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Body Fluid Compartments and Fluid Balance

Body Fluid Compartments and Fluid Balance. Water gain must = water loss. Source of metabolically formed water. Rate of formation of metabolic water is not regulated. Osmolarity of plasma is about 300 mOsm/L

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Body Fluid Compartments and Fluid Balance

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  1. Body Fluid Compartments • and • Fluid Balance

  2. Water gain must = water loss

  3. Source of metabolically formed water Rate of formation of metabolic water is not regulated

  4. Osmolarity of plasma is about 300 mOsm/L • (Measured by a method called “Freezing Point Depression) • TONICITY is Osmolarity in relation to that of plasma. It has the ability to change the VOLUME of a cell • Isotonic solution: same osmolarity as of plasma • Hypertonic solution • Hypotonic solution?

  5. ECF IS ISOTONIC Osmotic equilibrium is maintained rapidly between ICF and ECF • Isotonic fluid gain: e.g Intravenous infusion of isotonic saline (0.9 % NaCl, 5% glucose solution) • Isotonic fluid loss: Hemorrhage • No net fluid shift occur

  6. ECF IS HYPOTONIC What happens ? Water moves into the cells from ECF until osmotic equilibrium is achieved. Pronounced swelling of neurons leads to brain dysfunction Swelling of muscle leads to muscle weakness. Hypertension and edema caused by expansion of plasma volume. What Causes this? Over hydration: Renal failure with inability to excrete diluted urine Excessive drinking (transient) Increase in ADH secretion Water intoxication: condition of over hydration, hypotonicity and cellular swelling from excess water.

  7. ECF IS HYPERTONIC • What happens ? • Water moves out of the cells to ECF until osmotic equilibrium is achieved • Cells shrink as water leaves them • Neurons dehydration causes disturbances in brain function manifested as mental confusion to coma. • Dehydration due to: • Insufficient water intake • Excessive water loss (vomiting or diarrhea) • Diabetes Insipidus (deficiency of ADH) What Causes this? Accumulation of high levels of osmotically active solutes (urea in kidney failure)

  8. Tonicity & RBC

  9. Regulation of body water gain Water gain is regulated mainly by drinking through thirst mechanism Baroreceptor 2% decrease in body mass due to fluid loss causes mild dehydration Rate of formation of metabolic water is not regulated to maintain homeostasis

  10. 27_table_02

  11. 27_table_01

  12. Measurement of fluid volumes in the different body compartments Dye-dilution metod (Indicator-dilution)

  13. Measurements of various body volumes • Total body wateruse of a substance which when inserted in to the blood, will spread evenly through out all the body compartments. • Substances used i. Radio active water • ii. Antipyrine –because it is lipid soluble • Extra cellular volume Substance should disperse easily through water/plasma but NOT enter cells Substances used I Radioactive Na, Inulin These rapidly spread through out extra cellular space30-60 min Extra cellular volume is also called SODIUM space or INULIN space ICF = TBW-Extra cellular volume

  14. Plasma & blood volumes • Plasma Volume Substance must stay INSIDE blood compartment after it is injected in to the blood stream • Evans Blue (Dye) is the best choice • Blood volume = Plasma volume + hematocrit (cell volume) • Then BV = Plasma Volume/ (1-Hct) Given Plasma vol as 3 litres, and Hct as 0.45 BV = 3/(1-0.45) = 3/0.55 = 5.45 L

  15. Measurement of various body fluid compartments

  16. Summary • 1. Osmolarity, its normal value, and osmotic pressure • 2. Hydrostatic pressure • 3. Distribution of body water • 4. Intra and extra cellular fluids and their composition • 5. Tonicity of plasma and learnt what is iso, hypo and hyper tonicity • 6. Measurements of body fluid compartments

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