If you truly want to understand IV fluids, you must first understand where fluid lives in the body and how it moves.

Too often, IV fluid therapy is taught as a list:

• Normal saline
• Hartmann’s
• Dextrose
• Hypertonic saline

But without understanding intracellular, extracellular, intravascular spaces, and the forces that govern fluid movement, fluid prescribing becomes memorisation instead of clinical reasoning.

This guide will break down:

• Body fluid compartments
• Fluid shift mechanisms
• Osmosis, hydrostatic and oncotic pressures
• Starling forces
• How these explain real clinical scenarios

By the end, IV fluid therapy will make physiological sense.

Total Body Water: Where Does Fluid Live?

Water makes up a large proportion of the human body:

• ~60% of body weight in adult males
• ~50–55% in adult females
• Lower in elderly patients
• Even higher in infants

This water is not randomly distributed. It is divided into compartments.

The Two Major Compartments

1. Intracellular Fluid (ICF)

This is the fluid inside cells.

• Makes up about two-thirds of total body water
• Rich in potassium (K+)
• Low in sodium (Na+)

This is the fluid that maintains cellular function, metabolism, and electrical activity.

If intracellular balance is disturbed, cells suffer.

2. Extracellular Fluid (ECF)

This is fluid outside cells.

It makes up about one-third of total body water and is divided into:

A. Intravascular Fluid

• Fluid inside blood vessels (plasma)
• Responsible for blood pressure and tissue perfusion
• About 20–25% of ECF

B. Interstitial Fluid

• Fluid surrounding tissues and cells
• About 75–80% of ECF

This distinction is critical.

When we administer IV fluids, we place them directly into the intravascular space — but they do not always stay there.

Understanding where they move next is the key to safe practice.

Why Fluid Compartments Matter in Clinical Practice

When a patient is hypotensive, the immediate question is:

Is the intravascular space depleted?

When a patient is oedematous:

Is fluid leaking into the interstitial space?

When a patient is confused with low sodium:

Has fluid shifted into brain cells?

All of these questions are about fluid movement between compartments.

And fluid movement is governed by powerful physiological forces.

The Four Forces That Control Fluid Movement

Fluid does not move randomly. It obeys basic physical principles.

The four key mechanisms are:

1. Osmosis
2. Diffusion
3. Hydrostatic pressure
4. Oncotic pressure

Let us break them down clinically.

Osmosis: Water Follows Sodium

Osmosis is the movement of water across a semipermeable membrane toward an area of higher solute concentration.

In simple terms:

Water moves toward sodium.

This principle explains most sodium-related emergencies.

Hypernatremia (High Sodium)

• Extracellular sodium increases
• Water moves out of cells
• Cells shrink

In the brain, this can cause confusion and neurological symptoms.

Hyponatremia (Low Sodium)

• Extracellular sodium decreases
• Water moves into cells
• Cells swell

Brain cells are particularly vulnerable. Severe hyponatremia can lead to:

• Seizures
• Reduced consciousness
• Cerebral oedema

This is why rapid correction must be avoided — overly fast shifts can cause osmotic demyelination syndrome.

Hydrostatic Pressure: The Push Force

Hydrostatic pressure is the pressure exerted by fluid against vessel walls.

Think of it as:

The force pushing fluid out of capillaries.

It increases with:

• High blood pressure
• Fluid overload
• Heart failure

When hydrostatic pressure rises:

• Fluid moves into interstitial space
• Oedema develops

This is why giving excess IV fluids in heart failure can worsen peripheral and pulmonary oedema.

Oncotic Pressure: The Pull Force

Oncotic pressure (also called colloid osmotic pressure) is generated by plasma proteins — primarily albumin.

It acts as:

The force pulling fluid back into blood vessels.

Low albumin levels (hypoalbuminemia) reduce this pull effect.

Common causes:

• Liver disease
• Malnutrition
• Nephrotic syndrome
• Severe burns

When oncotic pressure drops:

• Fluid leaks into tissues
• Oedema and ascites develop

This explains why some patients are swollen but intravascularly depleted at the same time.

Starling Forces: The Balance of Push and Pull

At the capillary level, fluid movement is determined by the balance between:

• Hydrostatic pressure (push out)
• Oncotic pressure (pull in)

This relationship is known as Starling forces.

If push exceeds pull → fluid leaves vessels
If pull exceeds push → fluid returns to vessels

In health, there is balance.

In disease, that balance is disrupted.

Clinical scenarios explained through fluid shifts

Understanding compartments and forces transforms clinical reasoning.

Sepsis

In sepsis:

• Capillaries become leaky
• Oncotic pull weakens
• Fluid shifts into interstitial space

The patient may appear swollen but remain hypotensive.

This explains why aggressive crystalloid resuscitation is often required early.

Heart Failure

In heart failure:

• Hydrostatic pressure increases
• Fluid pushed into lungs and peripheral tissues

Giving large volumes of IV fluids can worsen pulmonary oedema.

Fluid therapy must be cautious.

Burns

Severe burns cause:

• Capillary leak
• Massive fluid shifts into interstitial space
• Intravascular depletion

These patients require structured fluid resuscitation (e.g., Parkland formula).

Diabetic Ketoacidosis (DKA)

In DKA:

• Osmotic diuresis leads to severe dehydration
• Intravascular volume falls

Fluid resuscitation is the first priority before insulin correction.

Hyponatremia

Water moves into brain cells
→ Cerebral oedema
→ Neurological compromise

Careful correction with hypertonic saline may be required in severe cases.

Applying this knowledge to IV fluid choice

Now IV fluids begin to make sense.

Ask yourself:

1. Which compartment is depleted?
2. Do I need to expand intravascular volume?
3. Is sodium abnormal?
4. Is there capillary leak?
5. Is albumin low?

Isotonic Crystalloids (e.g., 0.9% NaCl, Hartmann’s)

Remain mostly in extracellular space
Used for intravascular resuscitation

Hypotonic Fluids

Shift water into cells
Used cautiously in hypernatremia

Hypertonic Fluids

Pull water out of cells
Used in severe hyponatremia or cerebral oedema

Colloids (e.g., Albumin)

Increase oncotic pressure
Help retain fluid intravascularly

Common misconceptions about IV fluids

❌ All IV fluids stay in blood vessels
✔ Most crystalloids redistribute into interstitial space

❌ Oedema means the patient is fluid overloaded
✔ A patient can be oedematous but intravascularly dry

❌ More fluids always improve blood pressure
✔ Excess fluids can worsen outcomes in certain patients

The golden clinical question

Before prescribing or administering IV fluids, always ask:

What is happening at the cellular and capillary level?

This is the difference between task-based care and physiology-based practice.

Understanding fluid compartments and fluid shifts:

• Improves escalation decisions
• Strengthens sepsis recognition
• Supports safe cannulation and IV administration
• Enhances NEWS2 interpretation
• Reduces iatrogenic fluid overload

Final thoughts

IV fluids are not just bags of water.

They are powerful physiological interventions that influence:

• Cellular integrity
• Brain function
• Blood pressure
• Organ perfusion
• Mortality outcomes

When you understand intracellular, extracellular, intravascular and interstitial spaces — and the forces that move fluid between them — IV fluid therapy becomes logical, safe, and clinically intelligent.

The next time you hang a bag of fluid, you will not just be administering it.

You will understand exactly where it is going — and why.