Figure Mentioned:

Fig. 38.5 – Basic spirometer used to measure pulmonary ventilation.
Fig. 38.6 – Spirogram showing changes in lung volume during different breathing conditions.
Table Mentioned: Table 38.1 – Average pulmonary volumes and capacities in healthy young adult men and women.

Recording Changes in Pulmonary Volume—Spirometry

Spirometry is a method used to study pulmonary ventilation.
It records the volume of air moving into and out of the lungs.

Basic Spirometer

Fig. 38.5 shows a basic spirometer.
It consists of:
- A drum inverted over a chamber of water.
- A counterbalancing weight attached to the drum.
The drum contains a breathing gas, usually:
- Air
- Oxygen
A tube connects the person’s mouth to the gas chamber.

Working of the Spirometer

When the person breathes into the chamber:
- The drum moves.
When the person breathes out of the chamber:
- The drum moves in the opposite direction.
These movements are recorded to measure changes in lung volume.

Spirogram

Fig. 38.6 shows a spirogram.
A spirogram records changes in lung volume during different types of breathing.

Pulmonary Volumes and Capacities

For easier understanding, the air in the lungs is divided into:
- Four pulmonary volumes
- Four pulmonary capacities
The values shown represent the average for a young adult man.

Average Values

Table 38.1 summarizes the average pulmonary volumes and capacities for:
- Healthy young adult men
- Healthy young adult women

Key Concept

Spirometry measures pulmonary ventilation by recording the volume of air moving into and out of the lungs. A basic spirometer (Fig. 38.5) produces a spirogram (Fig. 38.6), which divides lung air into four pulmonary volumes and four pulmonary capacities. Table 38.1 summarizes their average values in healthy adults.

PULMONARY VOLUMES AND CAPACITIES

Figure Mentioned:

Fig. 38.5 – Basic spirometer used to measure pulmonary ventilation.
Fig. 38.6 – Spirogram showing changes in lung volume during different breathing conditions.
Table Mentioned: Table 38.1 – Average pulmonary volumes and capacities in healthy young adult men and women.

Recording Changes in Pulmonary Volume—Spirometry

Spirometry is a method used to study pulmonary ventilation.
It records the volume of air moving into and out of the lungs.

Basic Spirometer

Fig. 38.5 shows a basic spirometer.
It consists of:
- A drum inverted over a chamber of water.
- A counterbalancing weight attached to the drum.
The drum contains a breathing gas, usually:
- Air
- Oxygen
A tube connects the person’s mouth to the gas chamber.

Working of the Spirometer

When the person breathes into the chamber:
- The drum moves.
When the person breathes out of the chamber:
- The drum moves in the opposite direction.
These movements are recorded to measure changes in lung volume.

Spirogram

Fig. 38.6 shows a spirogram.
A spirogram records changes in lung volume during different types of breathing.

Pulmonary Volumes and Capacities

For easier understanding, the air in the lungs is divided into:
- Four pulmonary volumes
- Four pulmonary capacities
The values shown represent the average for a young adult man.

Average Values

Table 38.1 summarizes the average pulmonary volumes and capacities for:
- Healthy young adult men
- Healthy young adult women

Key Concept

Spirometry measures pulmonary ventilation by recording the volume of air moving into and out of the lungs. A basic spirometer (Fig. 38.5) produces a spirogram (Fig. 38.6), which divides lung air into four pulmonary volumes and four pulmonary capacities. Table 38.1 summarizes their average values in healthy adults.

Figure 38.6: Respiratory Excursions During Normal Breathing and During Maximal Inspiration and Expiration

STEP 1: What does the graph represent?

The graph records the amount of air inside the lungs at every second.

The red line is the lung volume.

Red line goes upward → Inspiration (air enters lungs)
Red line goes downward → Expiration (air leaves lungs)

Think of the red line like the water level inside a bottle.

More air enters → level rises.
Air leaves → level falls.

STEP 2: Understanding the Axes

Y-axis = Lung Volume (mL)

6000 mL
↑
│  More air inside lungs
│
│
1000 mL
↓
Less air inside lungs

Higher position = More air in lungs

Lower position = Less air in lungsX-axis = Time

Time simply shows breathing occurring continuously.

Time →

Breath 1

Breath 2

Breath 3

Deep Breath

Forceful Expiration

STEP 3: The First Small Waves (Normal Breathing)

Look carefully.

      /\      /\      /\      /\
_____/  \____/  \____/  \____/  \____

Each wave represents one normal breath.

Every wave has two parts.ard Part

↑

This is

Inspiration

Air enters lungs.

Lung volume increases.

Chest expands.

Diaphragm contracts.ownward Part

↓

This is

Expiration

Air leaves lungs.

Lung volume decreases.

Chest relaxes.

Diaphragm relaxes.

STEP 4: Why do the Small Waves Stay Between Two Dashed Lines?

Notice these dashed lines.

Upper dashed line
-----------------

Small breathing

-----------------
Lower dashed line

The lungs are only using about 500 mL of air during quiet breathing.

This small amount is called

Tidal Volume (TV)

Tidal Volume

Definition

Amount of air inspired or expired during one normal quiet breath.

Normal Value

≈500 mL

Easy Example

Imagine drinking water.

One normal sip

↓

One normal swallow

↓

That small amount

↓

Tidal Volume

STEP 5: What Happens Suddenly?

Look here.

        /\
       /  \
______/    \______

Suddenly the red line shoots upward.

Why?

Because the person takes

Maximum Inspiration

The lungs are filled almost completely.

Which volume is used?

After normal inspiration,

there is still a huge amount of air that can be inhaled.

That extra air is

Inspiratory Reserve Volume (IRV)

Definition

Extra air inspired after normal inspiration.

Normal Value

≈3000 mL

Easy Concept

Doctor says

“Take the deepest breath possible.”

Everything above the normal breath

↓

Inspiratory Reserve Volume

STEP 6: Look at the Highest Point

          ▲
          │

This is

Maximum amount of air inside lungs.

The lungs cannot hold any more air.

This is

Total Lung Capacity

Normal Value

≈5800–6000 mL

Formula

TLC

=

IRV + TV + ERV + RV

STEP 7: What Happens Next?

Immediately after maximum inspiration,

the red line falls sharply.

      /\
     /
____/

This means

Maximum Expiration

The person blows out as much air as possible.

STEP 8: Why Doesn’t the Red Line Reach Zero?

This is the MOST IMPORTANT CONCEPT.

Even after blowing out as hard as possible,

the lungs still contain air.

Why?

Because alveoli should never collapse.

Therefore

some air always remains.

This remaining air is

Residual Volume (RV)

Residual Volume

Definition

Air remaining in lungs after maximum forced expiration.

Normal Value

≈1200 mL

Why is RV important?

Without RV

↓

Alveoli collapse

↓

Next breath becomes extremely difficult

↓

Gas exchange stops temporarily

Think of a balloon.

Even after squeezing it,

a little air always remains.

That is Residual Volume.

STEP 9: Expiratory Reserve Volume (ERV)

Look here.

Normal expiration

↓

Still more air can come out

↓↓↓↓↓↓

That extra air is

Expiratory Reserve Volume

Normal Value

≈1100 mL

Definition

Extra air expired after normal expiration.

STEP 10: Functional Residual Capacity (FRC)

After a normal expiration,

lungs still contain air.

What remains?

Residual Volume

+

Expiratory Reserve Volume

This is called

Functional Residual Capacity

Formula

FRC

=

ERV + RV

Normal Value

≈2300 mL

Why important?

Between every two breaths,

oxygen continues moving into blood because lungs still contain air.

Without FRC,

oxygen delivery would stop between breaths.

STEP 11: Inspiratory Capacity (IC)

Look at this arrow.

Starts

End of normal expiration

Ends

Maximum inspiration

Everything you can inhale after a normal expiration

Inspiratory Capacity

Formula

IC

=

TV + IRV

Normal Value

≈3500 mLSTEP 12: Vital Capacity (VC)

Look at this long arrow.

Starts

Maximum inspiration

Ends

Maximum expiration

Everything that can actually move in and out of lungs

Vital Capacity

Formula

VC

=

IRV

+

TV

+

ERV

Normal Value

≈4600 mL

Easy Concept

Take the deepest breath possible.

Now blow out everything possible.

Total movable air

↓

Vital Capacity

STEP 13: Understanding Every Arrow

Arrow 1

Inspiratory Reserve Volume

Extra air inhaled after a normal inspiration.

Arrow 2

Tidal Volume

Normal breathing.

Arrow 3

Expiratory Reserve Volume

Extra air exhaled after normal expiration.

Arrow 4

Residual Volume

Air that never leaves lungs.

Arrow 5

Inspiratory Capacity

Normal expiration → Maximum inspiration

Arrow 6

Vital Capacity

Maximum inspiration → Maximum expiration

Arrow 7

Functional Residual Capacity

Air remaining after normal expiration.

Arrow 8

Total Lung Capacity

Maximum amount lungs can contain.

STEP 14: Complete Story of the Red Line

Small waves
↓

Normal breathing

↓

Deep inspiration

↓

Maximum lung filling

↓

Forceful expiration

↓

Only residual air remains

↓

Normal breathing starts again

STEP 15: Relationships (Very High-Yield)

Capacity	Formula	Normal Value
Tidal Volume (TV)	Normal breath	500 mL
Inspiratory Reserve Volume (IRV)	Extra inspired	3000 mL
Expiratory Reserve Volume (ERV)	Extra expired	1100 mL
Residual Volume (RV)	Air left after maximal expiration	1200 mL
Inspiratory Capacity (IC)	TV + IRV	3500 mL
Functional Residual Capacity (FRC)	ERV + RV	2300 mL
Vital Capacity (VC)	IRV + TV + ERV	4600 mL
Total Lung Capacity (TLC)	VC + RV	5800–6000 mL

STEP 16: Clinical Correlation (MBBS Viva)

Which lung volumes cannot be measured by spirometry?

Because spirometry measures only the air that moves in and out of the lungs, it cannot measure Residual Volume (RV). Therefore, any capacity that includes RV also cannot be measured directly.

❌ Residual Volume (RV)
❌ Functional Residual Capacity (FRC = ERV + RV)
❌ Total Lung Capacity (TLC = VC + RV)

Only volumes and capacities that do not include RV (such as TV, IRV, ERV, IC, and VC) can be measured by simple spirometry.

🩺 MBBS Memory Trick

Imagine your lungs as a water tank:

🟢 Tidal Volume (TV): The small amount of water that moves in and out with each normal pump cycle.
🔵 Inspiratory Reserve Volume (IRV): Extra water you can add when you open the valve fully.
🟠 Expiratory Reserve Volume (ERV): Extra water you can drain after normal emptying.
🔴 Residual Volume (RV): Water that always remains at the bottom of the tank—it cannot be drained.
🟣 Vital Capacity (VC): All the water you can move in and out.
⚫ Total Lung Capacity (TLC): Every drop the tank can hold, including the water that always remains.

Golden Rule:
The red line never reaches zero because healthy lungs are never completely empty. Residual volume keeps the alveoli open, prevents lung collapse, and ensures continuous gas exchange even between breaths.

Pulmonary Volumes

Figure: Fig. 38.6

Fig. 38.6 shows four pulmonary (lung) volumes.
When these four lung volumes are added together, they give the maximum volume to which the lungs can expand.
The lung volumes shown are for average adult men.
Lung volumes can vary greatly depending on:
- Physical fitness
- Age
- Height
- Sex
- Altitude where a person lives
The importance of each lung volume is explained below.

Tidal Volume

Tidal volume is the amount of air breathed in or out during one normal breath.
Normal tidal volume is about 500 mL.

Inspiratory Reserve Volume

Inspiratory reserve volume is the extra amount of air that can be inhaled after a normal inspiration.
This happens when a person breathes in with maximum effort.
Normal inspiratory reserve volume is about 3000 mL.

Expiratory Reserve Volume

Expiratory reserve volume is the maximum extra amount of air that can be exhaled after a normal expiration.
This requires forceful expiration.
Normal expiratory reserve volume is about 1100 mL.

Residual Volume

Residual volume is the amount of air that remains in the lungs after the strongest possible expiration.
Normal residual volume is about 1200 mL.

Key Concept

The four pulmonary volumes are tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume.
Together, these four volumes equal the maximum volume to which the lungs can expand.
Normal values: Tidal volume = 500 mL, Inspiratory reserve volume = 3000 mL, Expiratory reserve volume = 1100 mL, Residual volume = 1200 mL.

Pulmonary Capacities

Figure: Fig. 38.6

Sometimes, two or more lung volumes are combined to describe events during the pulmonary (breathing) cycle.
These combinations are called pulmonary capacities.
Fig. 38.6 shows the important pulmonary capacities.

Inspiratory Capacity

Inspiratory capacity is tidal volume + inspiratory reserve volume.
It is the amount of air a person can inhale after a normal expiration.
The lungs expand to their maximum size during this inspiration.
Normal inspiratory capacity is about 3500 mL.

Functional Residual Capacity

Functional residual capacity is expiratory reserve volume + residual volume.
It is the amount of air left in the lungs after a normal expiration.
Normal functional residual capacity is about 2300 mL.

Vital Capacity

Vital capacity is inspiratory reserve volume + tidal volume + expiratory reserve volume.
It is the maximum amount of air a person can breathe out after first filling the lungs as much as possible.
The air is then expired with maximum effort.
Normal vital capacity is about 4600 mL.

Total Lung Capacity

Total lung capacity is the maximum amount of air the lungs can hold after the greatest possible inspiration.
It is equal to vital capacity + residual volume.
Normal total lung capacity is about 5800 mL.

Pulmonary Capacities in Different People

Most pulmonary volumes and capacities are about 20% to 30% lower in women than in men.
Pulmonary volumes and capacities are greater in large and athletic people.
They are smaller in small and asthenic (thin) people.

Key Concept

Pulmonary capacities are formed by combining two or more pulmonary volumes.
Inspiratory Capacity (IC) = Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) = ~3500 mL
Functional Residual Capacity (FRC) = Expiratory Reserve Volume (ERV) + Residual Volume (RV) = ~2300 mL
Vital Capacity (VC) = Inspiratory Reserve Volume (IRV) + Tidal Volume (TV) + Expiratory Reserve Volume (ERV) = ~4600 mL
Total Lung Capacity (TLC) = Vital Capacity (VC) + Residual Volume (RV) = ~5800 mL
Women usually have 20%–30% lower pulmonary volumes and capacities than men.
Large, athletic people have greater lung capacities than small, asthenic people.

Abbreviations and Symbols Used in Pulmonary Function Studies

Figure: Fig. 38.6

Spirometry is one of the many methods used to measure pulmonary (lung) function.
Pulmonary physicians use these measurement methods every day.
Many of these methods depend on mathematical calculations.
Standard abbreviations and symbols are used to make calculations and presentation of pulmonary function data easier.
The important abbreviations and symbols are listed in Table 38.2.
The following equations show the relationships between pulmonary volumes and capacities.
Review Fig. 38.6 to understand and verify these relationships.

Equations

VC = IRV + VT + ERV
VC = IC + ERV
TLC = VC + RV
TLC = IC + FRC
FRC = ERV + RV

Key Concept

Standard abbreviations and symbols simplify pulmonary function calculations.
Important relationships are:
- VC = IRV + VT + ERV
- VC = IC + ERV
- TLC = VC + RV
- TLC = IC + FRC
- FRC = ERV + RV

etermination of Functional Residual Capacity, Residual Volume, and Total Lung Capacity—Helium Dilution Method

Functional Residual Capacity (FRC) is the amount of air left in the lungs after a normal expiration.
FRC is important for normal lung function.
FRC changes significantly in some lung diseases.
Therefore, measuring FRC is often necessary.

Why Spirometry Cannot Measure FRC Directly

A spirometer cannot measure FRC directly.
This is because the air in the residual volume (RV) cannot be exhaled into the spirometer.
Residual volume makes up about half of the FRC.
Therefore, FRC must be measured indirectly.
The helium dilution method is commonly used.

Helium Dilution Method

A spirometer with a known volume is filled with air mixed with helium of a known concentration.
Before breathing from the spirometer, the person expires normally.
At the end of normal expiration, the air remaining in the lungs equals the FRC.
The person then immediately starts breathing from the spirometer.
The gases in the spirometer mix with the gases in the lungs.
As the gases mix, helium becomes diluted by the FRC air.
The amount of helium dilution is used to calculate the FRC.

Formula for Functional Residual Capacity (FRC)

FRC = [(CiHe / CfHe) − 1] × ViSpir

Where:

FRC = Functional residual capacity
CiHe = Initial helium concentration in the spirometer
CfHe = Final helium concentration in the spirometer
ViSpir = Initial volume of the spirometer

Determination of Residual Volume (RV)

After calculating FRC, the Residual Volume (RV) can be determined.
Subtract the Expiratory Reserve Volume (ERV) from the FRC.

RV = FRC − ERV

Determination of Total Lung Capacity (TLC)

After calculating FRC, the Total Lung Capacity (TLC) can be determined.
Add the Inspiratory Capacity (IC) to the FRC.

TLC = FRC + IC

Abbreviations and Symbols for Pulmonary Function

Abbreviation	Function
VT	Tidal volume
FRC	Functional residual capacity
ERV	Expiratory reserve volume
RV	Residual volume
IC	Inspiratory capacity
IRV	Inspiratory reserve volume
TLC	Total lung capacity
VC	Vital capacity
Raw	Resistance of the airways to airflow
C	Compliance
VD	Volume of dead space gas
VA	Volume of alveolar gas
VI	Inspired ventilation per minute
VE	Expired ventilation per minute
VS	Shunt flow
V̇A	Alveolar ventilation per minute
V̇O₂	Oxygen uptake per minute
V̇CO₂	Carbon dioxide eliminated per minute
V̇CO	Carbon monoxide uptake per minute
DLO₂	Diffusing capacity of the lungs for oxygen
DLCO	Diffusing capacity of the lungs for carbon monoxide
PB	Atmospheric pressure
Palv	Alveolar pressure
Ppl	Pleural pressure
PO₂	Partial pressure of oxygen
PCO₂	Partial pressure of carbon dioxide
PN₂	Partial pressure of nitrogen
PaO₂	Partial pressure of oxygen in arterial blood
PaCO₂	Partial pressure of carbon dioxide in arterial blood
PAO₂	Partial pressure of oxygen in alveolar gas
PACO₂	Partial pressure of carbon dioxide in alveolar gas
PAH₂O	Partial pressure of water in alveolar gas
R	Respiratory exchange ratio
Q	Cardiac output
CaO₂	Oxygen concentration in arterial blood
CvO₂	Oxygen concentration in mixed venous blood
SO₂	Percentage saturation of hemoglobin with oxygen
SaO₂	Percentage saturation of hemoglobin with oxygen in arterial blood

Key Concept

FRC is the air remaining in the lungs after normal expiration.
Spirometry cannot measure FRC directly because residual volume cannot be exhaled.
Helium dilution is an indirect method used to measure FRC.
FRC = [(CiHe / CfHe) − 1] × ViSpir
RV = FRC − ERV
TLC = FRC + IC
Standard abbreviations and symbols make pulmonary function measurements and calculations easier.

Helium Dilution Formula

$\boxed{\text{FRC}=\left(\frac{Ci_{He}}{Cf_{He}}-1\right)\times Vi_{Sp}}$ FRC=(CfHeCiHe−1)×ViSp

Step 1: Before the Person Breathes

The spirometer contains:

Known volume = ViSp
Known helium concentration = CiHe

Example:

Spirometer volume = 3 L
Helium concentration = 10%

So,

Spirometer	Value
Volume	3 L
Helium	10%

Step 2: After the Person Breathes

The helium mixes with the air already inside the lungs (FRC).

Because helium spreads into a larger space, its concentration becomes lower.

Example:

Before	After
10%	5%

Notice:

Initial concentration = 10%
Final concentration = 5%

Helium became half as concentrated.

Step 3: Find How Many Times the Space Increased

Divide the initial concentration by the final concentration. $\frac{Ci_{He}}{Cf_{He}} = \frac{10}{5} = 2$ CfHeCiHe=510=2

Meaning:

The helium is now spread through a space that is 2 times larger than the spirometer alone.

Step 4: Why Do We Subtract 1?

The answer 2 includes:

1 spirometer volume
1 lung volume

We only want the lung volume (FRC).

So, $2-1=1$ 2−1=1

Now only the extra space (lungs) remains.

Step 5: Multiply by Spirometer Volume

Multiply by the known spirometer volume. $1\times3L=3L$ 1×3L=3L

Therefore, $\boxed{FRC=3L}$ FRC=3L

Another Example

Suppose

CiHe = 12%
CfHe = 4%
ViSp = 2 L

Step 1

$\frac{12}{4}=3$ 412=3

Meaning:

The helium spread into a space 3 times larger.

Step 2

Subtract 1. $3-1=2$ 3−1=2

Only the lung part remains.

Step 3

Multiply by spirometer volume. $2\times2L=4L$ 2×2L=4L

Answer

$\boxed{FRC=4L}$ FRC=4L

Why Does This Formula Work?

Imagine you pour 1 spoon of blue ink into 1 glass of water.

The color is dark.

Now pour the same ink into 2 glasses of water.

The color becomes lighter because the ink is diluted.

Helium behaves in the same way:

Small lung volume → Less dilution → Higher final helium concentration (CfHe).
Large lung volume → More dilution → Lower final helium concentration (CfHe).

So, by measuring how much helium is diluted, we can calculate how much air was already present in the lungs (FRC).

Easy Memory Formula

Known helium in spirometer
            ↓
Breath into spirometer
            ↓
Helium mixes with lung air
            ↓
Helium concentration falls
            ↓
Measure the fall
            ↓
Use the formula
            ↓
Calculate FRC

Key Concept

CiHe ÷ CfHe tells you how many times the helium-containing space increased after mixing.
Subtract 1 to remove the original spirometer volume.
Multiply by ViSp to convert that ratio into the actual lung volume (FRC).
Greater helium dilution (smaller CfHe) means a larger FRC.

Minute Respiratory Volume Equals Respiratory Rate Times Tidal Volume

Minute respiratory volume is the total amount of new air that enters the respiratory passages in one minute.
It is calculated by multiplying tidal volume by the respiratory rate per minute.

Formula

Minute Respiratory Volume = Tidal Volume × Respiratory RateWhat Each Symbol Means

Minute Respiratory Volume (MRV) = Total air entering the lungs in 1 minute
Tidal Volume (TV) = Air inhaled or exhaled in one normal breath
Respiratory Rate (RR) = Number of breaths taken in one minute

Easy Concept

Think of breathing like filling bottles with water.

Tidal Volume (TV) = Water in one bottle
Respiratory Rate (RR) = Number of bottles filled each minute
Minute Respiratory Volume (MRV) = Total water filled in one minute

So,

Air in one breath × Number of breaths = Total air breathed in one minute

Step-by-Step Calculation (Normal Person)

Given:

Tidal Volume = 500 mL
Respiratory Rate = 12 breaths/min

Step 1: Write the formula

MRV = TV × RR

Step 2: Substitute the values

MRV = 500 mL × 12 breaths/min

Step 3: Multiply

MRV = 6000 mL/min

Step 4: Convert to liters

6000 mL = 6 L

Final Answer

Minute Respiratory Volume = 6 L/min

Very Low Minute Respiratory Volume

A person can survive for a short time with:
- Minute Respiratory Volume = 1.5 L/min
- Respiratory Rate = 2–4 breaths/min

Maximum Minute Respiratory Volume

Respiratory rate can increase to 40–50 breaths/min.
Tidal volume can increase up to the Vital Capacity (4600 mL) in a young man.
This can increase the minute respiratory volume to more than 200 L/min.

Step-by-Step Understanding of Maximum Value

Given:

Tidal Volume ≈ 4600 mL
Respiratory Rate ≈ 50 breaths/min

Formula

MRV = TV × RR

Substitute

MRV = 4600 × 50

Multiply

MRV = 230,000 mL/min

Convert to liters

230,000 mL = 230 L/min

Final Answer

Maximum Minute Respiratory Volume ≈ 230 L/min

This is greater than 200 L/min, as mentioned in the text.

Comparison

Condition	Tidal Volume	Respiratory Rate	Minute Respiratory Volume
Normal	500 mL	12 breaths/min	6 L/min
Very Low	—	2–4 breaths/min	1.5 L/min
Maximum	4600 mL	50 breaths/min	230 L/min (>200 L/min)

Key Concept

Minute Respiratory Volume (MRV) = Tidal Volume (TV) × Respiratory Rate (RR)
Normal: 500 mL × 12 = 6000 mL = 6 L/min
Higher tidal volume or higher respiratory rate increases the minute respiratory volume.
Maximum breathing can produce more than 200 L/min, but most people can sustain only about half to two-thirds of this level for longer than 1 minute.

ALVEOLAR VENTILATION

The main purpose of pulmonary ventilation is to continuously replace the air in the gas exchange areas of the lungs.
In these areas, air is very close to the pulmonary blood, allowing gas exchange.
The gas exchange areas include:
- Alveoli
- Alveolar sacs
- Alveolar ducts
- Respiratory bronchioles
The rate at which fresh air reaches these gas exchange areas is called the alveolar ventilation rate.

Dead Space and Its Effect on Alveolar Ventilation

Some of the air we breathe does not reach the gas exchange areas.
Instead, it remains in the conducting airways, where gas exchange does not occur.
These airways include:
- Nose
- Pharynx
- Trachea
This air is called dead space air.
Dead space air does not take part in gas exchange, so it is not useful for exchanging oxygen and carbon dioxide.
During expiration, dead space air leaves the lungs first.
Air from the alveoli is exhaled only after the dead space air has been expelled.
Therefore, dead space reduces the efficiency of removing gases from the lungs during expiration.

Key Concept

Pulmonary ventilation keeps the gas exchange areas supplied with fresh air.
Alveolar ventilation rate is the amount of fresh air reaching the alveoli, alveolar sacs, alveolar ducts, and respiratory bronchioles.
Dead space air remains in the nose, pharynx, and trachea and does not participate in gas exchange.
During expiration, dead space air is exhaled first, followed by alveolar air.
Dead space decreases the efficiency of gas exchange by delaying the removal of alveolar air.

Measurement of Dead Space Volume

Figure: Fig. 38.7

Dead space volume can be measured using a simple oxygen–nitrogen method.
The method is illustrated in Fig. 38.7.

Procedure for Measuring Dead Space Volume

The person takes a deep breath of 100% oxygen (O₂).
This fills the entire dead space with pure oxygen.
Some oxygen also mixes with the alveolar air, but it does not completely replace it.
The person then breathes out through a nitrogen meter.
The nitrogen meter continuously records the nitrogen concentration during expiration.

What Happens During Expiration?

Formula for Dead Space Volume

Formula $\boxed{\text{VD}=\frac{\text{Gray Area}}{\text{Gray Area + Pink Area}}\times \text{VE}}$ VD=Gray Area + Pink AreaGray Area×VE

What Each Symbol Means

Easy Concept

Think of the expired air as two parts:

Gray Area = Dead space air (pure oxygen, no nitrogen)
Pink Area = Alveolar air (contains nitrogen)

The formula calculates:

Dead Space = (Dead Space Portion ÷ Total Expired Air Portion) × Total Expired Volume

Step-by-Step Calculation

Given

Gray Area = 30 cm²
Pink Area = 70 cm²
Total Expired Volume (VE) = 500 mL

Step 1: Add the Gray and Pink Areas

$30+70=100$ 30+70=100

Total graph area = 100 cm²

Step 2: Find the Fraction Representing Dead Space

$\frac{30}{100}=0.3$ 10030=0.3

This means 30% of the expired air is dead space air.

Step 3: Multiply by the Total Expired Volume

$0.3\times500=150\text{ mL}$ 0.3×500=150 mL

Final Answer

$\boxed{\text{VD}=150\text{ mL}}$ VD=150 mL

Normal Dead Space Volume

In a healthy young man, the normal dead space volume is about 150 mL.
Dead space volume increases with:
- Body size
- Height
- Lung size
Women usually have a smaller dead space volume because their airways are generally smaller.
Dead space volume increases slightly after 50–60 years of age.

Anatomical Dead Space

Anatomical dead space is the air present in the conducting airways.
It includes all respiratory passages except the alveoli and other gas exchange areas.
This air does not participate in gas exchange.

Physiological Dead Space

Sometimes, some alveoli do not function properly because blood flow through nearby pulmonary capillaries is absent or reduced.
These nonfunctional or partially functional alveoli also act as dead space.
When alveolar dead space is added to anatomical dead space, it is called physiological dead space.

Anatomical vs Physiological Dead Space

Anatomical Dead Space	Physiological Dead Space
Includes only the conducting airways	Includes conducting airways plus nonfunctional alveoli
Measured by the oxygen–nitrogen method	Includes anatomical dead space and alveolar dead space
Nearly equal to physiological dead space in healthy lungs	Can become much larger in lung disease

Physiological Dead Space in Health and Disease

In healthy lungs, anatomical dead space ≈ physiological dead space because all alveoli are functional.
In lung disease, many alveoli may become partially functional or nonfunctional.
As a result, physiological dead space can become up to 10 times greater than anatomical dead space.
It may increase to 1–2 liters.

Key Concept

Dead space volume is measured using the oxygen–nitrogen method shown in Fig. 38.7.
During expiration:
- Dead space air (pure O₂) comes out first.
- Alveolar air (contains nitrogen) comes out later.
Dead Space Formula: VD = (Gray Area ÷ (Gray Area + Pink Area)) × VE
Example:
- Gray Area = 30
- Pink Area = 70
- VE = 500 mL
- VD = (30 ÷ 100) × 500 = 150 mL
Normal dead space volume = 150 mL in a healthy young man.
Anatomical dead space includes only the conducting airways.
Physiological dead space includes the conducting airways plus nonfunctional alveoli.
In healthy lungs, anatomical and physiological dead spaces are nearly equal.
In lung disease, physiological dead space may increase to 1–2 liters.

This figure (Guyton Physiology Fig. 38.7) is one of the most important graphs for understanding Anatomical Dead Space. Most students memorize it without understanding why the curve looks this way. Once you understand where each milliliter (mL) of expired air comes from, the graph becomes very easy. Concept of this the Graph

Imagine you take one deep breath of 100% pure oxygen.

Step 1

Normally the lungs contain:

Oxygen (O₂)
Nitrogen (N₂)
Carbon dioxide (CO₂)

But after inhaling pure oxygen, the air inside your conducting airways (dead space) becomes almost 100% oxygen.

That means:

Dead space air contains almost NO nitrogen.

However,

The alveoli still contain nitrogen because it cannot disappear immediately.

So after one inspiration of pure oxygen we have:

Part of respiratory tract	Nitrogen present?
Nose, trachea, bronchi (Dead space)	Almost 0%
Alveoli	About 60%

This difference creates the graph.

Understanding the Axes

Y-axis (Vertical)

Percent Nitrogen (%)

This tells us

How much nitrogen is present in the expired air.

Higher point = more nitrogen.

Lower point = less nitrogen.

X-axis (Horizontal)

Air expired (mL)

This tells us

How much air has come out during expiration.

Example

0 mL → expiration just started

100 mL → first 100 mL expired

200 mL → next air expired

500 mL → almost the whole tidal volume expired

Now Understand the Graph Step by Step

① Left Side (Before Expiration)

The dotted vertical line at 0 mL

This marks

The exact moment expiration begins.

Nothing has come out yet.

② Dashed Vertical Line

This simply marks the starting point of expiration.

Everything to the left represents the previous inspiration.

③ Arrow

“Inspiration of pure oxygen”

⬅➡

This reminds you that

Just before expiration,

the person inhaled 100% oxygen.

This is the reason nitrogen becomes temporarily absent from the conducting airways.

④ Red Curve Starts Near Zero Nitrogen

At about 0–100 mL expired

Notice the curve stays almost at 0% nitrogen.

Why?

Because

The first expired air comes only from

Nose
Pharynx
Larynx
Trachea
Bronchi

These are conducting airways.

They contain the oxygen that was inhaled.

Since this air is almost pure oxygen,

Nitrogen ≈ 0%

Therefore

The graph begins at zero.

Clinical Meaning

This first expired air never reached alveoli.

It did not participate in gas exchange.

This volume is called

Anatomical Dead Space

⑤ Why Does the Curve Suddenly Rise?

Around 100–200 mL

The curve rises very rapidly.

This is the most important part.

Now alveolar air begins mixing with dead-space air.

Remember

Alveolar air still contains nitrogen.

So now expiration contains

Dead-space oxygen
Alveolar air with nitrogen

Because nitrogen-rich alveolar air is entering the expired air,

Nitrogen concentration rises rapidly.

Think of mixing:

Glass A = Pure water

Glass B = Colored juice

At first you pour only water.

Then you start pouring juice.

Color suddenly increases.

Exactly the same happens here.

⑥ Why Is the Curve S-Shaped?

The increase is not perfectly vertical because

Initially,

only a small amount of alveolar air mixes with dead-space air.

Then

More alveolar air enters.

Finally

Almost all expired air becomes alveolar air.

So nitrogen increases gradually,

then rapidly,

then slowly.

That produces the classic S-shaped curve.

⑦ Plateau (Flat Part)

After about 200–250 mL

The graph becomes almost horizontal.

Why?

Now almost every breath coming out is

Alveolar air

Since alveolar nitrogen concentration remains nearly constant,

Nitrogen no longer increases.

So the graph becomes flat.

This flat region is called the

Alveolar Plateau

Why Is the Plateau Around 60%?

Normal alveolar air contains approximately

Oxygen ≈ 104 mmHg
Carbon dioxide ≈ 40 mmHg
Water vapor
Nitrogen ≈ 569 mmHg

Nitrogen therefore forms roughly

60–75% of alveolar gas.

So after dead-space air has been washed out,

expired air reaches about 60% nitrogen.

Understanding the Shaded Areas

There are two shaded regions.

Gray Shaded Area

This is the upper-left shaded region.

It represents

The nitrogen that could have been present if alveolar air had been expired immediately.

But during early expiration,

dead-space air contains almost no nitrogen.

So this nitrogen is “missing.”

Clinical Meaning

The gray area represents

Nitrogen absent because dead-space air comes out first.

This area is used in the Fowler method to calculate anatomical dead space.

Pink Shaded Area

This is the area under the red curve.

It represents

The actual nitrogen measured in expired air.

Early expiration

Very little nitrogen

Later expiration

Much more nitrogen

Why Are Both Shaded Areas Important?

Imagine

If there were no dead space

The expired air would immediately contain 60% nitrogen.

The graph would look like this:

60%
│──────────────
│
│
│
└──────────────

But because dead space exists,

nitrogen appears late.

So the curve shifts to the right.

The missing nitrogen (gray area) exactly balances the extra nitrogen appearing later (pink area).

This equality is used to determine anatomical dead space.

What Does the Red Curve Mean?

The red curve shows

The nitrogen concentration in expired air at every moment during expiration.

Early expiration

↓

Air comes from dead space

↓

Nitrogen ≈ 0%

↓

Middle expiration

↓

Mixed dead-space + alveolar air

↓

Nitrogen rises rapidly

↓

Late expiration

↓

Pure alveolar air

↓

Nitrogen reaches a constant plateau

Easy Story to Remember

Imagine a long pipe connected to a balloon.

Pipe = Conducting airways (dead space)
Balloon = Alveoli

You fill the pipe with pure oxygen.

The balloon still contains nitrogen-rich air.

Now squeeze the balloon.

First

Only oxygen from the pipe comes out.

➡ Nitrogen = 0%

Oxygen from the pipe mixes with air from the balloon.

➡ Nitrogen begins to rise.

Finally

Only balloon air comes out.

➡ Nitrogen becomes constant (~60%).

That is exactly what the graph shows.

High-Yield MBBS Points

Y-axis: Percentage of nitrogen in expired air.
X-axis: Volume of expired air (mL).
0–100 mL: Dead-space air → almost 0% nitrogen.
100–200 mL: Mixed dead-space and alveolar air → nitrogen rises rapidly.
After ~200 mL: Mostly alveolar air → nitrogen reaches a plateau (~60%).
Gray shaded area: Missing nitrogen due to dead-space air; used in the Fowler method to estimate anatomical dead space.
Pink shaded area: Actual nitrogen measured in expired air.
Key concept: After inhaling pure oxygen, the conducting airways contain almost no nitrogen, while the alveoli still contain nitrogen. During expiration, dead-space air exits first, followed by alveolar air, creating the characteristic S-shaped nitrogen washout curve.

RATE OF ALVEOLAR VENTILATION

Alveolar ventilation per minute is the total amount of fresh air entering the alveoli and nearby gas exchange areas in one minute.
It is calculated by multiplying the respiratory rate by the amount of fresh air that actually reaches the alveoli with each breath.

Formula

VA = Freq × (VT − VD)

What Each Symbol Means

VA = Alveolar ventilation per minute
Freq = Respiratory rate (breaths per minute)
VT = Tidal volume (air taken in with one normal breath)
VD = Physiological dead space volume (air that does not participate in gas exchange)

Easy Concept

Think of each breath as carrying 500 mL of air.

However,

Not all 500 mL reaches the alveoli.
150 mL stays in the dead space (nose, pharynx, trachea, etc.).
Only the remaining air reaches the alveoli for gas exchange.

So,

Fresh air reaching alveoli in one breath = Total breath − Dead space air

Then,

Alveolar ventilation = Fresh alveolar air per breath × Number of breaths per minute

Step-by-Step Calculation

Given

Respiratory Rate (Freq) = 12 breaths/min
Tidal Volume (VT) = 500 mL
Dead Space Volume (VD) = 150 mL

Step 1: Write the Formula

VA = Freq × (VT − VD)

Step 2: Calculate the Fresh Air Reaching the Alveoli in One Breath

VT − VD

= 500 − 150

= 350 mL

Meaning:

Out of 500 mL inhaled,

150 mL remains in the dead space
350 mL reaches the alveoli

Step 3: Multiply by the Respiratory Rate

VA = 12 × 350

Step 4: Multiply

VA = 4200 mL/min

Step 5: Convert to Liters

4200 mL = 4.2 L/min

Final Answer

Alveolar Ventilation = 4200 mL/min (4.2 L/min)

Easy Visualization

In One Breath

500 mL Inhaled Air
│
├── 150 mL → Dead Space
│             (No gas exchange)
│
└── 350 mL → Alveoli
              (Gas exchange occurs)

In One Minute

350 mL reaches alveoli in one breath
            ×
12 breaths/min
            │
            ▼
4200 mL/min

Why Do We Subtract Dead Space?

Because dead space air never reaches the alveoli.

Only the air that actually enters the alveoli can:

Deliver oxygen (O₂) to the blood.
Remove carbon dioxide (CO₂) from the blood.

Therefore,

Dead space air must be subtracted before calculating alveolar ventilation.

Importance of Alveolar Ventilation

Alveolar ventilation is one of the main factors controlling the oxygen and carbon dioxide levels in the alveoli.
Therefore, discussions of gas exchange mainly focus on alveolar ventilation.

Key Concept

Alveolar Ventilation (VA) = Respiratory Rate × (Tidal Volume − Dead Space Volume)
Formula:
- VA = Freq × (VT − VD)
Normal Values:
- VT = 500 mL
- VD = 150 mL
- Respiratory Rate = 12 breaths/min
Step 1: Air reaching alveoli per breath = 500 − 150 = 350 mL
Step 2: Alveolar ventilation = 12 × 350 = 4200 mL/min = 4.2 L/min
Only the air reaching the alveoli participates in gas exchange.
Dead space air does not participate in gas exchange and must be subtracted from the tidal volume.