water and electrolyte balance

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  • Water and Electrolyte Balance

    R. C. GuptaM.D. (Biochemistry)

    Jaipur (Rajasthan), India

  • Water is the most abundant component ofour body

    Need for water is more urgent than that forany other nutrient

    Humans beings can live one month without food but only six days without water



    In adults, water accounts for:

    70% of the total body weight in males

    60% of the total body weight in females


    Water content depends on age:

    Infants: 75%

    Adults: 60-70%

    Elderly: 45%


    Water content differs in different tissues:

    Muscles: 70%

    Adipose tissue: 30%

    Bones: 10%

    Water content is more in muscular

    persons than in obese persons

  • Water:

    Bathes all cells

    Gives shape and form to cells

    Serves as a lubricant

    Is the solvent for all ions and molecules

    Transports materials to and from cells

    Is the medium for all biochemical reactions

  • Latent heat of evaporation

    Specific heat

    Dielectric constant

    Solvent power

    Some properties of water which make it

    an ideal medium for body fluids are its:

    Water has been chosen as the universal

    solvent for all living organisms

  • Solvent power

    Water is an efficient and suitable solvent for most of the solutes present in our


    Some compounds which do not dissolve readily in water can form colloidal


  • Water has a high dielectric constant

    A large number of oppositely charged particles can co-exist in water due to this

    Dielectric constant

  • Specific heat

    Water has a very high specific heat which means that a large amount of heat is

    required to raise the temperature of water

    Due to this, body temperature doesnt rise appreciably when thermal energy is

    released during oxidation of nutrients

  • Latent heat of evaporation

    Water has a high latent heat of evaporation relative to other liquids

    A large amount of thermal energy is required for evaporation of water

    When water evaporates from skin and lungs, a large amount of heat is lost

    This prevents a rise in body temperature

  • Distribution of water

    Compartment Water

    Total water in an

    average man 50 litres

    Water in intra-cellular

    compartment 35 litres

    Water in extra-cellular

    compartment 15 litres

  • Un-exchangeable fluid

    The water present outside the cells is

    known as extra-cellular fluid (ECF)

    The ECF is further distributed into some


    Trans-cellular fluid

    Interstitial fluid


  • Sub-compartment Volume

    3 litresPlasma (vascular compartment)

    Interstitial fluid (in between cells) 7 litres

    Trans-cellular fluid (in cavities) 1 litre

    4 litres

    Un-exchangeable fluid (in bones,

    cartilages, dense connective tissue etc)

  • Osmolality

    Concentration of solutes/particles in fluid, expressed in milliosmol (mosm) per kg

    Determines distribution of water in different compartments

    Water moves from lower to higher osmolality

  • The major osmotically active solutes in

    body fluids are:

    Electrolytes have more osmotic power asthey dissociate into at least two particles

    Non-electrolytes e.g. glucose, lipids etc

    Electrolytes e.g. inorganic salts and proteins

  • Intracellular Interstitial Plasma

    fluid fluid

    CATIONS (mEq/L)

    Sodium 10 137 142

    Potassium 160 5 5

    Magnesium 24 3 3

    Calcium 6 5 5

    Total 200 150 155

    ANIONS (mEq/L)

    Chloride 5 113 100

    Bicarbonate 5 27 27

    Sulphate 15 1 1

    Inorganic phosphate 25 2 2

    Organic phosphates 70

    Organic anions 15 5 5

    Proteins 65 2 20

    Total 200 150 155

  • Effective osmolality of a compartment isdetermined by the solutes restricted tothat compartment

    Effective osmolality of the compartment isalso known as its tonicity

  • Selective distribution of ions in differentcompartments is maintained by specificion channels and ion pumps

    A lot of energy is spent for maintaining thedifferential distribution of ions in differentcompartments

  • Cations

    Sodium is the major cation in extracellularfluid

    Potassium is the major cation in intracellularfluid

    This differential is maintained by Na+, K+-exchanging ATPase


  • Anions

    The major anions in extracellular fluid arechloride and bicarbonate

    The major anions in intracellular fluid arephosphates and proteins

  • Proteins

    Proteins are present in a:

    Fairly high concentration in intracellular fluid

    Smaller but significant concentration in plasma

    Negligible concentration in interstitial fluid

  • Effective osmolality is determined by:

    Sodium and its associated anions in the extracellular fluid

    Potassium and its associated anions in the intracellular fluid

  • The ions and molecules have specificdistribution in the intracellular fluid

    These are vital for the functioning of thecells, and are zealously maintained

  • Changes in osmolality are usually due toshift of salts (mainly sodium)

    When salts shift, water follows salts

  • Shrinkage of cells due to shifting of waterout of the cells can seriously affect thefunctioning of cells

    Swelling of cells due to shifting of waterinto the cells can also seriously affect thefunctioning of cells

  • Hyper-osmolality of extracellular fluiddraws water out of cells into the extra-cellular compartment

    Hypo-osmolality of extracellular fluiddrives water from extracellular compart-ment into the cells

  • Osmolality of plasma is 275-290 mosmol/kg

    A 0.9% solution of NaCl in water hasthe same osmolality (or tonicity) as plasma

    A 5% solution of glucose in water also hasthe same osmolality (or tonicity) as plasma

    These two are said to be isosmotic orisotonic with plasma

  • Oncotic pressure

    Osmotic pressure exerted by proteins iscalled oncotic pressure

    It is also known as colloid osmotic pressure

    The normal oncotic pressure of plasma isabout 25 mm of Hg

  • A decrease in the concentration of proteins inplasma decreases oncotic pressure of plasma

    Water is forced out of capillaries at the arterialend due to greater hydrostatic pressure

    It cannot re-enter at the venous end if the oncoticpressure is less than the hydrostatic pressure

    This will result in oedema

  • Water intake and output

    Water balance of the body depends uponthe relative intake and output of water

    Water is taken in as drinking water and inthe form of food and beverages

    Some water is formed in the body duringoxidative reactions (metabolic water)

  • Metabolic water

    Oxidation of 1 gm of carbohydrate produces 0.60 gm of water

    Oxidation of 1 gm of fat produces 1.07 gm of water

    Oxidation of 1 gm of protein produces 0.41 gm of water

  • In a temperate climate, intake of water is:

    Source Volume

    Drinking water about 1.5 L /day

    Water in food and beverages about 1.0 L /day

    Metabolic water about 0.3 L /day

    Total intake about 2.8 L /day

  • Route Volume

    Urine about 1.5 L /day

    Faeces about 0.1 L /day

    Water vapour in expired air about 0.4 L /day

    Water loss in the form of sweat about 0.8 L /day

    Total output about 2.8 L /day

    Water is lost from the body in the form of:

  • In hot climates, sweat loss is much more

    This is compensated by increased intake ofdrinking water

    If it is not compensated, urine output willdecrease

    However, urine output cannot decreasebelow a certain level

  • Normal excretion of solutes by the kidneysis about 600 milliosmol/day

    Minimum water required to dissolve 600milliosmol solutes is 500 ml

    If urine output is below 500 ml/day,excretion of metabolic waste decreases

    A urine output below 500 ml/day is calledoliguria

  • Regulation of water balance

    Water balance is maintained by:

    The thirst centre in hypothalamus

    Antiduretic hormone of posterior pituitary

    These two receive signals about osmolality of plasma from osmoreceptors located in

    the hypothalamus

  • Osmo-receptors can perceive a change of

    even 1-2% in the osmolality of plasma

    If there is an increase in the osmolality of plasma:

    Thirst centre is stimulated which increases water


    Posterior pituitary secretes anti-

    diuretic hormone which decreases

    urine output

  • ADH secretion begins when the osmolalityof plasma reaches about 285 mosmol/kg

    The thirst centre is stimulated when theosmolality of plasma reaches about 295mosmol/kg

  • When blood circulates through the kidneys,125 ml of glomerular filtrate is formed perminute

    About 180 litres of glomerular filtrate isformed in 24 hours

    Glomerular filtration rate

  • When the filtrate passes through the

    tubules, a large amount of solutes and

    water are absorbed

    The re-absorption can be divided into:

    Obligatory re-absorption

    Facultative re-absorption

    Tubular re-absorption

  • A large amount of solutes is absorbed

    when the filtrate passes through proximal

    convoluted tubules and loop of Henle

    A corresponding amount o