• Maintaining the tonicity (osmolarity) of the extracellular fluid (ECF) is mainly controlled by:
  • Vasopressin (ADH) secretion
  • The thirst mechanism
• Total body osmolality depends on:
  • Total body sodium (Na⁺)
  • Total body potassium (K⁺)
  • Total body water
• Body osmolality is directly proportional to: (Total Body Sodium + Total Body Potassium) ÷ Total Body Water
• Body fluid osmolality changes when there is an imbalance between:
  • The amount of electrolytes (Na⁺ and K⁺).
  • The amount of water taken into or lost from the body.
• When the plasma becomes hypertonic (osmotic pressure increases):
  • Vasopressin (ADH) secretion increases.
  • The thirst centre is stimulated.
  • The kidneys retain more water.
  • The retained water dilutes the concentrated (hypertonic) plasma.
  • Water intake increases because of thirst.
• When the plasma becomes hypotonic (osmotic pressure decreases):
  • Vasopressin (ADH) secretion decreases.
  • The kidneys excrete more solute-free water (water without excess electrolytes).
  • Excess water is removed from the body.
  • Plasma osmolarity returns toward normal.
• These mechanisms keep body fluid tonicity within a very narrow normal range.
• Normal plasma osmolality is about 280–295 mOsm/kg H₂O.
• Vasopressin (ADH) secretion is maximally suppressed at about 285 mOsm/kg H₂O.
• When plasma osmolality rises above 285 mOsm/kg H₂O, vasopressin secretion increases progressively.

Figure: Figure 38–1, Figure 38–2

KEY CONCEPT

• ECF tonicity is mainly regulated by vasopressin (ADH) and the thirst mechanism. When plasma becomes hypertonic, ADH secretion and thirst increase to conserve and replace water. When plasma becomes hypotonic, ADH secretion decreases, allowing excess solute-free water to be excreted. Together, these mechanisms maintain plasma osmolality within the normal range of 280–295 mOsm/kg H₂O.

Figure 38-2 – Relationship Between Plasma Osmolality and Plasma Vasopressin (ADH)

Easy Conceptual Summary for SELF LEARNERS

This graph explains one of the most important mechanisms in body fluid regulation:

As plasma osmolality increases, the secretion of vasopressin (ADH) increases almost linearly.

In simple words:

• Blood becomes concentrated (↑ osmolality) → ADH secretion increases.
• Blood becomes dilute (↓ osmolality) → ADH secretion decreases.

This helps the body maintain a constant plasma osmolality.

Basic Concept

Imagine your body as a water tank.

The brain continuously checks:

“Is my blood too concentrated or too dilute?”

If the blood becomes too concentrated (water loss),

the brain says:

“Save water!”

It releases ADH (vasopressin).

ADH tells the kidneys:

“Reabsorb more water and produce less urine.”

This dilutes the blood back toward normal.

Understanding the Axes

X-axis

Plasma Osmolality (mOsm/kg)

This shows:

How concentrated the blood is.

Moving to the right means:

➡️ Blood is becoming more concentrated.

Normal plasma osmolality:

≈ 285–295 mOsm/kg

Y-axis

Plasma Vasopressin (pmol/L)

This shows:

How much ADH is present in the blood.

Higher on the graph means:

➡️ More ADH secretion.

Understanding the Graph

The graph contains:

• Many red dots
• One blue line
• One dashed horizontal line (LD)

Each has a meaning.

Red Dots

Each red dot represents:

One healthy person

during infusion of hypertonic saline.

Hypertonic saline increases plasma osmolality,

causing ADH release.

Why are the dots scattered?

Different people respond slightly differently.

Some release:

• More ADH
• Less ADH

But the overall pattern is the same.

Key Point

Although individuals vary,

everyone shows the same general trend:

Higher osmolality

↓

Higher ADH

Blue Line

This is the average relationship.

It summarizes all the red dots.

What does it show?

Initially,

ADH secretion is almost zero.

Then,

once plasma osmolality reaches about

285–290 mOsm/kg

ADH secretion begins.

After that,

ADH increases almost linearly.

Easy Concept

Imagine a thermostat.

Below a certain temperature,

the heater stays OFF.

Once the temperature drops below the set point,

the heater turns ON and works harder.

ADH behaves exactly like this.

The Threshold

Notice that ADH does not increase immediately.

It first reaches a threshold.What is the threshold?

Approximately

285–290 mOsm/kg

Below this value,

very little ADH is secreted.

Above this value,

ADH secretion rises rapidly.

Easy Concept

Think of a fire alarm.

Small amounts of smoke do nothing.

Once enough smoke accumulates,

the alarm suddenly turns on.

The osmoreceptors work in the same way.

Why Does ADH Increase?

When plasma osmolality rises,

water has been lost from the body.

This causes:

Hypothalamic osmoreceptors shrink.

↓

They stimulate the:

Supraoptic nucleus

and

Paraventricular nucleus

↓

Posterior pituitary releases:

Vasopressin (ADH)

↓

ADH reaches the kidney.

What Does ADH Do?

ADH acts mainly on:

Collecting ducts

It inserts:

Aquaporin-2 channels

Water moves from urine back into blood.

Result:

• More water reabsorbed
• Less urine
• Concentrated urine
• Plasma osmolality decreases toward normal

Flow Diagram

↑ Plasma osmolality
        ↓
Hypothalamic osmoreceptors stimulated
        ↓
Posterior pituitary releases ADH
        ↓
Collecting duct inserts Aquaporin-2 channels
        ↓
↑ Water reabsorption
        ↓
↓ Urine volume
        ↓
↑ Urine concentration
        ↓
Plasma osmolality returns toward normal

What Happens if Plasma Osmolality Falls?

Suppose you drink 2 litres of water.

Blood becomes diluted.

Plasma osmolality decreases.

Now,

the osmoreceptors swell.

They reduce ADH secretion.

The kidneys stop reabsorbing excess water.

Large amounts of dilute urine are produced.

Plasma osmolality returns to normal.

Flow Diagram

↓ Plasma osmolality
        ↓
Osmoreceptors inhibited
        ↓
↓ ADH secretion
        ↓
Less water reabsorbed
        ↓
Large volume of dilute urine
        ↓
Plasma osmolality returns toward normal

What is LD?

The dashed horizontal line is labelled:

LD = Limit of Detection

Meaning

Very tiny amounts of ADH cannot be measured accurately.

Below this level,

ADH is present in such small amounts that it is almost undetectable.

Easy Concept

Imagine a weighing scale.

If something weighs only 0.1 gram,

the scale may not detect it.

Similarly,

very small amounts of ADH are below the laboratory’s detection limit.

Clinical Importance

1. Dehydration

Water loss

↓

Plasma osmolality increases

↓

ADH secretion increases greatly

↓

Small volume of concentrated urine

2. Excess Water Intake

Plasma becomes dilute

↓

ADH falls

↓

Large volume of dilute urine

3. Diabetes Insipidus

ADH is absent or ineffective.

Even when plasma osmolality rises,

ADH cannot work.

Patient develops:

• Polyuria
• Polydipsia
• Hypernatremia
• High plasma osmolality

4. SIADH

Too much ADH is secreted.

Even normal plasma osmolality causes excessive water retention.

Patient develops:

• Water retention
• Hyponatremia
• Low plasma osmolality
• Concentrated urine

Quick Comparison Table

Plasma Osmolality	ADH Secretion	Urine
Low	Very low	Large volume, dilute
Normal	Small amount	Normal
High	High	Small volume, concentrated

Easy Memory Trick

High Osmolality = High ADH 💧

Blood concentrated

↓

Save water

↓

ADH rises

Low Osmolality = Low ADH 🚰

Blood dilute

↓

Remove water

↓

ADH falls

One-Line Summary

“Saltier blood makes the brain release more ADH to save water.”

Key Concept

This graph demonstrates the direct relationship between plasma osmolality and vasopressin (ADH) secretion. When plasma osmolality is below the osmotic threshold (about 285–290 mOsm/kg), only very small amounts of ADH are released. Once this threshold is exceeded, hypothalamic osmoreceptors stimulate the posterior pituitary to release increasing amounts of ADH, producing an almost linear rise in plasma vasopressin concentration as plasma osmolality increases. ADH acts on the collecting ducts of the kidneys, increasing Aquaporin-2 water channels, which enhances water reabsorption, decreases urine volume, and produces concentrated urine. This negative feedback mechanism restores plasma osmolality toward normal. Thus, higher plasma osmolality leads to greater ADH secretion, whereas lower plasma osmolality suppresses ADH release and promotes the excretion of dilute urine.

VASOPRESSIN RECEPTORS

• There are three main types of vasopressin (ADH) receptors:
  • V1A receptor
  • V1B receptor
  • V2 receptor
• All vasopressin receptors are G-protein-coupled receptors (GPCRs).
• V1A and V1B receptors work through the phosphatidylinositol (IP₃/DAG) pathway.
• Activation of V1A and V1B receptors increases the intracellular calcium (Ca²⁺) concentration.
• The rise in intracellular Ca²⁺ produces the cellular response to vasopressin.
• V2 receptors work through the Gs protein pathway.
• Activation of V2 receptors stimulates adenylyl cyclase.
• This increases the production of cyclic adenosine 3′,5′-monophosphate (cAMP).
• The increase in cAMP produces the cellular effects of V2 receptor activation.

KEY CONCEPT

• Vasopressin (ADH) acts through three G-protein-coupled receptors. V1A and V1B receptors use the phosphatidylinositol (IP₃/DAG) pathway to increase intracellular Ca²⁺, whereas V2 receptors use the Gs–cAMP pathway to produce their effects.

EFFECTS OF VASOPRESSIN

• Vasopressin is also called Antidiuretic Hormone (ADH).
• Its main function is to help the kidneys conserve water.
• ADH increases the permeability of the collecting ducts in the kidneys to water.
• As a result, more water moves from the collecting ducts into the hypertonic renal medulla (renal pyramids).
• This causes the urine to become more concentrated.
• The volume of urine decreases.
• Overall, ADH retains more water than solute in the body.
• As more water is retained, the osmotic pressure (osmolality) of body fluids decreases toward normal.
• In the absence of ADH:
  • The collecting ducts become less permeable to water.
  • Less water is reabsorbed.
  • Urine becomes dilute (hypotonic compared with plasma).
  • Urine volume increases.
  • The body loses excess water.
  • Body fluid osmolality increases.
• The antidiuretic effect of ADH is mediated by V2 receptors.
• V2 receptors insert Aquaporin-2 (AQP2) water channels into the apical (luminal) membrane of principal cells in the collecting ducts.
• Aquaporin-2 channels allow water to move rapidly across the cell membrane.
• These water channels are normally stored inside the cell in endosomes.
• When ADH binds to V2 receptors, Aquaporin-2 channels quickly move to the luminal membrane.
• This greatly increases water reabsorption.
• V1A receptors produce the vasoconstrictor effect of ADH.
• Activation of V1A receptors causes vascular smooth muscle to contract.
• Although ADH is a strong vasoconstrictor, relatively high levels are required to increase blood pressure in the body.
• This is because ADH also acts on the brain to reduce cardiac output.
• This brain effect occurs mainly at the area postrema.
• Severe hemorrhage strongly stimulates ADH secretion.
• During hemorrhage, ADH helps maintain blood pressure.
• Blocking the vasoconstrictor action of ADH causes a greater fall in blood pressure after hemorrhage.
• Therefore, ADH plays an important role in maintaining blood pressure (blood pressure homeostasis).
• V1A receptors are also present in the liver.
• In the liver, ADH stimulates glycogen breakdown (glycogenolysis).
• V1A receptors are also found in the brain.
• In the brain and spinal cord, ADH functions as a neurotransmitter.
• V1B receptors (also called V3 receptors) are mainly found in the anterior pituitary gland.
• V1B receptors stimulate corticotroph cells to release adrenocorticotropic hormone (ACTH).

KEY CONCEPT

• Vasopressin (ADH) mainly conserves body water by acting on V2 receptors, which insert Aquaporin-2 channels into the collecting ducts and increase water reabsorption. V1A receptors cause vasoconstriction, stimulate liver glycogenolysis, and act in the brain, while V1B (V3) receptors stimulate ACTH secretion from the anterior pituitary.

METABOLISM
Circulating vasopressin is rapidly inactivated, principally in
the liver and kidneys. It has a biologic half-life of approxi￾mately 18 minutes in humans.

CONTROL OF VASOPRESSIN SECRETION: OSMOTIC STIMULI

• Vasopressin (ADH) is stored in the posterior pituitary gland.
• It is released into the bloodstream when nerve impulses travel through the neurons that contain ADH.
• Many factors affect ADH secretion.

Table: Table 38–1

• The most important osmotic stimulus for ADH secretion is an increase in plasma osmolality.
• When plasma osmolality rises above about 285 mOsm/kg, ADH-secreting neurons become more active.
• As a result, ADH is released into the bloodstream.
• At about 285 mOsm/kg, plasma ADH levels are very low or almost undetectable.
• When plasma osmolality falls below 285 mOsm/kg, ADH secretion decreases even further.
• ADH secretion is controlled by osmoreceptors.
• These osmoreceptors are located in the anterior hypothalamus.
• The osmoreceptors lie outside the blood–brain barrier.
• They are mainly located in the Organum Vasculosum of the Lamina Terminalis (OVLT), which is one of the circumventricular organs.
• The osmotic threshold for thirst is the same as or slightly higher than the threshold for ADH secretion.
• It is still uncertain whether the same osmoreceptors control both thirst and ADH secretion.

Figure: Figure 38–1, Figure 38–2

• ADH secretion is controlled by a very sensitive negative feedback mechanism.
• This feedback system continuously monitors plasma osmolality.
• Even a change of about 1% in plasma osmolality can significantly alter ADH secretion.
• Because of this precise regulation, normal plasma osmolality is maintained close to 285 mOsm/L.

KEY CONCEPT

• Vasopressin (ADH) secretion is mainly controlled by plasma osmolality. Osmoreceptors in the anterior hypothalamus (primarily the OVLT) detect small increases in plasma osmolality above about 285 mOsm/kg and stimulate ADH release. This highly sensitive negative feedback mechanism, together with the thirst response, maintains plasma osmolality close to 285 mOsm/L.

VOLUME EFFECTS ON VASOPRESSIN SECRETION

• Extracellular fluid (ECF) volume also regulates vasopressin (ADH) secretion.
• When ECF volume decreases, ADH secretion increases.
• When ECF volume increases, ADH secretion decreases.

Table: Table 38–1

• ADH secretion is inversely related to the activity of stretch receptors (baroreceptors).
• When stretch receptor activity decreases, ADH secretion increases.
• When stretch receptor activity increases, ADH secretion decreases.
• Stretch receptors are located in two parts of the circulation:
  • Low-pressure receptors
  • High-pressure receptors
• Low-pressure receptors are found in:
  • Great veins
  • Right atrium
  • Left atrium
  • Pulmonary blood vessels
• High-pressure receptors are found in:
  • Carotid sinuses
  • Aortic arch
• A fall in blood pressure causes a marked increase in plasma ADH levels.

Figure: Figure 38–3

• Low-pressure receptors monitor how full the vascular system is (blood volume).
• Even a moderate decrease in blood volume can increase ADH secretion.
• This can occur even when arterial blood pressure remains normal.
• Therefore, low-pressure receptors are the main receptors controlling the effect of blood volume on ADH secretion.
• Sensory impulses from these receptors travel through the vagus nerves.
• These impulses reach the Nucleus of the Tractus Solitarius (NTS) in the brainstem.
• The NTS sends inhibitory signals to the Caudal Ventrolateral Medulla (CVLM).
• The CVLM sends excitatory signals to the hypothalamus.
• These pathways help regulate ADH secretion.
• Angiotensin II further increases ADH secretion during hypovolemia (low blood volume) and hypotension (low blood pressure).
• It acts mainly on the circumventricular organs of the brain.
• Severe blood loss (hemorrhage) causes the release of large amounts of ADH.
• During hypovolemia, the osmotic response curve for ADH shifts to the left.
• This means ADH is released at lower plasma osmolality than normal.
• The response curve also becomes steeper.

Figure: Figure 38–4

• As a result:
  • Water reabsorption increases.
  • More water is retained in the body.
  • Plasma osmolality decreases.
  • Plasma sodium concentration also decreases (hyponatremia).
• Hyponatremia occurs because sodium (Na⁺) is the major osmotically active substance in plasma, and excess retained water dilutes its concentration.

KEY CONCEPT

• Besides plasma osmolality, ECF volume is an important regulator of ADH secretion. A decrease in blood volume or blood pressure reduces stretch receptor activity, increasing ADH release through brainstem and hypothalamic pathways. Angiotensin II further enhances this response. During hypovolemia, ADH is released more easily, promoting water retention and potentially causing dilutional hyponatremia.

Figure 38-3 – Relationship Between Mean Arterial Blood Pressure and Plasma Vasopressin (ADH)

Easy Conceptual Summary for self learners

This figure explains how changes in blood pressure affect the secretion of vasopressin (ADH).

The graph shows a very important physiological principle:

A small fall in blood pressure causes little change in ADH, but a large fall in blood pressure causes a massive increase in ADH secretion.

Unlike osmolality (which stimulates ADH in a linear manner), blood pressure stimulates ADH in an exponential manner.

Basic Concept

Imagine your body as a city water supply system.

There are two emergency levels:

Situation 1: Slight Water Shortage

The city continues normally.

No emergency measures are needed.

Situation 2: Major Water Pipe Burst

Now the city immediately:

• Saves water
• Closes unnecessary water use
• Starts emergency pumps

Your body behaves exactly the same.

When blood pressure falls only a little,

little ADH is released.

When blood pressure falls a lot,

the body enters emergency mode,

and ADH secretion increases dramatically.

Understanding the Axes

X-axis

% Change in Mean Arterial Blood Pressure

This shows:

How much blood pressure has fallen.

Notice the values are negative.

Example:

• 0% = Normal blood pressure
• −10% = Blood pressure decreased by 10%
• −20% = Blood pressure decreased by 20%
• −30% = Blood pressure decreased by 30%

The farther left you move,

the greater the fall in blood pressure.

Y-axis

Plasma Vasopressin (pmol/L)

This shows:

The amount of ADH in the blood.

Higher values mean:

➡️ More ADH secretion.

Understanding the Graph

The graph contains:

• Brown dots
• Blue curve

Brown Dots

Each dot represents:

One healthy person

whose blood pressure was gradually lowered using the ganglionic blocker trimethaphan.

Because every individual responds slightly differently,

the dots are scattered.

Key Point

Despite individual variation,

everyone follows the same overall pattern.

Blue Curve

This represents the average response.

Notice that it is curved, not straight.

This means:

The relationship is exponential, not linear.

What Happens at Normal Blood Pressure?

At

0% change

(blood pressure is normal)

ADH secretion is very low.

Only a small amount of ADH is needed.

Why?

Because the kidneys do not need to conserve extra water.

Small Fall in Blood Pressure (−5% to −10%)

Look near the right side of the graph.

Blood pressure decreases slightly.

ADH increases only a little.

Why?

The body can compensate using:

• Increased heart rate
• Vasoconstriction
• Slight ADH release

No emergency response is required.

Easy Concept

Losing a small glass of water from a bucket.

You hardly notice it.

Key Point

Small fall in blood pressure

↓

Small increase in ADH

Moderate Fall in Blood Pressure (−10% to −20%)

Now,

blood pressure decreases further.

The curve becomes steeper.

ADH secretion rises much faster.

Why?

Baroreceptors begin firing much less.

The brain interprets this as significant volume loss.

The posterior pituitary releases much more ADH.

Easy Concept

Now the bucket is half empty.

The body starts saving water seriously.

Key Point

Moderate hypotension

↓

Marked increase in ADH

Severe Fall in Blood Pressure (−20% to −30%)

Now look at the far left.

The curve rises almost vertically.

ADH levels become extremely high.

Why?

This is an emergency.

The body must preserve every possible drop of water.

ADH secretion becomes massive.

Easy Concept

Imagine a dam breaking.

Every emergency alarm activates.

Maximum water conservation begins.

Key Point

Severe hypotension

↓

Massive ADH release

↓

Maximum water conservation

Why Does Blood Pressure Affect ADH?

The body contains:

Baroreceptors

Located in:

• Carotid sinus
• Aortic arch
• Left atrium
• Pulmonary vessels

Normal Blood Pressure

Baroreceptors are stretched.

They continuously send impulses to the brain.

These impulses inhibit ADH release.

Blood Pressure Falls

Less stretch

↓

Less baroreceptor firing

↓

Hypothalamus becomes activated

↓

Posterior pituitary releases ADH

Flow Diagram

↓ Blood pressure
        ↓
↓ Stretch of baroreceptors
        ↓
↓ Baroreceptor firing
        ↓
Hypothalamus stimulated
        ↓
Posterior pituitary releases ADH
        ↓
Kidneys reabsorb more water
        ↓
Blood volume increases
        ↓
Blood pressure improves

What Does ADH Do During Hypotension?

ADH helps raise blood pressure in two ways.

1. Water Conservation

ADH acts on collecting ducts.

More water is reabsorbed.

↓

Blood volume increases.

2. Vasoconstriction

At high concentrations,

ADH binds to:

V₁ receptors

on blood vessels.

This causes:

Vasoconstriction

↓

Peripheral resistance increases

↓

Blood pressure rises.

Easy Memory

V₁ = Vessel

↓

Constriction

V₂ = Kidney Tubules

↓

Water reabsorption

Why Is This Curve Exponential?

Unlike osmolality,

blood pressure control is designed for emergencies.

A small fall in pressure is not dangerous.

A large fall may threaten life.

Therefore,

ADH secretion increases very rapidly once blood pressure falls significantly.

Easy Concept

Think of car brakes.

Press lightly,

small braking.

Press hard,

very powerful braking.

ADH behaves similarly during hypotension.

Comparison with Osmolality

Osmolality	Blood Pressure
Linear response	Exponential response
Very sensitive	Activated mainly during major hypotension
Controls water balance	Emergency volume preservation

Clinical Importance

1. Hemorrhage

Blood loss

↓

Blood pressure falls

↓

Massive ADH secretion

↓

Water retention

↓

Helps restore blood volume.

2. Severe Dehydration

Loss of water

↓

Blood volume decreases

↓

ADH secretion becomes very high.

3. Shock

Very low blood pressure

↓

Extremely high ADH

↓

Maximum vasoconstriction

↓

Maximum water conservation.

Quick Summary Table

Blood Pressure	ADH Level	Physiological Response
Normal	Low	Normal urine formation
Slight fall	Slight increase	Mild water conservation
Moderate fall	Moderate increase	Increased water reabsorption
Severe fall	Massive increase	Maximum water conservation + vasoconstriction

Easy Memory Trick

Normal BP = Normal ADH 🙂

Little water conservation.

Low BP = More ADH 💧

Save water.

Very Low BP = Maximum ADH 🚨

Emergency mode.

Water conservation + vasoconstriction.

One-Line Summary

“The lower the blood pressure, the higher the ADH secretion—and the increase becomes dramatic when blood pressure falls severely.”

Key Concept

This graph demonstrates the relationship between mean arterial blood pressure and plasma vasopressin (ADH) secretion. Under normal conditions or with only a small fall in blood pressure, plasma ADH levels remain low because baroreceptors continue to inhibit its release. As blood pressure decreases further, baroreceptor stretch and firing decrease, removing this inhibition and stimulating the hypothalamus and posterior pituitary to release increasing amounts of ADH. Unlike the response to plasma osmolality, this relationship is exponential rather than linear, meaning that large decreases in blood pressure produce disproportionately large increases in ADH secretion. ADH then acts on V₂ receptors in the kidneys to increase water reabsorption and, at high concentrations, on V₁ receptors in blood vessels to cause vasoconstriction. Together, these effects increase blood volume and systemic vascular resistance, helping restore arterial blood pressure during severe hypotension, dehydration, or hemorrhage.

Figure 38-4 – Effect of Hypovolemia and Hypervolemia on the Relationship Between Plasma Vasopressin (ADH) and Plasma Osmolality

Easy Conceptual Explanation for self learners

This graph explains a very important physiological principle:

Plasma osmolality is the main controller of ADH, but blood volume can “reset” the sensitivity of this control.

In simple words:

• When blood volume decreases (hypovolemia), the body releases ADH earlier and in larger amounts.
• When blood volume increases (hypervolemia), the body becomes less eager to release ADH.

Think of blood volume as the “volume control knob” that changes how strongly osmolality stimulates ADH.

Step 1: Understand the Axes

X-axis = Plasma Osmolality (mOsm/kg)

This shows how concentrated the blood is.

Moving to the right means:

➡️ Blood becomes more concentrated (less water).

Example:

• Drinking little water
• Sweating
• Dehydration

Y-axis = Plasma Vasopressin (pAVP) (pmol/L)

This shows the amount of ADH (vasopressin) in the blood.

Higher on the graph means:

➡️ More ADH is released.

Step 2: What are the Dots?

There are two groups of dots.

🟢 Green Dots

These represent:

Hypovolemia

↓

Low blood volume

Produced experimentally by:

Water deprivation (dehydration).

🟤 Brown/Red Dots

These represent:

Hypervolemia

↓

High blood volume

Produced experimentally by:

Infusion of hypertonic saline.

Although hypertonic saline increases osmolality, it also expands the extracellular fluid volume.Step 3: What Do the Two Lines Mean?

There are two regression lines.

🟣 Purple Dashed Line

Represents:

Hypovolemia

Notice:

• Starts further to the left.
• Is steeper.

These two features are the key to understanding the graph.

🔵 Blue Solid Line

Represents:

Hypervolemia

Notice:Understanding the Hypovolemia Line (Purple Dashed)

Imagine someone has:

• Lost blood
• Severe dehydration
• Vomiting
• Diarrhea

Blood volume becomes low.

The body becomes extremely protective of water.

Therefore:

Even a tiny increase in plasma osmolality causes a large ADH release.

Two Important Changes

1. Left Shift

Look where the purple line begins.

It starts earlier than the blue line.

That means:

The body begins secreting ADH at a lower osmolality.

Easy Concept

Normally:

The body waits before turning on the water-saving system.

During hypovolemia:

It presses the “Save Water” button much earlier.

2. Steeper Slope

The purple line rises more steeply.

This means:

For every small increase in osmolality,

ADH rises much more.

Easy Concept

A very sensitive alarm.

Small danger

↓

Huge response.Understanding the Hypervolemia Line (Blue Solid)

Now imagine:

The body already has plenty of fluid.

Examples:

• IV fluids
• Excess water intake
• Expanded blood volume

There is no urgency to save water.

Therefore:

The body becomes less sensitive.

Right Shift

The blue line begins later.

The body waits until osmolality becomes higher before releasing ADH.

Gentle Slope

The blue line rises more slowly.

ADH increases gradually.

Easy Concept

A relaxed alarm system.

Only responds when really necessary.

Compare Both Lines

At the Same Osmolality

Suppose plasma osmolality is:

295 mOsm/kg

Look vertically upward.

The purple line is much higher.

That means:

Hypovolemic person

↓

Produces much more ADH.

The hypervolemic person

↓

Produces much less ADH.

Why Does This Happen?

The body receives information from two systems.

1. Osmoreceptors

Located in the hypothalamus.

They monitor:

Plasma osmolality.

2. Baroreceptors

Located in:

• Carotid sinus
• Aortic arch
• Left atrium
• Pulmonary vessels

They monitor:

Blood volume and blood pressure.

During Hypovolemia

Baroreceptors detect:

Low stretch

↓

Low blood volume

↓

Brain says:

“Save every drop of water.”

↓

ADH secretion becomes much easier.

During Hypervolemia

Baroreceptors detect:

High stretch

↓

Enough blood volume

↓

Brain says:

“No urgent need to conserve water.”

↓

Less ADH is released.

Flow Diagram

During Hypovolemia

↓ Blood volume
        ↓
↓ Baroreceptor firing
        ↓
Hypothalamus stimulated
        ↓
ADH released earlier
        ↓
More water reabsorbed
        ↓
Blood volume restored

During Hypervolemia

↑ Blood volume
        ↓
↑ Baroreceptor firing
        ↓
Hypothalamus inhibited
        ↓
Less ADH released
        ↓
Less water reabsorbed
        ↓
Excess water excreted

What is “LD”?

At the bottom is:

LD = Limit of Detection

This means:

Below this level,

ADH is so low that laboratory tests cannot reliably detect it.

It does not mean ADH is completely absent.

Clinical Correlation

Severe Hemorrhage

Blood loss

↓

Hypovolemia

↓

ADH secretion begins at lower osmolality

↓

Maximum water conservation.

Severe Dehydration

Water loss

↓

Blood volume falls

↓

Steep ADH response

↓

Very concentrated urine.After Receiving IV Fluids

Blood volume increases

↓

ADH secretion decreases

↓

More dilute urine.

Easy Memory Trick

Imagine two security guards.

🟢 Guard 1 = Hypovolemia

Very nervous.

At the slightest warning,

he sounds the alarm.

Earlier + Stronger response

🔵 Guard 2 = Hypervolemia

Very relaxed.

He waits longer.

His response is weaker.

Later + Smaller response

Comparison Table

Feature	Hypovolemia (Purple Dashed)	Hypervolemia (Blue Solid)
Blood volume	Low	High
ADH threshold	Lower	Higher
Graph shift	Left	Right
Slope	Steeper	Less steep
ADH secretion	Much greater	Smaller
Water conservation	Maximum	Minimal

High-Yield Exam Points

• Plasma osmolality is the primary regulator of ADH secretion.
• Hypovolemia increases the sensitivity of osmoreceptors to plasma osmolality.
• Hypovolemia shifts the ADH–osmolality curve to the left and makes it steeper.
• Hypervolemia decreases the sensitivity of ADH secretion, shifting the curve to the right with a less steep slope.
• Baroreceptor input modifies, but does not replace, osmotic regulation of ADH.

One-Line Summary

Low blood volume makes the body release ADH earlier and more aggressively, whereas high blood volume delays and reduces ADH release, even at the same plasma osmolality.

Key Concept

This graph demonstrates how blood volume modifies the osmotic control of vasopressin (ADH) secretion. Under hypovolemic conditions (green dots, purple dashed line), decreased baroreceptor firing enhances hypothalamic sensitivity to osmolality, causing ADH secretion to begin at a lower plasma osmolality (leftward shift) and to increase more rapidly (steeper slope). This ensures maximal water conservation during dehydration or blood loss. Under hypervolemic conditions (brown/red dots, blue solid line), increased baroreceptor firing suppresses ADH release, so a higher plasma osmolality is required before ADH secretion begins (rightward shift), and the increase in ADH is less pronounced. Thus, osmolality is the primary stimulus for ADH release, while blood volume and pressure adjust the threshold and sensitivity of this response to maintain body fluid homeostasis.

OTHER STIMULI AFFECTING
VASOPRESSIN SECRETION
A variety of stimuli in addition to osmotic pressure changes
and ECF volume aberrations increase vasopressin secretion.
These include pain, nausea, surgical stress, and some emo￾tions (Table 38–1). Nausea is associated with particularly large
increases in vasopressin secretion. Alcohol decreases vaso￾pressin secretion.

CLINICAL IMPLICATIONS

• Volume changes and other nonosmotic factors can influence the normal osmotic control of vasopressin (ADH) secretion.
• After surgery, ADH levels are often increased.
• This increase is mainly due to:
  • Pain
  • Hypovolemia (decreased blood volume)
• High ADH levels cause the kidneys to retain more water.
• Excess water retention lowers plasma osmolality.
• This can produce dilutional hyponatremia (low plasma sodium due to excess water).

Clinical Box: Clinical Box 38–1

• Diabetes insipidus is a disorder caused by inadequate action of ADH.
• There are two main types of diabetes insipidus:
  • Central diabetes insipidus
  • Nephrogenic diabetes insipidus

Central Diabetes Insipidus

• Central diabetes insipidus occurs because the body does not produce enough ADH.
• ADH deficiency may result from diseases affecting:
  • Supraoptic nucleus
  • Paraventricular nucleus
  • Hypothalamo-hypophyseal tract
  • Posterior pituitary gland
• The major causes include:
  • About 30% due to tumors (primary or metastatic) involving the hypothalamus.
  • About 30% due to head trauma.
  • About 30% are idiopathic (unknown cause).
  • The remaining cases are caused by vascular diseases, infections, systemic diseases (such as sarcoidosis), or mutations of the prepropressophysin gene.
• Diabetes insipidus may temporarily occur after surgical removal of the posterior pituitary.
• If the hypothalamic nerve fibers are only injured and not destroyed, they can recover.
• The damaged nerve fibers form new vascular connections and begin releasing ADH again.

Symptoms of Diabetes Insipidus

• The main symptoms are:
  • Polyuria (passing large amounts of dilute urine).
  • Polydipsia (drinking large amounts of water).
• Polydipsia helps maintain normal body water and prevents dehydration.
• If the thirst mechanism is impaired or water intake decreases, severe dehydration can develop.
• This dehydration may become life-threatening.

Nephrogenic Diabetes Insipidus

• Nephrogenic diabetes insipidus occurs when the kidneys do not respond normally to ADH.
• There are two major forms:

1. V2 Receptor Defect

• Mutation of the V2 receptor gene makes the receptor unresponsive to ADH.
• The V2 receptor gene is located on the X chromosome.
• Therefore, this form is inherited as an X-linked recessive disorder.

2. Aquaporin-2 Defect

• Mutation of the Aquaporin-2 (AQP2) gene produces abnormal water channels.
• These defective water channels do not function properly.
• Many abnormal AQP2 channels fail to reach the apical membrane of collecting duct cells.
• Instead, they remain trapped inside the cells.
• As a result, water reabsorption is reduced despite normal ADH levels.

KEY CONCEPT

• Clinical conditions such as pain and hypovolemia increase ADH secretion and may cause dilutional hyponatremia. Diabetes insipidus results from either decreased ADH production (central diabetes insipidus) or failure of the kidneys to respond to ADH (nephrogenic diabetes insipidus). Both forms cause polyuria and polydipsia because the kidneys cannot conserve water effectively.

SYNDROME OF INAPPROPRIATE ANTIDIURETIC HORMONE (SIADH)

• SIADH occurs when vasopressin (ADH) secretion is abnormally high compared with the plasma (serum) osmolality.
• Excess ADH causes the kidneys to retain too much water.
• This excess water dilutes the blood.
• As a result, the serum sodium concentration falls (dilutional hyponatremia).
• Hyponatremia is defined as a serum sodium level below 135 mmol/L.
• When water retention expands the extracellular fluid (ECF) volume, aldosterone secretion decreases.
• Lower aldosterone levels reduce sodium reabsorption by the kidneys.
• This causes increased loss of sodium in the urine (salt wasting).
• Salt wasting may occur in:
  • Patients with brain diseases (cerebral salt wasting).
  • Patients with lung diseases (pulmonary salt wasting).
• In lung diseases, such as lung cancer, increased ADH secretion may occur because inhibitory signals from stretch receptors in the atria and great veins are reduced.
• Many lung tumors and some other cancers can produce and release ADH themselves.
• This abnormal hormone production further contributes to SIADH.
• The body has a protective mechanism called “vasopressin escape.”
• Vasopressin escape limits excessive water retention during prolonged high ADH levels.
• With prolonged exposure to high ADH, the kidneys reduce the production of Aquaporin-2 (AQP2) water channels.
• As the number of AQP2 channels decreases, less water is reabsorbed by the collecting ducts.
• Urine volume increases despite continued high ADH levels.
• This helps prevent severe water retention and limits the degree of hyponatremia.
• Therefore, the kidneys become less responsive to the water-retaining effect of ADH, a phenomenon known as vasopressin escape.

KEY CONCEPT

• SIADH is caused by excessive ADH secretion despite normal or low plasma osmolality. Excess ADH causes water retention, dilutional hyponatremia, and reduced aldosterone secretion, leading to urinary sodium loss. SIADH is commonly associated with brain disorders, lung diseases, and ADH-secreting tumors. During prolonged ADH excess, the kidneys reduce Aquaporin-2 expression, producing vasopressin escape, which limits further water retention and severe hyponatremia.

YNTHETIC AGONISTS & ANTAGONISTS

• Scientists have developed synthetic forms of vasopressin (ADH).
• These synthetic drugs are made by changing specific amino acids in the natural vasopressin molecule.
• The modified drugs have more selective actions than natural vasopressin.
• Some synthetic forms are also more potent than natural vasopressin.
• One important synthetic vasopressin analogue is Desmopressin (dDAVP).
• Its full name is 1-deamino-8-D-arginine vasopressin (dDAVP).
• Desmopressin has a very strong antidiuretic (water-retaining) effect.
• It has very little pressor (blood pressure–raising) effect.
• Because of this selective action, desmopressin is safer than natural vasopressin for increasing water reabsorption without causing significant vasoconstriction.
• Desmopressin is mainly used to treat vasopressin (ADH) deficiency.
• It is especially useful in patients with central diabetes insipidus.

KEY CONCEPT

• Synthetic vasopressin analogues are modified forms of ADH with selective actions. Desmopressin (dDAVP) is a synthetic ADH analogue that has a strong antidiuretic effect but minimal blood pressure–raising effect, making it the drug of choice for treating vasopressin deficiency, especially central diabetes insipidus.

THERAPEUTIC HIGHLIGHTS
Patients with inappropriate hypersecretion of vasopressin have been successfully treated with demeclocycline,
an antibiotic that reduces the renal response to
vasopressin.  

CLINICAL CONCEPT CHECK 38–1

Case Summary

• Age: 80-year-old woman
• Medical history:
  • Diabetes mellitus
  • Diabetic neuropathy
  • Hypertension
• Symptoms:
  • Abdominal pain
  • Nausea
  • No memory loss or dementia
  • No chest pain
• Medication:
  • Aspirin
  • Duloxetine (30 mg/day)

Laboratory Findings

• Serum sodium = 124 mmol/L
  • ↓ Hyponatremia
• Serum osmolality = 260 mmol/L
  • ↓ Low plasma osmolality (hypotonic plasma)
• Urine osmolality = 450 mmol/L
  • ↑ Concentrated urine
• Urine sodium = 155 mmol/L
  • ↑ High urinary sodium loss

Step-by-Step Interpretation

Step 1: Low serum sodium

• Serum sodium = 124 mmol/L
• This indicates hyponatremia.

Step 2: Low serum osmolality

• Serum osmolality = 260 mmol/L
• The plasma is hypotonic.

Step 3: Urine is concentrated

• Normally, when plasma osmolality is low, ADH secretion should be suppressed.
• The kidneys should produce large amounts of dilute urine.
• But this patient has concentrated urine (450 mmol/L).
• This means ADH is still acting on the kidneys.

Step 4: High urine sodium

• Urine sodium is 155 mmol/L, which is very high.
• Excess ADH causes:
  • Water retention
  • Expansion of ECF volume
  • Decreased aldosterone secretion
  • Increased sodium excretion in urine
• This pattern is typical of SIADH.

Step 5: Medication clue

• Duloxetine is a well-known medication that can induce SIADH, especially in elderly patients.

Most Likely Diagnosis

Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Why?

• Low serum sodium (124 mmol/L) → Dilutional hyponatremia.
• Low serum osmolality (260 mmol/L) → Excess body water.
• High urine osmolality (450 mmol/L) → ADH is inappropriately retaining water.
• High urine sodium (155 mmol/L) → Sodium wasting due to ECF expansion and reduced aldosterone.
• Duloxetine is a likely trigger for SIADH in this elderly patient.

KEY CONCEPT

• This patient most likely has SIADH. Normally, low plasma osmolality suppresses ADH secretion, producing dilute urine. In SIADH, ADH remains inappropriately elevated despite hypotonic plasma, causing water retention, dilutional hyponatremia, concentrated urine, and increased urinary sodium loss. In this case, duloxetine is the most likely precipitating factor.

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