Transport of carbon dioxide (CO₂) by the blood is easier than oxygen (O₂) transport.
Even under abnormal conditions, blood can transport much larger amounts of CO₂ than O₂.
The amount of CO₂ in the blood is closely related to the acid-base balance of the body fluids.
Under normal resting conditions, about 4 mL of CO₂ is transported from the tissues to the lungs by every 100 mL of blood.

CHEMICAL FORMS IN WHICH CO₂ IS TRANSPORTED

CO₂ is produced inside the tissue cells.
It first diffuses out of the cells in the dissolved molecular form (CO₂).
After entering the tissue capillaries, CO₂ undergoes very rapid physical and chemical reactions.
These reactions are essential for transporting CO₂ in the blood.
Fig. 41.13 illustrates these reactions.

Fig. 41.13

CO₂ diffuses from tissue cells into the blood.
Inside the blood, CO₂ undergoes rapid reactions that allow it to be transported efficiently.

KEY CONCEPT

CO₂ transport is easier than O₂ transport because blood can carry much larger amounts of CO₂. Under normal conditions, about 4 mL of CO₂ is transported by every 100 mL of blood, mainly after undergoing rapid chemical reactions inside the blood.

Transport of Carbon Dioxide (CO₂) in Blood (Guyton Fig. 41.13) superfast

🎯 One-Line Concept

Body cells produce CO₂ → Blood collects it → Blood carries it to the lungs → Lungs remove it by exhalation.

Think of CO₂ as the “waste product” of the body, just like garbage produced in a city. Blood acts as the garbage collection truck, carrying CO₂ from tissues to the lungs for disposal.

The Complete Story in 4 Simple Steps

Body Cells
      ↓
CO₂ enters Blood
      ↓
Blood transports CO₂
      ↓
Lungs remove CO₂

PART 1 – What Happens in the Tissues? (Left Side of Figure)

This is where CO₂ is loaded into the blood.

Step 1: Tissue Cells Produce CO₂

Every cell continuously makes energy (ATP).

During ATP production,

➡ CO₂ is produced as a waste product.

Cell
↓
ATP produced
↓
CO₂ produced

Since cells have the highest CO₂ pressure,

CO₂ diffuses into the blood.

Step 2: CO₂ Enters the Red Blood Cell (RBC)

CO₂ first dissolves in plasma and then quickly enters the RBC.

Cell
↓
Plasma
↓
Red Blood Cell

Inside the RBC, CO₂ has three possible pathways.

Pathway 1 — CO₂ Dissolved in Plasma (7%)

This is the simplest pathway.

Some CO₂ simply remains dissolved in the plasma.

Amount

≈ 7%

CO₂
↓
Plasma

No chemical reaction occurs.

Think of sugar dissolving in water.

Pathway 2 — CO₂ Binds to Hemoglobin (23%)

Some CO₂ attaches directly to hemoglobin.

CO₂
+
Hemoglobin
↓
Hb–CO₂

This compound is called

Carbaminohemoglobin (Hb–CO₂)

Amount transported

≈23%

This is different from oxygen binding because CO₂ binds mainly to the globin (protein) part of hemoglobin, not to the iron (heme) portion.

Pathway 3 — CO₂ Converted into Bicarbonate (70%) ⭐ (Most Important)

This is the major pathway.

About 70% of CO₂ travels this way.

Step A

CO₂ combines with water inside RBC.

CO₂ + H₂O

This reaction is very slow on its own.

Step B

The enzyme Carbonic Anhydrase speeds it up dramatically.

CO₂ + H₂O
      │
Carbonic Anhydrase
      ↓
H₂CO₃

Carbonic anhydrase is present in RBCs, making this reaction extremely fast.Step C

Carbonic acid (H₂CO₃) is unstable.

It immediately breaks into

H₂CO₃
↓
HCO₃⁻ + H⁺

Produces

Bicarbonate ion (HCO₃⁻)
Hydrogen ion (H⁺)

Step D

Hydrogen ions are buffered.

Hemoglobin acts like a sponge.

Hb + H⁺
↓
HHb

Hemoglobin prevents a large fall in blood pH.

Step E — Chloride Shift

Large amounts of bicarbonate accumulate inside RBC.

To maintain electrical neutrality,

Bicarbonate leaves the RBC.

At the same time,

Chloride ions (Cl⁻) enter.

HCO₃⁻
Moves Out

Cl⁻
Moves In

This exchange is called the

Chloride Shift (Hamburger Phenomenon)

Easy Memory

“Bicarbonate OUT → Chloride IN.”

Blood Now Travels to the Lungs

The blood carrying CO₂ reaches the lungs.

Everything now happens in reverse.

PART 2 – What Happens in the Lungs? (Right Side of Figure)

This is where CO₂ is unloaded from the blood.

Step 1

Bicarbonate returns into the RBC.

HCO₃⁻
Moves In

To balance the charge,

Chloride leaves.

Cl⁻
Moves Out

This is called the

Reverse Chloride Shift

Easy Memory

“Bicarbonate IN → Chloride OUT.”

Step 2

Hydrogen combines with bicarbonate.

H⁺ + HCO₃⁻
↓
H₂CO₃

Step 3

Carbonic anhydrase converts carbonic acid back.

H₂CO₃
↓
CO₂ + H₂O

Step 4

CO₂ leaves the blood.

Blood
↓
Alveolus
↓
Expired Air

You breathe it out.

What Happens to Hb–CO₂?

When blood reaches the lungs,

CO₂ separates from hemoglobin.

Hb–CO₂
↓
Hb + CO₂

CO₂ then diffuses into the alveoli and is exhaled.

Three Forms of CO₂ Transport (Most Important Exam Table)

Form of CO₂ Transport	Percentage	Easy Concept
Dissolved in plasma	7%	CO₂ simply dissolves in plasma.
Bound to hemoglobin (Carbaminohemoglobin)	23%	CO₂ binds to the globin part of hemoglobin.
As bicarbonate (HCO₃⁻)	70%	Major transport form after conversion inside RBC.

⭐ Remember: 70 → 23 → 7

70% = Bicarbonate (Most Important)
23% = Carbaminohemoglobin
7% = Dissolved in Plasma

Complete Flow Chart

BODY CELLS
     │
Produce CO₂
     │
     ▼
CO₂ enters Blood
     │
     ▼
Red Blood Cell
     │
     ├──► 7% Dissolved in Plasma
     │
     ├──► 23% Hb–CO₂
     │
     └──► 70%
            CO₂ + H₂O
                 │
       Carbonic Anhydrase
                 │
             H₂CO₃
                 │
           HCO₃⁻ + H⁺
                 │
     HCO₃⁻ leaves RBC
     Cl⁻ enters RBC
     (Chloride Shift)
                 │
        Blood reaches Lungs
                 │
     Reverse Chloride Shift
                 │
     HCO₃⁻ + H⁺ → H₂CO₃
                 │
     Carbonic Anhydrase
                 │
          CO₂ + H₂O
                 │
          CO₂ diffuses into alveoli
                 │
             CO₂ is exhaled

Everyday Analogy

Imagine a city’s garbage disposal system:

🏭 Body cells = Factories producing garbage (CO₂).
🚛 Blood = Garbage truck collecting the waste.
📦 Bicarbonate (HCO₃⁻) = The main container carrying most of the garbage (70%).
🏢 Lungs = Recycling center where the garbage is unloaded.
🌬️ Breathing out = Throwing the garbage away.

MBBS High-Yield Points

✅ CO₂ diffuses from tissue cells → blood because tissue cells have a higher PCO₂.
✅ Carbonic anhydrase inside RBC catalyzes the rapid conversion between CO₂ and carbonic acid.
✅ Hemoglobin buffers H⁺, helping maintain blood pH.
✅ Chloride shift: HCO₃⁻ out, Cl⁻ in (tissues).
✅ Reverse chloride shift: HCO₃⁻ in, Cl⁻ out (lungs).
✅ 70% of CO₂ is transported as bicarbonate (HCO₃⁻), making it the principal transport form.

🌟 Super Memory Summary

TISSUES (Loading CO₂)
Cells → Blood
CO₂ + H₂O
   │
Carbonic Anhydrase
   │
H₂CO₃
   │
HCO₃⁻ + H⁺
   │
HCO₃⁻ OUT
Cl⁻ IN
──────────────► Blood to Lungs
──────────────►
LUNGS (Unloading CO₂)
HCO₃⁻ IN
Cl⁻ OUT
   │
H₂CO₃
   │
Carbonic Anhydrase
   │
CO₂ + H₂O
   │
CO₂ → Alveoli → Exhaled

💡 Golden Rule

Most CO₂ (70%) is transported as bicarbonate (HCO₃⁻), formed inside red blood cells by the enzyme carbonic anhydrase. The chloride shift allows efficient bicarbonate transport in tissues, and the reverse chloride shift in the lungs enables CO₂ to be regenerated and exhaled.

Transport of CO₂ in a Dissolved State

A small amount of carbon dioxide (CO₂) is transported dissolved directly in the blood plasma.
The PCO₂ of venous blood is about 45 mm Hg.
The PCO₂ of arterial blood is about 40 mm Hg.
At a PCO₂ of 45 mm Hg, about 2.7 mL of CO₂ is dissolved in every 100 mL of blood.
At a PCO₂ of 40 mm Hg, about 2.4 mL of CO₂ is dissolved in every 100 mL of blood.

Easy Calculation

Dissolved CO₂ in venous blood = 2.7 mL/100 mL
Dissolved CO₂ in arterial blood = 2.4 mL/100 mL

CO₂ transported in dissolved form =

2.7 − 2.4 = 0.3 mL CO₂/100 mL blood

Therefore, only about 0.3 mL of CO₂ is transported in the dissolved state by every 100 mL of blood.
This represents about 7% of the total CO₂ normally transported.

Easy Concept

Imagine CO₂ travels in the blood by different methods.

One method is simply floating freely in the plasma.

Venous blood (from tissues):

Dissolved CO₂ = 2.7 mL

↓

Arterial blood (after lungs):

Dissolved CO₂ = 2.4 mL

↓

Difference delivered to the lungs:

2.7 − 2.4 = 0.3 mL CO₂

This 0.3 mL is the amount of CO₂ transported only in the dissolved form.

KEY CONCEPT

Only a small fraction (about 7%) of carbon dioxide is transported dissolved in the blood. Under normal conditions, venous blood contains 2.7 mL CO₂/100 mL and arterial blood contains 2.4 mL CO₂/100 mL, so about 0.3 mL CO₂/100 mL is transported in the dissolved state.

Transport of CO₂ in the Form of Bicarbonate Ion

Dissolved carbon dioxide (CO₂) reacts with water (H₂O) to form carbonic acid (H₂CO₃).
This reaction is very slow if it occurs by itself.
Inside red blood cells (RBCs), an enzyme called carbonic anhydrase is present.
Carbonic anhydrase catalyzes the reaction between CO₂ and water.
This enzyme increases the reaction speed by about 5000 times.
Because of this, the reaction occurs very rapidly inside RBCs.
The reaction reaches almost complete equilibrium within a fraction of a second.
This rapid reaction allows large amounts of CO₂ to react with the water inside red blood cells before the blood leaves the tissue capillaries.

Easy Concept

Think of carbonic anhydrase as a super-fast worker inside the red blood cell.

Without the worker:

CO₂ + H₂O

🐢 Very slow reaction

With carbonic anhydrase:

CO₂ + H₂O
      ↓
Carbonic Anhydrase ⚡
      ↓
H₂CO₃ (Carbonic acid)

🚀 Very fast reaction

The enzyme acts like a speed booster, allowing CO₂ to be converted almost instantly inside RBCs.

Easy Memory Trick

Carbonic Anhydrase

= CO₂ Speed Enzyme

Remember:

Without enzyme → Slow
With enzyme → 5000× Faster

KEY CONCEPT

Inside red blood cells, carbonic anhydrase rapidly converts CO₂ and water into carbonic acid. This enzyme speeds up the reaction about 5000-fold, allowing large amounts of CO₂ to be transported efficiently before the blood leaves the tissue capillaries.

Dissociation of Carbonic Acid Into Bicarbonate and Hydrogen Ions

Inside the red blood cells (RBCs), carbonic acid (H₂CO₃) quickly dissociates into:
- Hydrogen ions (H⁺)
- Bicarbonate ions (HCO₃⁻)
Most H⁺ ions combine with hemoglobin.
Hemoglobin acts as a powerful acid-base buffer.
Many HCO₃⁻ ions diffuse from the RBCs into the plasma.
At the same time, chloride ions (Cl⁻) move from the plasma into the RBCs.
This exchange occurs through a bicarbonate-chloride carrier protein in the RBC membrane.
The carrier rapidly moves:
- HCO₃⁻ out of the RBC
- Cl⁻ into the RBC
As a result, venous RBCs contain more chloride than arterial RBCs.
This process is called the chloride shift.
The reversible reaction of CO₂ with water inside RBCs, catalyzed by carbonic anhydrase, transports about 70% of the total CO₂ from the tissues to the lungs.
Therefore, bicarbonate transport is the most important form of CO₂ transport.
If carbonic anhydrase is inhibited (e.g., by acetazolamide):
- CO₂ transport becomes markedly reduced.
- Tissue PCO₂ may increase from the normal 45 mm Hg to about 80 mm Hg.

Fig. 41.13

CO₂ transport occurs in three forms:
- Bicarbonate (HCO₃⁻) = 70%
- Carbaminohemoglobin (Hb–CO₂) = 23%
- Dissolved CO₂ = 7%
Inside the RBC:
- CO₂ + H₂O → H₂CO₃
- H₂CO₃ → H⁺ + HCO₃⁻
- H⁺ binds to hemoglobin.
- HCO₃⁻ leaves the RBC.
- Cl⁻ enters the RBC (chloride shift).

Easy Concept

Tissue Cell
     │
     ▼
CO₂ enters RBC
     │
     ▼
CO₂ + H₂O
     │
Carbonic anhydrase
     ▼
H₂CO₃
     ▼
H⁺ + HCO₃⁻

H⁺ → Stays inside RBC (binds to hemoglobin)
HCO₃⁻ → Moves into plasma
Cl⁻ → Moves into RBC (chloride shift)

Easy Memory Trick

Remember the sequence:

CO₂ → H₂CO₃ → H⁺ + HCO₃⁻

Then:

H⁺ → Hemoglobin
HCO₃⁻ → Plasma
Cl⁻ → RBC

KEY CONCEPT

About 70% of CO₂ is transported as bicarbonate (HCO₃⁻). Inside red blood cells, carbonic anhydrase rapidly converts CO₂ into carbonic acid, which dissociates into H⁺ and HCO₃⁻. H⁺ is buffered by hemoglobin, while HCO₃⁻ enters the plasma in exchange for Cl⁻ (chloride shift), making bicarbonate the major form of CO₂ transport.

Transport of CO₂ in Combination With Hemoglobin and Plasma Proteins—Carbaminohemoglobin

In addition to reacting with water, CO₂ also reacts directly with the amine groups of hemoglobin.
This reaction forms carbaminohemoglobin (Hb–CO₂).
The combination of CO₂ and hemoglobin is reversible.
The bond is loose, so CO₂ is easily released in the alveoli, where alveolar PCO₂ is lower than pulmonary capillary PCO₂.
A small amount of CO₂ also combines with plasma proteins in the tissue capillaries.
This form of transport is less important because:
- Plasma proteins are only about one-fourth as abundant as hemoglobin.
Transport of CO₂ as carbaminohemoglobin and with plasma proteins accounts for about 30% of the total CO₂ transport.
This is approximately 1.5 mL of CO₂ per 100 mL of blood.
However, this reaction is slower than the reaction of CO₂ with water inside red blood cells.
Therefore, under normal conditions, the carbamino mechanism probably transports no more than about 20% of the total CO₂.

Easy Calculation

Total CO₂ transported = 4 mL/100 mL blood
CO₂ transported as carbamino compounds ≈ 30%

Calculation:

30% of 4 mL = 1.2 mL (approximately 1.5 mL as stated in the text)

Fig. 41.13

CO₂ is transported in three forms:
- Bicarbonate (HCO₃⁻) = 70%
- Carbaminohemoglobin (Hb–CO₂) = 23%
- Dissolved CO₂ = 7%

Easy Concept

Think of hemoglobin as a bus.

It can carry:

O₂ passengers
CO₂ passengers

For CO₂:

CO₂
   ↓
Hemoglobin
   ↓
Hb–CO₂ (Carbaminohemoglobin)

When the bus reaches the lungs:

Hb–CO₂
   ↓
CO₂ released into alveoli

Because the bond is weak (loose), CO₂ gets off the bus easily.

Easy Memory Trick

CO₂ has three ways to travel in blood:

HCO₃⁻ (Bicarbonate) → 70% ⭐ Most important
Hb–CO₂ (Carbaminohemoglobin) → About 20–30%
Dissolved CO₂ → 7%

KEY CONCEPT

CO₂ combines reversibly with hemoglobin to form carbaminohemoglobin, allowing easy release in the lungs. A small amount also binds to plasma proteins. This mechanism transports about 20–30% of total CO₂, while bicarbonate remains the major form of CO₂ transport.

CARBON DIOXIDE DISSOCIATION CURVE

Fig. 41.14 shows the carbon dioxide (CO₂) dissociation curve.
The curve shows the relationship between:
- Total CO₂ in the blood (all forms)
- Partial pressure of CO₂ (PCO₂)
Under normal conditions:
- Arterial blood PCO₂ = 40 mm Hg
- Venous blood PCO₂ = 45 mm Hg
Therefore, blood PCO₂ normally changes only within a small range (40–45 mm Hg).
The total CO₂ content of blood is about 50 volume percent (50 mL CO₂ per 100 mL blood).
During normal circulation, only about 4 volume percent of this CO₂ is exchanged between the tissues and the lungs.

Easy Concept of the CO₂ Exchange

In the tissues:

Cells produce CO₂.
Blood picks up CO₂.
Total CO₂ increases:

48 vol%
     ↓
52 vol%

In the lungs:

Blood releases CO₂ into the alveoli.
Total CO₂ decreases:

52 vol%
     ↓
48 vol%

Easy Calculation

Total CO₂ entering tissues = 48 vol%

Total CO₂ leaving tissues = 52 vol%

CO₂ exchanged =

52 − 48 = 4 vol%

Thus,

About 4 mL of CO₂ is transported by every 100 mL of blood.

Fig. 41.14

Arterial blood:
- PCO₂ = 40 mm Hg
- Total CO₂ = 48 vol%
Venous blood:
- PCO₂ = 45 mm Hg
- Total CO₂ = 52 vol%
Difference = 4 vol%
- This is the amount of CO₂ transported from the tissues to the lungs.

Easy Memory Trick

Tissues

48 → 52
(Blood gains CO₂)

↓

Lungs

52 → 48
(Blood loses CO₂)

KEY CONCEPT

The CO₂ dissociation curve shows that as blood PCO₂ increases, total blood CO₂ also increases. Normally, arterial blood contains about 48 vol% CO₂ (PCO₂ = 40 mm Hg) and venous blood contains about 52 vol% CO₂ (PCO₂ = 45 mm Hg), so about 4 vol% of CO₂ is exchanged between the tissues and the lungs.

This is Figure 41.14 from Guyton Physiology, and it explains how the amount of carbon dioxide (CO₂) carried in blood changes as PCO₂ changes.

This graph is called the:

⭐ Carbon Dioxide Dissociation Curve

It is the CO₂ equivalent of the oxygen-hemoglobin dissociation curve, but it is much simpler.

Unlike oxygen, most CO₂ is NOT carried by hemoglobin. It is mainly carried as bicarbonate (HCO₃⁻).

🎯 One-Line Concept

As blood PCO₂ increases, the amount of CO₂ carried in blood increases. As blood PCO₂ decreases, the amount of CO₂ carried decreases.

Simply,

High PCO₂ → Blood carries more CO₂
Low PCO₂ → Blood carries less CO₂

First Understand What the Graph Represents

Imagine blood moving between:

Body Tissues
      ↓
CO₂ enters blood
      ↓
Venous Blood
      ↓
Lungs
      ↓
CO₂ leaves blood
      ↓
Arterial Blood

This graph shows

How much CO₂ blood contains at different PCO₂ values.

Step 1: Understand the Axes

X-Axis (Horizontal)

Gas Pressure of Carbon Dioxide (PCO₂)

This is

Partial pressure of CO₂ in blood.

Starts from

0 mmHg

↓

120 mmHg

Easy Memory

Move right

↓

More CO₂ pressure

Move left

↓

Less CO₂ pressure

Y-Axis (Vertical)

CO₂ in Blood (Volumes %)

This means

How many milliliters of CO₂ are present in every 100 mL of blood.

Example

50 Vol%

50 mL CO₂

per

100 mL blood

Understanding the Red Curve

Notice

The curve rises continuously.

Unlike the oxygen curve,

it is almost a straight line over the normal physiological range.

Why Does It Rise?

As PCO₂ increases,

more CO₂ enters the blood.

Therefore,

blood stores more CO₂.

Simple.

Why Isn’t It S-Shaped Like Oxygen?

Because CO₂ is transported in three different forms, not mainly by hemoglobin.

Most CO₂ is carried as:

~70% as bicarbonate (HCO₃⁻)
~20–25% bound to proteins/hemoglobin (carbamino compounds)
~5–10% dissolved in plasma

Because of these multiple transport mechanisms, CO₂ content changes more smoothly with PCO₂.

The Yellow Shaded Area

⭐ Normal Operating Range

This is one of the most important parts of the graph.

The yellow band lies around

PCO₂ = 40–45 mmHg

These are the normal physiological values.

Left Side of Yellow Band

Arterial Blood

PCO₂

≈40 mmHg

Look at the graph.

CO₂ content

≈48 mL/100 mL blood

Why?

Blood has just passed through the lungs.

The lungs removed excess CO₂.

Therefore,

arterial blood contains less CO₂.

Right Side of Yellow Band

Venous Blood

PCO₂

≈45 mmHg

Now

Blood has returned from tissues.

The tissues produced CO₂.

Therefore,

venous blood contains more CO₂.

CO₂ content

≈52 mL/100 mL blood

What Does the Difference Mean?

Compare

Blood	CO₂ Content
Arterial	≈48 mL/100 mL
Venous	≈52 mL/100 mL

Difference

≈4 mL CO₂

Meaning

Every

100 mL blood

collects

about

4 mL of CO₂

while passing through tissues.

That CO₂ is later released in the lungs.

Why Does the Curve Become Less Steep at High PCO₂?

Look carefully.

At very high PCO₂,

the curve begins to flatten slightly.

Why?

Blood gradually approaches its capacity to store CO₂.

Additional CO₂ still enters,

but the increase becomes slower.

Compare Arterial and Venous Blood

Blood	PCO₂	CO₂ Content
Arterial	40 mmHg	≈48 mL/100 mL
Venous	45 mmHg	≈52 mL/100 mL

This small increase in PCO₂ (only 5 mmHg) allows blood to transport about 4 mL of additional CO₂ per 100 mL blood.

Why Can Blood Carry So Much CO₂?

Unlike oxygen,

CO₂ is chemically converted inside red blood cells.

The major reaction is:

CO₂ + H₂O
      ↓
H₂CO₃
      ↓
H⁺ + HCO₃⁻

This reaction is catalyzed by carbonic anhydrase.

Because CO₂ is converted into bicarbonate,

blood can transport large amounts of CO₂ without a huge rise in PCO₂.

Easy Analogy

Imagine

A warehouse.

Oxygen

Must stay inside boxes (hemoglobin).

Once boxes are full,

very little more oxygen can be stored.

Carbon Dioxide

Can be stored in

three different warehouses:

🏠 Dissolved in plasma

🏠 As bicarbonate

🏠 Bound to proteins and hemoglobin

Therefore,

blood has much greater flexibility in carrying CO₂.

Clinical Correlation

1. Exercise

Muscles produce much more CO₂.

Venous PCO₂ rises.

Blood carries more CO₂ back to the lungs.

Ventilation also increases to remove it. Hypoventilation

Examples

COPD
Opioid overdose
Neuromuscular weakness

CO₂ cannot be removed efficiently.

PCO₂ rises.

Blood CO₂ content increases.

This contributes to respiratory acidosis. Hyperventilation

Examples

Anxiety
Panic attack

Excess CO₂ is exhaled.

PCO₂ falls.

Blood CO₂ content decreases.

This contributes to respiratory alkalosis.

High-Yield MBBS Viva Points

Normal Values

Parameter	Value
Normal arterial PCO₂	40 mmHg
Normal venous PCO₂	45 mmHg
Arterial CO₂ content	≈48 mL/100 mL blood
Venous CO₂ content	≈52 mL/100 mL blood
CO₂ added in tissues	≈4 mL/100 mL blood

Carbon Dioxide Transport Forms

Form	Percentage
Bicarbonate (HCO₃⁻)	≈70%
Carbamino compounds	≈20–25%
Dissolved CO₂	≈5–10%

Compare Oxygen and Carbon Dioxide Dissociation Curves

Oxygen Curve	Carbon Dioxide Curve
S-shaped (sigmoid)	Nearly linear over the normal range
Mainly depends on hemoglobin binding	Depends largely on bicarbonate formation
Maximum oxygen content ≈20 mL/100 mL	Total CO₂ content ≈48–52 mL/100 mL in the normal range
Strong cooperative binding	No marked cooperative binding

Super Easy Memory Story

Imagine a garbage truck.

🏭 Body tissues

Factories produce garbage (CO₂).

The garbage truck (blood) collects it.

By the time it leaves the tissues:

➡️ Venous blood

PCO₂ = 45 mmHg

CO₂ content ≈ 52 mL/100 mL

🫁 Lungs

The garbage truck unloads the garbage.

Now:

➡️ Arterial blood

PCO₂ = 40 mmHg

CO₂ content ≈ 48 mL/100 mL

The truck is now ready to collect more garbage from the tissues.

🎯 MBBS Golden Rule

Three Numbers You Must Remember

Blood	PCO₂	CO₂ Content
Arterial Blood	40 mmHg	≈48 mL/100 mL
Venous Blood	45 mmHg	≈52 mL/100 mL
Difference	5 mmHg	≈4 mL CO₂/100 mL blood

Final Concept to Never Forget

The carbon dioxide dissociation curve shows that:

Higher PCO₂ → Blood carries more CO₂.
Lower PCO₂ → Blood carries less CO₂.
In the normal operating range (40–45 mmHg), a small increase of only 5 mmHg in PCO₂ allows blood to carry about 4 mL more CO₂ per 100 mL blood, mainly because most CO₂ is transported as bicarbonate (HCO₃⁻) rather than simply being dissolved or bound to hemoglobin. This efficient transport system allows the body to remove the CO₂ continuously produced by tissue metabolism.

When Oxygen Binds With Hemoglobin, CO₂ Is Released (the Haldane Effect) to Increase CO₂ Transport

When oxygen (O₂) binds to hemoglobin in the lungs, carbon dioxide (CO₂) is released from the blood.
This is called the Haldane effect.
The Haldane effect is more important for CO₂ transport than the Bohr effect is for O₂ transport.
In the lungs:
- O₂ combines with hemoglobin.
- Hemoglobin becomes a stronger acid.
- This helps remove CO₂ from the blood in two ways:

Less Carbaminohemoglobin Formation
- Acidic hemoglobin has less ability to bind CO₂.
- Therefore, CO₂ is released from carbaminohemoglobin.
- The released CO₂ diffuses into the alveoli.
Conversion of Bicarbonate Back to CO₂
- Acidic hemoglobin releases H⁺ ions.
- H⁺ combines with bicarbonate (HCO₃⁻).
- This forms carbonic acid (H₂CO₃).
- Carbonic acid quickly breaks down into:
  - Water (H₂O)
  - CO₂
- The newly formed CO₂ diffuses into the alveoli and is exhaled.

Fig. 41.15

The figure compares two CO₂ dissociation curves:
- PO₂ = 40 mm Hg (tissue capillaries)
- PO₂ = 100 mm Hg (lung capillaries)
Point A (Tissues):
- PCO₂ = 45 mm Hg
- Total CO₂ = 52 vol%
As blood reaches the lungs:
- PCO₂ decreases to 40 mm Hg
- PO₂ increases to 100 mm Hg

Without the Haldane Effect

CO₂ would decrease:

52 vol% → 50 vol%

CO₂ released =

52 − 50 = 2 vol%

With the Haldane Effect

The CO₂ dissociation curve shifts downward.
CO₂ decreases:

52 vol% → 48 vol%

CO₂ released =

52 − 48 = 4 vol%

Easy Concept of the Calculation

Without Haldane Effect:

52 → 50

Only 2 vol% CO₂ released

With Haldane Effect:

52 → 48

4 vol% CO₂ released

Result:

CO₂ released doubles
2 → 4 vol%

Easy Concept

Imagine hemoglobin is a bus.

In the Tissues

Bus carries CO₂ passengers.

↓

In the Lungs

Now oxygen passengers enter the bus.

O₂ gets on the bus 🚍

↓

CO₂ passengers must get off.

So:

O₂ enters
CO₂ leaves

This is the Haldane effect.

Easy Memory Trick

Lungs

O₂ IN
↓

CO₂ OUT

Haldane = Oxygen Helps Remove CO₂

KEY CONCEPT

The Haldane effect states that when oxygen binds to hemoglobin in the lungs, hemoglobin releases carbon dioxide more easily. This occurs by releasing CO₂ from carbaminohemoglobin and converting bicarbonate back into CO₂. As a result, the Haldane effect approximately doubles the amount of CO₂ released in the lungs and picked up in the tissues.

his is Figure 41.15 from Guyton Physiology, and it explains one of the most important concepts in CO₂ transport:

⭐ The Haldane Effect

This figure explains:

Why deoxygenated blood carries more CO₂ than oxygenated blood.

It is the opposite of the Bohr effect.

Bohr Effect: CO₂ helps unload O₂.
Haldane Effect: O₂ helps unload CO₂.

Both work together to maximize gas exchange.

🎯 One-Line Concept

When hemoglobin loses oxygen (in tissues), it can carry more CO₂. When hemoglobin gains oxygen (in lungs), it releases CO₂.

Simply,

Tissues:
↓ O₂ in Hb
→ ↑ CO₂ carrying

Lungs:
↑ O₂ in Hb
→ ↓ CO₂ carrying

This is the Haldane Effect.

Step 1: Understand the Axes

X-Axis (Horizontal)

PCO₂ (mmHg)

This shows the

Partial pressure of carbon dioxide in blood.

Starts from

35 mmHg

↓

50 mmHg

Easy Memory

Move right

↓

Higher CO₂ pressure

Y-Axis (Vertical)

CO₂ in Blood (Volumes %)

This shows

How much CO₂ is actually carried in every 100 mL of blood.

Example

50 Vol%

50 mL CO₂

per

100 mL blood

Why Are There Two Curves?

This is the key to understanding the graph.

Both curves represent the same PCO₂ values, but with different oxygen levels.

🔺 Upper Dashed Curve

PO₂ = 40 mmHg

This represents

Venous blood

coming from the tissues.

Characteristics:

Low oxygen
Deoxygenated hemoglobin
High capacity to carry CO₂

Therefore,

the curve is

higher.

🔴 Lower Solid Curve

PO₂ = 100 mmHg

This represents

Arterial blood

leaving the lungs.

Characteristics:

High oxygen
Oxygenated hemoglobin
Lower capacity to carry CO₂

Therefore,

the curve is

lower.

Why Is the Upper Curve Higher?

Suppose

PCO₂

45 mmHg.

Now compare

Both curves.

Upper Curve

PO₂

40 mmHg

Blood contains

more CO₂.

Lower Curve

PO₂

100 mmHg

Blood contains

less CO₂.

Why?

Because

deoxygenated hemoglobin binds CO₂ more readily than oxygenated hemoglobin.

This is the essence of the Haldane Effect.

Understanding Point A

Point A lies on the

upper dashed curve.

Conditions

PO₂

40 mmHg

PCO₂

45 mmHg

This represents

Venous blood returning from tissues.

What Has Happened?

Cells have produced

CO₂.

Hemoglobin has already released oxygen to the tissues.

Now

deoxygenated hemoglobin

binds more CO₂.

Therefore,

blood CO₂ content is high.

Understanding Point B

Point B lies on the

lower solid curve.

Conditions

PO₂

100 mmHg

PCO₂

40 mmHg

This represents

Arterial blood leaving the lungs.

What Has Happened?

Hemoglobin has picked up oxygen.

Once oxygen binds,

hemoglobin becomes less able to carry CO₂.

Therefore,

CO₂ is released into the alveoli and exhaled.

Blood CO₂ content falls.

Understanding the Arrow

The arrow is the

most important part of the graph.

It connects

Point A

↓

Point B.

What Does It Mean?

As blood passes through the lungs:

Hemoglobin binds oxygen.
Oxygenated hemoglobin loses its ability to carry CO₂.
CO₂ is released from the blood.
CO₂ diffuses into the alveoli.
CO₂ is exhaled.

This entire process is called the

⭐ Haldane Effect

Why Does Oxygen Cause CO₂ Release?

There are two main mechanisms.

Mechanism 1: Less Carbaminohemoglobin Formation

CO₂ can bind directly to hemoglobin.

Hemoglobin + CO₂
        ↓
Carbaminohemoglobin

Deoxygenated hemoglobin binds CO₂ well.

When oxygen binds,

this bond becomes unstable.

CO₂ is released.

Mechanism 2: Less Buffering of H⁺

Deoxygenated hemoglobin is a good buffer.

It binds hydrogen ions (H⁺).

When oxygen binds to hemoglobin,

H⁺ is released.

Those H⁺ ions combine with bicarbonate:

H⁺ + HCO₃⁻
      ↓
H₂CO₃
      ↓
CO₂ + H₂O

The newly formed CO₂ diffuses into the alveoli and is exhaled.

Why Does the Haldane Effect Help in Tissues?

Look at the upper curve.

In tissues:

Hemoglobin loses oxygen.
Hemoglobin becomes deoxygenated.
Deoxygenated hemoglobin can carry more CO₂.

Therefore,

blood efficiently collects the CO₂ produced by cells.

Why Does the Haldane Effect Help in Lungs?

Look at the lower curve.

In lungs:

Hemoglobin gains oxygen.
Oxygenated hemoglobin cannot hold as much CO₂.
CO₂ is released.

Therefore,

lungs efficiently eliminate CO₂.

Compare Tissues and Lungs

In Tissues	In Lungs
Hemoglobin loses O₂	Hemoglobin gains O₂
Deoxygenated Hb forms	Oxygenated Hb forms
CO₂ carrying increases	CO₂ carrying decreases
CO₂ enters blood	CO₂ leaves blood

Haldane Effect vs Bohr Effect

Students often confuse these.

Bohr Effect	Haldane Effect
CO₂ affects O₂ transport	O₂ affects CO₂ transport
↑ CO₂ shifts O₂ curve right	↑ O₂ reduces CO₂ carrying
Promotes oxygen unloading in tissues	Promotes CO₂ unloading in lungs

Easy Rule

Bohr: CO₂ helps O₂ leave hemoglobin.
Haldane: O₂ helps CO₂ leave hemoglobin.

Easy Story

Imagine

Hemoglobin is a

bus.

In the Tissues

Passengers

(Oxygen)

get off.

Now

many empty seats become available.

CO₂ passengers get on.

Bus carries lots of CO₂.

In the Lungs

New oxygen passengers enter.

The seats become occupied.

CO₂ passengers must get off.

CO₂ leaves through the lungs.

Exactly the same.

Clinical Correlation

1. Normal Physiology

The Haldane effect significantly increases the efficiency of CO₂ transport from tissues to lungs and its release into the alveoli.

2. Exercise

Working muscles release more oxygen from hemoglobin.

This increases the amount of deoxygenated hemoglobin, allowing blood to carry more CO₂ back to the lungs.

3. Oxygen Therapy in Severe COPD

In some patients with severe COPD and chronic CO₂ retention, giving high concentrations of oxygen can reduce the amount of CO₂ carried by deoxygenated hemoglobin (Haldane effect), contributing to a rise in blood CO₂. This is one mechanism among several (others include changes in ventilation-perfusion matching and reduced hypoxic respiratory drive in selected patients).

High-Yield MBBS Viva Points

Haldane Effect Definition

Oxygenation of hemoglobin decreases its ability to carry carbon dioxide, thereby promoting CO₂ release in the lungs. Deoxygenation of hemoglobin increases its ability to carry carbon dioxide, promoting CO₂ uptake in the tissues.

Remember

PO₂	CO₂ Carrying Capacity
Low PO₂	High
High PO₂	Low

Understanding the Whole Graph Step by Step

Point A

PO₂ = 40 mmHg
PCO₂ = 45 mmHg
Venous blood
Deoxygenated hemoglobin
Maximum CO₂ carrying capacity

↓

Blood enters lungs.

↓

Hemoglobin binds oxygen.

↓

CO₂ carrying capacity falls.

↓

Blood moves to

Point B

PO₂ = 100 mmHg
PCO₂ = 40 mmHg
Arterial blood
Less CO₂ remains
CO₂ has been exhaled

Super Easy Memory Trick

Imagine a taxi.

🚖 Taxi = Hemoglobin

🧳 Oxygen = VIP passenger

📦 CO₂ = Luggage

In tissues

VIP passenger gets out.

Now

the taxi has plenty of space.

It picks up lots of luggage (CO₂).

In lungs

VIP passenger gets back in.

The luggage must be unloaded.

Exactly the same happens with hemoglobin.

🎯 MBBS Golden Rule

Haldane Effect

Deoxygenated hemoglobin = More CO₂ carrying

Oxygenated hemoglobin = Less CO₂ carrying

Final Comparison

Feature	Bohr Effect	Haldane Effect
Main trigger	↑ CO₂ / ↑ H⁺	↑ O₂
Main result	Hemoglobin releases O₂	Hemoglobin releases CO₂
Occurs mainly in	Tissues	Lungs
Physiological purpose	Improve oxygen delivery	Improve carbon dioxide removal

Final Concept to Never Forget

The key message of Figure 41.15 is that oxygen and carbon dioxide transport are closely linked.

In the tissues, hemoglobin loses oxygen, becomes deoxygenated, and therefore binds more CO₂, helping transport metabolic waste.
In the lungs, hemoglobin binds oxygen, becomes oxygenated, and therefore releases CO₂, allowing it to be exhaled.

This coordinated exchange—the Haldane effect—makes carbon dioxide transport far more efficient and works alongside the Bohr effect to optimize gas exchange.

Change in Blood Acidity During CO₂ Transport

When CO₂ enters the blood in the peripheral tissues, it forms carbonic acid (H₂CO₃).
Carbonic acid lowers the blood pH, making the blood slightly more acidic.
However, the acid-base buffer systems of the blood prevent a large increase in H⁺ concentration.
Therefore, the blood pH changes only slightly.
Under normal conditions:
- Arterial blood pH = 7.41
- As blood picks up CO₂ in the tissue capillaries:
  - Venous blood pH = 7.37

Easy Calculation

Arterial pH = 7.41
Venous pH = 7.37

Change in pH =

7.41 − 7.37 = 0.04

Thus, during normal CO₂ transport, the blood pH decreases by only 0.04 units.
In the lungs:
- CO₂ leaves the blood.
- Carbonic acid decreases.
- The blood pH rises back to 7.41.
During heavy exercise, high metabolic activity, or slow tissue blood flow:
- Much more CO₂ is produced and retained.
- The decrease in tissue blood pH can be as much as 0.50 units.
- This is about 12 times greater than the normal pH change.
- Such a large fall in pH causes significant tissue acidosis.

Easy Concept

Think of CO₂ as an acid maker.

In the Tissues

CO₂ enters blood
        ↓
Carbonic acid forms
        ↓
Blood becomes slightly acidic

But the blood buffers immediately act like sponges.

Extra acid
      ↓
Blood buffers absorb it

So the pH changes only a little:

7.41
  ↓
7.37

In the Lungs

CO₂ leaves blood
      ↓
Less carbonic acid
      ↓
pH returns to 7.41

During Heavy Exercise

Muscles produce much more CO₂.

Much more CO₂
       ↓
Much more acid
       ↓
Buffers cannot completely prevent the fall
       ↓
Tissue acidosis

Easy Memory Table

Situation	Blood pH
Arterial blood	7.41
Venous blood	7.37
Normal pH decrease	0.04
Heavy exercise / slow blood flow	Up to 0.50 decrease

KEY CONCEPT

As CO₂ enters the blood, it forms carbonic acid, causing a slight fall in pH. Blood buffers limit this change, so pH normally decreases only from 7.41 to 7.37. During heavy exercise or poor blood flow, much more CO₂ accumulates, causing a much larger fall in pH and leading to tissue acidosis.

RESPIRATORY EXCHANGE RATIO

Under normal resting conditions:
- About 5 mL of O₂ is transported from the lungs to the tissues by every 100 mL of blood.
- About 4 mL of CO₂ is transported from the tissues to the lungs by every 100 mL of blood.
Therefore, the amount of CO₂ expired is about 82% of the O₂ taken up by the lungs.
The ratio of CO₂ output to O₂ uptake is called the Respiratory Exchange Ratio (R).

Formula

$R=\frac{\text{Rate of CO₂ output}}{\text{Rate of O₂ uptake}}$ R=Rate of O₂ uptakeRate of CO₂ output

Easy Calculation

Under normal conditions:

CO₂ output = 4 mL
O₂ uptake = 5 mL

$R=\frac{4}{5}=0.8$ R=54=0.8

≈ 0.82 (82%)

Therefore, under normal resting conditions, R is about 0.82.
The value of R changes depending on the type of food used for energy.
If the body uses only carbohydrates:
- R = 1.0
- One molecule of O₂ consumed produces one molecule of CO₂.
If the body uses only fats:
- R = 0.7
- Much of the O₂ combines with hydrogen to form water, so less CO₂ is produced.
On a normal mixed diet (carbohydrates + fats + proteins):
- Average R = 0.825

Easy Concept

Imagine the body is a factory.

O₂ = Raw material entering
CO₂ = Waste leaving

The Respiratory Exchange Ratio (R) tells us:

How much CO₂ waste comes out compared with the O₂ taken in.

Example 1: Mixed Diet (Normal)

O₂ In = 5 mL

CO₂ Out = 4 mL

↓

R = 4 ÷ 5

= 0.8 ≈ 0.82

Example 2: Carbohydrates Only 🍞

1 O₂ used

↓

1 CO₂ produced

↓

R = 1.0

Example 3: Fats Only 🧈

Some oxygen is used to make water, not CO₂.

More O₂ used

↓

Less CO₂ produced

↓

R = 0.7

Easy Memory Table

Fuel Used	Respiratory Exchange Ratio (R)
Carbohydrates only	1.0
Mixed diet	0.825
Fats only	0.7

KEY CONCEPT

The Respiratory Exchange Ratio (R) is the ratio of CO₂ output to O₂ uptake. Normally, R is about 0.82 because about 4 mL of CO₂ is produced for every 5 mL of O₂ consumed. R is highest (1.0) when carbohydrates are the main fuel and lowest (0.7) when fats are the main fuel.