Body water and Osmolality


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A Chemical Pathology lecture containing basic knowledge about Body water and Osmolality

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Body water and Osmolality

  1. 1. BODY WATER & OSMOLALITY Ola H. Elgaddar MBChB, MSc, MD, CPHQ, LSSGB Lecturer of Chemical Pathology Medical Research Institute Alexandria University Page 1
  2. 2. ILOs: After this lecture, you should be able to:  Understand the volume and distribution of body water among different body compartments.  Explain the reasons for composition differences of body fluids.  Understand Gibbs – Donnan Euilibrium. Page 2
  3. 3. ILOs: After this lecture, you should be able to:  Explain what osmotic pressure is.  Understand the colligative properties of a solution.  Calculate teh osmolality of a solution.  Know the different methods used in measuring osmolality  Recognize the significance of Osmolal gap. Page 3
  4. 4. BODY WATER Page 4
  5. 5. Total Body Water (TBW) • During gestation~ 90 % of fetal body wt • Decreases gradually • Adult male ~ 65 % of body weight • Adult females ~ 55 % of body weigth Page 5
  6. 6. Starling forces  Interstitial fluid is an ultrafiltrate from plasma and both are separated by caillary endothelial lining which acts as a semipermeable membrane. Page 8
  7. 7. Semipermeable membrane  A membrane that allows certain types of molecules to pass through but blocks others, based on characteristics such as the molecules size, chemistry, solubility, or other specific properties. Page 10
  8. 8. Starling forces  Starling forces formula demonstrates that the net movement of fluid across a capillary membrane is a function of membrane permeability and differences in hydrostatic and oncotic pressure on the two sides of the membrane. Page 13
  9. 9. Hydrostatic Pressure The pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. Page 14
  10. 10. Oncotic pressure (Colloid osmotic pressure) A form of osmotic pressure exerted by proteins in a blood vessel, that usually tends to pull water into the circulatory system. It is the opposing force to hydrostatic pressure. Page 15
  11. 11. Osmotic Pressure ??? Page 17
  12. 12. Electrolytes composition among Different body compartements The composition of ICF differs markedly from that of ECF because of the separation of these compartements by the cell membrane Page 19
  13. 13. Electrolytes composition among Different body compartements The composition differences are a consequence of both Gibbs-Donnan Equilibrium and active transport of ions. Page 21
  14. 14. Gibbs-Donnan Equilibrium when a semipermeable membrane separates nondiffusible substance (ex:protein), from diffusible substances (ex: electrolytes), the diffusible substances are distributed on the two sides of the membrane so that 1) the products of their concentrations are equal, and 2) the sum of the diffusible and non-diffusible anions on either side of the membrane is equal to the sum of the concentrations of diffusible and non-diffusible cations; the unequal distribution of diffusible ions thus produced creates a potential difference across Page 22 the membrane (membrane potential).
  15. 15. Active ions transportation The sodium-potassium pump, also known as the Na, K-ATPase, is a critical protein found in the membranes of all animal cells. It functions in the active transport of sodium and potassium ions across the cell membrane against their concentration gradients. For each ATP the pump breaks down, two potassium ions are transported into the cell Page 24 and three sodium ions out of the cell
  16. 16. OSMOLALITY Page 26
  17. 17. Osmotic pressure and osmosis  Osmotic pressure governs the movement of solvents (water) across membranes that separate two solutions.  Different membranes vary in pore size and shape (e.g: glomerular and capillary vessels) They are permeable to water, small molecules, and ions, but not permeable to macromolecules e.g. proteins. Page 27
  18. 18. Osmotic pressure and osmosis Differences in concentration of molecule that cannot cross membranes will cause those that can cross to move, thus establishing an osmotic equilibrium. This movement of solute and permeable ions exerts what is known as osmotic pressure Page 29
  19. 19. Osmotic pressure and osmosis Osmosis is the process that constitutes the movement of solvent across a membrane is response to differences in osmotic pressure across the 2 sides of the membrane.  Water migrates across the membrane toward the side containg more concentrated solute. Page 31
  20. 20. Osmolality and Osmolarity  Osmolality is a physical property of a solution that is based on the number of particles of the solute relative to mass of the solvent (expressed as mmols) / kg of solvent (w/w). (? Molality) Page 34
  21. 21. Osmolality and Osmolarity  Osmolarity is the no of particles of solute per liter of the solution, its units of measurement is mosmol/Liter or mmol/Liter. (? Molar conc.) Page 35
  22. 22. Osmolality and Osmolarity Which is the more exact expression; Osmolality or Osmolarity? Which has higher osmolality; Nacl or glucose? Page 36
  23. 23. Colligative properties of solutions When a solute is added to a solvent the following occurs: Increased osmotic pressure. Lowered vapour pressure. Increased boiling point. Decreased freezing point. Page 37
  24. 24. Colligative properties of solutions  Colligative properties are all directly related to the total number of solute particles per mass of solvent. 1 osmolal solution is defined to contain 1 osmol/k.g H2O. Page 38
  25. 25. Colligative properties of solutions  An electrolyte in solution dissociates into 2 (e.g. NaCl) or 3 (CaCl2) particles. The colligative effects of such solutions are multiplied by the no of dissociated ions formed/molecule.  Incomplete dissociation?? Page 39
  26. 26. Plasma and Urine Osmolality Clinical significance:  Assessment of acid-base disorders  Assessment of electrolyte disorders e.g. in diabetes inspidus or in syndrome of inappropriate secretion of anti-diuretic hormone (SIADH). Page 40
  27. 27. Calculation of Plasma Osmolality: mOsm/Kg = 1.86 [Na+(mmol/L)]+Glucose +urea (mmol/L)+9 (mmol/L) 1.86= Na+ and Cl- (incomplete dissociation) 9: The contribution of other osmotically active substances in plasma such as K+, Ca2+, and proteins. Page 42
  28. 28. Calculation of Plasma Osmolality: Ref values for osmolality: Plasma osmolality = 275-300 mosm/Kg Urine (24- hours) = 300-900 mosm/Kg N.B: Urine osmolality cannot be calculated Page 43
  29. 29. Measuring Plasma Osmolality: Comparison of measured osmolality to the calculated value can help identify the presence of an osmolal gap which can be important in determining the presence of exogenous osmotic substances that can lead to acid-base disturbances, ex: ethanol intoxication Page 44
  30. 30. Measuring Plasma Osmolality: The methods for determining osmolality are based on the collegative properties which are properties of a solution related to the number of molecules of solute per kilogram of solvent, such as changes in freezing point and vapor pressure. Page 45
  31. 31. Measuring Plasma Osmolality:  An increase in osmolality decreases the freezing point temperature and the vapor pressure.  Measurment of freezing point depression and vapor pressure decrease (Dew point) are the 2 mostly used methods of analysis  Freezing point depression is better?? Page 46
  32. 32. Freezing point depression osmometer: (Cryoscope) The components of an osmometer: 1. A thermostatically controlled cooling bath maintained at -7°C. 2. Stirring rod to initiate freezing of the sample. 3. Thermistor probe connected to a circuit to measure the temp of the sample. 4. Galvanometer that displays the freezing curve. 5. Potentiometer with direct read out. Page 47