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MLAB 2401: CLINICAL CHEMISTRY

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MLAB 2401: CLINICAL CHEMISTRY

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    1. MLAB 2401: CLINICAL CHEMISTRY WATER BALANCE & ELECTROLYTES Part One 1

    2. Water Balance Water 60% of total body mass Main Compartments Intracellular (ICF) inside cells 2/3 Extracellular (ECF) outside cells 1/3 2

    3. Water Balance 3

    4. More on the ECF… Extracellular Compartment(ECF) Composed of two sub-compartments Interstitial fluid (ISF) Surrounds cells in tissue Intravascular fluid (IVF) Volume of measurable fluid plasma 4

    5. Body Fluid Composition Plasma 55% of total blood volume Analytes measured directly Consists of ions, molecules, proteins Serum 5

    6. Water Balance Ions exist in all of these fluids, but the concentration varies depending on individual ion and compartment The body uses active and passive(diffusion) transport principles to keep water and ion concentration in place 6

    7. Water balance Sodium has a pulling effect on water More Na outside cells than inside, the water is pulled out of cells into the extracellular fluid. Na determines osmotic pressure of extracellular fluid Proteins (especially albumin) inside the capillaries strongly pulls/keeps water inside the vascular system Albumin provides oncotic pressure. By keeping Na & albumin in their place, the body is able to regulate its hydration. When there is a disturbance in osmolality, the body responds by regulating water intake, not by changing electrolyte balance 7

    8. Water Balance & Osmolality Osmolality - Physical property of a solution based on solute concentration Water concentration is regulated by thirst and urine output Thirst and urine production are regulated by plasma osmolality 8

    9. Water Balance & Osmolality Increased osmolality stimulates two responses that regulate water Hypothalamus stimulates the sensation of thirst Posterior pituitary secrets arginine vasopressin hormone (AVP) AVP increases H2O re-absorption by renal collection ducts In both cases, plasma water increases 9

    10. Osmolality Osmolality concentration of solute / kg reported as mOsm / kg another term: Osmolarity - mOsm / L - not often used 10

    11. Osmolality Determination 2 methods or principles to determine osmolality Freezing point depression (the preferred method) Vapor pressure depression Also called ‘dewpoint’ 11

    12. Specimen Collection: Osmolality Serum Urine Plasma not recommended due to osmotically active substances that can be introduced into sample Samples should be free of particulate matter..no turbid samples, must centrifuge 12

    13. Osmolality Calculated osmolality uses glucose, BUN, & Na values Formula: 2 (Na) + glucose/18 + BUN/2.8 = calculated osmolality Osmolal gap Difference between calculated and determined osmolality Formula: Determined Osm/kg-calculated Osm/ kg= osmolal gap Should be less than 10-15 units difference ( 13 ..

    14. Significance Increased osmolality are concerns for Infants Unconscious patients Elderly Decreased osmolality Diabetes insipidus AVP deficiency Because they have little / no water re-absorption, produce 10 – 20 liters of urine per day 14

    15. Reference Range: Osmolality Serum 275-295 mOsm/kg Urine 50-1200 mOsm/kg Osmolal gap <10-15 mOsm/ kg 15

    16. Regulation of Blood Volume Renin-angiotension-aldosterone system Aldosterone stimulates sodium reabsorption and potassium ion secretion Natriuretic peptides Glomerular filtration rate Volume receptors 16

    17. Renin-Angiotensin-Aldosterone System Series of events Body detects decreased blood volume Renin converts angiotensinogen to angiotension I Angiotension I converted to angiotension II by ACE Angiotension II causes Vasoconstriction Secretion of aldosterone Stimulates AVP secretion and thirst Enhances NaCl reabsorption 17 This system is initiated with a decrease in renal blood flow. Renin is secreted near the renal glomeruli inThis system is initiated with a decrease in renal blood flow. Renin is secreted near the renal glomeruli in

    18. References Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry: Techniques, principles, Correlations. Baltimore: Wolters Kluwer Lippincott Williams & Wilkins. http://thejunction.net/2009/04/11/the-how-to-authority-for-donating-blood-plasma/ http://www.nlm.nih.gov/medlineplus/ency/article/002350.htm Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson . 18

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