600 mL in an adult man.
• Functional residual capacity (FRC) = ERV + RV. The air left in the lungs at the end of a quiet exhalation, about 2,400 mL in an adult man. It is the resting volume of the lungs.
• Vital capacity (VC) = TV + IRV + ERV. The most air that can be breathed out after the deepest possible breath in, about 4,800 mL in an adult man. The vital capacity page explains what affects it.
• Total lung capacity (TLC) = TV + IRV + ERV + RV. All the air the lungs hold after the deepest possible breath in, about 6,000 mL in an adult man and about 4,200 mL in an adult woman.

A useful rule is that any capacity containing the residual volume, which means FRC and TLC, cannot be measured by spirometry alone. The table below gathers all eight values in one place.

Lung Volumes and Capacities Diagram: How Do They Fit Together?

On a lung volumes diagram or graph, the volumes are stacked on top of one another like layers, from residual volume at the bottom to inspiratory reserve volume at the top, and each capacity is drawn as a bracket that spans the layers it contains.

Reading a lung volumes graph

The standard lung volumes chart is a spirogram, a trace of volume over time recorded while a person breathes into a spirometer. Quiet breathing appears as small, regular waves; their height is the tidal volume. A maximal breath in raises the trace to its top, which marks total lung capacity, and a maximal breath out drops it to the lowest point the spirometer can record, which sits one residual volume above the true bottom of the lungs. Our spirometry page explains how the trace is recorded and how the timed measurements are read.

Three useful landmarks

1. End of a quiet breath out: the lungs are at functional residual capacity, the resting point of the whole system.
2. End of a maximal breath in: the lungs are at total lung capacity.
3. End of a maximal breath out: only the residual volume is left.

Every volume and capacity is the vertical distance between two of these landmarks or the top and bottom of a quiet breath. Once the landmarks are clear, the eight names become easy to place on any chart.

Why Do the Lungs Never Empty? Residual Volume and FRC

The lungs never empty completely because the chest wall cannot be squeezed small enough to push out all the air, and the smallest airways close before the alveoli can collapse, trapping the residual volume inside.

What residual volume does

Residual volume keeps the alveoli partly open between breaths, so they do not have to be reopened from a collapsed state each time, which would take far more effort. It also means air is always in contact with the blood, so gas exchange continues between breaths rather than stopping each time we breathe out. Residual volume rises with age, as the lungs lose some elastic recoil and small airways close earlier.

Functional residual capacity: the lungs at rest

Functional residual capacity is where the lungs settle when all the breathing muscles are relaxed. At that point the inward elastic pull of the lungs is exactly balanced by the outward spring of the chest wall, and the slightly negative pressure in the pleural cavity holds the two together. Each quiet breath adds only about 350 mL of fresh air to the roughly 2,400 mL already in the lungs, so the alveolar air changes only gradually and its oxygen and carbon dioxide levels stay steady.

How posture changes FRC

Functional residual capacity is smaller lying down than standing, because the abdominal organs push the diaphragm up into the chest. The same effect makes it smaller in pregnancy and in people with obesity.

What Is Minute Ventilation?

Minute ventilation is the total volume of air breathed in or out in one minute, calculated as tidal volume multiplied by the breathing rate, and it is about 6 liters per minute in a resting adult (500 mL × 12 breaths per minute).

Minute ventilation vs alveolar ventilation

Not all of that air reaches the alveoli. About 150 mL of each breath stays in the conducting airways, the anatomical dead space. Alveolar ventilation counts only the fresh air that reaches the gas exchange surfaces: (500 mL − 150 mL) × 12 breaths per minute, or about 4.2 liters per minute at rest.

Why deep breaths beat fast, shallow ones

Because the dead space is the same size in every breath, the depth of breathing matters more than its speed. Fast, shallow breathing can keep minute ventilation the same while sharply cutting alveolar ventilation, since a larger share of each small breath only fills the airways. Deeper breathing does the opposite, which is why the body raises tidal volume first when it needs more air.

Changes with activity

During hard exercise, both tidal volume and respiratory rate rise, and minute ventilation can increase more than tenfold over the resting value. How breathing adapts to effort and training is covered on our exercise and the respiratory system page.

How Are Lung Volumes Measured?

Lung volumes are measured with spirometry for the volumes a person can breathe in and out, and with gas dilution or body plethysmography for the residual volume and the capacities that include it.

• Spirometry: the person breathes through a mouthpiece into a device that records the volume moved. It measures tidal volume, both reserve volumes, inspiratory capacity and vital capacity, along with timed values used to judge airflow.
• Helium dilution: the person breathes from a closed container holding a known amount of helium, which does not cross into the blood. The dilution of the helium reveals how much air was already in the lungs, giving functional residual capacity.
• Nitrogen washout: the person breathes pure oxygen, and the nitrogen washed out of the lungs is collected and measured to calculate functional residual capacity.
• Body plethysmography: the person sits in a sealed booth, often called a body box, and pants against a closed shutter. Pressure changes in the booth and at the mouth, interpreted with Boyle's law, give the total gas volume in the chest.

Once functional residual capacity is known, residual volume is found by subtracting the expiratory reserve volume, and total lung capacity by adding the inspiratory capacity. Results are compared with predicted values for a person of the same age, sex and height, so a single number has little meaning on its own.

What Affects Lung Volumes and Capacities?

Lung volumes and capacities depend mainly on body size, sex and age, and also on posture, fitness, the altitude where a person grew up and the health of the lungs and chest wall.

• Height and body size: taller people have larger chests and larger lung volumes; height is the main factor in predicted values.
• Sex: on average, adult women have lung volumes about 20 to 30% smaller than men of the same age, partly because of smaller chest size.
• Age: vital capacity falls gradually in later adulthood, while residual volume rises; total lung capacity changes little.
• Posture: functional residual capacity and vital capacity are larger standing or sitting upright than lying flat.
• Altitude: people who grow up at high altitude tend to have larger lung volumes, an adaptation discussed on our breathing at high altitude page.
• Training: some athletes, such as swimmers, have larger than predicted vital capacities, although training changes lung size far less than it changes muscle strength and endurance.

The lungs themselves do not stretch much with practice; most differences between healthy people come from the size and shape of the chest and the strength of the muscles of respiration.

Common Conditions That Change Lung Volumes

Lung diseases change lung volumes in two typical patterns: obstructive diseases trap air and raise residual volume, while restrictive diseases shrink the lungs and lower most volumes. Our respiratory diseases section covers each condition, and anyone with breathing problems should see a doctor.

• Obstructive pattern: in COPD, emphysema and poorly controlled asthma, narrowed or floppy airways close early during exhalation. Air is trapped, so residual volume and functional residual capacity rise, total lung capacity may increase, and vital capacity often falls.
• Restrictive pattern: in pulmonary fibrosis, the lungs become stiff and cannot fill fully, so total lung capacity, vital capacity and functional residual capacity all decrease.
• Chest wall and muscle problems: severe curvature of the spine, weakness of the breathing muscles or marked obesity limit how far the chest can expand, giving a restrictive pattern with healthy lung tissue.
• Pleural disease: a large pleural effusion or a pneumothorax takes up space in the chest and reduces the volume the lung can occupy.

The full comparison of the two patterns is on our obstructive vs restrictive lung disease page.