Fluid Balance Flashcards
Fluid balance physiologic concept
Fluid balance is the balance between the amount of fluid in the ICF and the ECF, if more fluid than is necessary is in either compartment, edema, or too much pressure will occur. Fluid balance also pertains to the amount of fluid taken in should equal the amount excreted
Intracellular fluid (ICF)
fluid within the tissue cells. Contain 60% of fluid in the body
Extracellular fluid (ECF)
the interstitial fluid and the plasma
plasma (intravascular fluid)
extracellular fluid that is within the blood vessels
Interstitial fluid
the fluid in spaces between the tissue cells
Normal daily water intake and output (NONE)
Average urine output 1.5 L/day Intake should equal output Intake varies between people Averages: Beverage 1200 mL = urine 1500 Food 1000mL = feces 100 Metabolic 350 mL = sweat 50 skin and lungs 900 Total: 2550 in = 2550 out
Thirst mechanism (NONE)
- HYPOosmolality (low solute concentration) -> the brain downregulates ADH and thirst -> increased renal water excretion and decreased water intake -> dilute urine and normalized plasma Osm
- HYPERosmolality (high solute concentration) -> the brain upregulates ADH and thirst -> decreased renal water excretion and increased water intake -> concentrated urine and normalized plasma Osm
Hypotension or a low intravascular fluid volume are sensed by baroreceptors in the Aorta and Carotid arteries, and cause the release of ADH in the Hypothalamus (upregulates thirst).
Hyperosmolarity is sensed by chemoreceptors in the hypothalamus (release ADH, upregulates thirst).
Third spacing
Fluid in the body is normally limited to:
1st the intracellular space (cytoplasm) and
2nd the extracellular space of blood/lymph vessels and the interstitium.
“Third spacing” is the abnormal movement of substantial amounts of ECF to the non-functional, transcellular areas such as the eye, CNS, pleural and peritoneal cavities (ascites).
Ascites
Peritoneal cavity = filled with ECF
Interstitial edema
second-spacing: excess volume going to areas where there normally is fluid. Both conditions (&ascites), however, are the result of increased hydrostatic pressure on the vascular side of the capillary wall.
Osmosis
Movement of a solvent (H2O) across a membrane, driven by differences in osmolality (low to high)
Osmolality
concentration of a solution in number of particles of a solute per liter
Tonicity
relative account of osmolality which determines the direction of diffusion (osmosis)
Isotonic
concentration of solutes is equal inside and outside the cell
Hypertonic
concentration of solution is greater outside than inside (water leaves cell, cell shrinks)
Hypotonic
concentration of solution is greater inside than outside (water enters the cell, cells swell)
Distribution of salt water in the body and general process of volume homeostasis (NONE)
A small change in sodium can make big differences. Sodium content is related to total fluid volume (evaluated by a PE), sodium concentration is related to osmolality (evaluated by a blood test).
Sodium is the most abundant ion in the ECF, and its balance determines ECF volume in the body: water osmotically follows sodium:
- Hypervolemia reflects sodium (and water) retention, kidneys will excrete more (ADH DOWN).
- Hypovolemia reflects sodium (and water) deficit, kidneys will conserve more (ADH UP).
Sodium
Function: primary extracellular cation. moves water to control BP and total fluid volume
Regulation: excreted or retained by kidneys depending on tonicity. Renal absorption at multiple points
Potassium
Function: primary intracellular cation. Key for cell membrane excitability (electrical conduction)
Regulation: Ingested through food, regulated by aldosterone, acidosis/alkalosis mechanisms. Renal excretion at multiple points
Bicarbonate
Function: primary extracellular anion. Predominant buffer system in body
Regulation: By lungs and kidneys, acid/base balance. Absorbed as H+ in proximal tubule
Chloride
Function: important extracellular anion. Electroneutrality and acid/base
Regulation: By kidneys, coupled with Na absorption (salt NaCl)
Calcium
Function: predominantly inside cells, 99% stored in bones. Cell cycle, membrane excitability, bone formation, coagulation
Regulation: By parathyroid hormone, calcitriol, calcitonin, in Kidney, GI tract, bone. Reabsorption at multiple points
Hydrogen
Function: Low concentration in the body, determines pH (body function)
Regulation: By intracellular pH, aldosterone, and K+ concentration. Some excretion, but mostly is involved in formation of bicarb.
Phosphate
Function: Predominately in bones 86%, less than 1% in ECF. Vital component for ATP, DNA, RNA. Urinary buffer
Regulation: By PTH and calcitriol in GI tract, Kidney, bone (similar to CA++). Affected by ECV. Reabsorption at multiple points
Patient with a fluid/electrolyte disorder (NONE)
- Important to have an understanding of the basic fluid and electrolyte concepts.
- Important to know when to call for help.
- Perform a thorough history and physical exam
- Be aware that patient will usually present with non-specific symptoms
- Obtain a chemistry panel and other lab work or urine sample to determine the severity
- Know how to distinguish a chronic or an acute imbalance that may be life-threatening.
Electrolyte imbalances (4 questions)
see table
