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HOMEOSTASIS OF ECF: TONICITY, VOLUME & IONIC COMPOSITION

1. Big Picture (Why this matters)

• ECF = internal environment of cells
• Life depends on constant ECF tonicity, volume, and ionic composition
• Primary regulators:
• Kidneys → water + electrolyte handling
• Lungs → acid–base (CO₂ removal)
• Interstitial fluid = immediate environment of cells (“internal sea”)

2. Defense of ECF Tonicity (Core Concept)

What determines tonicity (osmolality)?

• Total body osmolality ∝ (Total Na⁺ + Total K⁺) / Total body water
• Therefore:
• Tonicity changes when water intake/loss ≠ electrolyte intake/loss

3. Main Defenders of Tonicity

A. Vasopressin (ADH)

B. Thirst mechanism

They act together to correct plasma osmolality.

4. Osmotic Control of Vasopressin & Thirst

When plasma becomes hypertonic:

• ↑ Effective osmotic pressure
• → ↑ Vasopressin secretion
• → ↑ Thirst
• → Water retention + increased intake
• → Plasma diluted → osmolality falls

When plasma becomes hypotonic:

• ↓ Vasopressin secretion
• → Excretion of solute-free water
• → Urine becomes dilute
• → Plasma osmolality rises back to normal

5. Normal Plasma Osmolality

• Normal range: 280–295 mOsm/kg H₂O
• Critical set point: ~285 mOsm/kg
• Vasopressin secretion:
• Maximally inhibited at 285
• Stimulated above 285
• Only 1% change in osmolality → significant ADH response
• This tight feedback keeps plasma osmolality very close to 285

6. Vasopressin Receptors (Mechanism Matters)

All are G-protein–coupled receptors.

7. Renal Effects of Vasopressin (Key Exam Core)

Mechanism

• Acts on V2 receptors in principal cells of collecting ducts
• Causes insertion of Aquaporin-2 (AQP2) into apical membrane
• AQP2 stored in intracellular endosomes
• Vasopressin → rapid translocation to membrane

Result

• ↑ Water permeability
• Water moves into hypertonic medullary interstitium
• Urine becomes concentrated
• Urine volume decreases
• Net retention of water > solute
• → ↓ Plasma osmolality

Absence of vasopressin:

• Collecting ducts impermeable to water
• Urine hypotonic
• Large urine volume
• Net water loss
• Plasma osmolality rises

8. Vascular & Extra-renal Effects of Vasopressin

Vasoconstriction (V1A)

• Potent vascular smooth muscle constrictor in vitro
• In vivo BP rise requires large amounts, because:
• Vasopressin also reduces cardiac output via brain action

Central action

• Acts on area postrema (circumventricular organ)
• Modulates cardiovascular response

Role in hemorrhage

• Hemorrhage → strong stimulus for vasopressin release
• Blocking V1 pressor effect → greater BP fall
• Therefore vasopressin contributes to BP homeostasis

Liver & CNS

• Liver: glycogenolysis
• Brain/spinal cord: acts as neurotransmitter

9. Vasopressin Metabolism

• Rapidly inactivated
• Mainly by liver and kidneys
• Half-life ≈ 18 minutes in humans

10. Osmoreceptors Controlling Vasopressin

• Located in anterior hypothalamus
• Outside BBB
• Mainly in circumventricular organs
• Especially OVLT (organum vasculosum of lamina terminalis)
• Same or closely related receptors likely mediate:
• Vasopressin release
• Thirst (exact identity still uncertain)

11. Volume Control of Vasopressin (Non-osmotic)

Principle

• Vasopressin secretion:
• ↑ when ECF volume ↓
• ↓ when ECF volume ↑

Receptors involved

Low-pressure receptors (primary):

• Great veins
• Right & left atria
• Pulmonary vessels
• Sense vascular fullness

High-pressure receptors:

• Carotid sinus
• Aortic arch

12. Neural Pathway for Volume Control

1. ↓ Stretch receptor firing
2. Afferents travel via vagus
3. → Nucleus tractus solitarius (NTS)
4. NTS → inhibitory pathway to CVLM(caudal ventrolateral medulla)
5. CVLM → excitatory input to hypothalamus
6. → ↑ Vasopressin secretion

13. Role of Angiotensin II

• Acts on circumventricular organs (OVLT)
• Reinforces vasopressin release during:
• Hypovolemia
• Hypotension

14. Hypovolemia Effects on Osmotic Control

• Severe hypovolemia (e.g., hemorrhage):
• Massive vasopressin release
• Osmotic response curve:
• Shifted left
• Steeper slope
• Result:
• Water retention
• ↓ Plasma osmolality
• → Dilutional hyponatremia
• (Na⁺ = main osmotically active solute)

15. Other Stimuli Increasing Vasopressin

• Pain
• Nausea (very strong stimulus)
• Vomiting
• Surgical stress
• Exercise
• Emotional stress
• Standing
• Drugs:
• Clofibrate
• Carbamazepine
• Angiotensin II

Decreases vasopressin:

• ↓ Plasma osmolality
• ↑ ECF volume
• Alcohol

16. Clinical Biasing of Vasopressin Control

• Non-osmotic stimuli override osmotic control
• Example:
• Post-surgical patients:
• Pain + hypovolemia
• ↑ Vasopressin→ ↓ Plasma osmolality→ Dilutional hyponatremia

17. Diabetes Insipidus (DI)

Definition

• Inability to concentrate urine due to:
• ↓ Vasopressin (central DI)
• Renal unresponsiveness (nephrogenic DI)

A. Central Diabetes Insipidus

Causes:

• Supraoptic / paraventricular nuclei disease
• Hypothalamo-hypophyseal tract lesions
• Posterior pituitary disease

Distribution:

• 30% neoplastic
• 30% post-traumatic
• 30% idiopathic
• Remainder:
• Vascular lesions
• Infections
• Sarcoidosis
• Genetic mutations (prepropressophysin)

Post-surgical DI:

• Often temporary
• Axons can regenerate & re-establish secretion

B. Nephrogenic Diabetes Insipidus

Type 1: V2 receptor mutation

• X-linked recessive
• Receptor unresponsive

Type 2: Aquaporin-2 mutation

• Autosomal Recessive
• Nonfunctional AQP2
• Many fail to reach apical membrane

Symptoms of DI

• Polyuria (large volumes of dilute urine)
• Polydipsia
• Thirst is protective
• If thirst impaired → severe dehydration → death

18. SIADH (Syndrome of Inappropriate ADH)

Core features

• Vasopressin inappropriately high relative to osmolality
• Causes:
• Dilutional hyponatremia
• Urinary sodium loss (↓ aldosterone due to ECF expansion)

Seen in

• Cerebral disease → “cerebral salt wasting”
• Pulmonary disease → “pulmonary salt wasting”
• Lung cancers & other tumors (ectopic ADH)

Mechanism in lung disease

• Interrupted inhibitory vagal input from atrial stretch receptors

19. Vasopressin Escape (Protective Mechanism)

Chronic high ADH → ↓ Aquaporin-2 expression

• Leads to:
• Sudden ↑ urine flow
• Limits severity of hyponatremia
• Demonstrated in animal studies

20. Therapeutic Highlights

• Demeclocycline
• Reduces renal response to vasopressin
• Used in SIADH

21. Synthetic Vasopressin Analogues

Desmopressin (dDAVP)

• Modified peptide
• High antidiuretic effect
• Minimal pressor activity
• Ideal for treating:
• Central diabetes insipidus

🔒 FINAL EXAM LOCK

Plasma osmolality is defended primarily by vasopressin and thirst, with vasopressin acting via V2 receptors to insert aquaporin-2 into collecting ducts, while volume status and non-osmotic stimuli can override osmotic control, producing conditions such as SIADH or diabetes insipidus..

DEFENSE OF ECF VOLUME

1. Core determinant of ECF volume

• ECF volume depends on total osmotically active solute in ECF, not water alone.
• Na⁺ and Cl⁻ are the dominant osmoles in ECF.
• Cl⁻ changes largely follow Na⁺, so Na⁺ content is the primary determinant of ECF volume.
• Therefore, mechanisms controlling Na⁺ balance are the main defense of ECF volume.

2. Hormonal control linking volume and water

• Although Na⁺ is primary, water excretion is also volume-regulated.
• ↑ ECF volume → inhibits vasopressin (ADH)
• ↓ ECF volume → increases vasopressin
• Volume signals override osmotic control of vasopressin.

3. Central role of angiotensin II in hypovolemia

• Angiotensin II actions:
• Stimulates aldosterone secretion
• Stimulates vasopressin
• Increases thirst
• Causes vasoconstriction
• These effects maintain blood pressure and volume in hypovolemia.
• Therefore, angiotensin II is a key hormone in volume depletion responses.

4. Natriuretic peptides in volume expansion

• ECF expansion → increased cardiac stretch
• Heart releases:
• Atrial natriuretic peptide (ANP)
• B-type natriuretic peptide (BNP)
• Effects:
• Natriuresis (Na⁺ excretion)
• Diuresis (water excretion)

5. Disease states affecting ECF volume

• Pure water loss (dehydration):
• Water lost from ICF + ECF
• → only moderate ECF volume reduction
• Na⁺ loss (diarrhea, severe acidosis, adrenal insufficiency, heat prostration):
• → marked ECF volume loss
• → can progress to shock
• Shock compensation:
• Prioritizes intravascular volume
• Also alters Na⁺ balance

6. Adrenal insufficiency — special mechanism

• ECF volume falls due to:
• Urinary Na⁺ loss
• Shift of Na⁺ into cells
• Reinforces why Na⁺ is central to volume homeostasis
• Hence, multiple redundant Na⁺-control mechanisms exist

KIDNEY ROLE IN Na⁺ & VOLUME CONTROL

7. Hemodynamic effects during ECF volume depletion

• ↓ ECF volume → ↓ blood pressure
• ↓ BP → ↓ glomerular capillary pressure
• ↓ pressure → ↓ GFR
• ↓ GFR → ↓ Na⁺ filtered

8. Tubular Na⁺ reabsorption

• Na⁺ reabsorption increases
• Partly due to ↑ aldosterone secretion
• Aldosterone secretion is regulated by ↓ mean intravascular pressure

9. Rapid Na⁺ retention independent of aldosterone

• Postural change (supine → standing):
• Aldosterone increases
• Na⁺ excretion decreases within minutes
• This occurs even in adrenalectomized subjects
• Therefore:
• Cannot be explained by aldosterone alone
• Likely due to:
• Hemodynamic changes
• Possibly ↓ ANP secretion

10. Hormones related to Na⁺ balance

• Kidneys produce:
• 1,25-dihydroxycholecalciferol
• Renin
• Erythropoietin
• Natriuretic substances:
• ANP & BNP → increase Na⁺ excretion
• Endogenous ouabain → inhibits Na⁺/K⁺-ATPase

RENIN–ANGIOTENSIN SYSTEM (RAS)

11. Renin — structure & synthesis

• Renin is an acid protease.
• Molecular weight: 37,326 Da
• Structure:
• Two lobes
• Active site cleft
• Asp104 + Asp292 essential
• Classified as an aspartyl protease.

12. Renin biosynthesis

• Synthesized as preprorenin (406 aa)→ prorenin (383 aa)→ renin (340 aa)
• Prorenin has little/no biological activity
• Active renin formed in JG cell secretory granules

13. Renin vs prorenin secretion

• Prorenin:
• Secreted constitutively
• Also produced by ovaries and other tissues
• Active renin:
• Produced almost exclusively by kidneys
• After nephrectomy:
• Prorenin remains
• Active renin ≈ zero

14. Function & kinetics of renin

• Half-life: ≤ 80 minutes
• Only function:
• Cleaves angiotensin I (decapeptide) from angiotensinogen

ANGIOTENSINOGEN

15. Properties

• Plasma α₂-globulin
• 453 amino acids
• 13% carbohydrate
• Synthesized in liver
• Levels ↑ with:
• Glucocorticoids
• Thyroid hormones
• Estrogens
• Cytokines
• Angiotensin II

ACE & ANGIOTENSIN II

16. ACE actions

• Converts angiotensin I → angiotensin II
• Inactivates bradykinin by ACE
• ACE inhibition → ↑ bradykinin → cough

17. ACE structure

• Somatic ACE:
• 170 kDa
• Two active sites
• Germinal ACE:
• 90 kDa
• One active site
• Both from same gene, different promoters

18. Angiotensin II metabolism

• Half-life: 1–2 minutes
• Metabolized by aminopeptidases:
• Angiotensin II → Angiotensin III→ Angiotensin IV
• Angiotensin III:
• ~40% pressor
• ~100% aldosterone-stimulating
• Most other fragments are inactive

19. Measurement

• PRA: angiotensin I generation
• PRC: renin concentration (angiotensinogen added)
• Normal:
• PRA ≈ 1 ng/mL/hr
• Angiotensin II ≈ 25 pg/mL

ACTIONS OF ANGIOTENSIN II

20. Vascular & renal actions

• Powerful arteriolar vasoconstrictor
• ↓ sensitivity in Na⁺ depletion due to receptor down-regulation
• Stimulates:
• Aldosterone
• Sympathetic norepinephrine release
• Mesangial contraction → ↓ GFR
• Direct tubular Na⁺ reabsorption (PCT NHE3)

21. Central nervous system actions

• Acts via circumventricular organs
• Effects:
• ↓ baroreflex sensitivity
• ↑ thirst (SFO + OVLT)
• ↑ vasopressin
• ↑ ACTH
• Does not cross BBB

TISSUE RENIN–ANGIOTENSIN SYSTEMS

22. Local RAS

• Present in:
• Blood vessels
• Uterus, placenta, fetal membranes
• Heart, brain, pancreas, fat, adrenal, gonads, pituitary
• Minimal contribution to circulating renin
• Angiotensin II acts as growth factor
• Explains benefit of ACE inhibitors / ARBs in heart failure

ANGIOTENSIN II RECEPTORS

23. AT1 receptors

• Gq-coupled → ↑ intracellular Ca²⁺
• Mediate most effects
• Subtypes:
• AT1A (vessels, brain)
• AT1B (adrenal, pituitary)
• In humans: chromosome 3

24. AT2 receptors

• Gene on X chromosome
• Activate phosphatases
• Open K⁺ channels
• ↑ NO → ↑ cGMP
• Prominent in fetal life
• Persist in adult brain

25. Differential regulation

• ↑ Angiotensin II:
• Down-regulates vascular AT1 A
• Up-regulates adrenal AT1 B

REGULATION OF RENIN SECRETION

26. Key regulators

• Stimulators:
• ↓ afferent arteriolar pressure
• ↓ NaCl at macula densa
• ↑ sympathetic activity (β₁ → ↑ cAMP)
• Prostaglandins
• Inhibitors:
• Angiotensin II
• Vasopressin
• ↑ NaCl delivery
• ↑ afferent pressure

27. Clinical conditions ↑ renin

• Na⁺ depletion
• Diuretics
• Hypotension
• Hemorrhage
• Upright posture
• Dehydration
• Cardiac failure
• Cirrhosis
• Renal artery stenosis
• Psychological stress

28. Renin & hypertension

• Goldblatt hypertension:
• Two -kidney-one-clip → ↑ renin
• One-kidney-one-clip → renin often normal ( vol dependant HTN)
• Many hypertensive patients respond to:
• ACE inhibitors
• AT1 blockers
• Even with normal renin

HORMONES OF THE HEART & OTHER NATRIURETIC FACTORS

1️⃣ Why natriuretic hormones exist (big picture)

• The body needs a counter-regulatory system to oppose:
• Salt and water retention
• Volume expansion
• High blood pressure
• The heart itself acts as an endocrine organ, sensing stretch and volume load and releasing hormones that:
• Promote natriuresis
• Reduce ECF volume
• Lower blood pressure
• Oppose RAAS and catecholamines

2️⃣ Structural basis: how the heart makes hormones

• Atrial myocytes (mainly) and ventricular myocytes (lesser extent) contain secretory granules
• These granules:
• Increase in number when NaCl intake rises
• Increase when ECF volume expands
• Extracts from atrial tissue directly cause natriuresis, proving hormonal activity

3️⃣ Atrial Natriuretic Peptide (ANP)

Structure

• Polypeptide hormone
• Characteristic 17–amino-acid ring
• Formed by a disulfide bond between two cysteines
• Circulating ANP:
• 28 amino acids
• Synthesized from a large precursor (151 amino acids):
• Includes a 24-amino-acid signal peptide
• Also present in brain tissue
• Brain forms are shorter than circulating ANP

Key logic

• Large inactive precursor → processed → active circulating hormone
• Ring structure is essential for biological activity

4️⃣ B-type Natriuretic Peptide (BNP)

Structure & source

• First isolated from porcine brain
• In humans:
• Present in brain
• Much higher concentration in the heart, especially ventricles
• Circulating BNP:
• 32 amino acids
• Shares the same 17-amino-acid ring as ANP
• But amino acid sequence differs within the ring

Key logic

• BNP reflects ventricular stretch
• Hence its major clinical role in heart failure

5️⃣ C-type Natriuretic Peptide (CNP)

Structure

• Exists as:
• 22-amino-acid form
• 53-amino-acid form
• Also contains the conserved ring structure

Distribution

• Found in:
• Brain
• Pituitary
• Kidneys
• Vascular endothelial cells
• Very little in the heart or circulation

Functional identity

• Acts mainly as a paracrine hormone
• Not a major circulating natriuretic hormone

6️⃣ Renal actions of natriuretic peptides (core exam mechanism)

Hemodynamic actions

• Dilate afferent arterioles
• Relax mesangial cells
• Both effects → ↑ glomerular filtration rate (GFR)

Tubular actions

• Direct inhibition of Na⁺ reabsorption in renal tubules

Net renal effect

• ↑ Na⁺ excretion
• ↑ Water excretion
• ↓ ECF volume

7️⃣ Vascular and systemic actions

• Increase capillary permeability
• Fluid shifts from intravascular to interstitial space
• Relax vascular smooth muscle
• Arterioles → ↓ peripheral resistance
• Venules → ↓ venous return
• CNP causes stronger venodilation than ANP or BNP
• Overall effect:
• ↓ Blood pressure

8️⃣ Interaction with other hormonal systems

• Inhibit renin secretion
• Oppose actions of:
• Angiotensin II
• Catecholamines
• Serve as physiological antagonists of RAAS

9️⃣ Central nervous system actions of ANP

• ANP is present in neurons
• ANP-containing pathway:
• Origin: anteromedial hypothalamus
• Projection: lower brainstem cardiovascular centers
• CNS effects:
• Oppose angiotensin II
• Counteract vasopressin
• Promote natriuresis

Exam logic

• Peripheral + central actions work together to reduce volume and pressure

🔟 Natriuretic peptide receptors (NPR)

NPR-A

• Transmembrane receptor
• Intracellular guanylyl cyclase domain
• Highest affinity for ANP

NPR-B

• Similar structure to NPR-A
• Guanylyl cyclase activity
• Highest affinity for CNP

NPR-C

• Binds ANP, BNP, and CNP
• Has a markedly truncated cytoplasmic domain
• Two proposed roles:
• Signaling role:
• Via G-proteins
• Activates phospholipase C
• Inhibits adenylyl cyclase
• Clearance receptor (strong evidence):
• Removes natriuretic peptides from blood
• Releases them later
• Helps stabilize plasma hormone levels

1️⃣1️⃣ Secretion patterns & physiology

Basal levels

• Plasma ANP ≈ 5 fmol/mL in normal individuals on moderate salt intake

Stimuli for secretion

• ANP:
• Atrial stretch
• ↑ Central venous pressure
• ↑ ECF volume
• BNP:
• Ventricular stretch

Physiologic demonstrations

• Neck-level water immersion:
• Removes gravitational pooling
• ↑ Central venous pressure
• ↑ Atrial stretch → ↑ ANP
• ↓ Renin and aldosterone
• Standing from supine position:
• ↓ Central venous pressure
• ↓ Plasma ANP

Disease relevance

• Both ANP and BNP are elevated in heart failure
• BNP measurement is widely used diagnostically

1️⃣2️⃣ Metabolism of ANP

• Short plasma half-life
• Broken down by neutral endopeptidase (NEP)
• Thiorphan inhibits NEP
• → ↑ circulating ANP levels

1️⃣3️⃣ Na⁺, K⁺-ATPase–inhibiting natriuretic factor

• Another circulating natriuretic substance exists
• Mechanism:
• Inhibits Na⁺, K⁺-ATPase
• Effects:
• Causes natriuresis
• Raises blood pressure (unlike ANP/BNP)
• Likely identity:
• Digitalis-like steroid (ouabain)
• Source:
• Adrenal glands
• Physiological role:
• Still uncertain

📊 Natriuretic Hormones & Related Factors — Consolidated Table

🧠 Receptor Summary (Quick Recall Table)

🔑 One-Line Exam Locks

• ANP = atrial stretch → natriuresis + ↓ RAAS
• BNP = ventricular stretch → heart failure marker
• CNP = endothelial, paracrine, venodilator
• NPR-C = clearance receptor
• Na⁺,K⁺-ATPase inhibitor = natriuresis + ↑ BP (digitalis-like)

ERYTHROPOIETIN (EPO)

1️⃣ Core function

• Matches oxygen-carrying capacity to tissue needs
• Increases RBC production when:
• Blood loss occurs
• Hypoxia develops
• Suppressed when RBC mass is excessive

2️⃣ Structure

• Glycoprotein hormone
• 165 amino acids
• Four oligosaccharide chains
• Essential for in-vivo activity
• Plasma level rises markedly in anemia

3️⃣ Action on bone marrow

• Acts on erythropoietin-sensitive committed stem cells
• Increases:
• Survival (anti-apoptotic effect)
• Proliferation
• Differentiation into erythrocytes

Receptor & signaling

• Single-pass transmembrane receptor
• Member of cytokine receptor superfamily
• Has tyrosine kinase activity
• Activates downstream:
• Serine and threonine kinases
• Final effect:
• ↓ Apoptosis
• ↑ RBC growth and maturation

4️⃣ Metabolism & timing

• Main site of inactivation: liver
• Plasma half-life: ~5 hours
• Rise in circulating RBCs:
• Appears after 2–3 days
• Due to slow maturation process

5️⃣ Sources of erythropoietin

Adults

• Kidneys → ~85%
• Liver → ~15%

Cellular origin

• Kidney:
• Interstitial cells of peritubular capillary bed
• Liver:
• Perivenous hepatocytes

Other sites

• Brain:
• Neuroprotective role against hypoxic injury
• Uterus & oviducts:
• Estrogen-induced
• Mediates estrogen-dependent angiogenesis

Clinical correlation

• Loss of renal mass → liver cannot compensate
• Leads to anemia in chronic kidney disease

6️⃣ Clinical use

• EPO gene cloned
• Recombinant form: epoetin alfa
• Used in:
• Anemia of chronic kidney disease
• Dialysis patients (very common deficiency)
• Autologous blood donation before elective surgery

7️⃣ Regulation of secretion

Primary stimulus

• Hypoxia

Additional stimulants

• Cobalt salts
• Androgens

Oxygen-sensing mechanism

• Likely a heme protein
• Deoxy form:
• Stimulates EPO gene transcription
• Oxy form:
• Inhibits transcription

Other facilitators

• Alkalosis at high altitude
• Catecholamines via β-adrenergic mechanism

Important distinction

• Renin–angiotensin system is separate
• No direct regulatory overlap with erythropoietin