WATER BALANCE AND ELECTROLYTES
MT4B: CLINICAL CHEMISTRY/ MIDTERM/2ND SEM
BODY WATER • Proteins, however, are important at the tissue-
• Bulk of the body Mass interstitial fluid interface where they provide the
• Constitutes the medium by which solutes are colloid osmotic pressure (oncotic pressure).
dissolved and by which metabolic reaction takes • When there is pure water loss an increase in the
place ECF osmolality occurs. This triggers the following
• Total Body Water – 40L physiological responses:
1. Antidiuretic hormone (ADH) or
• Distributed in TWO main compartments
vasopressin release via hypothalamic
- Intracellular fluid (ICF) – 24L
osmoreceptors
- Extracellular fluid (ECF)- 16L
2. Stimulation of the hypothalamic thirst
o Interstitial Fluid- 11.2L
center
o Plasma- 3.2L
3. Redistribution of water from the
o Transcellular- 1.6L
intracellular fluid compartment (ICF)
• Normally the amounts of water input and water
• Osmolality is defined as the concentration of
output are equal throughout the day.
solute (mmol) per kilogram of solvent.
• Water is obtained from the diet and oxidation
• The plasma osmolality, is increased in:
of foodstuff.
- Dehydration
• It is lost via the skin, lungs, kidneys and the - alcohol intoxication
intestine. - hypercalcemia
• The daily obligatory losses are shown as - diabetes insipidus
follows: - administration of osmotically active
- Skin 500 ml drugs e.g., mannitol.
- Lungs 400 ml • It is decreased in:
- Gut 100 mL - water intoxication (polydipsia)
- Kidneys 500 ml - inappropriate retention of water in the
- Total 1500 mL kidney (e.g., in SIADH or syndrome of
inappropriate secretion of ADH seen in
• These obligatory losses are compensated by asthma and pneumonia)
water taken from the following sources: - ectopic secretion of ADH e.g., in
bronchogenic cancer, lymphosarcoma,
- water from oxidative 400 ml etc.
metabolism
- minimum in diet 1100 mL ECF Volume
- Total 1500 mL • The ECF volume is directly dependent on the
sodium content. I
PHYSICAL PROPERTIES OF ECF • t is maintained by the following:
• The two important properties of the extracellular - Regulation of renal excretion of sodium or
fluid that is considered here are ECF osmolality glomerular filtration rate.
and ECF volume. - About 70% of filtered sodium is reabsorbed by
the earlier parts of the renal tubules.
ECF Osmolality - Aldosterone via the RAA system.
• ECF osmolality is regulated by the levels of - Aldosterone promotes sodium reabsorption
sodium and associated anions (e.g., Cl, HCO3), while stimulating the release of potassium and
glucose, urea, and proteins. hydrogen ions.
• Of these solutes, sodium is the major
determinant of plasma osmolality. RAA SYSTEM
• Note that of the different fluid compartments • Sodium depletion (hyponatremia), hypotension or
only plasma is accessible for analysis. potassium increase in blood (hyperkalemia)
• The fluids, however, are isoosmotic to each stimulates the juxtaglomerular (JG) cells of the
other that the nature of the plasma more or less kidneys to release renin.
reflects the nature of the other fluid • Renin converts angiotensinogen to angiotensin I
compartments. (AD).
• As mentioned in chapter 10, plasma osmolality is • Angiotensin I is converted to angiotensin II (All) in
92% due to sodium and its associated the lungs by the angiotensin-converting enzyme
electrolytes and only 8% is due to glucose, urea (ACE).
and proteins. • Angiotensin Il stimulates, the adrenal cortex to
• In fact, the proteins contribute only about produce aldosterone and is an active
, WATER BALANCE AND ELECTROLYTES
MT4B: CLINICAL CHEMISTRY/ MIDTERM/2ND SEM
• Aldosterone promotes retention of sodium and ANION GAP
excretion of potassium. • refers to the difference between the sums of the
concentrations of the principal cations (e.g., Na
DISORDERS OF WATER BALANCE and K) and of the principal anions (e.g., Cl and
HCO3).
Dehydration • It represents the unmeasured net negative charge
• Pure water loss or deficit on plasma proteins.
- leads to increased ECF osmolality. • Can be used in the quality control of automated
- will have a less effect on plasma or ECF volume analyzers.
because the body easily compensates for the • An abnormal anion gap in healthy individuals
fluid loss by recruiting water from the ICF suggests an instrument problem.
- lowers total body water but the total body
sodium remains normal. Methods of Calculating Anion Gap
• Water and sodium loss • The anion gap (AG) may be measured by any of
- has greater effect on ECF or plasma volume. the following formula:
- Hypovolemia readily takes place especially in o AG= Na (Cl + HCO3) NV: 7-14 mmol/L
isotonic fluid loss. o AG= (Na+K)-(Cl + HCO3-) NV: 10-18 mmol/L
- The different types of water and sodium loss • The anion gap is increased:
are: o Uremia
o Hypernatremic dehydration e.g.. water o ketoacidosis (starvation or diabetes)
and food deprivation, excessive sweating, o methanol aspirin, or ethylene glycol
osmotic diuresis poisoning (lots of acid metabolites
o Normonatremic dehydration e.g., vomiting, produced)
diarrhea o severe dehydration
o Hyponatremic dehydration e.g., diuretic o lactic acidosis.
therapy, salt-wasting renal disease,
• Decreased in :
adrenocortical insufficiency
o multiple myeloma
o protein error
Overhydration
o instrument error
• takes place when there is excessive intake of
water (polydipsia) or excessive reabsorption of
water such as in cases of SIADH and ectopic ADH
SODIUM
secretion. • This is the most abundant cation in the
extracellular fluid.
ELECTROLYTES • It accounts for about 92% of the osmotically
active solutes in the plasma.
• Ions capable of carrying an electrical charge
• Its amount also determines the ECF volume.
• The electrolytes in the body fluids carry diverse
• Seventy percent of sodium is freely
functions. These include the
exchangeable while 30% is complexed in the
1. Maintenance of osmotic pressure and
bone.
hydration e.g. sodium
2. Buffering functions e.g., HCO • The main functions of sodium are water pull e.g.,
3. Activators in enzyme reactions e.g. Mg osmotic activity of extracellular fluid, blood
4. Normal neuromuscular excitability e.g., Ca volume regulation, and neuromuscular
5. Redox reaction (electron transport) e.g., Fe excitability.
6. Myocardial rhythm and contractility
The levels of sodium are regulated by the
INSTERSTITIAL INTRACELLULAR following:
PLASMA
CATIONS
FLUID FLUID • Diet
Na 142 140 10
• Kidney: The renal threshold for sodium is 110-
K 5 4.1 150 130 mmol/L. About 70-80% of filtered sodium
Ca 5 4.1 -
Mg 5 3.0 40 is reabsorbed at the proximal tubule
Total 155 • RAA: As discussed above, aldosterone promotes
retention of sodium in exchange of secretion of
ANIONS
potassium or hydrogen ions.
C 103 115 15
HCO3 27 27 10
PO4 2 2 100
SO4 1 1.1 20
Organic acids 6 3.4 -
MT4B: CLINICAL CHEMISTRY/ MIDTERM/2ND SEM
BODY WATER • Proteins, however, are important at the tissue-
• Bulk of the body Mass interstitial fluid interface where they provide the
• Constitutes the medium by which solutes are colloid osmotic pressure (oncotic pressure).
dissolved and by which metabolic reaction takes • When there is pure water loss an increase in the
place ECF osmolality occurs. This triggers the following
• Total Body Water – 40L physiological responses:
1. Antidiuretic hormone (ADH) or
• Distributed in TWO main compartments
vasopressin release via hypothalamic
- Intracellular fluid (ICF) – 24L
osmoreceptors
- Extracellular fluid (ECF)- 16L
2. Stimulation of the hypothalamic thirst
o Interstitial Fluid- 11.2L
center
o Plasma- 3.2L
3. Redistribution of water from the
o Transcellular- 1.6L
intracellular fluid compartment (ICF)
• Normally the amounts of water input and water
• Osmolality is defined as the concentration of
output are equal throughout the day.
solute (mmol) per kilogram of solvent.
• Water is obtained from the diet and oxidation
• The plasma osmolality, is increased in:
of foodstuff.
- Dehydration
• It is lost via the skin, lungs, kidneys and the - alcohol intoxication
intestine. - hypercalcemia
• The daily obligatory losses are shown as - diabetes insipidus
follows: - administration of osmotically active
- Skin 500 ml drugs e.g., mannitol.
- Lungs 400 ml • It is decreased in:
- Gut 100 mL - water intoxication (polydipsia)
- Kidneys 500 ml - inappropriate retention of water in the
- Total 1500 mL kidney (e.g., in SIADH or syndrome of
inappropriate secretion of ADH seen in
• These obligatory losses are compensated by asthma and pneumonia)
water taken from the following sources: - ectopic secretion of ADH e.g., in
bronchogenic cancer, lymphosarcoma,
- water from oxidative 400 ml etc.
metabolism
- minimum in diet 1100 mL ECF Volume
- Total 1500 mL • The ECF volume is directly dependent on the
sodium content. I
PHYSICAL PROPERTIES OF ECF • t is maintained by the following:
• The two important properties of the extracellular - Regulation of renal excretion of sodium or
fluid that is considered here are ECF osmolality glomerular filtration rate.
and ECF volume. - About 70% of filtered sodium is reabsorbed by
the earlier parts of the renal tubules.
ECF Osmolality - Aldosterone via the RAA system.
• ECF osmolality is regulated by the levels of - Aldosterone promotes sodium reabsorption
sodium and associated anions (e.g., Cl, HCO3), while stimulating the release of potassium and
glucose, urea, and proteins. hydrogen ions.
• Of these solutes, sodium is the major
determinant of plasma osmolality. RAA SYSTEM
• Note that of the different fluid compartments • Sodium depletion (hyponatremia), hypotension or
only plasma is accessible for analysis. potassium increase in blood (hyperkalemia)
• The fluids, however, are isoosmotic to each stimulates the juxtaglomerular (JG) cells of the
other that the nature of the plasma more or less kidneys to release renin.
reflects the nature of the other fluid • Renin converts angiotensinogen to angiotensin I
compartments. (AD).
• As mentioned in chapter 10, plasma osmolality is • Angiotensin I is converted to angiotensin II (All) in
92% due to sodium and its associated the lungs by the angiotensin-converting enzyme
electrolytes and only 8% is due to glucose, urea (ACE).
and proteins. • Angiotensin Il stimulates, the adrenal cortex to
• In fact, the proteins contribute only about produce aldosterone and is an active
, WATER BALANCE AND ELECTROLYTES
MT4B: CLINICAL CHEMISTRY/ MIDTERM/2ND SEM
• Aldosterone promotes retention of sodium and ANION GAP
excretion of potassium. • refers to the difference between the sums of the
concentrations of the principal cations (e.g., Na
DISORDERS OF WATER BALANCE and K) and of the principal anions (e.g., Cl and
HCO3).
Dehydration • It represents the unmeasured net negative charge
• Pure water loss or deficit on plasma proteins.
- leads to increased ECF osmolality. • Can be used in the quality control of automated
- will have a less effect on plasma or ECF volume analyzers.
because the body easily compensates for the • An abnormal anion gap in healthy individuals
fluid loss by recruiting water from the ICF suggests an instrument problem.
- lowers total body water but the total body
sodium remains normal. Methods of Calculating Anion Gap
• Water and sodium loss • The anion gap (AG) may be measured by any of
- has greater effect on ECF or plasma volume. the following formula:
- Hypovolemia readily takes place especially in o AG= Na (Cl + HCO3) NV: 7-14 mmol/L
isotonic fluid loss. o AG= (Na+K)-(Cl + HCO3-) NV: 10-18 mmol/L
- The different types of water and sodium loss • The anion gap is increased:
are: o Uremia
o Hypernatremic dehydration e.g.. water o ketoacidosis (starvation or diabetes)
and food deprivation, excessive sweating, o methanol aspirin, or ethylene glycol
osmotic diuresis poisoning (lots of acid metabolites
o Normonatremic dehydration e.g., vomiting, produced)
diarrhea o severe dehydration
o Hyponatremic dehydration e.g., diuretic o lactic acidosis.
therapy, salt-wasting renal disease,
• Decreased in :
adrenocortical insufficiency
o multiple myeloma
o protein error
Overhydration
o instrument error
• takes place when there is excessive intake of
water (polydipsia) or excessive reabsorption of
water such as in cases of SIADH and ectopic ADH
SODIUM
secretion. • This is the most abundant cation in the
extracellular fluid.
ELECTROLYTES • It accounts for about 92% of the osmotically
active solutes in the plasma.
• Ions capable of carrying an electrical charge
• Its amount also determines the ECF volume.
• The electrolytes in the body fluids carry diverse
• Seventy percent of sodium is freely
functions. These include the
exchangeable while 30% is complexed in the
1. Maintenance of osmotic pressure and
bone.
hydration e.g. sodium
2. Buffering functions e.g., HCO • The main functions of sodium are water pull e.g.,
3. Activators in enzyme reactions e.g. Mg osmotic activity of extracellular fluid, blood
4. Normal neuromuscular excitability e.g., Ca volume regulation, and neuromuscular
5. Redox reaction (electron transport) e.g., Fe excitability.
6. Myocardial rhythm and contractility
The levels of sodium are regulated by the
INSTERSTITIAL INTRACELLULAR following:
PLASMA
CATIONS
FLUID FLUID • Diet
Na 142 140 10
• Kidney: The renal threshold for sodium is 110-
K 5 4.1 150 130 mmol/L. About 70-80% of filtered sodium
Ca 5 4.1 -
Mg 5 3.0 40 is reabsorbed at the proximal tubule
Total 155 • RAA: As discussed above, aldosterone promotes
retention of sodium in exchange of secretion of
ANIONS
potassium or hydrogen ions.
C 103 115 15
HCO3 27 27 10
PO4 2 2 100
SO4 1 1.1 20
Organic acids 6 3.4 -
