Download S03-clinicalbiochem1-Water

Water and Electrolytes

Water
 Sodium
 Potassium
 Acid base balance
 Chloride
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Composition of Body Fluids
• Water is the universal solvent
• Solutes are broadly classified into:
• Electrolytes–inorganic salts, all acids and bases, and some proteins
• Nonelectrolytes–examples include glucose, lipids, creatinine, and
urea
• Electrolytes have greater osmotic power than nonelectrolytes
• Water moves according to osmotic gradients
• The major electrolytes include Na+, K+, Ca2+, Mg2+, Cl-, SO42-,
bicarbonate and lactate, as well as few other organic anions and the trace
elements
• The major electrolytes occur primarily as free ions; the trace elements
occur primarily in combination with proteins
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Electrolyte Balance
• Electrolytes are important for:
• Neuromuscular excitability
• Secretory activity
• Membrane permeability
• Controlling fluid movements
• Fluid and electrolytes balance is central to the management of any patient
that is ill
•
Measurement of serum sod, pot, chloride and bicarbonate is the most
commonly requested biochemical profile
• The dietary requirements for electrolytes vary widely; some need to be
consumed only in small amounts. Others, such as calcium, potassium and
phosphorus, are excreted continuously and must be ingested regularly to
prevent deficiency
• Salts enter the body by ingestion and are lost via perspiration, feces, and
urine
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Body Water Content: Fluid Compartments
Water occupies two main fluid compartments
• Intracellular fluid (ICF)–about two thirds by volume, contained in
cells (28 L)
• Extracellular fluid (ECF) (14 L)–consists of two major
subdivisions
1. Plasma–the fluid portion of the blood (3.5L)
2. Interstitial fluid (IF)–fluid in spaces between cells (10.5L)
•
Ex. A man weight is
70kg
TBW = 70X 0.6 = 42L
ICF = 0.4 X 70= 28 L
ECF = 0.2X 70 = 14 L
Osmolality
•
Body compartments are separated by semipermeable membranes through which water moves freely
•
Osmotic pressure must be the same on both sides of a cell membrane and water moves to keep the
osmolality the same even this water causes cells to shrink or expand in volume
•
The osmolality of ICF is normally the same as the ECF  the two compartments contain isotonic
solution
•
Plasma osmolality = 1.86 ([Na+]) + [urea]/2.8 + [glucose]/18
•
Plasma osmolality = 2 ([Na+]) + [urea]/3 + [glucose]/20
•
Or simply plasma osmolality = 2 ([Na+])
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Extracellular and Intracellular Fluids
• Each fluid compartment of the body has a distinctive pattern of
electrolytes
• Extracellular fluids are similar (except for the high protein content
of plasma)
• Sodium is the chief cation
• Chloride is the major anion
• Intracellular fluids have low sodium and chloride
• Potassium is the chief cation
• Phosphate is the chief anion
• Sodium and potassium concentrations in extra- and intracellular
fluids are nearly opposites
• This reflects the activity of cellular ATP-dependent sodiumpotassium pumps
Electrolyte Composition of Body Fluids
Figure 26.2
Water Balance and ECF Osmolality
•
To remain properly hydrated, water intake must equal water output
•
Water intake sources
•
Ingested fluid (60%) and solid food (30%)
•
Metabolic water or water of oxidation (10%)
•
•
Water output
•
Urine (60%) and feces (4%)
•
Insensible losses (Skin and lung) (28%), sweat (8%)
•
Obligatory water losses include:
1.
Insensible water losses from lungs and skin
2.
Water that accompanies undigested food residues in feces
Increases in plasma osmolality trigger thirst and release of antidiuretic
hormone (ADH)
What Determine Your Need for Water?
•
Amount needed to
give the proper
osmotic
concentration
•
Amount needed to
replace water lost
excretion
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Water Intake and Output
• Under normal conditions , the
amounts of water taken into the
body and lost from it are equal over
a period of time.
• Water is obtained from the diet
and oxidative metabolism and is
lost through the kidneys, skin,
lungs and gut.
• The minimum volume of urine
necessary for normal excretion of
waste products is about 500 ml/24
hr but, as a result of obligatory
losses by other routes, the
minimum daily water intake
necessary for the maintenance of
water balance is approximately
1100 ml.
• To survive,
multicellular
organisms must
maintain their ECF
volume.
• Humans deprived of
fluids die after a few
days from circulatory
collapse as a result of
the reduction in the
total body water.
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Water Intake and Output
 Water intake largely depends on social
habits and is very variable. Some people
drink less than half a liter each day, and
others may drink more than 5 L in 24
hours without harm

Water losses are equally variable and are
normally seen as changes in the volume
of urine produced. The kidneys can
respond quickly to meet the body's need
to get rid of water. The urine flow rate can
vary widely in a very short time
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Water and ECF osmolality
Changes in body water content will alter the osmolality:
• Loss of water from the ECF  increase its osmolality  movement of water from the ICF
to the ECF.
• A Slight increase in ECF osmolality  stimulating the hypothalamus thirst centre, which
promotes a desire to drink, and the hypothalamic osmoreceptors, which causes the release of
Arginine vasopressin (AVP) (antidiuretic hormone or ADH).
• Vasopressin (ADH) makes the renal collecting ducts permeable to water  water reabsorption and concentration of the urine
• If the ECF osmolality falls, there is no sensation of thirst and vasopressin secretion is
inhibited. A dilute urine is produced  water loss and restoration of the ECF osmolality
• Other stimuli affecting vasopressin secretion include angiotensin II, arterial and venous
baroreceptors and volume receptors. If there is a decrease in plasma volume of more than
10%, hypovolaemia becomes a powerful stimulus to vasopressin release
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Regulation of Water Intake: Thirst Mechanism
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Influence and Regulation of ADH
• Water reabsorption in collecting ducts is proportional to
ADH release
• Low ADH levels produce dilute urine and reduced volume
of body fluids
• High ADH levels produce concentrated urine
• Hypothalamic osmoreceptors trigger or inhibit ADH
release
• Factors that specifically trigger ADH release include
prolonged fever; excessive sweating, vomiting, or
diarrhea; severe blood loss; and traumatic burns
Disorders of Water Balance: Dehydration
• Water loss exceeds water intake and the body is in
negative fluid balance
• Causes include: hemorrhage, severe burns, prolonged
vomiting or diarrhea, profuse sweating, water
deprivation, and diuretic abuse
• Signs and symptoms: cottonmouth, thirst, dry flushed
skin, and oliguria
• Prolonged dehydration may lead to weight loss, fever,
and mental confusion
• Other consequences include hypovolemic shock and
loss of electrolytes
Disorders of Water Balance: Dehydration
1 Excessive loss of H2O from
ECF
(a) Mechanism of dehydration
2 ECF osmotic
pressure rises
3 Cells lose H2O
to ECF by
osmosis; cells
shrink
Disorders of Water Balance: Hypotonic
Hydration
• Renal insufficiency or an extraordinary amount of water
ingested quickly can lead to cellular overhydration, or
water intoxication
• ECF is diluted–sodium content is normal but excess
water is present
• The resulting hyponatremia promotes net osmosis into
tissue cells, causing swelling
• These events must be quickly reversed to prevent severe
metabolic disturbances, particularly in neurons
Disorders of Water Balance:
Hypotonic Hydration
1 Excessive H2O enters
the ECF
(b) Mechanism of hypotonic hydration
2 ECF osmotic
pressure falls
3 H2O moves into
cells by osmosis;
cells swell
Overhydration
• Symptoms
• Weight gain & edema
• Cough, dyspnea [fluid congestion in lungs]
• bounding pulse, neck vein engorgement [fluid excess in
the vascular system]
•  Hg and Hct
• Nausea & vomiting
• Management
• Restrict fluids to lower fluid volume
• Diuretics or hypertonic saline
• Continuous assessments to prevent skin breakdown
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