Measuring Lung Capacity with Spirometry

 

Spirometry is a simple, noninvasive breathing assessment that can be used to determine overall lung health. During a spirometry assessment, an individual will inhale and exhale through a device called a spirometer. The spirometer will measure the volume of air that is passed through a sensor. If our airway is obstructed or if cells within are respiratory system are damaged, volumes in the test will be lower than what we might expect.

In this way, spirometry can be particularly useful in helping assess and diagnose respiratory conditions like asthma, COPD, chronic bronchitis or enphysema.

 


TL; DR

  • Spirometry is a noninvasive breathing test to evaluate overall lung health
  • Lung volumes and capacities can be classified by the their function during respiration.
  • Spirometry test data can determine how an individual lung capacities compare to who have individuals a healthy respiratory system.
 

 

Gas Exchange in the Lungs

The structure of the lung maximizes its surface area to increase gas diffusion. Because of the enormous number of alveoli (anywhere from 300-700 million in each human lung), the surface area of the lung is incredible (roughly 75m2 or ~800 sqft). To put that into perspective, that’s an area larger than the floor space of typical 1 bedroom apartment!

Having such a large surface area is one reason why aveoli are so efficient allowing for gas exchange. The large surface area increases the amount of gas that can diffuse into and out of the lungs.

Diffusion is a process in which transport is driven by a concentration gradient. Gas molecules move from a region of high concentration to a region of low concentration. Blood that is low in oxygen concentration and high in carbon dioxide concentration undergoes gas exchange with air in the lungs. The air in the lungs has a higher concentration of oxygen than that of oxygen-depleted blood and a lower concentration of carbon dioxide. This concentration gradient allows for gas exchange during respiration.

Author: OpenStax

Lung Volume & Lung Capacity

Different animals have different lung capacities based on their activities. Cheetahs have evolved a much higher lung capacity than humans; it helps provide oxygen to all the muscles in the body and allows them to run very fast. Elephants also have a high lung capacity. In this case, it is not because they run fast but because they have a large body and must be able to take up oxygen in accordance with their body size.

Human lung size is determined by genetics, sex, and height. At maximal capacity, an average lung can hold almost six liters of air, but lungs do not usually operate at maximal capacity.

Air in the lungs is measured in terms of lung volumes and lung capacities. Volume measures the amount of air for one function (such as inhalation or exhalation). Capacity is any two or more volumes (for example, how much can be inhaled from the end of a maximal exhalation). Below is a list of classifications for different lung volumes and lung capacities.

Author: OpenStax

What is Spirometry?

To evaluate overall lung performance, health care provides might carry out a series of pulmonary function tests. The tests that is used to measure lung volumes and lung capacities, is called spirometry. Spirometry can be particularly useful in assess lung health in individuals who have respiratory conditions.

Spirometry is a simple, non-invasive procedure where an individual breathes into a device that measures the volume of air that passes through it. Each of the terms outlined above can be measured using spirometry.

Author: National Heart Lung and Blood Institute

Evaluating Results from Spirometry Test

The graph shows the volume of air that was breathed during a typical spirometry test. You may notice which resembles a wave. When the inidivial inhales, the line will increase, and when the patient exhales, the line will move in the reverse direction.

Lung Volumes

During normal breathing – outlined in yellow – only about eight percent of air in the lungs is exchanged, and the amount of air in the lungs is one-half the total lung capacity. The volume of air we breath in and out while breaking normally is call the tidal volume.

The amount of air we can breathe while taking a deep breath is  in is called the inspiratory capacity. This is a combination of the volume we can breathe in during normal inhalation (tidal volume) plus the reserve volume available for us to take a deep breath (inspiratory reserve volume).

Similarly, we also have a reserve volume that is available for exhalation. You’ll notice that even after a normal exhalation, you still have the ability to exhale more air. This air that is available for forceful exhalaing is called expiratory reserve volume.

After forceful exhalation, a residual volume of air is still left in the lungs and is important for keeping the lungs – particularly the alveoli – partially inflated. This allows for continual gas exchange, even after maximal exhalation. Note, since you can’t exhale you residual volume, it cannot be measured directly using spirometry.

 

 

Lung Capacity

Lung capacity is a measurement of two or more lungs volumes. For example, our total lung capacity is the volume you get when you add up all your lung volumes.

$$ TLC = IRV + TV + ERV + RV $$

Our inspiratory capacity would be the volume that we inhale with normal breaths, plus the reserve volume we have for inhalation.

$$ IC = IRV + TV $$

The vital capacity is the difference between the total lung capacity and the residual volume. Or put another way, the total amount of air we can inhale and exhale.

$$ VC = TLC – RV $$

$$ VC = IRV + TV + ERV $$

The functional residual capacity is the amount of air in the lungs after normal exhalation. That would mean we still have our expiratory reserve volume and the residual volume left in our lungs.

$$ FRC = ERV + RV $$

Author: OpenStax
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  2. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P. “22.3 The Process of Breathing ” Anatomy and Physiology. OpenStax, 2013. Houston, TX. https://openstax.org/books/anatomy-and-physiology/pages/22-3-the-process-of-breathing. License Terms: Edited & Adapted | Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.