Extracellular fluid osmolarity and sodium concentration are controlled together.
Sodium is the most abundant ion in the extracellular fluid.
Normal plasma sodium concentration = 135–145 mEq/L.
Average plasma sodium concentration = 142 mEq/L.
Average osmolarity = 300 mOsm/L.
Corrected osmolarity = 282 mOsm/L.
Osmolarity usually changes by only ±2% to 3%.
These values must be kept very precise.
They control fluid distribution between intracellular and extracellular compartments.

Estimating Plasma Osmolarity From Plasma Sodium Concentration

Plasma osmolarity is not routinely measured in most clinical laboratories.
Sodium and its associated anions make up about 94% of extracellular solutes.
Plasma osmolarity can be estimated from plasma sodium concentration.

Formula:

Plasma Osmolarity = 2.1 × Plasma Sodium Concentration

Example:

Plasma Sodium = 142 mEq/L
Plasma Osmolarity = 2.1 × 142
Plasma Osmolarity = 298 mOsm/L
For greater accuracy, glucose and urea concentrations are also included.
Plasma Osmolarity = (2 × Plasma Sodium) + Glucose + Urea
These estimates are usually within a few percent of directly measured osmolarity.
Sodium and associated anions normally contribute about 94% of extracellular osmoles.
Glucose and urea contribute about 3% to 5% of total osmoles.
Urea crosses most cell membranes easily.
Urea produces little effective osmotic pressure under steady-state conditions.
Sodium and its associated anions are the main determinants of fluid movement across cell membranes.
Control of osmolarity and control of sodium concentration can be discussed together.
Many mechanisms regulate sodium and water excretion by the kidneys.
Two major systems regulate extracellular fluid sodium concentration and osmolarity.

Osmoreceptor–ADH System
Thirst Mechanism

KEY CONCEPT

Sodium is the main extracellular ion.
Normal plasma sodium = 135–145 mEq/L.
Average plasma sodium = 142 mEq/L.
Estimated Plasma Osmolarity = 2.1 × Plasma Sodium.
Example: 142 × 2.1 = 298 mOsm/L.
Sodium and associated anions determine most extracellular osmolarity.
The two main regulators are ADH and Thirst.

OSMORECEPTOR-ADH FEEDBACK SYSTEM

The osmoreceptor-ADH feedback system controls extracellular fluid sodium concentration and osmolarity.
This system becomes active when extracellular fluid osmolarity rises above normal.
A water deficit can increase extracellular fluid osmolarity.

Step 1

Extracellular fluid osmolarity increases.
In practical terms, plasma sodium concentration increases.
Special nerve cells called osmoreceptors detect this change.
Osmoreceptor cells are located in the anterior hypothalamus.
They are located near the supraoptic nuclei.
Increased osmolarity causes osmoreceptor cells to shrink.

Step 2

Shrinkage stimulates the osmoreceptor cells.
Osmoreceptor cells begin firing nerve signals.
These signals are sent to nerve cells in the supraoptic nuclei.
The supraoptic nuclei relay the signals down the pituitary stalk.
The signals reach the posterior pituitary gland.

Step 3

Action potentials arrive at the posterior pituitary.
The posterior pituitary releases ADH.
ADH is stored in secretory granules (vesicles) in nerve endings.

Step 4

ADH enters the bloodstream.
ADH is transported to the kidneys.
ADH increases water permeability of the late distal tubules.
ADH increases water permeability of the cortical collecting tubules.
ADH increases water permeability of the medullary collecting ducts.

Step 5

Increased water permeability occurs in distal nephron segments.
More water is reabsorbed.
A smaller volume of urine is excreted.
The urine becomes more concentrated.

Result

Water is conserved in the body.
Sodium continues to be excreted in urine.
Other solutes continue to be excreted in urine.
Extracellular fluid becomes diluted.
The initial high osmolarity is corrected.

When Extracellular Fluid Becomes Too Dilute

The opposite sequence occurs when extracellular fluid becomes hypo-osmotic.
Excess water intake decreases extracellular fluid osmolarity.
Less ADH is produced.
Renal tubules become less permeable to water.
Less water is reabsorbed.
A larger volume of urine is excreted.
The urine becomes dilute.
Body fluids become more concentrated.
Plasma osmolarity returns toward normal.

KEY CONCEPT

↑ Extracellular Osmolarity → Osmoreceptor Shrinkage
Osmoreceptor Shrinkage → ↑ ADH Release
↑ ADH → ↑ Water Permeability in Distal Tubules & Collecting Ducts
↑ Water Reabsorption → ↓ Urine Volume
Water Conservation → ↓ Extracellular Osmolarity

Easy Flow

Water Deficit → ↑ Osmolarity → Osmoreceptors Shrink → ↑ ADH → ↑ Water Reabsorption → Concentrated Urine → Osmolarity Returns Toward Normal

Excess Water Intake → ↓ Osmolarity → ↓ ADH → ↓ Water Reabsorption → Dilute Urine → Osmolarity Returns Toward Normal

ADH Is Synthesized in Supraoptic and Paraventricular Nuclei of Hypothalamus and Released From the Posterior Pituitary

ADH is synthesized in the hypothalamus.
ADH is released from the posterior pituitary gland.
The hypothalamus contains two types of magnocellular (large) neurons.
These neurons synthesize ADH in the supraoptic and paraventricular nuclei.

Sites of ADH Production

About 5/6 (≈83%) of ADH is synthesized in the supraoptic nuclei.
About 1/6 (≈17%) of ADH is synthesized in the paraventricular nuclei.
Both nuclei send axons to the posterior pituitary.

Transport of ADH

After synthesis, ADH is transported down the axons.
ADH moves to the nerve endings in the posterior pituitary gland.

Release of ADH

Increased osmolarity stimulates the supraoptic and paraventricular nuclei.
Other factors can also stimulate these nuclei.
Nerve impulses travel down the axons to the nerve endings.
Membrane permeability of the nerve endings changes.
Calcium entry into the nerve endings increases.
ADH is stored in secretory granules (vesicles).
Increased calcium entry triggers ADH release.
Released ADH enters the capillary blood of the posterior pituitary.
ADH then enters the systemic circulation.

Response to Osmotic Stimulus

ADH secretion occurs rapidly.
Plasma ADH levels can increase severalfold within minutes.
This provides a rapid way to alter renal water excretion.

AV3V Region

Another important area for osmolarity and ADH control is the AV3V region.
The AV3V region is located along the anteroventral region of the third ventricle.

Components of the AV3V Region

The upper part contains the subfornical organ.
The lower part contains the organum vasculosum of the lamina terminalis.
Between them is the median preoptic nucleus.

Connections of the Median Preoptic Nucleus

It has multiple nerve connections with the subfornical organ.
It has multiple nerve connections with the organum vasculosum.
It connects with the supraoptic nuclei.
It connects with blood pressure control centers in the medulla.

Functions of the AV3V Region

Damage to the AV3V region causes defects in ADH secretion control.
Damage to the AV3V region causes defects in thirst control.
Damage to the AV3V region causes defects in sodium appetite control.
Damage to the AV3V region causes defects in blood pressure regulation.

Stimulation of the AV3V Region

Electrical stimulation of the AV3V region increases ADH secretion.
Electrical stimulation increases thirst.
Electrical stimulation increases sodium appetite.
Angiotensin II stimulation also increases ADH secretion.
Angiotensin II stimulation also increases thirst.
Angiotensin II stimulation also increases sodium appetite.

Osmoreceptors

Neurons near the AV3V region and supraoptic nuclei act as osmoreceptors.
These neurons are stimulated by small increases in extracellular fluid osmolarity.
Osmoreceptors send nerve signals to the supraoptic nuclei.
These signals regulate firing of the supraoptic nuclei.
These signals regulate ADH secretion.
Osmoreceptors likely stimulate thirst when extracellular fluid osmolarity increases.

Special Feature of the Subfornical Organ and Organum Vasculosum

Their blood vessels lack the typical blood-brain barrier.
Ions and other solutes can move easily between blood and local interstitial fluid.
Osmoreceptors rapidly detect changes in extracellular fluid osmolarity.
Osmoreceptors strongly control ADH secretion.
Osmoreceptors strongly control thirst.

KEY CONCEPT

ADH is synthesized in the hypothalamus.
ADH is released from the posterior pituitary.
Supraoptic nuclei produce ≈83% of ADH.
Paraventricular nuclei produce ≈17% of ADH.
↑ Osmolarity → ↑ Nerve Activity → ↑ Calcium Entry → ↑ ADH Release.
AV3V region helps control ADH, thirst, sodium appetite, and blood pressure.
Osmoreceptors detect osmolarity changes and stimulate ADH secretion and thirst.

Easy Flow

↑ Osmolarity → Osmoreceptors Activated → Supraoptic & Paraventricular Nuclei Stimulated → ↑ Calcium Entry → ADH Released from Posterior Pituitary → Bloodstream → Kidneys → Water Conservation

ADH (Antidiuretic Hormone) Regulation — Simplest Concept

🎯 Main Idea

The body constantly checks:

How concentrated the blood is (osmolarity)
How much blood volume/pressure is present

If the body senses dehydration, high osmolarity, or low blood volume, it releases ADH.

ADH tells the kidneys:

➡️ “Save water, don’t lose it in urine.”

Step 1: Sensors Detect a Problem

A. Osmoreceptors (Most Important)

📍 Located in the hypothalamus

They monitor blood concentration.

When activated?

Water loss (sweating)
Dehydration
High plasma osmolarity
High sodium concentration

Message:

🗣️ “Blood is too concentrated! Save water!”

B. Baroreceptors & Cardiopulmonary Receptors

📍 Located in:

Large blood vessels
Heart

They monitor:

Blood pressure
Blood volume

When activated?

Hemorrhage (blood loss)
Severe dehydration
Low blood pressure

Message:

🗣️ “Blood volume is falling! Save water!”

Step 2: Hypothalamus Produces ADH

Two special neurons in the hypothalamus make ADH:

1. Supraoptic Nucleus

⭐ Major source of ADH

2. Paraventricular Nucleus

⭐ Also produces ADH

Think:

Osmoreceptors/Baroreceptors → Hypothalamic Neurons → ADH Production

Step 3: ADH Travels to Posterior Pituitary

ADH is:

✅ Synthesized in hypothalamic neurons

❌ Not synthesized in pituitary

It travels down nerve fibers to the:

📍 Posterior Pituitary

where it is stored and released into blood.

Step 4: ADH Acts on Kidneys

ADH reaches:

📍 Distal Tubules
📍 Collecting Ducts

Effect:

More water channels (Aquaporins) are inserted.

Result:

💧 More water reabsorbed into blood
💧 Less water lost in urine

Final Result

With ADH

✅ Water retained in body
✅ Blood volume increases
✅ Plasma osmolarity decreases
✅ Blood pressure improves

Urine becomes:

❌ Small volume

✅ Highly concentrated

Easy Flow Chart

Dehydration / ↑ Osmolarity / ↓ Blood Volume
                    ↓
      Osmoreceptors & Baroreceptors
                    ↓
           Hypothalamus
   (Supraoptic + Paraventricular Nuclei)
                    ↓
              ADH Produced
                    ↓
          Posterior Pituitary
                    ↓
            ADH Released
                    ↓
               Kidneys
                    ↓
         ↑ Water Reabsorption
                    ↓
      ↓ Urine Volume + ↑ Concentration

KEY CONCEPT

ADH is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus, stored and released from the posterior pituitary, and acts on the distal tubules and collecting ducts of the kidneys to increase water reabsorption, producing a small volume of concentrated urine.

Stimulation of ADH Release By Decreased Arterial Pressure and/or Decreased Blood Volume

ADH release is also controlled by cardiovascular reflexes.
These reflexes respond to decreases in blood pressure.
These reflexes respond to decreases in blood volume.

Types of Reflexes Involved

Arterial Baroreceptor Reflexes
Cardiopulmonary Reflexes

Origin of These Reflexes

These reflex pathways begin in high-pressure areas of the circulation.
High-pressure areas include the aortic arch.
High-pressure areas include the carotid sinus.
These reflex pathways also begin in low-pressure areas of the circulation.
Low-pressure areas are mainly located in the cardiac atria.

Nerve Pathway

Sensory (afferent) signals travel through the vagus nerve.
Sensory (afferent) signals also travel through the glossopharyngeal nerve.
These signals synapse in the nuclei of the tractus solitarius.
Signals are then relayed to hypothalamic nuclei.
These hypothalamic nuclei control ADH synthesis.
These hypothalamic nuclei control ADH secretion.

Stimuli That Increase ADH Secretion

Increased osmolarity increases ADH secretion.
Decreased arterial pressure increases ADH secretion.
Decreased blood volume increases ADH secretion.

During Hemorrhage

Hemorrhage decreases blood pressure.
Hemorrhage decreases blood volume.
ADH secretion increases.
Increased ADH causes more water reabsorption by the kidneys.
More water is retained in the body.
Blood pressure moves back toward normal.
Blood volume moves back toward normal.

KEY CONCEPT

Three Major Stimuli for ADH Release

↑ Osmolarity
↓ Arterial Pressure
↓ Blood Volume

Easy Flow

Hemorrhage → ↓ Blood Pressure + ↓ Blood Volume → Cardiovascular Reflexes Activated → Hypothalamus Stimulated → ↑ ADH Release → ↑ Water Reabsorption by Kidneys → Blood Pressure & Blood Volume Return Toward Normal

Quantitative Importance of Osmolarity and Cardiovascular Reflexes in Stimulating ADH Secretion

A decrease in effective blood volume stimulates ADH secretion.
An increase in extracellular fluid osmolarity stimulates ADH secretion.

Sensitivity of ADH Secretion

ADH is much more sensitive to changes in osmolarity than to changes in blood volume.
Even a small change in osmolarity can increase ADH secretion.

Effect of Osmolarity

A 1% increase in plasma osmolarity is enough to increase ADH levels.

Effect of Blood Volume

After blood loss, ADH levels do not increase significantly at first.
Blood volume must decrease by about 10% before ADH levels rise appreciably.

Severe Blood Volume Loss

When blood volume decreases further, ADH levels rise rapidly.
In severe blood volume depletion, cardiovascular reflexes become very important for ADH secretion.

Daily Regulation of ADH

During simple dehydration, ADH secretion is controlled mainly by changes in plasma osmolarity.
Osmolarity is the major day-to-day regulator of ADH release.

Combined Effect

Decreased blood volume enhances the ADH response to increased osmolarity.
Decreased blood pressure enhances the ADH response to increased osmolarity.

KEY CONCEPT

ADH Sensitivity Comparison

1% ↑ Plasma Osmolarity → ↑ ADH Secretion
10% ↓ Blood Volume → Significant ↑ ADH Secretion

Main Controller in Daily Life

Simple Dehydration → Osmolarity is the main stimulus for ADH release

During Severe Blood Loss

Severe ↓ Blood Volume → Cardiovascular Reflexes become the major stimulus for ADH release

Easy Comparison

Stimulus	ADH Response
1% ↑ Osmolarity	ADH increases
10% ↓ Blood Volume	ADH begins to increase significantly
Severe ↓ Blood Volume	Rapid and marked ADH increase

Easy Flow

Simple Dehydration → ↑ Osmolarity → ↑ ADH

Severe Hemorrhage → Large ↓ Blood Volume → Cardiovascular Reflexes Activated → Marked ↑ ADH

Figure 29.11 — ADH Secretion: Osmolarity vs Blood Volume Regulation (Simplest Explanation)

🎯 What Is This Graph Showing?

This graph compares two different stimuli that increase ADH secretion:

⚪ White Circles

Increase in Plasma Osmolarity

Blood becomes more concentrated.
Water is lost but blood volume remains nearly normal.
Called Isovolemic Osmotic Increase.

⚫ Black Circles

Decrease in Blood Volume

Blood volume falls.
Example: hemorrhage, severe dehydration.
Called Isotonic Volume Depletion.

The graph shows how much ADH (AVP) is released in each situation.

Understanding the Axes

X-Axis (Horizontal)

Percent Change

This shows how much the stimulus changes.

For example:

Value	Meaning
0%	Normal condition
5%	Small increase/decrease
10%	Moderate change
20%	Large change

Y-Axis (Vertical)

Plasma ADH (pg/mL)

Shows the amount of ADH in blood.

ADH Level	Meaning
Low	Little water conservation
High	Strong water conservation
Very High	Emergency water conservation

⚪ WHITE CIRCLES

Effect of Increased Osmolarity

What do the white circles represent?

Each white circle is an experimental measurement showing:

➡️ How much ADH was released when plasma osmolarity increased.

What happens?

As osmolarity increases:

Blood becomes more concentrated.
Osmoreceptors detect it.
ADH secretion increases.

The increase is:

📈 Almost a straight-line increase

White Line Equation

$P_{AVP}=2.5\Delta Osm + 2.0$ PAVP=2.5ΔOsm+2.0

What does this mean?

PAVP

Amount of ADH in blood.

Δ Osm

Change in osmolarity.

2.5

For every 1-unit increase in osmolarity:

ADH increases by approximately 2.5 pg/mL.

+2.0

Even at normal osmolarity there is a small baseline ADH secretion.

Easy Concept

↑ Osmolarity
      ↓
Osmoreceptors stimulated
      ↓
ADH increases steadily

Key Point

Small changes in osmolarity produce noticeable increases in ADH.

⚫ BLACK CIRCLES

Effect of Decreased Blood Volume

What do black circles represent?

Each black circle is an experimental measurement showing:

➡️ How much ADH was released when blood volume decreased.

What happens initially?

Look at the left side of black circles.

Even when blood volume decreases slightly:

ADH changes very little.

Example:

0–5% volume loss

ADH remains around:

2–5 pg/mL

Why?

Baroreceptors are not very sensitive to small volume changes.

The body can tolerate minor volume loss.

What Happens After Significant Volume Loss?

When volume loss becomes larger:

Around 10–15%
Baroreceptors fire strongly
Hypothalamus becomes highly stimulated

ADH secretion rises dramatically.

Look at the Red Curved Line

The red curve through the black circles represents the average response.

Notice:

First Part

Almost flat.

Small volume loss
      ↓
Little ADH increase

Later Part

Very steep.

Large volume loss
      ↓
Massive ADH increase

Black Line Equation

$P_{AVP}=1.3e^{-0.17(vol)}$ PAVP=1.3e−0.17(vol)

(Demonstrates an exponential relationship.)

What does exponential mean?

ADH does NOT increase evenly.

Instead:

5% volume loss
     ↓
Small ADH rise

10% volume loss
     ↓
Moderate ADH rise

15–20% volume loss
     ↓
Huge ADH rise

The response becomes increasingly steep.

Why Does This Happen?

The body considers:

Increased Osmolarity

Important

But

Severe Blood Volume Loss

Life-threatening

Because:

Blood pressure may collapse.
Organs may not receive blood.
Shock can occur.

Therefore:

When volume loss becomes severe,

🚨 ADH secretion skyrockets.

Comparison of White Circles vs Black Circles

Feature	⚪ White Circles	⚫ Black Circles
Stimulus	Increased Osmolarity	Decreased Blood Volume
Receptors	Osmoreceptors	Baroreceptors
Pattern	Linear	Exponential
Sensitivity	Very High	Low initially
Small Changes Cause ADH Release?	Yes	No
Large Changes Cause Huge ADH Release?	Moderate	Yes
Main Function	Control Osmolarity	Emergency Volume Preservation

Most Important Guyton Concept

ADH is more sensitive to osmolarity than blood volume.

A tiny increase in osmolarity (1–2%) can increase ADH secretion.

However, blood volume usually must fall by about 10% or more before ADH rises dramatically.

Final Exam Point

↑ Plasma Osmolarity
        ↓
Small change produces ADH release
        ↓
Linear increase (White Circles)

↓ Blood Volume
        ↓
Small losses cause little effect
        ↓
Large losses (>10–15%)
        ↓
Explosive ADH secretion
        ↓
Exponential increase (Black Circles)

KEY CONCEPT

White circles show that ADH secretion increases almost linearly with small increases in plasma osmolarity, demonstrating very high sensitivity of osmoreceptors. Black circles show that ADH changes little with small blood volume losses but rises exponentially during severe volume depletion, demonstrating the powerful emergency response of baroreceptors to protect blood pressure and circulation.

Other Stimuli for ADH Secretion

ADH secretion can be increased by other stimuli acting on the central nervous system.
ADH secretion can also be decreased by other stimuli acting on the central nervous system.
Various drugs can increase or decrease ADH secretion.
Various hormones can increase or decrease ADH secretion.
Nausea is a powerful stimulus for ADH release.
After vomiting, ADH levels may increase up to 100 times the normal level.
Nicotine stimulates ADH release.
Morphine stimulates ADH release.
Alcohol inhibits ADH release.
Marked diuresis can occur after alcohol ingestion.
This increased urine production is partly due to inhibition of ADH release.

KEY CONCEPT

Central nervous system stimuli can alter ADH secretion.
Nausea → Strong increase in ADH release.
Vomiting → ADH may increase up to 100× normal.
Nicotine → ↑ ADH release.
Morphine → ↑ ADH release.
Alcohol → ↓ ADH release.
Alcohol ingestion → Increased urine output (diuresis).

IMPORTANCE OF THIRST IN CONTROLLING EXTRACELLULAR FLUID OSMOLARITY AND SODIUM CONCENTRATION

During water deficit, the kidneys minimize fluid loss through the osmoreceptor-ADH feedback system.
Adequate fluid intake is also necessary.
Fluid intake replaces fluid lost through sweating.
Fluid intake replaces fluid lost through breathing.
Fluid intake replaces fluid lost through the gastrointestinal tract.
Fluid intake is regulated by the thirst mechanism.
The thirst mechanism works together with the osmoreceptor-ADH mechanism.
Together they maintain precise control of extracellular fluid osmolarity.
Together they maintain precise control of sodium concentration.
Many of the same factors that stimulate ADH secretion also stimulate thirst.
Thirst is the conscious desire for water.

Central Nervous System Centers for Thirst

The same area along the anteroventral wall of the third ventricle that promotes ADH release also stimulates thirst.
Another small area is located anterolaterally in the preoptic nucleus.
Electrical stimulation of this area causes immediate drinking.
Drinking continues as long as the stimulation continues.
All these areas together are called the thirst center.
Neurons in the thirst center respond to increased extracellular fluid sodium concentration.
These neurons stimulate drinking behavior.
These neurons likely function as osmoreceptors.
They stimulate thirst in the same way that osmoreceptors stimulate ADH release.
Increased osmolarity of the cerebrospinal fluid in the third ventricle promotes drinking.
The organum vasculosum of the lamina terminalis is likely involved in this response.
This structure lies beneath the ventricular surface.
It is located at the inferior end of the AV3V region.

Stimuli for Thirst

1. Increased Extracellular Fluid Osmolarity

Increased extracellular fluid osmolarity causes intracellular dehydration in the thirst centers.
Intracellular dehydration stimulates the sensation of thirst.
This response helps dilute extracellular fluid.
This response helps return osmolarity toward normal.

2. Decreased Extracellular Fluid Volume and Arterial Pressure

Decreased extracellular fluid volume stimulates thirst.
Decreased arterial pressure stimulates thirst.
This pathway is different from the pathway activated by increased plasma osmolarity.
Blood loss due to hemorrhage stimulates thirst.
Thirst can occur even when plasma osmolarity does not change.
This stimulation probably occurs through neural input from cardiopulmonary baroreceptors.
This stimulation probably occurs through neural input from systemic arterial baroreceptors.

3. Angiotensin II

Angiotensin II is an important stimulus for thirst.
Angiotensin II acts on the subfornical organ.
Angiotensin II acts on the organum vasculosum of the lamina terminalis.
These regions are outside the blood-brain barrier.
Angiotensin II can diffuse into these tissues.
Angiotensin II is stimulated by factors associated with hypovolemia.
Angiotensin II is stimulated by factors associated with low blood pressure.
Its effect on thirst helps restore blood volume toward normal.
Its effect on thirst helps restore blood pressure toward normal.
Angiotensin II also acts on the kidneys to decrease fluid excretion.

4. Dryness of the Mouth and Esophagus

Dryness of the mouth can cause the sensation of thirst.
Dryness of the esophageal mucous membranes can cause the sensation of thirst.
A thirsty person may feel relief immediately after drinking water.
This relief occurs before water is absorbed from the gastrointestinal tract.
This relief occurs before extracellular fluid osmolarity changes.

5. Gastrointestinal and Pharyngeal Stimuli

Gastrointestinal stimuli influence thirst.
Pharyngeal stimuli influence thirst.
In animals with an esophageal opening to the exterior, water is not absorbed into the blood.
Partial relief of thirst still occurs after drinking.
This relief is only temporary.
Gastrointestinal distention may partially reduce thirst.
Inflation of a balloon in the stomach can relieve thirst.
Relief of thirst through gastrointestinal mechanisms is short-lived.
Relief of thirst through pharyngeal mechanisms is short-lived.
Thirst is completely satisfied only when plasma osmolarity returns to normal.
Thirst is completely satisfied only when blood volume returns to normal.
Animals and humans can accurately regulate fluid intake.
This ability prevents overhydration.
After drinking water, 30 to 60 minutes may be required for absorption and distribution throughout the body.
If thirst were not temporarily relieved after drinking, excessive drinking could continue.
Excessive drinking could lead to overhydration.
Excessive drinking could cause excessive dilution of body fluids.

KEY CONCEPT

Thirst works with the osmoreceptor-ADH system to regulate body fluid balance.
The thirst center is located near the anteroventral wall of the third ventricle.
Increased osmolarity is the most important stimulus for thirst.
Decreased blood volume and blood pressure stimulate thirst.
Angiotensin II stimulates thirst.
Dry mouth stimulates thirst.
Gastrointestinal and pharyngeal signals provide temporary relief of thirst.
Thirst is completely satisfied only when osmolarity and/or blood volume return to normal.
Temporary relief of thirst helps prevent overhydration.

Threshold for Osmolar Stimulus of Drinking

The kidneys must continuously excrete a minimum amount of water.
This occurs even in a dehydrated person.
Water is needed to remove excess solutes taken in through food.
Water is needed to remove excess solutes produced by metabolism.
Water is lost by evaporation from the lungs.
Water is lost through the gastrointestinal tract.
Water is lost by evaporation from the skin.
Water is lost through sweating.
Therefore, the body always has a tendency toward dehydration.
Dehydration increases extracellular fluid sodium concentration.
Dehydration increases extracellular fluid osmolarity.
The thirst mechanism is activated when sodium concentration rises about 2 mEq/L above normal.
Activation of the thirst mechanism creates a desire to drink water.
This level is called the threshold for drinking.
Even small increases in plasma osmolarity stimulate drinking.
Drinking helps restore extracellular fluid osmolarity toward normal.
Drinking helps restore extracellular fluid volume toward normal.
In this way, extracellular fluid osmolarity is precisely controlled.
In this way, extracellular fluid sodium concentration is precisely controlled.

KEY CONCEPT

The body continuously loses water through the kidneys, lungs, gastrointestinal tract, skin, and sweating.
This creates a constant tendency toward dehydration.
A rise in plasma sodium of about 2 mEq/L above normal activates the thirst mechanism.
This level is called the threshold for drinking.
Drinking restores extracellular fluid osmolarity and volume toward normal.
Thirst helps maintain precise control of osmolarity and sodium concentration.

Disorders of Thirst and Water Intake

Increased thirst occurs in various medical disorders.
These disorders are often associated with increased urine volume (polyuria).
These disorders are often associated with reduced extracellular fluid volume.
Poorly controlled diabetes mellitus can cause increased thirst.
Diabetes insipidus can cause increased thirst.
Increased water intake acts as a compensatory response.
Increased water intake helps correct increased plasma osmolarity.
Increased water intake helps correct extracellular fluid volume depletion.
Polydipsia means excessive thirst.
Polydipsia can occur without known physiological stimuli for thirst.
Psychogenic polydipsia is one type of excessive thirst.
Psychogenic polydipsia may be caused by mental illnesses.
Schizophrenia may cause psychogenic polydipsia.
Obsessive-compulsive disorders may cause psychogenic polydipsia.
Psychogenic polydipsia can lead to significant hyponatremia.
Adipsia means absence of thirst.
Adipsia can occur even when hypernatremia is present.
Adipsia can occur even when volume depletion is present.
Adipsia is rare.
Adipsia usually results from lesions of the hypothalamic thirst centers.
These lesions may be caused by trauma.
These lesions may be caused by infection.
These lesions may be caused by surgery.
Partial deficiency of the thirst mechanism is called hypodipsia.
Hypodipsia causes inadequate water intake.
Inability to access water may also reduce water intake.
Hypodipsia may occur in patients who have had a stroke.
Hypodipsia may occur in older patients with dementia.
Hypodipsia may occur in critically ill patients.
Inadequate water intake leads to dehydration.
Inadequate water intake leads to hypernatremia.
Dehydration and hypernatremia occur despite large increases in ADH levels.

KEY CONCEPT

Polyuria and extracellular fluid volume depletion → Increased thirst.
Diabetes mellitus and diabetes insipidus → Increased thirst and water intake.
Polydipsia = Excessive thirst.
Psychogenic polydipsia → May occur in schizophrenia or obsessive-compulsive disorders.
Psychogenic polydipsia → Can cause hyponatremia.
Adipsia = Absence of thirst.
Hypodipsia = Reduced thirst causing inadequate water intake.
Hypothalamic lesions can cause adipsia.
Inadequate water intake → Dehydration + Hypernatremia despite high ADH levels.

INTEGRATED RESPONSES OF OSMORECEPTOR-ADH AND THIRST MECHANISMS

In a healthy person, the osmoreceptor-ADH mechanism and thirst mechanism work together.
These mechanisms regulate extracellular fluid osmolarity precisely.
These mechanisms regulate sodium concentration precisely.
They continue to function despite constant dehydration challenges.
These feedback systems can also handle additional challenges.
High salt intake is one example of an additional challenge.
These systems usually keep plasma osmolarity reasonably constant.
Sodium intake can increase up to six times the normal amount.
This causes only a small change in plasma sodium concentration.
This occurs when both the ADH and thirst mechanisms are functioning normally.
If the ADH mechanism fails, the thirst mechanism can still help control extracellular osmolarity.
If the thirst mechanism fails, the ADH mechanism can still help control extracellular osmolarity.
Either mechanism alone can control sodium concentration with reasonable effectiveness.
Adequate fluid intake must be available.
Fluid intake must replace the obligatory daily urine volume.
Fluid intake must replace water lost through respiration.
Fluid intake must replace water lost through sweating.
Fluid intake must replace water lost through the gastrointestinal tract.
Plasma sodium concentration becomes poorly controlled if both mechanisms fail at the same time.
Plasma osmolarity becomes poorly controlled if both mechanisms fail at the same time.
Increased sodium intake causes large changes in plasma sodium concentration when the ADH-thirst system is blocked.
Without the ADH-thirst system, plasma sodium concentration cannot be adequately regulated.
Without the ADH-thirst system, plasma osmolarity cannot be adequately regulated.
No other feedback mechanism can adequately control plasma sodium concentration and osmolarity in the absence of both mechanisms.

KEY CONCEPT

Osmoreceptor-ADH mechanism and thirst mechanism work together to control osmolarity and sodium concentration.
These systems maintain near-normal plasma osmolarity even during high salt intake.
If one mechanism fails, the other can still provide reasonable control.
Adequate water intake is required for effective regulation.
If both ADH and thirst mechanisms fail, plasma sodium concentration and osmolarity become poorly controlled.
No other feedback system can adequately replace the combined ADH-thirst mechanisms.

Figure 29.12 — Why ADH and Thirst Keep Plasma Sodium Almost Constant

🎯 Main Message of This Graph

This graph demonstrates:

Even when sodium intake changes enormously, plasma sodium concentration changes very little because the ADH and thirst mechanisms automatically maintain water balance.

Without ADH and thirst, plasma sodium concentration becomes poorly controlled.

Understanding the Axes

X-Axis (Horizontal)

Sodium Intake (mEq/day)

This shows how much sodium is consumed each day.

Left Side

30 mEq/day

Low sodium intake

Right Side

180 mEq/day

High sodium intake

So moving from left to right means:

➡️ Eating much more salt.

Y-Axis (Vertical)

Plasma Sodium Concentration (mEq/L)

This shows sodium concentration in extracellular fluid (blood plasma).

Normal range:

✅ About 135–145 mEq/L

Higher values mean:

More concentrated body fluids
Higher osmolarity

🔴 RED LINE = NORMAL PERSON

What does the red line represent?

A person with:

✅ Normal ADH system

AND

✅ Normal thirst mechanism

First Red Point

Sodium Intake = 30 mEq/day

Plasma sodium ≈ 142 mEq/L

This is normal.

Second Red Point

Sodium Intake = 180 mEq/day

Plasma sodium ≈ 143 mEq/L

Still normal.

What Happened?

Sodium intake increased:

30 → 180 mEq/day

That is about:

6 times more sodium

Yet plasma sodium changed only:

142 → 143 mEq/L

Almost no change!

Why Did Sodium Stay Constant?

When extra sodium is eaten:

Step 1

Plasma osmolarity rises slightly.

More sodium
      ↓
Higher osmolarity

Step 2

Osmoreceptors detect it.

↑ Osmolarity
      ↓
Osmoreceptors stimulated

Step 3

ADH secretion increases.

↑ ADH
      ↓
Kidneys retain more water

Step 4

Thirst increases.

Person drinks more water

Step 5

Added water dilutes sodium.

More water retained
        +
More water consumed
        ↓
Sodium concentration returns toward normal

🔵 BLUE LINE = ADH and Thirst Blocked

What does the blue line represent?

Scientists experimentally blocked:

❌ ADH system

AND

❌ Thirst mechanism

The body could no longer correct sodium concentration.

First Blue Point

Sodium Intake = 30 mEq/day

Plasma sodium ≈ 137 mEq/L

Slightly lower than normal.

Second Blue Point

Sodium Intake = 150–180 mEq/day

Plasma sodium ≈ 151–152 mEq/L

Very high.

This represents:

⚠️ Hypernatremia

⚠️ Increased osmolarity

⚠️ Dehydrated cells

Why Did Sodium Rise So Much?

Normally:

Extra sodium
      ↓
ADH released
      ↓
Water retained

But now:

Extra sodium
      ↓
No ADH response
      ↓
No extra water retained

And normally:

Extra sodium
      ↓
Thirst stimulated
      ↓
Drink water

But now:

Extra sodium
      ↓
No thirst
      ↓
No extra water intake

Result

Sodium accumulates.

Water does not increase.

Therefore:

More sodium
     +
Less water compensation
          ↓
Plasma sodium rises sharply

Why Is the Blue Line So Steep?

Because there is no feedback control.

Normally:

↑ Sodium
      ↓
ADH + Thirst Activated
      ↓
Problem Corrected

Blocked condition:

↑ Sodium
      ↓
No ADH
No Thirst
      ↓
No Correction
      ↓
Sodium Keeps Rising

Therefore the blue line climbs steeply.

Comparison of the Two Lines

Feature	🔴 Normal	🔵 ADH & Thirst Blocked
ADH Present	Yes	No
Thirst Present	Yes	No
Water Retention	Yes	No
Water Drinking	Yes	No
Plasma Sodium Control	Excellent	Poor
Change in Sodium Concentration	Very Small	Very Large
Homeostasis	Maintained	Lost

The Most Important Guyton Concept

The body does not primarily control sodium concentration by controlling sodium intake.

Instead, it controls sodium concentration by controlling water balance.

ADH Controls Water Output

ADH
   ↓
Water Reabsorption

Thirst Controls Water Input

Thirst
   ↓
Water Intake

Together they keep plasma sodium nearly constant.

Easy Flowchart

↑ Sodium Intake
        ↓
Slight ↑ Plasma Osmolarity
        ↓
Osmoreceptors Activated
        ↓
↑ ADH + ↑ Thirst
        ↓
More Water Retained
        +
More Water Drunk
        ↓
Plasma Sodium Returns Near Normal

KEY CONCEPT

The red line shows that even a sixfold increase in sodium intake causes only a tiny change in plasma sodium concentration because ADH and thirst mechanisms automatically increase water retention and water intake. The blue line shows that when ADH and thirst are blocked, plasma sodium rises dramatically, proving that these two feedback systems are the body’s most important regulators of extracellular fluid osmolarity and sodium concentration.

Role of Angiotensin II and Aldosterone in Controlling Extracellular Fluid Osmolarity and Sodium Concentration

Angiotensin II and aldosterone play important roles in regulating sodium reabsorption by the renal tubules.
Low sodium intake increases levels of angiotensin II and aldosterone.
Increased levels of these hormones stimulate sodium reabsorption by the kidneys.
This prevents large sodium losses.
Sodium intake may decrease to as low as 10% of normal.
Even then, sodium losses are minimized.
High sodium intake decreases formation of angiotensin II and aldosterone.
Reduced hormone levels allow the kidneys to excrete large amounts of sodium.
Angiotensin II and aldosterone are important in regulating sodium excretion.
They do not play a major role in regulating extracellular fluid sodium concentration.
These hormones increase the amount of sodium in the extracellular fluid.
They also increase extracellular fluid volume.
They increase water reabsorption along with sodium reabsorption.
Therefore, sodium concentration changes very little.
Angiotensin II and aldosterone have little effect on sodium concentration under normal conditions.
Plasma sodium concentration remains well regulated even when the aldosterone feedback system is blocked.
Increasing sodium intake more than 6-fold changes plasma sodium concentration by only about 1% to 2%.
This occurs under normal conditions.
This also occurs when the aldosterone feedback system is blocked.
Plasma sodium concentration can remain well regulated without a functional aldosterone feedback system.
Similar results occur when angiotensin II formation is blocked.
There are two main reasons why angiotensin II and aldosterone have little effect on plasma sodium concentration.

First Reason

Angiotensin II and aldosterone increase sodium reabsorption.
Angiotensin II and aldosterone increase water reabsorption.
Extracellular fluid volume increases.
Total body sodium increases.
Sodium concentration changes very little.

Second Reason

The ADH-thirst mechanism remains functional.
Any tendency for plasma sodium concentration to increase stimulates compensation.
Water intake increases.
ADH secretion increases.
Extracellular fluid becomes diluted.
Plasma sodium concentration moves back toward normal.
The ADH-thirst system is much more important than the angiotensin II and aldosterone systems for regulating sodium concentration under normal conditions.
Patients with primary aldosteronism have very high aldosterone levels.
Plasma sodium concentration usually increases only about 3 to 5 mEq/L above normal.
Complete loss of aldosterone secretion can occur after adrenalectomy.
Complete loss of aldosterone secretion can occur in Addison disease.
These conditions cause tremendous sodium loss by the kidneys.
Large sodium losses can markedly reduce plasma sodium concentration.
Large sodium losses eventually cause severe volume depletion.
Large sodium losses eventually cause decreased blood pressure.
These changes activate the thirst mechanism through cardiovascular reflexes.
Activation of thirst increases water intake.
Increased water intake causes further dilution of plasma sodium concentration.
Increased water intake helps reduce the decrease in body fluid volume.
Under extreme conditions, plasma sodium concentration may change significantly.
This can occur even when the ADH-thirst mechanism is functioning.
The ADH-thirst mechanism is the most powerful feedback system for controlling extracellular fluid osmolarity.
The ADH-thirst mechanism is the most powerful feedback system for controlling sodium concentration.

KEY CONCEPT

Angiotensin II and aldosterone regulate sodium reabsorption by the kidneys.
Low sodium intake → ↑ Angiotensin II + ↑ Aldosterone → ↑ Sodium reabsorption.
High sodium intake → ↓ Angiotensin II + ↓ Aldosterone → ↑ Sodium excretion.
These hormones increase both sodium and water reabsorption.
Therefore, extracellular fluid volume increases but sodium concentration changes very little.
The ADH-thirst system is the main regulator of plasma osmolarity and sodium concentration.
Primary aldosteronism usually increases plasma sodium by only 3–5 mEq/L.
Loss of aldosterone can cause severe sodium loss and decreased plasma sodium concentration.
The ADH-thirst mechanism is the most powerful system controlling extracellular fluid osmolarity and sodium concentration.

Figure 29.13 — Aldosterone vs Plasma Sodium Concentration (Easiest Concept)

🎯 Main Message of This Graph

This graph shows:

Even if the aldosterone system is completely blocked, plasma sodium concentration remains almost normal.

This means:

✅ Aldosterone is NOT the main regulator of plasma sodium concentration.

Instead:

✅ ADH and thirst are the major regulators of plasma sodium concentration and osmolarity.nderstanding the Axes

X-Axis (Horizontal)

Sodium Intake (mEq/day)

Shows how much sodium is eaten each day.

Left Side

30 mEq/day

Low sodium intake

Right Side

180 mEq/day

High sodium intake

So moving from left to right means:

➡️ Eating much more salt.

Y-Axis (Vertical)

Plasma Sodium Concentration (mEq/L)

Shows sodium concentration in blood plasma.

Normal value:

135–145 mEq/L

The body tries to keep this constant.

🔴 RED LINE = Normal Person

This line represents:

✅ Normal aldosterone system

✅ Normal kidneys

First Red Dot

Sodium Intake = 30 mEq/day

Plasma sodium ≈ 143 mEq/L

Normal.d Red Dot

Sodium Intake = 180 mEq/day

Plasma sodium ≈ 145 mEq/L

Still normal.

What Does This Mean?

Even though sodium intake increased about:

30 → 180 mEq/day

(about 6 times higher)

Plasma sodium changed only:

143 → 145 mEq/L

Very little change.

🔵 BLUE LINE = Aldosterone System Blocked

This line represents:

❌ No aldosterone feedback

The kidneys cannot use aldosterone to regulate sodium reabsorption normally.

First Blue Dot

Sodium Intake = 30 mEq/day

Plasma sodium ≈ 140–141 mEq/L

Slightly lower.

Second Blue Dot

Sodium Intake = 180 mEq/day

Plasma sodium ≈ 144–145 mEq/L

Still almost normal.

What Is Surprising?

Even without aldosterone:

Low sodium intake
        ↓
Plasma sodium ≈ Normal

High sodium intake
        ↓
Plasma sodium ≈ Normal

The blue line stays almost flat.

Compare the Two Lines

Notice:

Red Line (Normal)

143 → 145 mEq/L

Small change.

Blue Line (No Aldosterone)

140 → 144 mEq/L

Also a small change.

The two lines are very close together.

This means:

➡️ Aldosterone has only a small effect on plasma sodium concentration.

Why Doesn’t Plasma Sodium Change Much?

Many students think:

More aldosterone
      ↓
More sodium retained
      ↓
Huge rise in plasma sodium

This is WRONG.hat Actually Happens?

When aldosterone retains sodium:

Na+ retained
      ↓
Water follows sodium
      ↓
Both increase together

So:

Sodium amount ↑
Water amount ↑

Because both increase together:

Concentration changes very little

Easy Example

Imagine:

Before

100 sodium particles
100 water molecules

Concentration remains balanced.

After Aldosterone

120 sodium particles
120 water molecules

Both increased together.

Result:

Concentration nearly unchanged

Then What Does Aldosterone Actually Control?

Aldosterone mainly controls:

Total Body Sodium Content

Aldosterone
      ↓
Na+ retention

Extracellular Fluid Volume

Na+ retained
      ↓
Water retained
      ↓
ECF volume increases

Blood Volume

ECF volume ↑
      ↓
Blood volume ↑

Blood Pressure

Blood volume ↑
      ↓
Blood pressure ↑

What Controls Sodium Concentration?

The major regulators are:

ADH

Controls:

Water output

Thirst

Controls:

Water intake

Together they determine:

Plasma osmolarity
Plasma sodium concentration

Comparison: Aldosterone vs ADH

Feature	Aldosterone	ADH
Main Action	Retains Sodium	Retains Water
Controls Plasma Sodium Concentration?	Very Little	Major Control
Controls ECF Volume?	Yes	Slightly
Controls Blood Volume?	Yes	Yes
Controls Osmolarity?	Minimal	Major
Controls Sodium Content?	Major	Minimal

Important Guyton Concept

Aldosterone regulates:

Total body sodium
        ↓
ECF volume
        ↓
Blood volume
        ↓
Blood pressure

ADH regulates:

Water balance
        ↓
Plasma osmolarity
        ↓
Plasma sodium concentration

Easy Flowchart

Aldosterone ↑
       ↓
Na+ Reabsorption ↑
       ↓
Water Follows Na+
       ↓
ECF Volume ↑
       ↓
Blood Volume ↑
       ↓
Blood Pressure ↑

But

Na+ and Water increase together
          ↓
Plasma Sodium Concentration
Changes Very Little

KEY CONCEPT

This graph shows that plasma sodium concentration remains nearly constant whether the aldosterone system is functioning or blocked. Aldosterone primarily regulates total body sodium content, extracellular fluid volume, blood volume, and blood pressure—not plasma sodium concentration. Plasma sodium concentration is mainly controlled by ADH and the thirst mechanism through regulation of body water.

Salt-Appetite Mechanism for Controlling Extracellular Fluid Sodium Concentration and Volume

Maintenance of normal extracellular fluid volume requires a balance between sodium intake and sodium excretion.
Maintenance of normal extracellular fluid sodium concentration requires a balance between sodium intake and sodium excretion.
In most people, sodium intake is much greater than required for homeostasis.
People in industrialized societies usually consume processed foods.
Average sodium intake is usually 100–200 mEq/day.
Humans can survive normally while consuming only 10–20 mEq/day of sodium.
High sodium intake may contribute to cardiovascular disorders.
High sodium intake may contribute to hypertension.
Salt appetite occurs partly because humans and animals naturally like salt.
Salt may be consumed even when there is no sodium deficiency.
Salt appetite also has a regulatory component.
A sodium deficiency creates a behavioral drive to obtain salt.
This behavioral drive is especially important in herbivores.
Herbivores naturally consume a low-sodium diet.
Salt craving may also be important in humans.
Salt craving is especially important during severe sodium deficiency.
Severe sodium deficiency occurs in Addison disease.
Addison disease causes deficiency of aldosterone secretion.
Aldosterone deficiency causes excessive sodium loss in urine.
Excessive sodium loss decreases extracellular fluid volume.
Excessive sodium loss decreases sodium concentration.
Both changes stimulate the desire for salt.
The main stimuli for increased salt appetite are associated with sodium deficiency.
Decreased blood volume increases salt appetite.
Decreased blood pressure increases salt appetite.
Circulatory insufficiency increases salt appetite.
The neuronal mechanism for salt appetite is similar to the thirst mechanism.
Some neuronal centers in the AV3V region are involved in salt appetite.
Lesions in the AV3V region often affect thirst and salt appetite together.
Low blood pressure activates circulatory reflexes.
Decreased blood volume activates circulatory reflexes.
These reflexes influence thirst and salt appetite simultaneously.

KEY CONCEPT

Normal sodium balance requires sodium intake = sodium excretion.
Average sodium intake = 100–200 mEq/day.
Humans can survive on 10–20 mEq/day of sodium.
High sodium intake may contribute to hypertension.
Salt appetite increases when sodium deficiency occurs.
Addison disease → ↓ Aldosterone → ↑ Sodium loss → ↓ ECF Volume + ↓ Sodium Concentration → ↑ Salt Craving.
Major stimuli for salt appetite:
- Sodium deficiency
- ↓ Blood volume
- ↓ Blood pressure
- Circulatory insufficiency
Salt appetite and thirst share similar brain mechanisms in the AV3V region.