This document discusses body fluids and electrolyte balance. It begins by outlining the major body fluid compartments - intracellular fluid, interstitial fluid, and plasma. The functions of body fluids are then described. Key points include body fluids acting as a medium for metabolic reactions and transporting nutrients and waste. The document outlines various indicators used to measure body fluid compartments and discusses homeostasis of body fluids and electrolytes. Major electrolytes like sodium, potassium, calcium, and chloride are explained in depth, including their roles, concentrations in different fluid compartments, and regulatory mechanisms.

1. Body Fluids & Electrolyte Balance
Department of Human Physiology
| Nnamdi Azikiwe University Nnewi Campus, Nigeria
Lecture Slide
David C. Ikwuka

2. Outline
 Body Fluids
 Functions
 Compartments
 Measurement
 Indicators for measurement
 Body Fluid Homeostasis
 Electrolytes
 Regulation
 Clinical Significance
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3. Body Fluids
 In Lean adults, Fluids make up ~55% of female &
60% of male total body mass
 Inorganic substances (Minerals) are dissolved in fluids
& form Ions called Electrolytes.
– Intracellular fluid (ICF) inside cells ~ 2/3 of body fluid
– Extracellular fluid (ECF) outside cells
• Interstitial fluid between cell ~ 80% of ECF
• Plasma in blood is ~ 20% of ECF
ICF & ECF differ in their Electrolyte Composition
Examples also includes lymph, cerebrospinal fluid,
synovial fluid, aqueous humor, vitreous body,
endolymph, perilymph, and pleural, pericardial, and
peritoneal fluids. They are referred to as Transcellular
Fluids
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4. Functions of Body Fluids
1. Necessary for normal cellular function
2. Medium for metabolic reactions
3. Transports nutrients, waste products
4. Acts as lubricant,
5. Acts as insulator,
6. Act as shock absorber
7. Helps to regulate & maintain body
temperature
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5. 5
Body Fluid Compartment
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6. Movement of Body Fluids
 The movement of body fluid across cell and
capillary membranes accomplished
through:
– Osmosis
– Diffusion
– Filtration
– Active transport
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7. Terminologies Associated with Body Fluids
 Osmolality (Osm): Concentration of Solutes per kg of water
(Osm/kg)
i. Greatest determinants of osmolality in ECF: sodium,
glucose, urea
ii. Greatest determinants of osmolality in ICF: potassium,
glucose, urea
 Tonicity: Osmolality of solution
i. Isotonic solution: same osmolality as body fluids
ii. Hypertonic solution: higher osmolality than body fluids
iii. Hypotonic solution: lower osmolality than body fluids
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8. Continua.
 When RBCs are placed in Hypotonic solutn
they gain water & may burst (Hemolysis).
 When RBCs are placed in Hypertonic solution
which has higher osmotically active solutes at
higher osmolality & osmotic pressure relative
to plasma, they shrink because of the osmosis
of water out of the cells (Crenation)
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9. Measurement of Body Fluid Compartment
1. Indicator-Dilution Method: Administer Indicator
confined to BF compartm. of interest, allow sufficient
time for uniform distribution throughout the
compartm., collect plasma sample. At equilibrium, the
Indicator Conc. Will be same in the entire compartm.
𝑉𝑜𝑙 =
𝐴𝑚𝑡. 𝑜𝑓 𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑜𝑟
𝐶𝑜𝑛𝑐. 𝑜𝑓 𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑜𝑟
If indicator was lost from fluid compartment, the amt. lost
is subtracted from the amt. administered.
 To measure TBW, Heavy Water (D2O), Tritiated water
(HTO) or Antipirine (drug that distributes throughout all
of body water) is used as Indicator.
9

10.  TBW Volume: Tritium (3H2O), Deuterium
(2H2O), Antipyrine.
 ECF Volume: Inulin, Thiosulphate ion,
Radioactive Sodium (22Na), Radioactive
Iothalamate (125I-iothalamate) & Radioactive
Chloride
 ICF Volume: TBW-ECF
 Plasma Volume: Evans blue dye (T-1824),
(125I-albumin)
 Interstitial Fluid Vol: ECF-Plasma Vol.
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Indicators of Body Fluid Compartment

11. Body Fluid Homeostasis
 A steady-state balance where the volume &
composition of body fluids remain constant
despite the addition & elimination of water &
solutes from the body. It’s a complex process
involves all organs systems of the body.
 Transport by epithelia cells of the GIT, kidneys
& Lungs controls both the intake & excretion
of numerous substances & water.
 The CVS delivers nutrients to & removes
waste products from cells & tissues
 Nervous & Endocrine systems provide
regulation & integration of these important
functions
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12. Fluid Homeostasis
 The body maintains fluid homeostasis by
ensuring that the amount of water added to
the body each day equals or is balanced by
the amount lost or excreted.
 Although the amount of water added to the
body each day may not be constant
 There are several routes for water excretion
but the Kidney is the only regulated route.
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13. Fluid Homeostasis
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14. Body Fluid Homeostasis
 2 barriers separate ICF, interstitial fluid and plasma
– Plasma membrane separates ICF from
surrounding interstitial fluid
– Blood vessel wall divide interstitial fluid from
plasma
 Body is in fluid balance when required amounts of
water and solutes are present and correctly
proportioned among compartments.
 The conc. Of Virtually all substances in the body
must be maintained within narrow range ie there is
a Set Point & mechanisms for monitoring
deviations from set point & Effector Mechanism to
maintain the conc. Of that substance in the body
constant
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15. Intracellular
fluid (ICF)
Extracellular
fluid (ECF)
The ECF and ICF are in
balance, with the two
solutions isotonic.
ECF water loss
Water loss from ECF
reduces volume and
makes this solution
hypertonic with respect
to the ICF.
Increased
ECF volume
Decreased ICF volume
An osmotic water shift
from the ICF into the
ECF restores osmotic
equilibrium but
reduces the ICF
volume.
Changes to the ICF and ECF when water losses outpace
water gains

16. – Very different composition
– Are at osmotic equilibrium
– Loss of water from ECF is replaced by water
in ICF
• = Fluid shift
– Occurs in minutes to hours and restores osmotic
equilibrium
• Dehydration
– Results in long-term transfer that cannot replace ECF
water loss
– Homeostatic mechanisms to increase ECF fluid volume
will be employed
ICF and ECF compartments balance
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17. ELECTROLYTES
 Are ionized substances in Body fluids
which contributes most to the total solute
conc. (Osmolality) of Body Fluids. Body
fluids also contains uncharged particles
(Glucose, Urea etc)
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18. Electrolytes In Body Fluids
18
 Ions form when electrolytes dissolve ad
dissociate
 4 general functions
– Control osmosis of water between body fluid
compartments
– Help maintain the acid-base balance
– Carry electrical current
– Serve as cofactors
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19. Protein & Electrolyte Conc. In Fluids
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20. Concentrations in Body Fluids
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– Concentration of ions typically expressed in
milliequivalents per liter (mEq/liter)
• Na+ or Cl- number of mEq/liter = mmol/liter
• Ca2+ or HPO4
2- number of mEq/liter = 2 x
mmol/liter
– Major difference between 2 ECF
compartments (plasma and interstitial fluid) is
plasma contains many more protein anions
• Largely responsible for blood colloid osmotic
pressure
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21. ICF & ECF
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–ECF most abundant cation is Na+, anion
is Cl-
–ICF most abundant cation is K+, anion are
proteins and phosphates (HPO4
2-)
–Na+ /K+ pumps play major role in keeping
K+ high inside cells and Na+ high outside
cell
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22. Na+
22
 Most abundant ECF ion (Serum level:135-
145mEq/L)
 Together with Cl- constitute 90% of solute in ECF
 Plays pivotal role in fluid and electrolyte balance
because it account for more than half of the
osmolarity of ECF
 Level in blood controlled by
– Aldosternone – increases renal reabsorption
– ADH – if sodium too low, ADH release stops
– Atrial natriuretic peptide – increases renal excretion
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23. K+
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– Most abundant cations in ICF (Serum level:
3.5-5.3mEq/L)
– Key role in establishing RMP in neurons and
muscle fibers
– Also helps maintain normal ICF fluid volume
– Helps regulate pH of body fluids when
exchanged for H+
– Controlled by aldosterone – stimulates
principal cells in renal collecting ducts to
secrete excess K+.
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24. 24

25. Factors That Alter K+ Distribution Between Fluid
Compartments
Factors That Shift K+ into
Cells
(Decrease Extracellular [K+])
Factors That Shift K+ Out of Cells
(Increase Extracellular [K+])
Insulin Insulin deficiency (DM)
Aldosterone Aldosterone deficiency
b-adrenergic stimulation b-adrenergic blockade
Alkalosis Acidosis
Cell lysis
↑ed ECF Osmolarity
Strenuous exercise 25

26. Ca2+
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– Most abundant mineral in body (Serum level: 9-
11mg/dL)
– In body fluids mainly an ECF cation
– 98% in adults in skeleton and teeth & contributes
to hardness of teeth and bones
 Plays vital roles in blood clotting, neurotransmitter
release, muscle tone, & excitability of nervous and
muscle tissue
– Regulated by parathyroid hormone
• Stimulates osteoclasts to release calcium from bone – resorption
• Also enhances reabsorption from glomerular filtrate
• Increases production of calcitrol to increase absorption for GI tract
– Calcitonin lowers blood calcium levels
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27. 27

28. Magnesium
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– In adults, ~ 54% of total body magnesium is part
of bone as magnesium salts, remaining 46% as
Mg2+ in ICF (45%) or ECF (1%)
– 2nd most common ICF cation (Serum levels 1.5-
2.5mEq/L)
– Cofactor for certain enzymes and sodium-
potassium pump, essential for normal
neuromuscular activity, synaptic transmission,
myocardial function, Protein & DNA synthesis.
– Secretion of parathyroid hormone depends on Mg2+
– Regulated in blood plasma by varying rate excreted in
urine Nnamdi Azikiwe University Nnewi Campus

29. 29

30. Cl-
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– Most prevalent anion in ECF (Serum level: 95-
105mEq/L)
– Moves relatively easily between ECF and ICF
because most plasma membranes contain Cl-
leakage channels and antiporters
– Can help balance levels of anions in different
fluids
• Chloride shift in RBCs
– Regulated by
• ADH – governs extent of water loss in urine
• Processes that increase or decrease renal reabsorption
of Na+ also affect reabsorption of Cl-
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31. HCO3-
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– 2nd most prevalent ECF anion (Also in ICF)
– Conc. increases in blood passing through systemic
capillaries picking up carbon dioxide
• CO2 combines with H2O to form carbonic acid
which dissociates
• Drops in pulmonary capillaries when CO2 exhaled
– Chloride shift helps maintain correct balance of anions
in ECF and ICF
– Kidneys are main regulators of blood HCO3
-
• Can form and release HCO3
- when low or excrete
excess
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32. 32

33. Phosphates
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 ~ 85% in adults present as calcium phosphate
salts in bone & teeth, remaining 15% ionized –
H2PO4
-, HPO4
2-, and PO4
3- are major ICF anion
(Serum level: 2.4-4.5mg/dL)
– HPO4
2- important buffer of H+ in body fluids and
urine
– Same hormones governing calcium homeostasis
also regulate HPO4
2- in blood
• Parathyroid hormone – stimulates resorption of bone
by osteoclasts releasing calcium and phosphate but
inhibits reabsorption of phosphate ions in kidneys
• Calcitrol promotes absorption of phosphates and
calcium from GI tract
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34. Electrolyte Absorption Mechanism Via GIT
34

35. Ion Absorption Ion Excretion
ICF ECF
Ion absorption occurs across the
epithelial lining of the small intestine
and colon.
Ion reserves (primarily
in the skeleton)
Ion pool in body fluids
Sweat gland
secretions
(secondary
site of ion loss)
Kidneys
(primary site
of ion loss)
Mineral balance, the balance between ion absorption (in the
digestive tract) and ion excretion (primarily at the kidneys)

36. Regulation of Water & Solute Loss
 Elimination of excess body water through urine
 Extent of urinary salt (NaCl) loss is the main factor
that determines body fluid volume
 Main factor that determines body fluid osmolarity is
extent of urinary water loss
 3 hormones regulate renal Na+ and Cl- reabsorption
(or not)
– Angiotensin II and aldosterone promote urinary
Na+ and Cl- reabsorption of (and water by
osmosis) when dehydrated
– Atrial natriuretic peptide (ANP) promotes excretion
of Na+ and Cl- followed by water excretion to
decrease blood volume 36
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37. The mechanisms that regulate sodium balance
when sodium concentration in the ECF changes
Rising plasma
sodium levels
The secretion of ADH
restricts water loss and
stimulates thirst, promoting
additional water
consumption.
Osmoreceptors
in hypothalamus
stimulated
HOMEOSTASIS
DISTURBED
Increased Na
levels in ECF
If you consume large
amounts of salt without
adequate fluid, as when
you eat salty potato
chips without taking a
drink, the plasma Na
concentration rises
temporarily.
ADH Secretion Increases
Recall of Fluids
Because the ECF
osmolarity increases,
water shifts out of the
ICF, increasing ECF
volume and lowering
ECF Na concentrations.
HOMEOSTASIS
RESTORED
Decreased Na
levels in ECF
HOMEOSTASIS
Normal Na
concentration
in ECF
Start

38. Two types of Thirst
38
1. Osmotic/Osmometric Thirst:
Occurs when solute conc. Of ECF ↑ses>it draws
water out of the cells (by osmosis) & they shrink in
volume causing dehydration>triggers CVOs
(Osmoreceptors: Vascular organ of lamina terminalis
& Subfornical organ)> Median preoptic nucleus
(MPN) in hypothalamus>initiates water seeking &
ingestive behaviour.
 Destruction of MPN of hypothalamus in
humans/animals results in partial or total loss in
desire to drink even with extremely high salt conc.
In ECF. Thirst sensation is also reduce or diminish
in elderly.

39. 2. Hypovolemic Thirst
Caused by Blood loss, vomiting & diarrhea
- TBV drops too low↓>vascular system responds by
constricting blood vessels> Renin release>RAS
Also provoke the CVOs-Osmoreceptors (Area
postrema, subfornical organs, vascular organ of
lamina terminalis) >MPN which initiates water seeking
& ingestive behaviour.
Others include:
- Arterial baroreceptors sense decreased arterial
pressure & signals CNS
- Cardiopulmonary receptors sense decreased blood
volume & signals area postrema & nucleus tractus
solitarii
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40. Antidiuretic Hormone
40
– Also known as vasopressin
– Produced by hypothalamus (Supraoptic &
Paraventricular nuclei), released from posterior
pituitary
– Promotes insertion of aquaporin-2 into
principal cells of collecting duct
– Permeability to water increases
– Produces concentrated urine
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41. 41
ADH Feedback for H2O Regulation

42. Thirst Quenching
 Homeostatic mechanism to stop drinking has 2 phases
1. Preabsorptive Phase-evoke quench thrist signals
before fluid is absorbed from stomach & distributed to
body via circulatn
- Relies on sensory inputs in mouth, pharynx,
oesophagus & upper GIT to anticipate amt of fluid
needed providing rapid signals to brain to stop drinking
2. Postabsorptive Phase- occurs via blood monitoring for
osmolality, fluid volume & Na+ balance which are
sensed in the CVOs linked via neural networks to
terminate thirst when fluid homeostasis is established.
42

43. Renin-Angiotensin-Aldosterone System
It’s a Hormone System that is essential for
fluid balance & blood regulation.
A. Stage 1: Release of Renin (Granular cells
of JGA) in response to
a. ↓ed Na+ delivery to DCT by Macula densa
cells
b. ↓ed perfusion pressure in kidney detected
by barorecptors in Afferent arteriole.
c. Sympathetic stimulation of JGA via β1
adrenoceptors.
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44. 44
B. Stage 2: Production of Angiotensin II
a. Angiotensinogen (liver) is
cleaved→Angiotensin I by Renin
b. Angiotensin I →Angiotensin II by ACE (Lungs &
Kidney)
c. Angiotensin II exerts its effect by binding to
receptors (AT1 & AT2) throughout the body.
It mediates its effect majorly via AT1 on
i. Neural Effects: It acts on hypothalamus to
stimulate thirst sensation & ADH secretion from
neurohypophysis resulting to fluid consumption
& kidney H20 retention.
ii. Cardiovascular Effects: Acts on AT1 in
endothelium of arterioles to cause
vasoconstriction, thus, ↑ing total peripheral
resistance & blood pressure.

45. 45
iii. Renal Effects: acts on kidneys to cause afferent &
efferent arteriole constriction & ↑ed Na+
reabsorption in PCT. Also important in
Tubuloglomerular feedback by release of
prostaglandins which results in preferential afferent
arteriole vasodilation in glomerulus.
C. Stage 3-Aldosterone Release: Angiotensin II
stimulate the release of Aldosterone
(mineralocorticoid) from zona glomerulosa of Adrenal
cortex.
i. Aldosterone acts on principal cells of the CD in
nephrons to ↑ expression of apical epithelium Na+
channels (ENAC) to reabsorb urinary Na, it also
↑ses the activity of basolateral Na+/K+/ATPase.
These causes ↑ed reabsorption of Na+ by ENAC &
Na+/K+ exchange by Na+/K+/ATPase pump. i.e, ↑ed
aldosterone level can ↓ K+ levels in the blood.

46. 46

47. Clinical Significance
 Intravenous Fluid: Must be isotonic to
blood in order to maintain the correct
Osmotic pressure & prevent cells from
shrinking or expanding.
Eg. Normal saline + 5%
Dextrose~300mOsm= Plasma osmolality
& Ringer’s lactate: Glucose+ Lactic Acid
+different salts
Reduce Swelling in Cerebral Edema:
Hypertonic solutions of Mannitol
47

48. Clinical Significance
Cerebral Edema: when neurosurgical
procedures & strokes result in
accumulation of interstitial fluid in the brain
& swelling of neurons, raising intercranial
pressure disrupting neuronal fxn & leading
to coma and death.
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49. Clinical Significance
 Diabetes insipidus: A rare disorder, a persons kidney
is passes abnormaly large urine vol. that is insipid
 Types are
i. Central DI (Neurogenic, hypothalamic or Pituatary):
damage to Pituatary gland, hypothalamus from
surgery, tumor, head injury or illness that affects
production, storage & release of ADH. Can also be
autoimmune rxn ie immune cells damage cells that
produce ADH.
ii. Nephrogenic DI: defect in kidney tubules (Collcting
Ducts), make them unable to respond to ADH (may be
genetic-Aquaporin-2 channels mutations, or caused
by certain drugs like lithium or foscavir)
iii. Gestational DI: occurs in pregnancy, enzyme from
placenta destroys ADH in the mother.
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50. Clinical Significance
 Syndrome of Inappropriate ADH (SIADH): Caused by
Bronchogenic tumor which produces ADH in
uncontrolled fashion. Plasma ADH levels are very high
& Plsma Osmolality Low because Kidneys form
Conc.Urine & save water.
 Primary Polydipsia (or Psychogenic polydipsia)
characterized by intake of excessive fluid in absence of
physiological stimuli to drink could be as a result of
psychiatric disorders (Schizophrenia) associated with
dry mouth
-Symptoms include: excessive thirst & xerostomia> large
water consumption; hyponatraemia (causing headache,
vomiting, muscular weakness, twitching);
hypervolemia(leading to Oedema, hypertension & weight
gain); behavioural changes (fluid seeking behaviour)
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51. Clinical Significance
Diarrhea: characterized by loose & watery
stool. In most cases the cause is unknown
cause (idiopathic origin) & it goes away on its
own. Diarrhea can cause dehydration,
electrolyte imbalance (Loss of Na+, K+ & Mg+
vital to body fxn) & kidney failure (no enough
blood supplied to kidneys)
 It can be acute (loose watery diarrhea lasting
1-2 days) persistent (lasting 2-4 weeks) &
chronic diarrhea (lasting >4 weeks or comes &
goes regularly over a long period of time)
51

52. Clinical Significance
A. Hyponatremia: Plasma Na+ levels <135mEq/L. In
H2O & Na+ Loss seen in Vomiting, diarrhea &
overuse of diuretics: the decrease in ECF stimulates
thirst & ADH release which causes increase H2O
ingestion & kidney concentration of urine and
plasma Hyposmolality & hyponatremia results.
 Could result from Addison’s Disease.
 Symptoms: muscle cramps, lethargy, fatigue,
disorientation, headache, anorexia, nausea, agitation,
hypothermia, seizures, and coma. These symptoms,
mainly neurologic, are a consequence of the swelling
of brain cells as plasma osmolality falls.
52

53. Clinical Significance
 Hypernatremia: is ↑ed plasma Na+ conc. & H2O
loss caused by excess Na+ added to ECF
(hyperosmotic overhydration) or primary loss of
H2O from ECF (hyperosmotic dehydration) as a
result of lack of ADH secretion.
 Hypercalcaemia: Problem in Parathyroid glands
producing excess parathormone causing ↑ed blood
calcium levels.
 Hypocalcaemia: caused by hypoparathyroidism
leading to ↓ed levels of calcium in blood.
53

54. Clinical Significance
Edema: Accumulation of Fluid in body tissues.
Could be Extracellular or Intracellular.
 Extracellular: Excess fluid accumulation in
extracellular spaces caused by either failure of
lymphatics to return fluid from Interstitium
back to blood or abnormal fluid leakage from
plasma in Interstitium across capillaries.
 Edema becomes severe in Lymphatic
blockage
 Edema is also implicated in heart failure.
54

55. Closing Remarks and
Questions

56. Closing Remarks
56

57. Questions?
57