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Ventilation Mechanics
& Lung Volumes

Explore how pressure gradients drive breathing, how the diaphragm and intercostals generate airflow, and how we measure the volumes and capacities of the respiratory system.

Anatomy & Physiology

What Is Pulmonary Ventilation?

Pulmonary ventilation (breathing) is the mechanical process of moving air into and out of the lungs. It consists of two phases: inspiration (inhalation) and expiration (exhalation). Unlike gas exchange at the alveolar membrane, ventilation is purely about bulk airflow driven by pressure differences.

The Muscles of Breathing

Diaphragm
The dome-shaped primary muscle of inspiration. When it contracts, it flattens downward, increasing thoracic volume.
External Intercostals
Elevate the ribs upward and outward during inspiration, further expanding the thoracic cavity.
Internal Intercostals
Depress the ribs during forced expiration. Quiet expiration is mostly passive (elastic recoil).

Pressure Relationships in the Thorax

Atmospheric (\( P_{\text{atm}} \))
Pressure of the external environment: \( 760 \text{ mmHg} \) at sea level. The reference point for all other pressures.
Intrapulmonary (\( P_{\text{alv}} \))
Pressure inside the alveoli. Fluctuates above and below \( P_{\text{atm}} \) during the breathing cycle.
Intrapleural (\( P_{\text{ip}} \))
Pressure in the pleural cavity. Always subatmospheric (about \( -4 \text{ cmH}_2\text{O} \) at rest), preventing lung collapse.

Boyle's Law: The Engine of Breathing

Small V High P Expand Large V Low P Volume increases → Pressure decreases

When the diaphragm contracts and the thoracic cavity expands, intrapulmonary pressure drops below atmospheric pressure, and air rushes in.

Mechanics of Inspiration

1
Neural signal: The medullary respiratory center sends impulses via the phrenic nerve (diaphragm) and intercostal nerves.
2
Muscle contraction: The diaphragm flattens; external intercostals elevate the ribs (bucket-handle and pump-handle motions).
3
Volume increase: Thoracic volume increases in all three dimensions (vertical, lateral, anteroposterior).
4
Pressure drop: By Boyle's Law, \( P_{\text{alv}} \) falls below \( P_{\text{atm}} \) (about \( -1 \text{ cmH}_2\text{O} \)).
5
Airflow in: Air moves down the pressure gradient into the lungs until \( P_{\text{alv}} = P_{\text{atm}} \).

Mechanics of Expiration

Quiet Expiration
Passive process. Inspiratory muscles relax, elastic recoil of lungs and chest wall compresses alveoli. \( P_{\text{alv}} \) rises above \( P_{\text{atm}} \) by about \( +1 \text{ cmH}_2\text{O} \), pushing air out.
Forced Expiration
Active process. Internal intercostals and abdominal muscles contract, forcefully compressing the thorax. Used during exercise, coughing, and sneezing.

Pressure Changes During One Breath

0 +1 -1 \( \text{Pressure (cmH}_2\text{O)} \) Time Inspiration Expiration \( P_{\text{atm}} \) \( P_{\text{alv}} \) drops \( P_{\text{alv}} \) rises

Measuring Lung Volumes

A spirometer measures the volumes of air moved during breathing. These measurements are divided into four primary volumes and four lung capacities (each capacity is the sum of two or more volumes).

\( \text{Volume (mL)} \) Time Inspiration Expiration TV IRV ERV RV

The Four Primary Lung Volumes

Tidal Volume (TV)
Air moved in one quiet breath. About \( 500 \text{ mL} \) in a healthy adult.
Inspiratory Reserve (IRV)
Extra air that can be inhaled beyond a normal TV. About \( 3,100 \text{ mL} \).
Expiratory Reserve (ERV)
Extra air that can be forcefully exhaled beyond a normal TV. About \( 1,200 \text{ mL} \).
Residual Volume (RV)
Air remaining in the lungs after maximum expiration. About \( 1,200 \text{ mL} \). Cannot be measured by spirometry.

Lung Capacities: Sums of Volumes

1
Inspiratory Capacity (IC) = TV + IRV = \( 500 + 3{,}100 = 3{,}600 \text{ mL} \)
2
Functional Residual Capacity (FRC) = ERV + RV = \( 1{,}200 + 1{,}200 = 2{,}400 \text{ mL} \)
3
Vital Capacity (VC) = TV + IRV + ERV = \( 500 + 3{,}100 + 1{,}200 = 4{,}800 \text{ mL} \)
4
Total Lung Capacity (TLC) = VC + RV = \( 4{,}800 + 1{,}200 = 6{,}000 \text{ mL} \)

Volumes vs. Capacities

Lung Volumes Lung Capacities
Definition Single, non-overlapping measurements Sum of two or more volumes
Count 4 (TV, IRV, ERV, RV) 4 (IC, FRC, VC, TLC)
Spirometry All except RV All except those including RV
Overlap Never overlap each other Can share component volumes

How to Calculate Lung Capacities

1
Identify what you need. Read the question: which capacity or volume is being asked for?
2
Recall the formula. Each capacity is a specific sum: IC = TV + IRV, FRC = ERV + RV, VC = TV + IRV + ERV, TLC = VC + RV.
3
Plug in values. Substitute the given volumes into the formula.
4
Check units. Lung volumes are in mL or L. Verify your answer is physiologically reasonable (TLC is approximately \( 6{,}000 \text{ mL} \) in a healthy adult male).

Example 1: Finding Vital Capacity

A patient has the following spirometry results: TV = \( 500 \text{ mL} \), IRV = \( 2,900 \text{ mL} \), ERV = \( 1,100 \text{ mL} \). Calculate the vital capacity.

Example 2: Finding Total Lung Capacity

Given: VC = \( 4,500 \text{ mL} \) and RV = \( 1,300 \text{ mL} \). What is the total lung capacity?

Example 3: Clinical Scenario

A patient with emphysema has increased RV (\( 2,500 \text{ mL} \)) because air gets trapped. Their VC is \( 3,200 \text{ mL} \) (reduced due to disease). What is their TLC?

Common Mistakes

✕ Confusing Volumes and Capacities
Saying "tidal capacity" or "vital volume." Remember: a volume stands alone; a capacity is always a sum of volumes.
✕ Thinking Expiration Is Always Active
Quiet expiration is passive (elastic recoil). Active muscular effort only occurs during forced expiration.
✕ Forgetting RV Is Unmeasurable by Spirometry
Any capacity that includes RV (FRC, TLC) also cannot be determined by spirometry alone. Special methods (plethysmography) are needed.

Summary of Volumes & Capacities

Measurement Formula Typical Value Spirometry?
TV Primary volume \( 500 \text{ mL} \) Yes
IRV Primary volume \( 3,100 \text{ mL} \) Yes
ERV Primary volume \( 1,200 \text{ mL} \) Yes
RV Primary volume \( 1,200 \text{ mL} \) No
IC TV + IRV \( 3,600 \text{ mL} \) Yes
FRC ERV + RV \( 2,400 \text{ mL} \) No
VC TV + IRV + ERV \( 4,800 \text{ mL} \) Yes
TLC VC + RV \( 6,000 \text{ mL} \) No

Question 1 of 5

During quiet expiration, which of the following best describes the role of the respiratory muscles?

Question 2 of 5

A patient has TV = \( 450 \text{ mL} \), IRV = \( 2,800 \text{ mL} \), and ERV = \( 1,000 \text{ mL} \). What is their vital capacity?

Question 3 of 5

Which lung measurement cannot be determined using a standard spirometer?

Question 4 of 5

According to Boyle's Law, what happens to intrapulmonary pressure when thoracic volume increases during inspiration?

Question 5 of 5

A patient with emphysema has an RV of \( 2,800 \text{ mL} \) (nearly double normal) and a VC of \( 3,000 \text{ mL} \). What does this tell us about their breathing?