Gas exchange between the alveolar air and the pulmonary capillary blood is discussed in the next chapter.
Here, an important feature of the pulmonary capillaries is explained.

Arrangement of Pulmonary Capillaries

The alveolar walls contain a very large number of pulmonary capillaries.
In most areas, the capillaries lie so close together that they almost touch each other side by side.

Easy Concept

Think of the alveolar wall as being covered by a dense network of capillaries.

Alveolar Wall

══════════════════════════

||||||||||||||||||||||||||
Capillary Capillary Capillary
Capillary Capillary Capillary
Capillary Capillary Capillary

(Almost Touching Each Other)

Concept:

The capillaries are packed very closely together.
There is very little space between adjacent capillaries.

Sheet of Flow

Because the capillaries are so closely packed, the blood does not appear to flow through separate individual capillaries.
Instead, the blood flows across the alveolar wall like a continuous sheet.
Therefore, pulmonary capillary blood flow is often called a “sheet of flow.”

Easy Concept

Ordinary Capillary Flow

Capillary     Capillary     Capillary

   │             │             │
   ▼             ▼             ▼

Blood flows separately
in each capillary

Pulmonary Capillary Flow

══════════════════════════
Alveolar Wall

██████████████████████████
Continuous Blood Flow
(Sheet of Flow)

Concept:

In most tissues, blood flows through individual capillaries.
In the alveolar wall, the capillaries are so close together that the blood behaves like a continuous sheet of flow.

Key Concept

Pulmonary capillary dynamics involve the exchange of gases between alveolar air and pulmonary capillary blood, which is discussed in the next chapter.
The alveolar walls contain numerous pulmonary capillaries.
In most areas, these capillaries almost touch one another side by side.
Because of this close arrangement, blood flows in the alveolar walls as a continuous “sheet of flow” rather than through separate individual capillaries.

Pulmonary Capillary Pressure

Direct measurement of pulmonary capillary pressure has not been reported.
Therefore, the pulmonary capillary pressure is estimated indirectly.
These indirect estimates suggest that the mean pulmonary capillary pressure is about 7 mm Hg.

Why Is the Pulmonary Capillary Pressure About 7 mm Hg?

The mean left atrial pressure is about 2 mm Hg.
The mean pulmonary arterial pressure is about 15 mm Hg.
The pulmonary capillaries lie between the pulmonary artery and the left atrium.
Therefore, the pulmonary capillary pressure must lie somewhere between these two pressures.
For this reason, the mean pulmonary capillary pressure is about 7 mm Hg.

Easy Concept

Think of the pulmonary capillaries as being between the pulmonary artery and the left atrium.

Pulmonary Artery
(15 mm Hg)
        │
        ▼
Pulmonary Capillaries
(?)
        │
        ▼
Left Atrium
(2 mm Hg)

Concept:

Blood flows from the pulmonary artery to the left atrium.
Therefore, the capillary pressure must be between 15 mm Hg and 2 mm Hg.

Step-by-Step Understanding

Step 1

Mean Pulmonary Arterial Pressure = 15 mm Hg

↓

Step 2

Mean Left Atrial Pressure = 2 mm Hg

↓

Step 3

Pulmonary capillaries are located between these two structures.

↓

Step 4

Therefore,

Pulmonary Capillary Pressure must lie between 15 mm Hg and 2 mm Hg.

↓

Final Value

Mean Pulmonary Capillary Pressure ≈ 7 mm Hg

Easy Memory Trick

Pulmonary Artery
15 mm Hg
      │
      ▼
Pulmonary Capillary
7 mm Hg
      │
      ▼
Left Atrium
2 mm Hg

Remember:

15 → 7 → 2

(Artery → Capillary → Left Atrium)

Key Concept

Pulmonary capillary pressure is not measured directly.
It is estimated indirectly.
The mean pulmonary capillary pressure is about 7 mm Hg.
This value is reasonable because:
- Mean pulmonary arterial pressure = 15 mm Hg
- Mean left atrial pressure = 2 mm Hg
Since the pulmonary capillaries are located between the pulmonary artery and the left atrium, their pressure must lie between these two values, giving an average of about 7 mm Hg.

Length of Time Blood Stays in the Pulmonary Capillaries

Histological studies of the total cross-sectional area of all pulmonary capillaries are used to calculate how long blood remains in the pulmonary capillaries.
When the cardiac output is normal, blood stays in the pulmonary capillaries for about 0.8 second.

During Increased Cardiac Output

When the cardiac output increases, the time blood stays in the pulmonary capillaries decreases.
It may shorten to about 0.3 second.

Easy Concept

Normal Cardiac Output

Normal Cardiac Output
        │
        ▼
Blood Enters
Pulmonary Capillaries
        │
        ▼
Stays for
0.8 Second
        │
        ▼
Gas Exchange Occurs

Concept:

At normal cardiac output, blood has about 0.8 second for gas exchange.

During Exercise (Increased Cardiac Output)

Cardiac Output Increases
        │
        ▼
Blood Flows Faster
        │
        ▼
Time in Pulmonary Capillaries
Decreases
        │
        ▼
About 0.3 Second

Concept:

During exercise, blood moves faster.
Therefore, it stays in the pulmonary capillaries for only about 0.3 second.

Why Doesn’t the Time Become Even Shorter?

The time does not become much shorter because additional pulmonary capillaries open.
These capillaries are normally collapsed.
When they open, they accommodate the increased blood flow.

Easy Concept

Exercise
        │
        ▼
Cardiac Output Increases
        │
        ▼
Normally Collapsed
Capillaries Open
        │
        ▼
More Pathways for Blood
        │
        ▼
Blood Still Has Enough Time
for Gas Exchange

Concept:

Instead of forcing all blood through the same capillaries, the lungs open additional capillaries.
This helps maintain enough time for gas exchange, even when blood flow increases.

Gas Exchange

In only a fraction of a second, blood passing through the alveolar capillaries:
- Becomes oxygenated (gains oxygen)
- Loses excess carbon dioxide

Easy Concept

Blood Enters
Pulmonary Capillary
        │
        ▼
O₂ Enters Blood
        │
        ▼
CO₂ Leaves Blood
        │
        ▼
Blood Leaves Oxygenated

Concept:

Even though blood stays in the pulmonary capillaries for only a short time, gas exchange is completed efficiently.

Key Concept

At normal cardiac output, blood remains in the pulmonary capillaries for about 0.8 second.
During increased cardiac output, this time decreases to about 0.3 second.
The time does not become much shorter because additional normally collapsed pulmonary capillaries open, allowing more pathways for blood flow.
Within a fraction of a second, blood in the alveolar capillaries becomes oxygenated and loses excess carbon dioxide.

Capillary Exchange of Fluid in the Lungs and Pulmonary Interstitial Fluid Dynamics

Fluid exchange across the pulmonary capillary membrane is qualitatively similar to fluid exchange in peripheral tissues.
However, there are important quantitative differences between the lungs and peripheral tissues.

1. Pulmonary Capillary Pressure

The pulmonary capillary pressure is low, about 7 mm Hg.
In comparison, the functional capillary pressure in many peripheral tissues is about 17–25 mm Hg.

Easy Concept

Pulmonary Capillary
Pressure
= 7 mm Hg

↓

Peripheral Tissue
Capillary Pressure
= 17–25 mm Hg

Concept:

The lungs have much lower capillary pressure than most peripheral tissues.

2. Pulmonary Interstitial Fluid Pressure

The interstitial fluid pressure in the lungs is slightly more negative than in peripheral subcutaneous tissue.
This pressure has been measured in two ways.

Method 1: Micropipette Measurement

A micropipette inserted into the pulmonary interstitium measures a pressure of about −5 mm Hg.

Method 2: Alveolar Fluid Absorption Measurement

Measuring the absorption pressure of fluid from the alveoli gives a value of about −8 mm Hg.

Easy Concept

Pulmonary Interstitium

Method 1
Micropipette
        │
        ▼
−5 mm Hg

Method 2
Fluid Absorption
from Alveoli
        │
        ▼
−8 mm Hg

Concept:

Both methods show that the interstitial pressure in the lungs is negative.

3. Colloid Osmotic Pressure of Pulmonary Interstitial Fluid

The colloid osmotic pressure of the pulmonary interstitial fluid is about 14 mm Hg.
In most peripheral tissues, the colloid osmotic pressure is less than half of this value.

Easy Concept

Pulmonary Interstitial
Colloid Osmotic Pressure

= 14 mm Hg

↓

Peripheral Tissues

Less Than Half
of 14 mm Hg

Concept:

4. Thin Alveolar Walls

The alveolar walls are extremely thin.
The alveolar epithelium covering the alveolar surface is very weak.
If the interstitial pressure becomes greater than the alveolar air pressure (>0 mm Hg), the alveolar epithelium can rupture.
This allows fluid to move from the interstitial space into the alveoli.

Easy Concept

Interstitial Pressure
>0 mm Hg
        │
        ▼
Alveolar Epithelium
Ruptures
        │
        ▼
Fluid Moves
Into Alveoli

Concept:

Positive interstitial pressure can break the thin alveolar wall.
As a result, fluid enters the alveoli.

Step-by-Step Understanding

Step 1

Normal Pulmonary Capillary Pressure = 7 mm Hg

↓

Step 2

Pulmonary Interstitial Pressure = −5 to −8 mm Hg

↓

Step 3

Pulmonary Interstitial Colloid Osmotic Pressure = 14 mm Hg

↓

Step 4

If Interstitial Pressure > 0 mm Hg

↓

Alveolar Epithelium Ruptures

↓

Fluid Moves Into the Alveoli

Key Concept

Pulmonary capillary fluid exchange is qualitatively similar to peripheral tissues, but important quantitative differences exist.
Pulmonary capillary pressure = about 7 mm Hg, compared with 17–25 mm Hg in many peripheral tissues.
Pulmonary interstitial fluid pressure is negative:
- Micropipette measurement = about −5 mm Hg
- Alveolar fluid absorption measurement = about −8 mm Hg
Pulmonary interstitial colloid osmotic pressure = about 14 mm Hg, which is higher than in most peripheral tissues.
The alveolar walls are extremely thin, and the alveolar epithelium is weak.
If interstitial pressure rises above alveolar air pressure (>0 mm Hg), the alveolar epithelium can rupture, allowing fluid to move from the interstitial space into the alveoli.

Interrelationships Between Interstitial Fluid Pressure and Other Pressures in the Lung

Figure: Fig. 39.7

Fig. 39.7 shows:
- A pulmonary capillary
- An alveolus
- A lymphatic capillary
The lymphatic capillary drains the interstitial space between the pulmonary capillary and the alveolus.
Fig. 39.7 also shows the balance of forces acting across the pulmonary capillary membrane.

Forces Causing Fluid to Move Out of the Capillary

1. Capillary Hydrostatic Pressure

Capillary hydrostatic pressure = 7 mm Hg
This pressure pushes fluid out of the pulmonary capillary into the interstitial space.

2. Interstitial Fluid Colloid Osmotic Pressure

Interstitial fluid colloid osmotic pressure = 14 mm Hg
This pressure pulls fluid out of the pulmonary capillary into the interstitial space.

3. Negative Interstitial Fluid Hydrostatic Pressure

Negative interstitial fluid hydrostatic pressure = 8 mm Hg
This negative pressure pulls fluid outward from the capillary into the interstitial space.

Step-by-Step Calculation of Total Outward Force

Formula

Total Outward Force

= Capillary Hydrostatic Pressure
+ Interstitial Fluid Colloid Osmotic Pressure
+ Negative Interstitial Fluid Hydrostatic Pressure

Substitute the Values

= 7 + 14 + 8

Solve

= 29 mm Hg

Final Answer

Total Outward Force = 29 mm Hg

Easy Concept

Fluid Inside Capillary

        │
        ▼

Capillary Hydrostatic Pressure
        7 mm Hg

+

Interstitial Colloid Osmotic Pressure
       14 mm Hg

+

Negative Interstitial Pressure
        8 mm Hg

        │
        ▼

Total Outward Force

29 mm Hg

Concept:

These three forces work together.
They move fluid from the pulmonary capillary into the interstitial space.

Force Causing Fluid to Move Into the Capillary

Plasma Colloid Osmotic Pressure

Plasma colloid osmotic pressure = 28 mm Hg
This pressure pulls fluid back into the pulmonary capillary.

Easy Concept

Interstitial Space

        │
        ▼

Plasma Colloid Osmotic Pressure

28 mm Hg

        │
        ▼

Fluid Pulled Back
Into the Capillary

Concept:

Plasma proteins attract water back into the blood.

Step-by-Step Calculation of Net Filtration Pressure

Formula

Net Filtration Pressure

= Total Outward Force
− Total Inward Force

Substitute the Values

= 29 − 28

Solve

= +1 mm Hg

Final Answer

Net Filtration Pressure = +1 mm Hg

Easy Concept

Outward Force
29 mm Hg

        │
        ▼

Inward Force
28 mm Hg

        │
        ▼

Net Filtration Pressure

+1 mm Hg

Concept:

The outward force is only slightly greater than the inward force.
Therefore, only a small amount of fluid leaves the pulmonary capillary.

Effect of the +1 mm Hg Filtration Pressure

The +1 mm Hg filtration pressure causes a small continuous movement of fluid from the pulmonary capillaries into the interstitial spaces.
A small amount of this fluid evaporates into the alveoli.
Most of the filtered fluid is returned to the circulation through the pulmonary lymphatic system.

Easy Concept

Pulmonary Capillary
        │
        ▼
Small Amount of Fluid
Leaves the Capillary
(+1 mm Hg)
        │
        ▼
Interstitial Space
        │
        ├──────────────┐
        ▼              ▼
Small Amount      Pulmonary
Evaporates        Lymphatics
Into Alveoli      Return Fluid
                  to Blood

Concept:

A tiny amount of fluid normally leaves the capillary.
The lymphatic vessels continuously remove this fluid, preventing fluid accumulation.

Key Concept

Fig. 39.7 shows a pulmonary capillary, alveolus, and lymphatic capillary, along with the forces controlling fluid movement.
Outward forces:
- Capillary hydrostatic pressure = 7 mm Hg
- Interstitial fluid colloid osmotic pressure = 14 mm Hg
- Negative interstitial fluid hydrostatic pressure = 8 mm Hg
- Total outward force = 7 + 14 + 8 = 29 mm Hg
Inward force:
- Plasma colloid osmotic pressure = 28 mm Hg
Net filtration pressure = 29 − 28 = +1 mm Hg
This +1 mm Hg pressure causes a small continuous filtration of fluid into the pulmonary interstitium.
Most of this fluid is returned to the circulation by the pulmonary lymphatic system, while a small amount evaporates into the alveoli.

Figure 39.7: Pressures Causing Fluid Movement Across Pulmonary Capillaries (Guyton Physiology 15th Edition)

🎯 One-Line Core Concept

Normally, only a very tiny amount of fluid filters out of pulmonary capillaries into the lung interstitium (+1 mmHg), and this fluid is immediately removed by the lymphatic system. This keeps the alveoli dry and prevents pulmonary edema.

🧠 First Understand the Big Picture

Imagine an alveolus (air sac) beside a pulmonary capillary (blood vessel).

Pulmonary Capillary
        │
        │
 Interstitial Space
        │
        │
     Alveolus

Fluid is continuously trying to move from the blood into the lung tissue.

At the same time,

other forces pull the fluid back.

The balance between these forces determines whether:

Fluid stays inside blood vessels ✅
Fluid enters lung tissue ❌
Pulmonary edema develops ❌

This figure explains all these forces.

First Learn Four Important Pressures

There are 4 Starling forces acting across the pulmonary capillary.

Think of them as two forces pushing fluid out and two forces pulling fluid back in.

Force 1

Pulmonary Capillary Hydrostatic Pressure

Value = +7 mmHg

Shown in the capillary.

What is Hydrostatic Pressure?

This is simply

Blood pressure inside the pulmonary capillary.

Blood pushes against the capillary wall.

It tries to push fluid outward.

Imagine squeezing a water-filled balloon.

Water wants to escape.

Exactly the same thing happens here.

Direction

Capillary
   │
   │
Fluid pushed OUT

Therefore

Hydrostatic pressure

➡ Pushes fluid OUT

Force 2

Interstitial Hydrostatic Pressure

Value = −8 mmHg

This is written outside the capillary.

Many students find the negative sign confusing.

Why is it Negative?

The pressure in the lung interstitium is actually slightly below atmospheric pressure.

Think of it as a gentle vacuum.

Instead of pushing fluid back,

it pulls fluid outward.

Imagine gently sucking on a straw.

Fluid is pulled toward the suction.

The negative interstitial pressure does the same.

Direction

Interstitial Space
      ↑
Fluid pulled outward

So,

although it is called hydrostatic pressure,

because it is negative, it helps pull fluid out of the capillary.

Total Outward Hydrostatic Force

There are now two outward forces.

Capillary Hydrostatic = +7

Interstitial Pressure = -8

Total outward force

7 + 8 = 15 mmHg outward

Force 3

Plasma Colloid Osmotic Pressure

Value = 28 mmHg

This is shown as

−28

What is this?

Blood proteins (especially albumin) cannot leave the capillary.

They attract water.

Think of albumin as a sponge inside the capillary.

It continuously pulls water back.

Direction

Fluid
← Back into capillary

Therefore,

Plasma proteins

➡ Pull fluid INTO the capillary.

Force 4

Interstitial Colloid Osmotic Pressure

Value = 14 mmHg

Shown as

−14

Why does it exist?

A small amount of protein is present in the lung interstitium.

These proteins also attract water.

Therefore,

they pull fluid out of the capillary.

Direction

Capillary
      │
Fluid pulled outward

Calculate the Net Pressure (Step by Step)

This is the easiest way to remember.

Step 1

Outward Forces

Capillary hydrostatic

+7

Plus

Negative interstitial pressure

+8

Total

15 mmHg outward

Step 2

Inward Force

Plasma oncotic pressure

28 mmHg inward

Step 3

Outward Osmotic Force

Interstitial oncotic pressure

14 mmHg outward

Final Equation

Outward

15 + 14 = 29

Inward

Net movement

29 − 28 = +1 mmHg outward

This is exactly why the figure shows

Net Pressure = +1 mmHg

What Does +1 mmHg Mean?

It means

Only a very tiny amount of fluid leaves the pulmonary capillary.

This is completely normal.

Without this tiny filtration,

lung tissues would not receive nutrients.

Where Does This Fluid Go?

Look at the yellow structure.

This is the

Lymphatic Vessel

Numbers

Outside the lymphatic

−5 mmHg

Inside the lymphatic

−4 mmHg

These negative pressures create a suction effect.

The lymphatic pump continuously removes filtered fluid.

Why Is the Lymphatic System So Important?

Imagine a floor where someone spills a few drops of water every minute.

A vacuum cleaner immediately removes those drops.

The floor stays dry.

The lymphatic system acts as that vacuum cleaner.

It prevents fluid from accumulating around the alveoli.

Now Look at the Alveolus

On the right side is the alveolus.

Notice several numbers.

Alveolar Pressure

Approximately

−8 mmHg

This negative pressure is mainly due to:

Surface tension
Interstitial forces around the alveolus

It helps keep the interstitial space under slight suction.

Surface Tension at the Pore

Surface tension also contributes to maintaining a negative pressure around the alveolus.

This assists in preventing fluid from flooding the alveolar air space.

Evaporation (0 mmHg)

Inside the alveolus,

water continuously evaporates into inspired air.

This evaporation helps keep the alveolar surface almost dry.

The figure marks this as approximately 0 mmHg because evaporation itself is not a major Starling pressure, but it contributes to keeping alveolar fluid minimal.

Why Don’t Alveoli Fill With Water?

This is the most important concept.

Three protective mechanisms keep alveoli dry:

① Very Low Net Filtration

Only

+1 mmHg

So only a tiny amount of fluid leaves capillaries.

② Lymphatic Drainage

Any filtered fluid is immediately removed.

③ Continuous Evaporation

Tiny amounts of fluid on the alveolar surface evaporate into inspired air.

What Happens in Left Heart Failure?

Suppose pulmonary capillary pressure rises.

Instead of

7 mmHg

it becomes

20 mmHg

Now,

Outward force becomes much greater.

Large amounts of fluid leave the capillary.

The lymphatics cannot remove all of it.

Fluid accumulates.

↓

Pulmonary edema develops.

↓

Alveoli fill with fluid.

↓

Severe breathlessness occurs.

Easy Flowchart

Normal Lung
        ↓
Only +1 mmHg filtration
        ↓
Small amount of fluid enters interstitium
        ↓
Lymphatic pump removes it
        ↓
Alveoli remain dry
        ↓
Normal gas exchange

Memory Trick

Think of a leaking water tank.

🚰 Capillary = Water tank
💧 Small leak = +1 mmHg filtration
🪣 Lymphatic = Drain removing leaked water
🌬️ Alveolus = Floor kept dry by evaporation

As long as the drain works, the floor stays dry.

If the leak becomes too large (e.g., increased pulmonary capillary pressure), the drain is overwhelmed, water accumulates, and the floor floods—just like pulmonary edema.

📚 High-Yield MBBS Points

Forces pushing fluid OUT of the pulmonary capillary

Pulmonary capillary hydrostatic pressure = +7 mmHg
Negative interstitial hydrostatic pressure = −8 mmHg (acts as an outward suction)

Forces opposing or modifying filtration

Plasma colloid osmotic pressure = 28 mmHg → pulls fluid into the capillary.
Interstitial colloid osmotic pressure = 14 mmHg → pulls fluid out into the interstitium.

Net filtration pressure

Outward forces = 7 + 8 + 14 = 29 mmHg
Inward force = 28 mmHg
Net = +1 mmHg outward, producing only minimal filtration.

Why alveoli stay dry

Very low net filtration pressure (+1 mmHg).
Pulmonary lymphatics continuously remove filtered fluid.
Evaporation from the alveolar surface limits fluid accumulation.

Clinical significance

An increase in pulmonary capillary hydrostatic pressure (e.g., left-sided heart failure or mitral valve disease) increases filtration, overwhelms lymphatic drainage, and leads to pulmonary edema, impairing gas exchange.

Negative Pulmonary Interstitial Pressure and Mechanism for Keeping Alveoli Dry

Under normal conditions, the alveoli do not fill with fluid.

What Keeps the Alveoli Dry?

The pulmonary capillaries and the pulmonary lymphatic system maintain a slight negative pressure in the pulmonary interstitial spaces.

Effect of Negative Interstitial Pressure

Whenever extra fluid appears in the alveoli, the negative interstitial pressure mechanically pulls (sucks) the fluid into the lung interstitium.
The fluid passes through the small openings between the alveolar epithelial cells.

Easy Concept

Extra Fluid
Inside Alveolus
        │
        ▼
Negative Interstitial
Pressure
        │
        ▼
Fluid Is Pulled Into
Lung Interstitium

Concept:

The negative pressure acts like a gentle suction.
It pulls extra fluid out of the alveoli, helping to keep them dry.

Removal of the Excess Fluid

The excess fluid in the lung interstitium is carried away by the pulmonary lymphatic vessels.

Easy Concept

Alveolus
        │
Extra Fluid
        │
        ▼
Lung Interstitium
        │
        ▼
Pulmonary Lymphatics
        │
        ▼
Fluid Returned
to the Circulation

Concept:

After fluid leaves the alveoli, the lymphatic system removes it.
This prevents fluid accumulation in the lungs.

Normal Condition of the Alveoli

Easy Concept

Normal Alveolus
        │
        ▼
Mostly Dry
        │
        ▼
Small Amount of Fluid
Covers the Surface
        │
        ▼
Keeps the Alveoli Moist

Concept:

The alveoli are not completely dry.
A thin layer of fluid is normally present to keep the alveolar surface moist.

Step-by-Step Understanding

Step 1

Extra fluid enters the alveoli.

↓

Step 2

Negative interstitial pressure pulls the fluid into the lung interstitium.

↓

Step 3

Pulmonary lymphatics remove the excess fluid.

↓

Step 4

Only a thin moist layer remains on the alveolar surface.

↓

Final Result

Alveoli remain dry and ready for efficient gas exchange.

Key Concept

Pulmonary capillaries and the pulmonary lymphatic system maintain a slight negative pressure in the pulmonary interstitium.
This negative pressure pulls extra fluid from the alveoli into the lung interstitium through small openings between the alveolar epithelial cells.
The pulmonary lymphatics remove the excess interstitial fluid.
Under normal conditions, the alveoli remain dry, except for a thin layer of fluid that keeps the alveolar surface moist.