finals ia - water and electrolyte balance

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  • Water Electrolyte Balance

    Ricardo R. Santos, M.D.

  • Normal daily intake and output of water (in ml/day)


    Fluid ingested 2100

    From metabolism 200

    Total intake 2300


    Insensible skin 350

    Insensible lungs 350

    Sweat 100

    Feces 100

    Urine 1400

    Total output 2300

  • Sources of water

    1. Preformed water

    *Water found in food and drink

    2. Metabolic water

    *Water produced through the catabolic breakdown of nutrients during cellular


  • Body Fluid Compartments

    Extracellular Fluid - 14.0 L (20%)

    Plasma - 3.0 L Interstitial fluid 11.0 L

    Intracellular fluid 28.0 L (40%)

    Transcellular fluid 1 to 2 L

  • Other fluids

    1. Lymph

    2. Transcellular fluids

    - Cerebrospinal fluid

    - Gastrointestinal fluids (digestive juices)

    - Synovial fluid

    - Eye fluids (aqueous & vitreous humors)

    - Ear fluids (perilymph & endolymph)

    - Pleural, pericardial and peritoneal fluids

  • General Considerations

    In a 70-kg man, total body water is 60% of body weight or 42 L.

    As person grows older %age of body fluid gradually decreases.

    Increase in fat decreases body water.

    Women have less %age of body water than men

  • Obligatory H20 reabsorption

    Facultative H20 reabsorption

    Site in the nephron


    DCT and CD

    Hormone involved


    ADH (vasopressin)

    High plasma osmolarity

    no effect


    Urine volume depends decreased

    Types of Water Reabsorption in the Kidney

  • The mechanisms for the regulation of body fluids are centered in the hypothalamus.

    The regulation of body fluid volume and extracellular osmolarity is under the control of ADH and aldosterone.

    Primary factors that trigger release of ADH:


    2.Baroreceptors (pressure receptors)

    Secondary factors: stress, pain, hypoxia, K

  • Thirst response

    Connected to the response of the osmoreceptors.

    - increased plasma osmolarity stimulates osmosreceptors which in turn stimulates sensation of thirst

    Thirst center is in the hypothalamus.

    Other factors involved:

    1. degree of dryness of mucosal linings of mouth and pharynx

    2. stretch receptors in the GIT

  • Major Electrolytes

    Extracellular Intracellular

    (mEq/L) (mEq/L)

    Cations sodium potassium

    calcium magnesium

    Anions chloride phosphate

    bicarbonate proteins

  • Sodium

    Normal plasma level = 135 145 mEq/L

    Predominant cation in the ECF

    Plays a crucial role in the excitability of muscles & neurones

    Important in regulating fluid balance

    Sodium regulation at the cellular level is controlled by the Na-K pump

    Body levels of Na (retention/excretion) are controlled by aldosterone

    Aldosterone is controlled by renin-angiotensin system

  • Secretion, Transport and Metabolism of Aldosterone

    Renin-angiotensin system - involved in regulation of blood pressure and electrolyte metabolism - primary substance in this process is angiotensin II

    Angiotensinogen Angiotensin I renin converting enzyme

    Angiotensin II


    Angiotensin III


    Degradation products

  • Potassium

    Normal plasma level = 3.5 5.5 mEq/L

    Critical for electrical conduction of nerve impulses - particularly cardiac electrical conduction

    Major cation in the ICF

    Potassium balance at the cellular level is maintained by the Na-K pump

    Kidney can excrete K and in exchange for Na controlled by aldosterone

    Body is much more sensitive to small changes in serum K levels than to small changes in other serum electrolytes

  • Calcium

    Normal level: 4.0 5.5 mEq/L or 8.5 10 mg/dl

    Required for normal skeletal, cardiac and smooth muscle contraction

    Needed for blood clotting

    Intestinal absorption of dietary calcium requires vitamin D

    Calcium metabolism is regulated by parathyroid hormone and vitamin D

    Calcitonin from the thyroid gland causes ECF levels of calcium levels to decrease by inhibition of bone resorption, inhibits vitamin D absorption and increases renal excretion of calcium.

  • Magnesium

    Normal value: 1.5 2.5 mEq/L

    Needed to prevent overexcitability of muscles

    Has a sedative effect on neuromuscular junction, inhibits acetylcholine release, and diminishes muscle cell excitability

    Acts as a cofactor in enzyme reactions

    Participates in bone and teeth production

  • Ionic composition of plasma and interstitial fluid

    Ionic compositions are similar.

    Protein concentration is higher in the plasma.

    Because of Donnans effect, concentrations of cations is slightly greater in the plasma by 2%.

    Proteins are negatively charge more proteins in the plasma bind more cations.

    Interstitial fluids have more anions than plasma.

  • Basic principles osmosis and osmotic pressure

    Osmosis is the net diffusion of water across a semipermeable membrane from a region of high water conc. to one that has a lower water conc.

    In osmosis, water diffuses from a region of low solute conc. to one that has a high solute concentration.

    Cell membrane is relatively impermeable to most solutes but highly permeable to water.

  • Concentration terms

    Important in order to describe the concentration of solute particles.

    Total number of particles in a solution is measured in terms of osmoles.

    One osmole (osm) is equal to 1 mole (mol) (6.02 x 1023) of solute particles.

    A soln. containing 1 mole of glucose in each liter has a conc. of 1 osm/L.

  • Concentration terms

    A soln with 1 mole of NaCl per liter will have an osmolar conc of 2 osm/L

    Osmole is too large a unit for expressing osmotic activity of solutes in the body.

    Therefore, the term milliosmole (mOsm), which equals 1/1000 osmole, is commonly used.

  • Concentration terms

    Osmolal concentration of soln is called:

    1. Osmolality - when conc is expressed as

    osmoles per kg of water.

    2. Osmolarity when conc is expressed as

    osmoles per liter of solution

    The two terms can be used synonymously

    It is easier to express body fluid quantities in liters of fluid than in kg of water.

  • Concentration Terms

    Osmotic pressure the precise amount of pressure required to prevent the osmosis.

    It is an indirect measurement of the water and solute concentrations of a solution.

    The higher the osmotic conc of a solution, the lower the water conc but the higher the solute conc.

    Osmotic pressure of a solution is directly proportional to the concentration of osmotically active particles in that solution.

  • Concentration Terms

    Total osmolarity of the body fluid compartments is 300 mOsm/L

    80% of total osmolarity of the ECF is due to sodium and chloride.

    Corrected osmolar activity 282 mOsm/L

    Total osmotic pressure 5443 mm Hg

    Impermeant solute one that will not permeate the cell membrane

  • Concentrations Terms

    The terms isotonic, hypotonic and hypertonic refer to whether solutions will cause a change in cell volume.

    Tonicity of solutions depends on the conc of impermeant solutes.

    Isosmotic solutions with an osmolarity the same as the cell.

    Hyperosmotic and hypo-osmotic refers to solutions that have a higher osmolarity or lower osmolarity, respectively compared with normal ECF.

  • Concentrations Terms

    Isotonic solutions 0.9% soln of NaCl or a 5% glucose solution.

    - have osmolarity of 282 mOsm/L

    - cell will not change in volume

    Hypotonic solution < than 0.9% NaCl

    - osmolarity less than normal

    - water will diffuse into the cell (swell)

    Hypertonic solution - > than 0.9% NaCl

    - osmolarity more than normal

    - water will flow out of the cell (shrink)

  • Clinical problems with fluid balance

    Hypotonic dehydration

    Isotonic dehydration

    Hypertonic dehydration

    Hypotonic overhydration

    Isotonic overhydration

    Hypertonic overhydration

  • Hypotonic dehydration

    Hypotonic contraction of ECF

    Fluid has fewer solutes than normal plasma

    Relatively uncommon loss of more solute (usually Na) than water

    Causes fluid to shift from the blood stream into the cells, leading to decreased vascular volume and eventual shock

    Increased cellular swelling

    Cerebral edema causes increased intracranial pressure, headache and confusion

    Seen in heat stroke or exhaustion

  • Isotonic dehydration

    Isotonic contraction of ECF

    Fluid has the same osmolarity as normal plasma

    Most common form of dehydration

    Occurs when fluids and electrolytes are lost in even amounts

    There are no intercellular shifts

  • Isotonic dehydration


    - diuretic therapy

    - excessive vomiting

    - excessive ur