FLUID BALANCE D. C. Mikulecky

FLUID BALANCE zThe Balance Concept: Input -Output = Storage/Depletion zBody Fluid Compartments zRegulation of fluid balance zRelation between Salt Balance and ECF Volume

The Balance Concept: Input -Output = Storage/Depletion Internal Pool (ECF Concentration) of substance Storage Metabolic Production Metabolic Consumption Inside Body Inputs from environment Excretion

Body Fluid Compartments

BODY FLUID COMPARTMENT COMPOSITION PlasmaInterstitial Fluid Intracellular Fluid

Regulation of fluid balance zECF Volume: Maintains blood pressure zECF Osmolarity: Prevents swelling or shrinking of cells

ECF Volume: Maintains blood pressure - Short Term zA Change in ECF Volume can raise or lower blood pressure zBarorecepter reflexes: Alter Cardiac output and Total Peripheral Resistance leading to a compensatory alteration in blood pressure zFluid automatically shifts between plasma and interstitial fluid with rises or fall in blood pressure

Relation between Salt Balance and ECF Volume zThe total sodium load (quantity of sodium, not its concentration) in the ECF determines the total amount of water that will be osmotically retained zThe total sodium load is determined by the balance relation

The Balance Concept: Input -Output = Storage/Depletion Internal Pool (ECF Concentration) of substance Storage Metabolic Production Metabolic Consumption Inside Body Inputs from environment Excretion

Control of sodium load zControl of amount filtered by regulating GFR (Glomerular Filtration Rate) zControl of sodium reabsorption through the Renin-Angiotensin-Aldosterone system

ECF Osmolarity: Prevents swelling or shrinking of cells zECF hypertonicity is associated with dehydration zECF hypotonicity is associated with overhydration zControl of water balance is by thirst and vasopressin

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

DAILY WATER BALANCE (LITERS) zFLUID INTAKE 1.25 zIN FOOD 1.0 zMETABOLIC 0.35 z INSENSIBLE 0.9 z SWEAT 0.1 z FECES 0.1 z URINE 1.5 TOTAL 2.6 INPUT (ml/day) OUTPUT (ml/day) TOTAL 2.6

WATER DEFICIT zINCREASED OSMOLARITY SENSED BY HYPOTHALAMIC WATER RECEPTORS z FALL IN ECF VOLUME z FALL IN ARTERIAL BLOOD PRESSURE STIMULATION OF HYPOYHALAMIC 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