Module 3: Fluids and Electrolytes Flashcards
Total Body Water
- ICF = 2/3, ECF = 1/3
- ECF: Interstitial Fluid = 3/4, plasma 1/4
Total Body Water Percentages average male
- 70 kg male
- Total Body Water = 60%
- ICF= 40%
- ECF = 20%
- Interstitial fluid = 15%
- Plasma = 5%
weight conversion lbs to kg
1lb = 0.45kg
Total Body Water Estimates for average female
- Total Body Water = 50% body weight
- ICF = 33%
- ECF = 17%
- Interstitial Fluid = 12.75%
- Plasma = 4.25%
Body Water Across the Ages
- newborn infants: risk due to low body fat, high metabolism, increased SA for evaporation
- All infants: risk due to low body weight, immature renal conservation of fluids
- Older adults: risk due to declining renal function, increased water loss through skin, decreased thirst
Osmolality of the blood
~300mOsm/kg
Isotonic
0.9%NaCl
no change in cells
Aquaporins
allow water to move freely between ECF and ICF
- increase in number in distal convoluted tubule due to ADH
What will happen to NET filtration if capillary permability increases?
- decreased venous reabsorption and increased arterial filtration
- loss of proteins from the blood will decrease capillary oncotic pressure and reduce reabsorption of fluid to the capillary
- it will also increase oncotic pressure of the interstitium and increase filtration
Reabsorption
fluid re-enters blood/lymph in interstitium
Net Filtration Equation
- Net filtration = [OUT] - [IN]
- Net = [HPc + OPi] - [OPc + HPi]
- there is always a net 1% left behind that gets reabsorbed into the lympatic system
- Positive number? Filtration, more fluid entering the interstitium
- Negative Number? Reabsorption into capillaries
Filtration
Hydrostatic vs Oncotic Pressure
- Hydrostatic pressure: fluid pushed away from the source
- Oncotic Pressure: pressure of proteins and solutes that draw fluid toward them
- **these forces are also called Starling Forces**
Dependent Edema
fluid accumulaties in gravity dependent areas
- standing = feet and legs
- supine = sacrum, buttock
Swelling and fluid accumulation can:
- limit nutrients, oxygen, and waste diffusion
- limit blood flow = ischemia
- cause pain due to increased pressure on nerves
**Most Common Mechanisms of Edema**
- increased capillary hydrostatic pressure
- increased capillary membrane permeability
- decreased capillary oncotic pressure
- lymphatic obstruction
Increased capillary hydrostatic pressure
Common mechanism of edema
- venous obstruction, salt and water retention, heart failure
- causes: fluid movement into the tissues
Increasesd capillary membrane permeability
Common mechanism of edema
- burns, inflammation
- causes: loss of plasma proteins to interstitial space
Decreased capillary oncotic pressure
common mechanism of edema
- decreased syntehsis of plasma proteins (cirrhosis, malnutrition)
- increased loss of plasma proteins (nephrotic syndrome)
- increased plasma Na+ and H2O retention (dilution of plasma proteins)
Polydypsia
excessive thirst
- common in diabetes mellitus
- certain brain pathologies but psychogenic mechanism is unclear
Thirst is triggered by:
- high serum osmolality
- sensed by peripheral and central osmoreceptors
- Low blood pressure
- sensed by peripheral baroreceptors
- hormone and medication effects
- i.e. Angiotensin II
Natriuretic Peptides
Natriuretic = urine excretion of sodium
- secreted by myocardial cells in atrai and ventricles
- ex. during volume overload and heart failure
- ANP and BNP
ANP and BNP
Natriuretic Peptides
- secreted by the myocardial cells in atria and ventricles
- Actions within the kidney:
- inhibit secretion of renin, angiotensin II, aldosterone, and ADH
- increases GFR by vasodilating the afferent arterioles
- Result:
- increased urine formation
- decreased blood volume
- decreased blood pressure
Normal Function of Sodium
- 90% of cations in ECF are Na+
- determines osmolality of ECF
- osmotic balance
- electrical graidents
- acid/base role
- cellular reactions
- transport mechanisms
Normal Range of Sodium
135-145 mEq/L
Causes of Hyponatremia
- Low Sodium
- water intoxication/dilution
- sodium loss
- inadequate intake
What happens to the fluid compartments of hyponatremia
- gain intracellular fluid , cells swell
- water leaves the ECF, hypovolemia
Causes of Hypernatremia
- loss of water
- gain of sodium
What happens to the fluid compartments during hypernatremia?
loss of intracellular fluid, cells shrink
- water moves into the ECF –> hypervolemia
Normal Function of Potassium
K+ is the major intracellular electrolyte (high in cells, low in body)
- intracellular osmolality
- cells resting electrical potential
- H+ and Na+ levels
- Metabolic functions
- Glycogen deposition in the liver and skeletal muscle
Causes of hypokalemia
- insulin related K+ cellular uptake
- indaquate intake (rare but can happen in poor diet, alcoholism, anorexia nervosa)
- redistribution of K+ due to acid/base imbalance (H/K shift)
- GI and renal loss due to increased fluid flow
Hypokalemia and cell excitability
- low K+ in the ECF = more K+ gradient
- resting membrane potential is hyperpolarized and less excitable
Normal Levels of Potassium
3.5 -5.0 mEq/L
Causes of Hyperkalemia
High levels of potassium
- excessive intake (rare: ingestion of salt substitutes)
- redistribution of K+ due to acid base
- decreased renal excretion, renal failure
- cellular trauma, expecially trauma to the muscles that releases potassium from inside the cell
- insulin deficit especially when coupled with high intake of K+
- insulin usually helps to bring K+ into the cells
Hyperkalemia and cell excitability
high ECF K+ = less gradient
- hyperexcitability followed by lack of depolarization
- loss of “reset” membrane potential
Why do we see fatigue with both high and low K+?
- Low K+ = difficult to generate action potentials
- High K+ = difficult to reset to get new action potentials
Regulation of Potassium
- Cellular uptake of K+ (into ICF out of ECF)
- insulin
- aldosterone
- beta-adrenergic stimulation
- alkalosis (high blood pH, blood is too basic)
- Cellular release of K+ (from ICF to ECF)
- alpha-adrenergic stimulation
- acidosis
- Excretion:
- urine
- stool
- sweat
Functions of Calcium
- bone and tooth structure
- co-factor for enzymes
- cellular signaling pathway
- plasma membrane stability
- neurotransmitter release
- muscle contraction
Mechanisms of Hypocalcemia
- inadequate dietary intake
- removal of parathyroid gland, low PTH
- vitamind D deficiency
- bone metastasis (increased Ca2+ deposition)
- pancreatitis (free fatty acids bind calcium)
- high pH, alkalosis (less free Ca2+)
- low albumin –> hypoalbuminemia
- end-stage renal disease
Mechanisms of Hypercalcemia
- high PTH, PTH secreting tumor
- bone metastases from breast, prostate, blood, cervical cancers
- excess vitamin D
- prolonged immobilzation (bone breakdown)
- low pH, acidosis (more free Ca2+)
Hypercalcemia and cell excitability
- Calcium has a stabilizing effect on membrane voltage
- higher threshold, decreased excitability
Hypocalcemia and cell excitability
- Calcium is a membrane voltage stabilizer
- decreases the threshold, increases excitability
Parathyroid hormone: Ca2+ and HPO42-
increases calcium, decreases phosphate
Vitamin D: calcium and phosphate
increases calcium, increases phosphate
Calcitonin and Calcium
decreases calcium
Hormonal mechanisms of calcium and phosphate regulation
- PTH: increases calcium, decreases phosphate
- Vitamin D: increases calcium, decreases phosphate
- Calcitonin: decreases calcium
pH and Calcium Regulation
- low pH (aciditic) increases ionization of calcium
- more free Ca2+
- high pH (basic) decreases ionization of calcium
- less free Ca2+
Phosphate as it relates to Calcium regulation
- high levels of phosphate bind calcium (less free Ca2+)
- excretion of phopshate increases free Ca2+
Hyperphosphatemia is caused by:
- low GFR/ renal function
- breakdown of metastatic tumors
- laxative abuse
- low PTH
- **Results in low serum calcium**
Hypophosphatemia is caused by:
- vitamin D deficiency
- high antacid use
- alcohol abuse
- malabsorption syndromes
- respiratory alkalosis
Function and Location of Magnesium
- intracellularly along with K+
- cofactor in many enzymatic reactions
- interacts with calcium and potassium channels
Location and Function of Phosphate
- 85% in the bone
- intracellularly as inorganic phosphate
- part of phospholipids and ATP
- highly related to Calcium
Normal Levels of Calcium in the body
9.0-10.5 mg/dL, ionized 5.5-5.6mg/dL
Normal Levels of Magnesium
1.5-2.5mg/dL
Normal Levels of Phosphate (HPO24-)
2.5-4.5
