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Disturbances of fluid and electrolyte
balance
LECTURE 1
Homeostasis
- (^) The maintenance of normal volume and normal composition of the extracellular fluid is vital to life.
- (^) Homeostasis: the various physiologic arrangements which serve to restore the normal state, once it has been disturbed - (^) Fluid balance - (^) Electrolyte balance - (^) Osmotic balance - (^) Acid-base balance
Body Fluid Volumes
Compartment (^) Fluid Vol. (l) % Body Fluid % Body Wgt. Total Body Fluid
I ntracellular Fluid (I CF)
Extracellular Fluid (ECF)
Plasma
(20% ECF)
I nterstitial Fluid
(80% ECF)
Lymph & Transcellular Fluid
Neg. Neg. Neg.
General principles
- (^) Osmolarity : number of osmoles per liter of solution
- (^) Osmolality: number of osmoles per kilogram of solvent
- (^) Measurement: depression of freezing point
- (^) Calculation (plasma): 2x(Na+K)+glucose+BUN (mmol/l)
- (^) Tonicity: effective osmolality of a solution relative to plasma
- (^) Colloids: high molecular weight particles (> 20000 D)
- (^) Oncotic pressure (colloid osmotic pressure): the pressure necessery to prevent diffusion of solvent molecules (water) into region in which there is a higher concentration of a colloid to which the membrane is impermeable
Extracellular space Intracellular space Intravasal Interstitial space space Capillary wall Oncotic pressure Colloids Electrolytes Water Cell membrane Osmotic pressure 8
Fluid Na+ K + HCO3- Cl - ICF 12 140 12 4 Plasma 140 4-5 26 112 ISF 145 4. 4 27 117
Composition of fluids (mmol/l)
Fluid Balance
- (^) The main way to regulate body water gain is by adjusting the volume of water intake, mainly by drinking more or less fluid. The thirst center in the hypothalamus governs the urge to drink.
- (^) Although increased amounts of water and solutes are lost through sweating and exhalation during exercise, loss of excess water or excess solutes depends mainly on regulating excretion in the urine.
- (^) The extent of urinary Na (and Cl) loss is the main determinant of body fluid volume, whereas the extent of urinary water loss is the main determinant of body fluid osmolarity.
Water Deficit
- (^) INCREASED OSMOLARITY SENSED BY HYPOTHALAMIC RECEPTORS - FALL IN ECF VOLUME - (^) FALL IN ARTERIAL BLOOD PRESSURE STIMULATION OF HYPOTHALAMIC NEURONS INCREASED THIRST INCREASED WATER INTAKE DECREASED PLASMA OSMOLARITY INCREASED VASOPRESSIN OPEN PORES IN COLLECTING DUCT MORE WATER REABSORBED FALL IN URINE OUTPUT ARTERIOLAR VASOCONSTRICTION DECREASED PLASMA OSMOLARITY
RELIEVES
RELIEVES
Blood pressure and renal handling of
sodium
FALL IN SODIUM LOAD FALL IN ARTERIAL PRESSURE FALL IN GFR FALL IN FILTERED SODIUM INCREASE IN ALDOSTERONE SECRETION INCREASE IN SODIUM ABSORPTION FALL IN EXCRETION OF SODIUM, CHLORIDE, AND FLUID INCREASED CONSERVATION OF SODIUM AND FLUID RELIEVES
- (^) Disturbance of fluid balance (intake≠output)
- Dehydraton
- Overhydration (hyperhydration)
- (^) Disturbance of osmolarity (electrolyte intake≠water intake) 1. Isonatremic (isotonic) 2. Hyponatremic (hypotonic) 3. Hypernatremic (hypertonic)
Disturbances of fluid homeostasis
Diagnosis:
- (^) Physical signs: skin turgor, oedema, mucous membranes, neck veins pulse, level of consciousness, capillary refill, fontanel (children)
- (^) Vital signs: blood pressure; heart rate; (respiratory rate); body temperature; CVP; urine output; serum and urine Na, osmolarity; serum total protein
Dehydration
Isotonic Hypertonic Hypotonic Se Na n + - Se osmolarity n + - Hb + + + Ur + + + Blood volume - - - Thirst mod. increase increased no
Isotonic dehydration
- (^) Intracellular fluid volume remains constant
- (^) External loss: vomiting, diarrhoea, haemorrhage, burning
- (^) Internal loss: ileus, ascites, pleural effusion
- (^) Therapy: volume replacement with isotonic solution
