fluids and electrolytes. body fluid composition water – largest body component –55-65% total...
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FLUIDS AND ELECTROLYTES
Body Fluid Composition
• Water – largest body component– 55-65% total body weight
• Solutes –dissolved in body water
Solutes– Electrolytes
• Cations – positive– Sodium (Na+)
– Potassium (K+)
– Calcium (Ca+2)
• Anions – negative– Chloride (Cl-)
– Bicarbonate (HCO3-)
– Phosphate (HPO4-)
– Non-electrolytes Proteins • Urea
• Glucose
• Oxygen (O2)
• Carbon dioxide (CO2)
Body Maintains Charge and Osmolality
• Body fluids – Electrically neutral
– Osmotically maintained• Specific # solute molecules per volume fluid
• Homeostasis of charge, osmolality maintained by – Ion transport
– Water movement between fluid compartments
– Kidney function
Units of Solute Measurement (A Review)
• MW (molecular weight) = sum of weights of atoms • mEq (milliequivalents) = MW (in mg) / valence
– Valence = # charges of ion• Na+ valence = 1• Cl- valence = 1• Ca+2 valence = 2
– Allows comparisons of charge # of solutes in fluids, without molecular wts
• Important to monitor overall charge of body fluids
• mOsm (milliosms) = # particles in solution– Particles = atoms or molecules, charged or uncharged– Measures concent of overall # of solute particles in fluid
• Important to monitor overall concentrations of body fluids
Fluid Compartments
• ICF = IntraCellular Fluid– Inside cells– 65% total body weight
• ECF = ExtraCellular Fluid– Not inside cells– 35% total body weight– Further divided
ECF – cont’d
– IVF = IntraVascular Fluid• In blood vessels
• 8% total body weight
– ISF = InterStitial Fluid• Bathes cells + lymph
• 25% of total body weight
Movement of Body Fluids (WATER FOLLOWS SALT!)
• Solutes may be in higher concentration on one side of the cell membrane
• Fluid (water) can move to equilibrate concentration and/or charge on both sides of the membrane
Definitions
• Diffusion = net movement of particles (solutes) down concentration gradient to establish equilibrium between two sides of membrane– Passive (no energy needed)– Assisted (energy must be added by the cell)
• Osmosis = diffusion of water – Freely passes through cell membranes.
Movement of Body Fluids – cont’d
• ICF to ECF– Osmolality changes in ICF not rapid
• Cell strives to maintain fluid and ion concentrations
• Cell very dependent on relatively constant water/solute amounts
– BUT if ECF osmolality changes so water moves among compartments, both compartments affected and equilibrated over time
Movement of Body Fluids – cont’d
• IVF to ISF to IVF– Happens constantly due to changes in fluid
pressures and osmotic forces at arterial and venous ends of the capillaries
– Necessary to move oxygen and nutrients toward metabolizing cells, and wastes and carbon dioxide away from metabolizing cells
Movement of Body Fluids – cont’d• Arteriolar end of the capillary:
– Highest fluid pressure force -- Blood Hydrostatic Pressure (BHP)• Pressing out against the capillary walls• Direction toward cells• Encourages movement out • Due to fluid pressures and heart contractions
– Colloid Osmotic Pressure (COP)• Lower pressure at arteriolar end • Pulls inward from capillary wall• Encourages fluid to stay inside • Due to large proteins, cells in capillary (too large to move through capillary
walls, so remain in the capillary), can’t leave the bloodstream– Overall at arteriolar end of capillary, BHP > COP
• Greater force encouraging fluid out of capillary than encouraging fluid to stay inside capillary
• So fluid (from heart; oxygenated; w/ nutrients) encouraged to move toward metabolizing cells
Movement of Body Fluids – cont’d
• At the venous end of the capillary:– BHP decreased
• Fluid “lost” from vessel • Don’t have same high fluid pressure pushing against vessel walls
– COP stays the same• Same amt large proteins, blood cells
– Now rel higher pressure “pulling in” away from cells– Overall at venous end of capillary, COP > BHP
• Greater force encourages fluid into capillary than encourages fluid out of capillary
• Overall, fluid (from around cells; containing wastes and CO2) encouraged to move into capillary
– Now returned to lungs to excrete CO2 and to kidneys to excrete wastes
Movement of Body Fluids – cont’d
• ECF to environment– Fluid intake = fluid output
• Intake is -- water, food, beverages
• Output -- urine, feces, sweat and water vapor
Regulation of Body Water• Works through ADH (AntiDiuretic Hormone)
• If there is – Decreased amount water in the body, or– Increased amount Na+ in the body, or– Increased blood osmolality
• So the blood is too concentrated, or
– Decreased circulating blood volume
• All of these lead to:
ADH Release – cont’d
• Stim’n hypothalamic osmoreceptors, • Release ADH, and
• Stimulation of thirst response– Thirst response increased drinking
• Overall, water volume within the body increases
ADH – cont’d
• ADH works at kidney incr’d permeability of kidney tubules to water incr’d reabsorption of water from kidney
tubules back into blood vasculature• Water WOULD have been released to urine
• So water is conserved, not excreted
ADH – cont’d• Overall:
– Incr’d water consumption, and– Incr’d water conservation, so– Incr’d amt water in body
• Relieves decr’d water, decr’d circulation blood volume
– Decr’d blood osmolality• Relieves increased body Na+, increased blood osmolality
• Note: the various conditions leading to ADH release can all be caused by different dysfunctions or traumas, but are all related physiologically – If either hemorrhage (decr’d blood volume) or sweating (decr’d water in body)
decr’d fluid available decr’d IVF decr’d blood pressure• Compensation: body tries to increase fluids in body
– If endocrine disorder incr’d Na+ incr’d blood osmolality• Compensation: body tries to increase fluids to bring blood osmolality back to
normal range (since body can’t n;ormalize osmolality through solute amount)• So if sodium LOAD has doubled to 290 mEq/L: if fluid VOLUME can double, will
now have 290 mEq/2L, which = 145 mEq/L (normal sodium CONCENTRATION is 145 mg/L)
ADH – cont’d
• Note: conditions ADH release, regardless of cause, related physiologically – Hemorrhage (decr’d blood volume) or sweating (decr’d
water in body) decr’d fluid available decr’d IVF decr’d blood pressure• Compensation: body tries to increase fluids in body
– Endocrine disorder incr’d Na+ incr’d blood osmolality
• Compensation: body tries to increase fluids (body can’t normalize osmolality through solute amount)
• Ex: Na+ LOAD doubled (290 mEq/L); by doubling fluid VOLUME 290 mEq/2L= 145 mEq/L (normal sodium CONCENTRATION is 145 mg/L)
Important Cations Contributing to Body Fluid Osmolality
• Sodium (Na+)– About 90% ECF cations– Normal range = 136-145 mEq/L in ECF
– Pairs with Cl-, HCO3- to neutralize charge
– Low in ICF (~10 mEq/L)– Most impt ion in regulating water balance
Sodium – cont’d
– Regulation in ECF -- Renal tubule reabsorption, modulated by hormones:
• Aldosterone– Rel’d from adrenal gland when body senses decr’d Na+ load
– Works at renal tubule to incr renal tubule reabsorption of Na+
• Renin/angiotensin– Affects aldosterone release (also controls Na+ reabsorption)
• Natriuretic hormone– Works at kidney decr’d renal reabsorption of Na+
Potassium• Major INTRAcellular cation• ICF concentration = 150-160 mEq/L• Lower in ECF (3.5-4.5 mEq/L)
– K+ concentration INSIDE cells approximates Na+ concentration OUTSIDE
– Na+ concentration INSIDE cells approximates K+ concentration OUTSIDE
– Body keeps electrical charge constant in ICF and ECF, but uses diff cations inside/outside cells
• Cell moves two cations differently, uses each differently – Overall cells strive to maintain high K+ inside and high Na+
outside
Potassium – cont’d
• Why keep K+ high inside and Na+ high outside cells?– Resting membrane potential in neurons, muscles
• Na+, K+ move into/out of these cells depolarization action potential
• If imbalanced, can neurological, muscle contraction problems
• REMEMBER: heart is an important muscle!
Potassium – cont’d• K+ also important:
– Regulates fluid, ion balance inside the cell• Sim to Na+ regulation outside the cell
– pH regulation• K+ can move across the cell membrane for H+ when H+ is in
excess (body fluids are too acidic)
• Regulation of body K+ is through the kidney– Aldosterone regulates K+
• If body senses decr’d K+ in plasma aldosterone release• Works at kidney tubule incr’d reabsorption of K+ from tubule
back to blood (similar to Na+ mechanism)
– Insulin increased K+ taken up by cells• So K+ must be monitored in diabetic patients
Isotonic Alterations
• Volume of fluid changes, but numbers and types of electrolytes remain at normal levels (Table 4-5)– Loss volume depletion (hypovolemia); occurs
with• Hemorrhage• Severe wound drainage• Excess sweating• Burns• Third spacing
Hypovolemia – cont’d
– Fluid lost from blood vessels, but remains in body– Receptors in the vessels sensitive to pressure– Interpreted as fluid loss, so decr’d ECF volume
• Decr’d urine output
• Weight loss (through fluid weight)
• Can hypovolemic shock
– Symptoms of hypovolemia• Decr’d blood pressure
• Incr’d heart rate
Hypervolemia
• Excess body fluid; occurs with:– Excessive IV fluids – Overproduction aldosterone
• Why should this lead to hypervolemia?
– Some drugs (ex: cortisol)– With incr’d ECF volume
• Weight gain (fluid weight)
• Diluted urine
• Incr’d blood pressure
• Can also edema
Edema -- ECF Isotonic Volume Excess
• Accum’n isotonic fluid in interstitial space (incr’d ISF)
• Forces that favor incr’d ISF also favor edema:– Incr’d BHP, if
• Chronic hypertension• Venous obstruction• Water retention
– Decr’d COP if• Not enough proteins/cells in the blood:
– Protein synthesis disorders of liver– Blood cell disorders decreased # of blood cells
Edema – cont’d– Increased capillary permeability, which can occur with
• Trauma
• Inflammation
• Decreased lymph drainage with– Blocked lymph node
– Surgical removal of lymph vessels
– REMEMBER: ISF drains into lymph vessels, which returns ISF to the bloodstream
Edema – cont’d
• Clinical– Pitting – Weight gain (water weight)– Neck vein distension– Incr’d blood pressure
• Treatment– Treat underlying conditions (tumor, blood cell
disorder, etc.)– Relieve symptoms
Electrolyte Imbalances (Table 4-6): Sodium
• Hypernatremia (a hypertonic imbalance)– Plasma Na+ > 145 mEq/L– Too much Na+ or too little water– “Tonicity”: # of solute particles in solution
• Hypertonic -- high amt solute
• Hypotonic = dilute
– Characteristics of hypernatremia: movement of water from ICF to ECF, so
– Cells dehydrate – Overall incr’d ECF vol (at expense of the cell vol)
Sodium Imbalances – cont’d
• Hypernatremia – cont’d– Due to
• Admin hypertonic IV sol’ns
• Oversecretion aldosterone
• Loss of pure water
• Long term sweating w/ chronic fever
• Respiratory infection water vapor loss
• Diabetes polyuria
• Insufficient water ingested
Sodium Imbalances – cont’d
• Hypernatremia – cont’d– Clinical
• Thirst
• Lethargy
• Neurological dysfunction (dehydration of brain cells)
– Treatment• Lower serum Na+
• Use isotonic salt-free IV fluid (5% glucose) to replace body water; returns Na+ concentration to normal levels
Sodium Imbalances – cont’d
• Hyponatremia – Overall Na+ decr ECF– Two types
• Depletional: Too little Na+• Dilutional: Too much water
– Causes of each type• Depletional (Na+ lost out of body or insufficient)
– Diuretics– Chronic vomiting– Chronic diarrhea
» Though electrolytes loss – Decr’d aldosterone– Decr’d Na+ intake
Sodium Imbalances – cont’d
• Hyponatremia – cont’d• Dilutional
– Renal dysfunction w/ incr’d hypotonic fluid intake– Excessive sweating increased thirst intake excessive pure water
• Syndrome of Inappropriate ADH (SIADH) impaired renal excretion of water
• Excessive beer drinking (???)– Clinical
• Neurological symptoms– Now improper Na+ concentration outside cell improper
depolarization/action potential/neuron conduction– Seizures possible
– Treatment• Restrict water, or• Administer Na+
Electrolyte Imbalances: Potassium
• Hypokalemia– Characteristics
• Serum K+ < 3.5 mEq/L
• Beware if diabetes:– Insulin plays a role in K+ into the cell
– Ketoacidosis incr’d H+ in ECF (so ECF too highly + charged)
» H+ enters cells
» Body tries to equilibrate + charges by moving K+ out of cells
» Now ECF high in K+ (lost through urine)
» Overall whole body K+ deficit
Potassium Imbalances – cont’d• Hypokalemia – cont’d
– Causes• Decr’d K+ intake (rare)• Incr’d K+ loss
– With chronic diuretics, g.i. disturbance– Acid/base imbalance K+ out of cells into ECF, then lost through
urine
• Clinical– Neuromuscular disorders– Cardiac arrest
• REMEMBER: Na+/K+ gradient for proper action potentials in neurons/muscles
– Treatment• Increase K+ intake• BUT slowly to avoid abrupt Na+/K+ gradient change
Potassium Imbalances – cont’d
• Hypokalemia – cont’d• Clinical
– Neuromuscular disorders– Cardiac arrest– REMEMBER: body maintains Na+/K+ gradient for proper
action potentials in neurons/muscles so proper neuron/muscle function
– Treatment• Increase K+ intake• BUT must be increased slowly to avoid abrupt Na+/K+ gradient
change
Potassium Imbalances – cont’d• Hyperkalemia
– Serum K+ > 5.5 mEq/L
– Importance/causes/when to check• Renal disease (kidney regulates K+)
• Massive cellular trauma– High intracellular K+ is released into ECF
• Insulin deficiency– Insulin plays a role in K+ uptake into cells
• Addison’s disease altered aldosterone secretion
• Decr’d blood pH (high blood acidity) H+ into cells K+ out of cells ECF High K+ in ECF
– BUT over time K+ will be lost to urine overall decrease in body K+
Potassium Imbalances – cont’d
• Hyperkalemia – cont’d– Clinical
• Muscle weakness, paralysis
• Change in ECG pattern
– Treatment• Insulin + glucose K+ taken into the cells (out of
ECF)
• Bicarbonate – buffers H+ ions if hyperkalemia due to acidosis
• Ca+2 counteracts K+ effects on heart