2 normalmetabolismofwaterandsodium

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Chapter 3: Disorders of Water and
Electrolytes Metabolism
 Section
 I. Normal Metabolism of Water and Sodium
II. Disorders of Water and Sodium
 Metabolism
III. Disorders of Potassium Metabolism

2
significance
 1.Evidence of the evolution
 2.Common problems in different disciplines
and diseases
 3.Volume, variety and order of infusion

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Section I. Normal Metabolism of
Water and Sodium
 ( 一 ) Some basic concepts:
 1. Homeostasis
 Homeostasis indicates a stable internal environment
within the body, which means:
 (a) body fluid: normal volume, composition and pH ;
 (b) normal temperature, blood pressure, cardiac output;
 (c) normal level of hormones, et al.

 Normal body fluid is the most important part in the
homeostasis.

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2. Body fluid (water, no pure water )
Body fluid is the fluids in the body.
Body fluid is a solution with
solutes (electrolytes, glucose, amino acids,
fatty substances and other constituents) and
solvent (water).

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Body fluid constitutes 60% of body weight.

 Intracellular fluid
 Body fluid (40%)ICF
 (60% of body IVF(5%)
 weight) Extracellular fluid
 (20%)ECF EVF (15%)

(1)Volume and distribution
of body fluid

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ECF includes:
IVF: plasma, 5%
 EVF: 15%
(1)interstitial fluid,
(2)lymph fluid
(3)transcellular fluid.
 跨细胞液
(1~2%)

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The transcellular fluid is the fluids in cavities:
cerebrospinal fluid (in ventricles of brain),
pleural fluid (in pleural cavity),
pericardial fluid (in pericardial cavity)
joint fluid (in articular cavity)
Transcellular fluid comes from epithelial cells.


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99% of interstitial fluid is fixed to collagen,
mucopolysaccharide and hyaluronic acid (gel),
(connective tissue), which called fixed water.
1% of interstitial fluid is free water (moving
freely).

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Influential factors on the volume of body fluid
--------------------------------------------------
 age % of BW
 -----------------------------------------------
 new born(0~1mon) 80%
 infant (1~12mon) 70%
 school age(1~17 year) 65%
 adult 60%
 ----------------------------------------------
 Very old person.
 tolerance?

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Influential factors on the volume of body fluid
------------------------------------------------
organ (tissue) water content
------------------------------------------------
fat 25%~30%
muscle 76%
bone 14%~46%
liver 70%
skin 72%
------------------------------------------------
A fatty person and a thin person with the same body
weight lost the same volume of body fluid, whose
condition is worse?

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(2)Composition of body
fluid 1) plasma (ECF)
 --------------------------------------
-
 Cations (mEq/L) Anions (mEq/L)
 ----------------------------------------------
-
 Na+
142 HCO3¯ 27
 K+
5 CI¯ 103
 Ca2+
5 HPO42
¯ 2
 Mg2+
2 SO42
¯ 1
 organic acid 5
 proteinate 16
 ----------------------------------------------
 total 154 154
(a)The major cation is Na+
, which cannot be replaced.
(b) The major anions are Cl¯ and HCO3¯.(can be replaced from
each other.
(c) Electrical neutral is present according to the numerals of mEq/L
in the normal state.

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2)interstitial fluid
The main
difference of
composition
between plasma
and interstitial
fluid is the protein
concentration in
interstitial fluid.
(for electrical
neutral ??)

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3)ICF
 Cations(mEq/L) Anions(mEq/L)
 ----------------------------------------------
 Na+
15 HCO3¯ 10
 K+
150 CI¯ 1
 Ca2+
2 HPO42
¯ 100
 Mg2+
27 SO42
¯ 20
 proteinate 63
 ---------------------------------------------
 total 194 194
Characteristics:
(1)The major cation is K+
, which cannot be replaced.
(2) The major anions are phosphate(HPO42
¯ )and sulphate
(SO42
¯).
(3) Electrical neutral is present according to the numerals of
mEq/L in the normal state.

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(3) Osmotic
pressure
 Osmosis is
the net diffusion
of water through
a selectively
permeable
membrane

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 The osmotic pressure
is formed on the
membrane as the water
moves from the sides with
lesser number of non-
diffusible particles to the
side with greater number
of non-diffusible particles.

 The osmotic pressure
pulls the water to the side
with more number of non-
diffusible particles.

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 The osmotic pressure is decided by the
number of particles. It is not determined by the
molecular weight or electric charge of the particle..
 The greater the number of non-diffusible
particles in the solution, the greater its osmotic
pressure is.
 Question: Do a Na+
and a molecule of protein
form the same osmotic pressure?

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 The total osmolality is 280 ～ 320 mOsm/L
(average 300 mOsm/L) in body fluid.
 90% ～ 95% of osmotic pressure is determined
by electrolytes (anions+cations).
 The anions (negative ions) constitute 139 mOsm/L.
 The cations (positive ions) constitute 151 mOsm/L.
 Glucose, urea, etc constitute 10 mOsm/L.

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Classification of osmotic pressure
 The crystal osmotic pressure is mainly
formed mainly by particles of electrolytes.
 The colloidal osmotic pressure is formed
by particles of proteins.

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(4) Characters of isotonicity
H2O exchange across cell membrane
The osmotic pressure in
the cells and out the
cells usually are the
same.
There is a tendency to
keep the isotonicity
between intracellular
and extracellular spaces.
Which type of molecule
move for keeping
isotonicity?

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 An isotonic solution
does not causes the
change in cell size.
 Water will move from
a hypotonic solution to
a hypertonic solution.
 Hypotonic solution
causes a cell to swell.
 Hypertonic solution
causes a cell to shrink.

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 The cell membrane is
highly permeable to water
but restricts the movement
of solute molecules.
 The movement of
electrolytes largely depends
on the activity of transport
proteins (channel, Na+
-
K+
pump) in the (fat-
soluble) cell membrane.

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H2O exchange across capillary
 crystal osmotic pressure??
 colloidal osmotic pressure??

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2.Function of water
 (1) Acts as a medium and reactant for chemical
reactions.
 (2) Acts as a transport vehicle for nutrients,
electrolytes, blood gases and metabolic wastes.
 (3) Importance for the regulation of body
temperature.
 1)The water can absorb more heat with less
increase of body temperature. (specific heat)
 2)More heat can be eliminated with sweat.
（蒸发热 enthalpy of vaporization ）
 3)Heat within the body is easy to be
eliminated from the surface because of the high
mobility of water.

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 (4)Acts as lubricant
 (joint fluid in joint cavity)
 (5) Acts as a cushion
 (CSF, amniotic fluid for fetus)
 A man with hunger strike can survive for 1
months.
 A man with “water strike” can only survive for
about 10 days.

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3. Gain and loss of water
 Water intake and water loss must be
balanced in order to keep the volume of the body
fluid constant and to maintain the homeostasis.
 ---------------------------------------------
 gains (ml/day) loss (ml/day)
 ----------------------------------------------
 drink 1200 lung 300
 food 1000 skin 500
 metabolic feces 200
 water 300 urine 1500
 ----------------------------------------------
 total 2500 2500
 ---------------------------------------------

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(1) Gain of water
 The major source
of body fluid is the oral
ingestion of water or soup
in food.
 Another source is the
water formed within the
body from the metabolism
of carbohydrate, protein and
fat (catabolism), which is
called metabolic water
(endogenous water).
gains (ml/day) loss (ml/day)
------------------------------------------
-
drink 1200 lung 300
food 1000 skin 500
metabolic feces 200
water 300 urine 1500
------------------------------------------
-
total 2500 2500

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
 Catabolism is
unavoidable.
 1g of protein
generates 0.41ml
of water.
 1 g of glucose
generates 0.60 ml
of water.
 1 g of fat
generates 1.07 ml
of water.
----------------------------------------------
----------------------------------------------
total 2500 2500

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(2) Loss of water
 Water is lost from the
body in several pathways.
1) Urine is the major
pathway to loss the water.
The volume of urine per
day is at least 500 ml for
eliminating waste
substances of metabolism.
 There will be 35 g of
urea produced each day.
The urea should be
eliminated at most in 6~8
% of concentration in
urine, so the 500 ml of
urine is needed at least
each day. More urea in the
body is poisonous.
---------------------------------------------
-
---------------------------------------------
total 2500 2500

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2 ） The expired air
contains water
vapour.
The 300 ml of
loss from lung is
unavoidable in
normal respiration.
Pure water is
expired by lung.
----------------------------------------------
----------------------------------------------
total 2500 2500

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 3) The insensible
vapor from skin loses
500ml of pure water per
day. It is unavoidable.
 Sweat is not
insensible evaporation.
 Sweat is
avoidable.
 Sweat is not pure
water. Sweat contains
0.2~0.3% of NaCl.(salty)
 4) A small amount
(100~200 ml) of water is
normally lost in the
feces.

----------------------------------------------
----------------------------------------------
total 2500 2500

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 Generally
speaking, for an
adult, the loss of
water from skin,
lung, gastrointestinal
tract and kidney is at
least 1500 ml per
day, so 1500 ? ml of
water should be
replaced into the
body via different
ways
----------------------------------------------
----------------------------------------------
total 2500 2500

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4. Function of sodium
 (1) Maintain the
osmolality of ECF.
Sodium and its
attendant anions (Cl-
and HCO3-
) account
for 90% to 95% of
osmotic pressure in
ECF.
 The effect of Na+
cannot be replaced by
K+
, Ca 2+
or Mg2+
.

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 (2) Determine the membrane potential and
 neuromuscular excitability.
 (3)Influence the acid-base balance
 NaHCO3 is alkalinity, NaCI is acidity after
infusion into the body. (neutrality outside the body)
 (4)Participate the metabolic process.
 Sodium influences the activity of enzymes.


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5. Balance of sodium
 (1) Content of sodium
 The content of sodium is 40~50 mmol /Kg of BW.
 The sodium can be divided into two parts:
 1)Exchangeable pool
 Exchangeable pool includes sodium in
extracellular fluid (50%) and intracellular fluid(10%).
60% of sodium is exchangeable.
 2)non-exchangeable pool.
 40% of Na+
is bound within bone mass which is
non-exchangeable.

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 The concentration of Na+
in ICF,
([Na+
]i) is 10 mmol/L.
 The concentration of Na+
in ECF,
([Na+
]e) is 130~150 mmol/L.


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(2) Na+
balance of intake and loss
 Body needs 500mg each day. Average daily
intake from food in China is 6~15g/day. The
Chinese diet contains too much sodium,
especially in north China.
 Increased Na+
in the body (H2O and Na
retention) is the precipitating factor of
hypertension.


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Loss of sodium
 (1)The kidneys are extremely efficient in
regulating sodium balance.
 The kidneys will eliminate more if the intake of
sodium is more.
 The kidneys will eliminate less if the intake of
sodium is less.
 The kidneys will not eliminate sodium if there is
no intake of sodium.
 (2)Gastrointestinal tract is the second way to lose
sodium. Less than 10% of the sodium intake is lost
from gastrointestinal tract (with feces ).


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6. Regulation of water and sodium
metabolism
 Normal metabolism of water and sodium means:
 (1) normal volume,
 (2) normal composition
 (3) normal osmotic pressure of body fluid.
 It is regulated by nervous system and hormones:
 (1) Thirst (neuro-regulation)
(2) Antidiuretic hormone, ADH
 (3) Aldosterone
 (4)Atrial natriuretic peptide ( ANP).
 (5) Renal regulation

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(1) Thirst (neuro-regulation)
Thirst is a kind of sense (feeling) coming from
thirst center, with which we will ask for drinking
water.

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Causes of thirst:
1)Increase of ECF osmolality (1~2%)
stimulates the thirst center via
osmoreceptor in anterior hypothalamus.
Then there will be the sense of thirst.
 After the drink of water, the
increased ECF osmolality will decrease to
normal, then the sense of thirst will
disappear.

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 2) Decrease of effective blood volume
(hypovolemia) stimulates the thirst center via volume
receptor in venae cavae and atrium.
 3) Elevated level of angiotensin II can
stimulates the thirst center.
 (Hypovolemia→renal ischemia→renin release→angitensin
I→angiotensin II)
 4) Dryness of mouth can also stimulates the
thirst center.
 After the drinking of water, the blood volume
will increase to normal.

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 no thirst
increase of ECF
osmolality (1~2%)
hypovolemia elevated
angiotensin II
dryness of
mouth
osmoreceptor
(anterior
hypothalamus
)
volume receptor
in venae cavae
and atrium
thirst center (anterior hypothalamus)
sense of thirst and drink of water
decrease of ECF
osmolality
increase of
ECF volume
decrease of
angiotensin
concentration II
disappear of dryness
Is it enough?

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 Thirst only is not enough to regulate the
balance of water and sodium.
 The defects of (neuro-regulation) thirst are:
 1)There will be no obvious thirst in patients with
hypovolemia if the osmolality is not increased.
 2)A patient with coma will not have the sense
of thirst.
 (too young baby.too old man)

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Hormoral regulation
(2) Antidiuretic hormone, ADH
 ADH is synthesized
by cells in the
supraoptic and
paraventricular nuclei of
hypothalamus, and
stored in and released
from posterior pituitary.


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The following factors will increase the
secretion of ADH.
 1) Increased osmolality of ECF(1% ～ 2%) increases ADH
secretion by stimulating the osmoreceptor ( in hypothalamus).
 2) Decreased volume of ECF （ 5% ～ 10% ） via volume
receptor (in venae cavae and atrium).
 3) Reduction of blood pressure via baroreceptors (in arch of
aorta and carotis)
 4) Stress situations ( patients during surgery, severe
trauma and pain)
 5) Increased angiotensin Ⅱ
 6) Some drugs: anesthetics (stimulate), alcohol (inhibit)

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increase of ECF osmolality
(1~2%) via osmoreceptor
hypovolemia via volume
receptor
reduction of BP via
baroreceptors
synthesis
and
release
of ADH
drugs anesthetics
(stimulate), alcohol (inhibit)
Stress
elevated angiotensin II
+
+
+
+
+

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 ADH is the
major regulator for
water output and
reabsorption in kidney.
 A rise of ADH
increases the
permeability of the
tubular wall to water,
then increases the
reabsorption of water
from the distal tubules
and collecting dusts of
the kidneys.

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(3) Aldosterone (hormoral regulation)
 Aldosterone is the major regulator of sodium excretion
and reabsorption.
 Aldosterone is a steroid hormone produced by the
adrenal cortex. The following factors are known to
stimulate the secretion of aldosterone.
 1) Angiotensin II is the major stimulator.
 2) A decrease of sodium level in ECF will stimulate
aldosterone secretion.
 3) An increase of potassium level in ECF will stimulate
aldosterone secretion.

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renal
blood flow
[Na+] in
macula densa
excitement of
sympathetic nerve
renin release from the
juxtaglomerular cells
increase of angiotensin II
releases
[K+], [Na+]
blood flow in plasma
aldosterone secretion from
adrenal cortex
Na+ reaborption in
renal tubules
K+ and H+ excretion
from kidneys

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（ 4 ） Atrial natriuretic peptide( ANP) 房钠肽
 ANP is a 28-amino acid peptide released from the
atrium in response to increased atrial stretch via
mechanoreceptors.
 The effect of ANP are to:
 1)dilate the renal blood vessels to increase the GFR,
 2) inhibit the reabsorption of Na+
from collecting dusts..
 3) inhibit the release of renin, aldosterone and ADH.
 4) be a endogenous antagonist to angiotensin II.
 The result is to increase the kidney’s ability to excrete both
water and sodium.

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(5) Renal regulation
 1) Glomerular filtration
 2) Reabsorption and excretion of renal
tubules

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Glomerular filtration
GFR= 125ml/min=180L/day
GFR depends on:
(a) filtration pressure
(b) permeability of glomerular membrane
(c) surface area of filtration.
60~70% of filtrate is reabsorbed at proximal
tubules, finally 99%~99.5% of filtrate is reabsorbed by
renal tubules, which is called glomerular-tubular
balance.

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Summary of regulating water
and sodium metabolism:
(1) Neuro-regulation
 Thirst
(2) Hormoral regulation
 ADH
 Aldosterone
 ANP

2 normalmetabolismofwaterandsodium

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