2 - Lung Volumes & Capacity

Tidal volume (VT)

the amount of air inhaled or exhaled during normal, quiet breathing

• 500 mL

• Amount of air inhaled or exhaled during normal, quiet breathing

• Represents baseline ventilation at rest

Residual Volume (RV)

air that remains in the lungs after maximal expiration, cannot be exhaled

• 1200 mL

Role of Residual Volume

• Prevents lung collapse.

• Maintains continuous gas exchange between breaths.

• Stabilizes alveoli.

Vital Capacity (VC)

• Maximum amount of air that can be moved in a single breath.

• Reflects overall lung health and ventilatory ability.

• 4700 mL

• Vt + IRV + ERV

Total Lung Capacity (TLC)

• Total air the lungs can hold.

• Demonstrates the maximum structural and functional limit of the lungs.

• VC + RV

Functional Residual Capacity (FRC)

Volume remaining in lungs at the end of normal expiration.

• Represents the equilibrium point between lung recoil and chest wall expansion.

• 2400 mL

• Serves as an oxygen reservoir

• ERV + RV

Importance of Functional Residual Capacity

• Acts as a reservoir for oxygen.

• Allows continuous gas exchange between breaths.

• Prevents large fluctuations in blood oxygen levels.

Dead space

Dead space refers to air that does not participate in gas exchange.

• The lungs must move extra air to ensure enough reaches gas-exchange surfaces.

• It explains why moving air ≠ effective respiration

• lung function depends on matching ventilation with perfusion

Functional importance of dead space

• Dead space reduces the efficiency of breathing

• The respiratory system must increase ventilation to compensate for wasted air

• Dead space directly affects:

  • Alveolar ventilation

  • CO₂ elimination

  • Oxygen uptake

Anatomic Dead Space

• Volume of the conducting airways

• Approximately 150 mL

• Roughly 1/3 of tidal volume

• Contains no alveoli → no gas exchange

Physiologic Dead Space

• Total lung volume that does not participate in gas exchange

• Includes:

  • Anatomic dead space

  • Alveoli that are ventilated but not perfused

• Normally ≈ anatomic dead space

• Increases in pathology

Breathing Pattern Effect on Dead Space

• With rapid, shallow breathing, a larger fraction of each breath is dead space

• With slow, deep breathing, dead space becomes a smaller fraction of ventilation

Determinants of physiological dead space

tidal volume, [CO2] of arterial blood, [CO2] of expired air

What is the limitation of determining physiological dead space?

cannot measure alveolar [CO2], but it is exchanged with arterial blood [CO2]

What disease states would be reason for increased physiologic dead space?

pulmonary embolism, reduced pulmonary blood flow, lung regions that are ventilated but not perfused

Alveolar Ventilation (VA)

The volume of air that actually reaches alveoli per minute.

Alveolar Ventilation determines:

• Oxygen uptake

• Carbon dioxide elimination

Minute Ventilation

• Total air moved per minute depending on both tidal volume and respiratory rate

• Increases with:

  • Exercise

  • Increased metabolic demand

Conventional Spirometry

patient inhales deeply, then exhales as much air as possible into the spirometry tube. With the mouth still on the tube, patient inhales as deeply as possible. This is done under a closed circuit

• measures airflow and lung volumes

Incentive Spirometry

measures vital capacity only; patient exhales fully before test, then inhales forcefully into spirometry tube, taking in as much air as possible

Spirometry purpose

Assesses:

• Lung volumes

• Lung capacities

• Airflow limitation

Functional significance:

• Distinguishes normal lung function from obstructive or restrictive disease.

• Demonstrates that lung function is not just volume, but flow over time.

Forced Expiratory Volumes (FEV)

• Measure how quickly air can be expelled after a maximal inspiration

• evaluates airflow, airway resistance, and mechanical limitation to expiration

• FEV₁/FVC ratio reflects airway function.

Inspiratory Reserve Volume (IRV)

• 3000 mL

• Additional air that can be inhaled after a normal inspiration

• Demonstrates the lungs’ ability to increase ventilation when needed

Expiratory Reserve Volume (ERV)

• 1200 mL

• Additional air that can be exhaled after a normal expiration

• Reflects ability to actively force air out of the lungs

Inspiratory Capacity (IC)

• 3500 mL

• Maximum amount of air that can be inhaled after a normal expiration

Forced Vital Capacity (FVC)

total volume forcibly exhaled after a maximal inspiration

FEV₁

Volume of air exhaled in the first second of forced expiration

FEV₁/FVC Ratio

• Normal ≈ 0.8

• Interpretation:

  • < 0.8 → obstructive disease

  • > 0.8 → restrictive disease

Helium dilution method

• Subject breathes known concentration of helium to measure functional residual capacity

• Limitation:

  • Underestimates lung volumes in obstructive disease

Body Plethysmography

• Subject sits in airtight chamber

• Measures functional residual capacity and total lung gas volume

• More accurate in obstruction

Minute ventilation formula

minute ventilation = tidal volume * breaths/min

Why is rapid, shallow breathing inefficient?

disproportionately ventilates dead space resulting in poor CO2 elimination and reduced effective oxygen delivery

Alveolar ventilation formula

VA = VT - VD * breaths/min

Why is ventilation, while necessary, not sufficient for oxygenation?

it must match perfusion for improved gas exchange

Obstructive lung disease

• Airway narrowing or collapse

• Increased resistance to airflow

• results in a markedly reduced FEV1

Restrictive lung disease

• Lungs are stiff

• Total lung volumes are reduced, difficulty breathing in

• FVC is markedly reduced

A decrease in which lung parameter is most characteristic of restrictive lung disease?

vital capacity

A patient with pulmonary fibrosis shows reduced lung compliance. What parameter is most likely decreased?

total lung capacity

In emphysema, which lung volume is typically increased due to air trapping?

residual volume

A bronchodilator improves airflow in an asthmatic patient. which lung parameter is most directly affected?

airway resistance

Functional dead space measures the volume of ________ air not engaged in gas exchange.

alveolar

Physiologic dead space can be considered “total dead space” and measures __________ + ________ dead space.

anatomic, functional

What type of abnormal lung condition would increase physiologic dead space volume?

restrictive; pulmonary embolism